Greenland ice sheet

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Greenland ice sheet
Grønlands indlandsis Sermersuaq
Type	Ice sheet
Coordinates	76°42′N 41°12′WCoordinates: 76°42′N 41°12′W
Area	1,710,000 km2 (660,000 sq mi)
Length	2,400 km (1,500 mi)
Width	1,100 km (680 mi)
Thickness	2,000–3,000 m (6,600–9,800 ft)

The Greenland ice sheet (Danish: Grønlands indlandsis, Greenlandic: Sermersuaq) is a vast body of ice covering 1,710,000 square kilometres (660,000 sq mi), roughly 79% of the surface of Greenland.

Greenland ice sheet

It is the second largest ice body in the world, after the Antarctic ice sheet. The ice sheet is almost 2,900 kilometres (1,800 mi) long in a north–south direction, and its greatest width is 1,100 kilometres (680 mi) at a latitude of 77°N, near its northern margin. The mean altitude of the ice is 2,135 metres (7,005 ft). The thickness is generally more than 2 km (1.2 mi) and over 3 km (1.9 mi) at its thickest point. In addition to the large ice sheet, isolated glaciers and small ice caps cover between 76,000 and 100,000 square kilometres (29,000 and 39,000 sq mi) around the periphery. If the entire 2,850,000 cubic kilometres (684,000 cu mi) of ice were to melt, it would lead to a global sea level rise of 7.2 m (24 ft). The Greenland Ice Sheet is sometimes referred to under the term inland ice, or its Danish equivalent, indlandsis. It is also sometimes referred to as an ice cap.

General

The presence of ice-rafted sediments in deep-sea cores recovered from northwest Greenland, in the Fram Strait, and south of Greenland indicated the more or less continuous presence of either an ice sheet or ice sheets covering significant parts of Greenland for the last 18 million years. From about 11 million years ago to 10 million years ago, the Greenland Ice Sheet was greatly reduced in size. The Greenland Ice Sheet formed in the middle Miocene by coalescence of ice caps and glaciers. There was an intensification of glaciation during the Late Pliocene. Ice sheet formed in connection to the uplift of the West Greenland and East Greenland uplands. The Western and Eastern Greenland mountains constitute passive continental margins that were uplifted in two phases, 10 and 5 million years ago, in the Miocene epoch. Computer modelling shows that the uplift would have enabled glaciation by producing increased orographic precipitation and cooling the surface temperatures. The oldest known ice in the current ice sheet is as much as 1,000,000 years old.

The weight of the ice has depressed the central area of Greenland; the bedrock surface is near sea level over most of the interior of Greenland, but mountains occur around the periphery, confining the sheet along its margins. If the ice suddenly disappeared, Greenland would most probably appear as an archipelago, at least until isostasy lifted the land surface above sea level once again. The ice surface reaches its greatest altitude on two north–south elongated domes, or ridges. The southern dome reaches almost 3,000 metres (10,000 ft) at latitudes 63°–65°N; the northern dome reaches about 3,290 metres (10,800 ft) at about latitude 72°N (the fourth highest "summit" of Greenland). The crests of both domes are displaced east of the centre line of Greenland. The unconfined ice sheet does not reach the sea along a broad front anywhere in Greenland, so that no large ice shelves occur. The ice margin just reaches the sea, however, in a region of irregular topography in the area of Melville Bay southeast of Thule. Large outlet glaciers, which are restricted tongues of the ice sheet, move through bordering valleys around the periphery of Greenland to calve off into the ocean, producing the numerous icebergs that sometimes occur in North Atlantic shipping lanes. The best known of these outlet glaciers is Jakobshavn Glacier (Greenlandic: Sermeq Kujalleq), which, at its terminus, flows at speeds of 20 to 22 metres or 66 to 72 feet per day.

On the ice sheet, temperatures are generally substantially lower than elsewhere in Greenland. The lowest mean annual temperatures, about −31 °C (−24 °F), occur on the north-central part of the north dome, and temperatures at the crest of the south dome are about −20 °C (−4 °F). On 22 December 1991, a temperature of −69.6 °C (−93.3 °F) was recorded at an automatic weather station near the topographic summit of the Greenland Ice Sheet, making it the lowest temperature ever recorded in the Northern Hemisphere. The record went unnoticed for more than 28 years and was finally recognized in 2020.

Change of the ice sheet

Melting ice during July 2012, images created by NASA show the process in the summer

 

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NASA scientist Eric Rignot provides a narrated tour about Greenland's ice sheet.

The ice sheet as a record of past climates

The ice sheet, consisting of layers of compressed snow from more than 100,000 years, contains in its ice today's most valuable record of past climates. In the past decades, scientists have drilled ice cores up to 4 kilometres (2.5 mi) deep. Scientists have, using those ice cores, obtained information on (proxies for) temperature, ocean volume, precipitation, chemistry and gas composition of the lower atmosphere, volcanic eruptions, solar variability, sea-surface productivity, desert extent and forest fires. This variety of climatic proxies is greater than in any other natural recorder of climate, such as tree rings or sediment layers.

The melting ice sheet

Summary

Many scientists who study the ice ablation in Greenland consider that an increase in temperature of two or three degrees Celsius would result in a complete melting of Greenland's ice and leave Greenland completely submerged in water. Positioned in the Arctic, the Greenland ice sheet is especially vulnerable to climate change. Arctic climate is believed to be now rapidly warming and much larger Arctic shrinkage changes are projected. The Greenland Ice Sheet has experienced record melting in recent years since detailed records have been kept and is likely to contribute substantially to sea level rise as well as to possible changes in ocean circulation in the future. The area of the sheet that experiences melting has been argued to have increased by about 16% between 1979 (when measurements started) and 2002 (most recent data). The area of melting in 2002 broke all previous records. The number of glacial earthquakes at the Helheim Glacier and the northwest Greenland glaciers increased substantially between 1993 and 2005. In 2006, estimated monthly changes in the mass of Greenland's ice sheet suggest that it is melting at a rate of about 239 cubic kilometers (57 cu mi) per year. A more recent study, based on reprocessed and improved data between 2003 and 2008, reports an average trend of 195 cubic kilometers (47 cu mi) per year. These measurements came from the US space agency's GRACE (Gravity Recovery and Climate Experiment) satellite, launched in 2002, as reported by BBC. Using data from two ground-observing satellites, ICESAT and ASTER, a study published in Geophysical Research Letters (September 2008) shows that nearly 75 percent of the loss of Greenland's ice can be traced back to small coastal glaciers.

If the entire 2,850,000 km³ (684,000 cu mi) of ice were to melt, global sea levels would rise 7.2 m (24 ft). Recently, fears have grown that continued climate change will make the Greenland Ice Sheet cross a threshold where long-term melting of the ice sheet is inevitable. Climate models project that local warming in Greenland will be 3 °C (5 °F) to 9 °C (16 °F) during this century. Ice sheet models project that such a warming would initiate the long-term melting of the ice sheet, leading to a complete melting of the ice sheet (over centuries), resulting in a global sea level rise of about 7 metres (23 ft). Such a rise would inundate almost every major coastal city in the world. How fast the melt would eventually occur is a matter of discussion. According to the IPCC 2001 report, such warming would, if kept from rising further after the 21st Century, result in 1 to 5 meter sea level rise over the next millennium due to Greenland ice sheet melting. Some scientists have cautioned that these rates of melting are overly optimistic as they assume a linear, rather than erratic, progression. James E. Hansen has argued that multiple positive feedbacks could lead to nonlinear ice sheet disintegration much faster than claimed by the IPCC. According to a 2007 paper, "we find no evidence of millennial lags between forcing and ice sheet response in paleoclimate data. An ice sheet response time of centuries seems probable, and we cannot rule out large changes on decadal time-scales once wide-scale surface melt is underway."

The melt zone, where summer warmth turns snow and ice into slush and melt ponds of meltwater, has been expanding at an accelerating rate in recent years. When the meltwater seeps down through cracks in the sheet, it accelerates the melting and, in some areas, allows the ice to slide more easily over the bedrock below, speeding its movement to the sea. Besides contributing to global sea level rise, the process adds freshwater to the ocean, which may disturb ocean circulation and thus regional climate. In July 2012, this melt zone extended to 97 percent of the ice cover. Ice cores show that events such as this occur approximately every 150 years on average. The last time a melt this large happened was in 1889. This particular melt may be part of cyclical behavior; however, Lora Koenig, a Goddard glaciologist suggested that "...if we continue to observe melting events like this in upcoming years, it will be worrisome." Global warming is increasing growth of algae on the ice sheet. This darkens the ice causing it to absorb more sunlight and potentially increasing the rate of melting.

Meltwater around Greenland may transport nutrients in both dissolved and particulate phases to the ocean. Measurements of the amount of iron in meltwater from the Greenland ice sheet show that extensive melting of the ice sheet might add an amount of this micronutrient to the Atlantic Ocean equivalent to that added by airborne dust. However much of the particles and iron derived from glaciers around Greenland may be trapped within the extensive fjords that surround the island and, unlike the HNLC Southern ocean where iron is an extensive limiting micronutrient, biological production in the North Atlantic is subject only to very spatially and temporally limited periods of iron limitation. Nonetheless high productivity is observed in the immediate vicinity of major marine terminating glaciers around Greenland and this is attributed to meltwater inputs driving the upwelling of seawater rich in macronutrients.

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  Until 2007, rate of decrease in ice sheet height in cm per year.

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  Modelling results of the sea-level rise under different warming scenarios.

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  Satellite image of dark melt ponds.

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  Albedo change in Greenland

Observation and research since 2010

The cold blob visible on NASA's global mean temperatures for 2015, the warmest year on record up to 2015 (since 1880) – Colors indicate temperature evolution (NASA/NOAA; 20 January 2016).

In a 2013 study published in Nature, 133 researchers analyzed a Greenland ice core from the Eemian interglacial. They concluded that during this geological period, roughly 130,000–115,000 years ago, the GIS (Greenland Ice Sheet) was 8 degrees C warmer than today. This resulted in a thickness decrease of the northwest Greenland ice sheet by 400 ± 250 metres, reaching surface elevations 122,000 years ago of 130 ± 300 metres lower than at present.

Researchers have considered that clouds may enhance Greenland ice sheet melt. A study published in Nature in 2013 found that optically thin liquid-bearing clouds extended this July 2012 extreme melt zone, while a Nature Communications study in 2016 suggests that clouds in general enhance Greenland ice sheet's meltwater runoff by more than 30% due to decreased meltwater refreezing in the firn layer at night.

A 2015 study by climate scientists Michael Mann of Penn State and Stefan Rahmstorf from the Potsdam Institute for Climate Impact Research suggests that the observed cold blob in the North Atlantic during years of temperature records is a sign that the Atlantic Ocean's Meridional overturning circulation (AMOC) may be weakening. They published their findings, and concluded that the AMOC circulation shows exceptional slowdown in the last century, and that Greenland melt is a possible contributor.

In August 2020 scientists reported that melting of the Greenland ice sheet is shown to have passed the point of no return, based on 40 years of satellite data. The switch to a dynamic state of sustained mass loss resulted from widespread retreat in 2000–2005.

 

In August 2020 scientists reported that the Greenland ice sheet lost a record amount of ice during 2019.

A study published in 2016, by researchers from the University of South Florida, Canada and the Netherlands, used GRACE satellite data to estimate freshwater flux from Greenland. They concluded that freshwater runoff is accelerating, and could eventually cause a disruption of AMOC in the future, which would affect Europe and North America.

The United States built a secret nuclear powered base, called Camp Century, in the Greenland ice sheet. In 2016, a group of scientists evaluated the environmental impact and estimated that due to changing weather patterns over the next few decades, melt water could release the nuclear waste, 20,000 liters of chemical waste and 24 million liters of untreated sewage into the environment. However, so far neither US or Denmark has taken responsibility for the clean-up.

A 2018 international study found that the fertilizing effect of meltwater around Greenland is highly sensitive to the glacier grounding line depth it is released at. Retreat of Greenland's large marine-terminating glaciers inland will diminish the fertilizing effect of meltwater- even with further large increases in freshwater discharge volume.

On 13 August 2020, Communications Earth and Environment, a Nature Research journal published a study on "Dynamic ice loss from the Greenland Ice sheet driven by sustained glacier retreat". The situation was described as being past the "point of no return" and attributed to two factors, "increased surface meltwater runoff and ablation of marine-terminating outlet glaciers via calving and submarine melting, termed ice discharge."

On 20 August 2020, scientists reported that the Greenland ice sheet lost a record amount of 532 billion metric tons of ice during 2019, surpassing the old record of 464 billion metric tons in 2012 and returning to high melt rates, and provide explanations for the reduced ice loss in 2017 and 2018.

On 31 August 2020, scientists reported that observed ice-sheet losses in Greenland and Antarctica track worst-case scenarios of the IPCC Fifth Assessment Report's sea-level rise projections.

Melting process since 2000

• Between 2000 and 2001: Northern Greenland's Petermann glacier lost 85 square kilometres (33 sq mi) of floating ice.
• Between 2001 and 2005: Sermeq Kujalleq broke up, losing 93 square kilometres (36 sq mi) and raised awareness worldwide of glacial response to global climate change.
• July 2008: Researchers monitoring daily satellite images discovered that a 28-square-kilometre (11 sq mi) piece of Petermann broke away.
• August 2010: A sheet of ice measuring 260 square kilometres (100 sq mi) broke off from the Petermann Glacier. Researchers from the Canadian Ice Service located the calving from NASA satellite images taken on August 5. The images showed that Petermann lost about one-quarter of its 70 km-long (43 mile) floating ice shelf.
• July 2012: Another large ice sheet twice the area of Manhattan, about 120 square kilometres (46 sq mi), broke away from the Petermann glacier in northern Greenland.
• In 2015, Jakobshavn Glacier calved an iceberg about 4,600 feet (1,400 m) thick with an area of about 5 square miles (13 km²).

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  Satellite measurements of Greenland's ice cover from 1979 to 2009 reveals a trend of increased melting.

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  NASA's MODIS and QuikSCAT satellite data from 2007 were compared to confirm the precision of different melt observations.

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  This narrated animation shows the accumulated change in the elevation of the Greenland ice sheet between 2003 and 2012.

Meltwater creates rivers caused by cryoconite on July 21, 2012

 

Meltwater rivers may flow down into moulins

Two mechanisms have been utilized to explain the change in velocity of the Greenland Ice Sheets outlet glaciers. The first is the enhanced meltwater effect, which relies on additional surface melting, funneled through moulins reaching the glacier base and reducing the friction through a higher basal water pressure. (Not all meltwater is retained in the ice sheet and some moulins drain into the ocean, with varying rapidity.) This idea was observed to be the cause of a brief seasonal acceleration of up to 20% on Sermeq Kujalleq in 1998 and 1999 at Swiss Camp. (The acceleration lasted between two and three months and was less than 10% in 1996 and 1997 for example. They offered a conclusion that the "coupling between surface melting and ice-sheet flow provides a mechanism for rapid, large-scale, dynamic responses of ice sheets to climate warming". Examination of recent rapid supra-glacial lake drainage documented short term velocity changes due to such events, but they had little significance to the annual flow of the large outlet glaciers.

The second mechanism is a force imbalance at the calving front due to thinning causing a substantial non-linear response. In this case an imbalance of forces at the calving front propagates up-glacier. Thinning causes the glacier to be more buoyant, reducing frictional back forces, as the glacier becomes more afloat at the calving front. The reduced friction due to greater buoyancy allows for an increase in velocity. This is akin to letting off the emergency brake a bit. The reduced resistive force at the calving front is then propagated up-glacier via longitudinal extension because of the backforce reduction. For ice streaming sections of large outlet glaciers (in Antarctica as well) there is always water at the base of the glacier that helps lubricate the flow.

If the enhanced meltwater effect is the key, then since meltwater is a seasonal input, velocity would have a seasonal signal and all glaciers would experience this effect. If the force imbalance effect is the key, then the velocity will propagate up-glacier, there will be no seasonal cycle, and the acceleration will be focused on calving glaciers. Helheim Glacier, East Greenland had a stable terminus from the 1970s–2000. In 2001–2005 the glacier retreated 7 km (4.3 mi) and accelerated from 20 to 33 m or 70 to 110 ft/day, while thinning up to 130 meters (430 ft) in the terminus region. Kangerdlugssuaq Glacier, East Greenland had a stable terminus history from 1960 to 2002. The glacier velocity was 13 m or 43 ft/day in the 1990s. In 2004–2005 it accelerated to 36 m or 120 ft/day and thinned by up to 100 m (300 ft) in the lower reach of the glacier. On Sermeq Kujalleq the acceleration began at the calving front and spread up-glacier 20 km (12 mi) in 1997 and up to 55 km (34 mi) inland by 2003. On Helheim the thinning and velocity propagated up-glacier from the calving front. In each case the major outlet glaciers accelerated by at least 50%, much larger than the impact noted due to summer meltwater increase. On each glacier the acceleration was not restricted to the summer, persisting through the winter when surface meltwater is absent.

An examination of 32 outlet glaciers in southeast Greenland indicates that the acceleration is significant only for marine-terminating outlet glaciers—glaciers that calve into the ocean. A 2008 study noted that the thinning of the ice sheet is most pronounced for marine-terminating outlet glaciers. As a result of the above, all concluded that the only plausible sequence of events is that increased thinning of the terminus regions, of marine-terminating outlet glaciers, ungrounded the glacier tongues and subsequently allowed acceleration, retreat and further thinning.

Warmer temperatures in the region have brought increased precipitation to Greenland, and part of the lost mass has been offset by increased snowfall. However, there are only a small number of weather stations on the island, and though satellite data can examine the entire island, it has only been available since the early 1990s, making the study of trends difficult. It has been observed that there is more precipitation where it is warmer, up to 1.5 meters per year on the southeast flank, and less precipitation or none on the 25–80 percent (depending on the time of year) of the island that is cooler.

Rate of change

Arctic Temperature Trend 1981–2007

Several factors determine the net rate of growth or decline. These are

1. Accumulation and melting rates of snow in the central parts
2. Melting of surface snow and ice which then flows into moulins, falls and flows to bedrock, lubricates the base of glaciers, and affects the speed of glacial motion. This flow is implicated in accelerating the speed of glaciers and thus the rate of glacial calving.
3. Melting of ice along the sheet's margins (runoff) and basal hydrology,
4. Iceberg calving into the sea from outlet glaciers also along the sheet's edges

Explanation of accelerated glacial coastward movement and iceberg calving fails to consider another causal factor: increased weight of the central highland ice sheet. As the central ice sheet thickens, which it has for at least seven decades, its greater weight causes more horizontal outward force at the bedrock. This in turn appears to have increased glacial calving at the coasts Visual evidence for increased central highland ice sheet thickness exists in the numerous aircraft that have made forced landings on the icecap since the 1940s. They landed on the surface and later disappeared under the ice. A notable example is the Lockheed P-38F Lightning World War II fighter plane Glacier Girl that was exhumed from 268 feet of ice in 1992 and restored to flying condition after being buried for over 50 years. It was recovered by members of the Greenland Expedition Society after years of searching and excavation, eventually transported to Kentucky and restored to flying condition.

The IPCC Third Assessment Report (2001) estimated the accumulation to 520 ± 26 Gigatonnes of ice per year, runoff and bottom melting to 297±32 Gt/yr and 32±3 Gt/yr, respectively, and iceberg production to 235±33 Gt/yr. On balance, the IPCC estimates −44 ± 53 Gt/yr, which means that the ice sheet may currently be melting. Data from 1996 to 2005 shows that the ice sheet is thinning even faster than supposed by IPCC. According to the study, in 1996 Greenland was losing about 96 km³ or 23.0 cu mi per year in volume from its ice sheet. In 2005, this had increased to about 220 km³ or 52.8 cu mi a year due to rapid thinning near its coasts, while in 2006 it was estimated at 239 km³ (57.3 cu mi) per year. It was estimated that in the year 2007 Greenland ice sheet melting was higher than ever, 592 km³ (142.0 cu mi). Also snowfall was unusually low, which led to unprecedented negative −65 km³ (−15.6 cu mi) Surface Mass Balance. If iceberg calving has happened as an average, Greenland lost 294 Gt of its mass during 2007 (one km³ of ice weighs about 0.9 Gt).

The IPCC Fourth Assessment Report (2007) noted, it is hard to measure the mass balance precisely, but most results indicate accelerating mass loss from Greenland during the 1990s up to 2005. Assessment of the data and techniques suggests a mass balance for the Greenland Ice Sheet ranging between growth of 25 Gt/yr and loss of 60 Gt/yr for 1961 to 2003, loss of 50 to 100 Gt/yr for 1993 to 2003 and loss at even higher rates between 2003 and 2005.

Analysis of gravity data from GRACE satellites indicates that the Greenland ice sheet lost approximately 2900 Gt (0.1% of its total mass) between March 2002 and September 2012. The mean mass loss rate for 2008–2012 was 367 Gt/year.

Glaciologist at work

A study published in 2020 estimated, by combining 26 individual estimates of mass balance derived by tracking changes in Greenland's ice sheet volume, speed and gravity as part of the Ice Sheet Mass Balance Inter-comparison Exercise, that the Greenland Ice Sheet had lost a total of 3,902 gigatons (Gt) of ice between 1992 and 2018. The rate of ice loss has increased over time from 26 ± 27 Gt/year between 1992 and 1997 to 244 ± 28 Gt/year between 2012 and 2017 with a peak mass loss rate of 275 ± 28 Gt/year during the period 2007 and 2012.

A paper on Greenland's temperature record shows that the warmest year on record was 1941 while the warmest decades were the 1930s and 1940s. The data used was from stations on the south and west coasts, most of which did not operate continuously the entire study period.

While Arctic temperatures have generally increased, there is some discussion concerning the temperatures over Greenland. First of all, Arctic temperatures are highly variable, making it difficult to discern clear trends at a local level. Also, until recently, an area in the North Atlantic including southern Greenland was one of the only areas in the World showing cooling rather than warming in recent decades, but this cooling was replaced by strong warming in the period 1979–2005.

Near-Earth supernova

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Near-Earth_supernova 

The Crab Nebula is a pulsar wind nebula associated with the 1054 supernova. It is located about 6,500 light-years from the Earth.

A near-Earth supernova is an explosion resulting from the death of a star that occurs close enough to the Earth (roughly less than 10 to 300 parsecs (30 to 1000 light-years) away) to have noticeable effects on Earth's biosphere.

Historically, each near-Earth supernova explosion has been associated with a global warming of around 3–4 °C (5–7 °F). An estimated 20 supernova explosions have happened within 300 pc of the Earth over the last 11 million years. Type II supernova explosions are expected to occur in active star-forming regions, with 12 such OB associations being located within 650 pc of the Earth. At present, there are six near-Earth supernova candidates within 300 pc.

Effects on Earth

On average, a supernova explosion occurs within 10 parsecs (33 light-years) of the Earth every 240 million years.^[a] Gamma rays are responsible for most of the adverse effects that a supernova can have on a living terrestrial planet. In Earth's case, gamma rays induce radiolysis of diatomic N₂ and O₂ in the upper atmosphere, converting molecular nitrogen and oxygen into nitrogen oxides, depleting the ozone layer enough to expose the surface to harmful solar and cosmic radiation (mainly ultra-violet). Phytoplankton and reef communities would be particularly affected, which could severely deplete the base of the marine food chain.

Odenwald discusses the possible effects of a Betelgeuse supernova on the Earth and on human space travel, especially the effects of the stream of charged particles that would reach the Earth about 100,000 years later than the initial light and other electromagnetic radiation produced by the explosion.

Risk by supernova type

Candidates within 300 pc

Star designation	Distance (pc)	Mass (M☉)
IK Pegasi	46	1.65/1.15
Spica	80	10.25/7.0
Alpha Lupi	141	10.1
Antares	169	12.4/10
Betelgeuse	197	7.7–20
Rigel	264	18

Speculation as to the effects of a nearby supernova on Earth often focuses on large stars as Type II supernova candidates. Several prominent stars within a few hundred light years of the Sun are candidates for becoming supernovae in as little as a millennium. Although they would be spectacular to look at, were these "predictable" supernovae to occur, they are thought to have little potential to affect Earth.

It is estimated that a Type II supernova closer than eight parsecs (26 light-years) would destroy more than half of the Earth's ozone layer. Such estimates are based on atmospheric modeling and the measured radiation flux from SN 1987A, a Type II supernova in the Large Magellanic Cloud. Estimates of the rate of supernova occurrence within 10 parsecs of the Earth vary from 0.05–0.5 per billion years to 10 per billion years. Several studies assume that supernovae are concentrated in the spiral arms of the galaxy, and that supernova explosions near the Sun usually occur during the approximately 10 million years that the Sun takes to pass through one of these regions. Examples of relatively near supernovae are the Vela Supernova Remnant (c. 800 ly, c. 12,000 years ago) and Geminga (c. 550 ly, c. 300,000 years ago).

Type Ia supernovae are thought to be potentially the most dangerous if they occur close enough to the Earth. Because Type Ia supernovae arise from dim, common white dwarf stars, it is likely that a supernova that could affect the Earth will occur unpredictably and take place in a star system that is not well studied. The closest known candidate is IK Pegasi. It is currently estimated, however, that by the time it could become a threat, its velocity in relation to the Solar System would have carried IK Pegasi to a safe distance.

Past events

Evidence from daughter products of short-lived radioactive isotopes shows that a nearby supernova helped determine the composition of the Solar System 4.5 billion years ago, and may even have triggered the formation of this system. Supernova production of heavy elements over astronomic periods of time ultimately made the chemistry of life on Earth possible.

Past supernovae might be detectable on Earth in the form of metal isotope signatures in rock strata. Subsequently, iron-60 enrichment has been reported in deep-sea rock of the Pacific Ocean by researchers from the Technical University of Munich. Twenty-three atoms of this iron isotope were found in the top 2 cm of crust (this layer corresponds to times from 13.4 million years ago to the present). It is estimated that the supernova must have occurred in the last 5 million years or else it would have had to happen very close to the solar system to account for so much iron-60 still being here. A supernova occurring so close would have probably caused a mass extinction, which did not happen in that time frame. The quantity of iron seems to indicate that the supernova was less than 30 parsecs away. On the other hand, the authors estimate the frequency of supernovae at a distance less than D (for reasonably small D) as around (D/10 pc)³ per billion years, which gives a probability of only around 5% for a supernova within 30 pc in the last 5 million years. They point out that the probability may be higher because the Solar System is entering the Orion Arm of the Milky Way. In 2019, the group in Munich found interstellar dust in Antarctic surface snow not older than 20 years which they relate to the Local Interstellar Cloud. The detection of interstellar dust in Antarctica was done by the measurement of the radionuclides Fe-60 and Mn-53 by highly sensitive Accelerator mass spectrometry, where Fe-60 is again the clear signature for a recent near-Earth supernova origin.

Gamma ray bursts from "dangerously close" supernova explosions occur two or more times per billion years, and this has been proposed as the cause of the end Ordovician extinction, which resulted in the death of nearly 60% of the oceanic life on Earth.

In 1998 a supernova remnant, RX J0852.0-4622, was found in front (apparently) of the larger Vela Supernova Remnant. Gamma rays from the decay of titanium-44 (half-life about 60 years) were independently discovered emanating from it, showing that it must have exploded fairly recently (perhaps around the year 1200), but there is no historical record of it. The flux of gamma rays and X-rays indicates that the supernova was relatively close to us (perhaps 200 parsecs or 660 ly). If so, this is an unexpected event because supernovae less than 200 parsecs away are estimated to occur less than once per 100,000 years.

Rare Earth hypothesis

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Rare_Earth_hypothesis 

 

The Rare Earth hypothesis argues that planets with complex life, like Earth, are exceptionally rare

In planetary astronomy and astrobiology, the Rare Earth hypothesis argues that the origin of life and the evolution of biological complexity such as sexually reproducing, multicellular organisms on Earth (and, subsequently, human intelligence) required an improbable combination of astrophysical and geological events and circumstances.

According to the hypothesis, complex extraterrestrial life is an improbable phenomenon and likely to be rare. The term "Rare Earth" originates from Rare Earth: Why Complex Life Is Uncommon in the Universe (2000), a book by Peter Ward, a geologist and paleontologist, and Donald E. Brownlee, an astronomer and astrobiologist, both faculty members at the University of Washington.

In the 1970s and 1980s, Carl Sagan and Frank Drake, among others, argued that Earth is a typical rocky planet in a typical planetary system, located in a non-exceptional region of a common barred-spiral galaxy. From the principle of mediocrity (extended from the Copernican principle), they argued that we are typical, and the universe teems with complex life. However, Ward and Brownlee argue that planets, planetary systems, and galactic regions that are as friendly to complex life as the Earth, the Solar System, and our galactic region are rare.

Requirements for complex life

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Pongola

Huronian

Cryogenian

Andean

Karoo

Quaternary

Ice Ages

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Earliest multicell life

(million years ago)

The Rare Earth hypothesis argues that the evolution of biological complexity requires a host of fortuitous circumstances, such as a galactic habitable zone, a central star and planetary system having the requisite character, the circumstellar habitable zone, a right-sized terrestrial planet, the advantage of a gas giant guardian like Jupiter and a large natural satellite, conditions needed to ensure the planet has a magnetosphere and plate tectonics, the chemistry of the lithosphere, atmosphere, and oceans, the role of "evolutionary pumps" such as massive glaciation and rare bolide impacts, and whatever led to the appearance of the eukaryote cell, sexual reproduction and the Cambrian explosion of animal, plant, and fungi phyla. The evolution of human intelligence may have required yet further events, which are extremely unlikely to have happened were it not for the Cretaceous–Paleogene extinction event 66 million years ago removing dinosaurs as the dominant terrestrial vertebrates.

In order for a small rocky planet to support complex life, Ward and Brownlee argue, the values of several variables must fall within narrow ranges. The universe is so vast that it could contain many Earth-like planets. But if such planets exist, they are likely to be separated from each other by many thousands of light years. Such distances may preclude communication among any intelligent species evolving on such planets, which would solve the Fermi paradox: "If extraterrestrial aliens are common, why aren't they obvious?"

The right location in the right kind of galaxy

Rare Earth suggests that much of the known universe, including large parts of our galaxy, are "dead zones" unable to support complex life. Those parts of a galaxy where complex life is possible make up the galactic habitable zone, primarily characterized by distance from the Galactic Center. As that distance increases:

1. Star metallicity declines. Metals (which in astronomy means all elements other than hydrogen and helium) are necessary to the formation of terrestrial planets.
2. The X-ray and gamma ray radiation from the black hole at the galactic center, and from nearby neutron stars, becomes less intense. Thus the early universe, and present-day galactic regions where stellar density is high and supernovae are common, will be dead zones.
3. Gravitational perturbation of planets and planetesimals by nearby stars becomes less likely as the density of stars decreases. Hence the further a planet lies from the Galactic Center or a spiral arm, the less likely it is to be struck by a large bolide which could extinguish all complex life on a planet.

Dense centers of galaxies such as NGC 7331 (often referred to as a "twin" of the Milky Way^[3]) have high radiation levels toxic to complex life.

 

According to Rare Earth, globular clusters are unlikely to support life.

Item #1 rules out the outer reaches of a galaxy; #2 and #3 rule out galactic inner regions. Hence a galaxy's habitable zone may be a ring sandwiched between its uninhabitable center and outer reaches.

Also, a habitable planetary system must maintain its favorable location long enough for complex life to evolve. A star with an eccentric (elliptic or hyperbolic) galactic orbit will pass through some spiral arms, unfavorable regions of high star density; thus a life-bearing star must have a galactic orbit that is nearly circular, with a close synchronization between the orbital velocity of the star and of the spiral arms. This further restricts the galactic habitable zone within a fairly narrow range of distances from the Galactic Center. Lineweaver et al. calculate this zone to be a ring 7 to 9 kiloparsecs in radius, including no more than 10% of the stars in the Milky Way, about 20 to 40 billion stars. Gonzalez, et al. would halve these numbers; they estimate that at most 5% of stars in the Milky Way fall in the galactic habitable zone.

Approximately 77% of observed galaxies are spiral, two-thirds of all spiral galaxies are barred, and more than half, like the Milky Way, exhibit multiple arms. According to Rare Earth, our own galaxy is unusually quiet and dim (see below), representing just 7% of its kind. Even so, this would still represent more than 200 billion galaxies in the known universe.

Our galaxy also appears unusually favorable in suffering fewer collisions with other galaxies over the last 10 billion years, which can cause more supernovae and other disturbances. Also, the Milky Way's central black hole seems to have neither too much nor too little activity.

The orbit of the Sun around the center of the Milky Way is indeed almost perfectly circular, with a period of 226 Ma (million years), closely matching the rotational period of the galaxy. However, the majority of stars in barred spiral galaxies populate the spiral arms rather than the halo and tend to move in gravitationally aligned orbits, so there is little that is unusual about the Sun's orbit. While the Rare Earth hypothesis predicts that the Sun should rarely, if ever, have passed through a spiral arm since its formation, astronomer Karen Masters has calculated that the orbit of the Sun takes it through a major spiral arm approximately every 100 million years. Some researchers have suggested that several mass extinctions do correspond with previous crossings of the spiral arms.

Orbiting at the right distance from the right type of star

According to the hypothesis, Earth has an improbable orbit in the very narrow habitable zone (dark green) around the Sun.

The terrestrial example suggests that complex life requires liquid water, requiring an orbital distance neither too close nor too far from the central star, another scale of habitable zone or Goldilocks Principle: The habitable zone varies with the star's type and age.

For advanced life, the star must also be highly stable, which is typical of middle star life, about 4.6 billion years old. Proper metallicity and size are also important to stability. The Sun has a low 0.1% luminosity variation. To date no solar twin star, with an exact match of the sun's luminosity variation, has been found, though some come close. The star must have no stellar companions, as in binary systems, which would disrupt the orbits of planets. Estimates suggest 50% or more of all star systems are binary. The habitable zone for a main sequence star very gradually moves out over its lifespan until it becomes a white dwarf and the habitable zone vanishes.

The liquid water and other gases available in the habitable zone bring the benefit of greenhouse warming. Even though the Earth's atmosphere contains a water vapor concentration from 0% (in arid regions) to 4% (in rain forest and ocean regions) and – as of February 2018 – only 408.05 parts per million of CO
2, these small amounts suffice to raise the average surface temperature by about 40 °C, with the dominant contribution being due to water vapor, which together with clouds makes up between 66% and 85% of Earth's greenhouse effect, with CO
2 contributing between 9% and 26% of the effect.

Rocky planets must orbit within the habitable zone for life to form. Although the habitable zone of such hot stars as Sirius or Vega is wide, hot stars also emit much more ultraviolet radiation that ionizes any planetary atmosphere. They may become red giants before advanced life evolves on their planets. These considerations rule out the massive and powerful stars of type F6 to O (see stellar classification) as homes to evolved metazoan life.

Small red dwarf stars conversely have small habitable zones wherein planets are in tidal lock, with one very hot side always facing the star and another very cold side; and they are also at increased risk of solar flares (see Aurelia). Life probably cannot arise in such systems. Rare Earth proponents claim that only stars from F7 to K1 types are hospitable. Such stars are rare: G type stars such as the Sun (between the hotter F and cooler K) comprise only 9% of the hydrogen-burning stars in the Milky Way.

Such aged stars as red giants and white dwarfs are also unlikely to support life. Red giants are common in globular clusters and elliptical galaxies. White dwarfs are mostly dying stars that have already completed their red giant phase. Stars that become red giants expand into or overheat the habitable zones of their youth and middle age (though theoretically planets at a much greater distance may become habitable).

An energy output that varies with the lifetime of the star will likely prevent life (e.g., as Cepheid variables). A sudden decrease, even if brief, may freeze the water of orbiting planets, and a significant increase may evaporate it and cause a greenhouse effect that prevents the oceans from reforming.

All known life requires the complex chemistry of metallic elements. The absorption spectrum of a star reveals the presence of metals within, and studies of stellar spectra reveal that many, perhaps most, stars are poor in metals. Because heavy metals originate in supernova explosions, metallicity increases in the universe over time. Low metallicity characterizes the early universe: globular clusters and other stars that formed when the universe was young, stars in most galaxies other than large spirals, and stars in the outer regions of all galaxies. Metal-rich central stars capable of supporting complex life are therefore believed to be most common in the quiet suburbs of the larger spiral galaxies—where radiation also happens to be weak.

The right arrangement of planets

Depiction of the Sun and planets of the Solar System and the sequence of planets. Rare Earth argues that without such an arrangement, in particular the presence of the massive gas giant Jupiter (fifth planet from the Sun and the largest), complex life on Earth would not have arisen.

Rare Earth proponents argue that a planetary system capable of sustaining complex life must be structured more or less like the Solar System, with small and rocky inner planets and outer gas giants. Without the protection of 'celestial vacuum cleaner' planets with strong gravitational pull, a planet would be subject to more catastrophic asteroid collisions.

Observations of exo-planets have shown that arrangements of planets similar to the Solar System are rare. Most planetary systems have super Earths, several times larger than Earth, close to their star, whereas the Solar System's inner region has only a few small rocky planets and none inside Mercury's orbit. Only 10% of stars have giant planets similar to Jupiter and Saturn, and those few rarely have stable nearly circular orbits distant from their star. Konstantin Batygin and colleagues argue that these features can be explained if, early in the history of the Solar System, Jupiter and Saturn drifted towards the Sun, sending showers of planetesimals towards the super-Earths which sent them spiralling into the Sun, and ferrying icy building blocks into the terrestrial region of the Solar System which provided the building blocks for the rocky planets. The two giant planets then drifted out again to their present position. However, in the view of Batygin and his colleagues: "The concatenation of chance events required for this delicate choreography suggest that small, Earth-like rocky planets – and perhaps life itself – could be rare throughout the cosmos."

A continuously stable orbit

Rare Earth argues that a gas giant must not be too close to a body where life is developing. Close placement of gas giant(s) could disrupt the orbit of a potential life-bearing planet, either directly or by drifting into the habitable zone.

Newtonian dynamics can produce chaotic planetary orbits, especially in a system having large planets at high orbital eccentricity.

The need for stable orbits rules out stars with systems of planets that contain large planets with orbits close to the host star (called "hot Jupiters"). It is believed that hot Jupiters have migrated inwards to their current orbits. In the process, they would have catastrophically disrupted the orbits of any planets in the habitable zone. To exacerbate matters, hot Jupiters are much more common orbiting F and G class stars.

A terrestrial planet of the right size

Planets of the Solar System to scale. Rare Earth argues that complex life cannot exist on large gaseous planets like Jupiter and Saturn (top row) or Uranus and Neptune (top middle) or smaller planets such as Mars and Mercury

It is argued that life requires terrestrial planets like Earth and as gas giants lack such a surface, that complex life cannot arise there.

A planet that is too small cannot hold much atmosphere, making surface temperature low and variable and oceans impossible. A small planet will also tend to have a rough surface, with large mountains and deep canyons. The core will cool faster, and plate tectonics may be brief or entirely absent. A planet that is too large will retain too dense an atmosphere like Venus. Although Venus is similar in size and mass to Earth, its surface atmospheric pressure is 92 times that of Earth, and surface temperature of 735 K (462 °C; 863 °F). Earth had a similar early atmosphere to Venus, but may have lost it in the giant impact event which formed the Moon.

With plate tectonics

The Great American Interchange on Earth, around ~ 3.5 to 3 Ma, an example of species competition, resulting from continental plate interaction

 

An artist's rendering of the structure of Earth's magnetic field-magnetosphere that protects Earth's life from solar radiation. 1) Bow shock. 2) Magnetosheath. 3) Magnetopause. 4) Magnetosphere. 5) Northern tail lobe. 6) Southern tail lobe. 7) Plasmasphere.

Rare Earth proponents argue that plate tectonics and a strong magnetic field are essential for biodiversity, global temperature regulation, and the carbon cycle. The lack of mountain chains elsewhere in the Solar System is direct evidence that Earth is the only body with plate tectonics, and thus the only nearby body capable of supporting life.

Plate tectonics depend on the right chemical composition and a long-lasting source of heat from radioactive decay. Continents must be made of less dense felsic rocks that "float" on underlying denser mafic rock. Taylor emphasizes that tectonic subduction zones require the lubrication of oceans of water. Plate tectonics also provides a means of biochemical cycling.

Plate tectonics and as a result continental drift and the creation of separate land masses would create diversified ecosystems and biodiversity, one of the strongest defences against extinction. An example of species diversification and later competition on Earth's continents is the Great American Interchange. North and Middle America drifted into South America at around 3.5 to 3 Ma. The fauna of South America evolved separately for about 30 million years, since Antarctica separated. Many species were subsequently wiped out in mainly South America by competing Northern American animals.

A large moon

Tide pools resulting from tidal interaction of the Moon are said to have promoted the evolution of complex life.

The Moon is unusual because the other rocky planets in the Solar System either have no satellites (Mercury and Venus), or only tiny satellites which are probably captured asteroids (Mars).

The Giant-impact theory hypothesizes that the Moon resulted from the impact of a Mars-sized body, dubbed Theia, with the young Earth. This giant impact also gave the Earth its axial tilt (inclination) and velocity of rotation. Rapid rotation reduces the daily variation in temperature and makes photosynthesis viable. The Rare Earth hypothesis further argues that the axial tilt cannot be too large or too small (relative to the orbital plane). A planet with a large tilt will experience extreme seasonal variations in climate. A planet with little or no tilt will lack the stimulus to evolution that climate variation provides. In this view, the Earth's tilt is "just right". The gravity of a large satellite also stabilizes the planet's tilt; without this effect the variation in tilt would be chaotic, probably making complex life forms on land impossible.

If the Earth had no Moon, the ocean tides resulting solely from the Sun's gravity would be only half that of the lunar tides. A large satellite gives rise to tidal pools, which may be essential for the formation of complex life, though this is far from certain.

A large satellite also increases the likelihood of plate tectonics through the effect of tidal forces on the planet's crust. The impact that formed the Moon may also have initiated plate tectonics, without which the continental crust would cover the entire planet, leaving no room for oceanic crust. It is possible that the large scale mantle convection needed to drive plate tectonics could not have emerged in the absence of crustal inhomogeneity. A further theory indicates that such a large moon may also contribute to maintaining a planet's magnetic shield by continually acting upon a metallic planetary core as dynamo, thus protecting the surface of the planet from charged particles and cosmic rays, and helping to ensure the atmosphere is not stripped over time by solar winds.

Earth's atmosphere

Atmosphere

A terrestrial planet of the right size is needed to retain an atmosphere, like Earth and Venus. On Earth, once the giant impact of Theia thinned Earth's atmosphere, other events were needed to make the atmosphere capable of sustaining life. The Late Heavy Bombardment reseeded Earth with water lost after the impact of Theia. The development of an ozone layer formed protection from ultraviolet (UV) sunlight. Nitrogen and carbon dioxide are needed in a correct ratio for life to form. Lightning is needed for nitrogen fixation. The carbon dioxide gas needed for life comes from sources such as volcanoes and geysers. Carbon dioxide is only needed at low levels (currently at 400 ppm); at high levels it is poisonous. Precipitation is needed to have a stable water cycle. A proper atmosphere must reduce diurnal temperature variation.

One or more evolutionary triggers for complex life

This diagram illustrates the twofold cost of sex. If each individual were to contribute to the same number of offspring (two), (a) the sexual population remains the same size each generation, where the (b) asexual population doubles in size each generation

Regardless of whether planets with similar physical attributes to the Earth are rare or not, some argue that life usually remains simple bacteria. Biochemist Nick Lane argues that simple cells (prokaryotes) emerged soon after Earth's formation, but since almost half the planet's life had passed before they evolved into complex ones (eukaryotes) all of whom share a common ancestor, this event can only have happened once. In some views, prokaryotes lack the cellular architecture to evolve into eukaryotes because a bacterium expanded up to eukaryotic proportions would have tens of thousands of times less energy available; two billion years ago, one simple cell incorporated itself into another, multiplied, and evolved into mitochondria that supplied the vast increase in available energy that enabled the evolution of complex life. If this incorporation occurred only once in four billion years or is otherwise unlikely, then life on most planets remains simple. An alternative view is that mitochondria evolution was environmentally triggered, and that mitochondria-containing organisms appeared soon after the first traces of atmospheric oxygen.

The evolution and persistence of sexual reproduction is another mystery in biology. The purpose of sexual reproduction is unclear, as in many organisms it has a 50% cost (fitness disadvantage) in relation to asexual reproduction. Mating types (types of gametes, according to their compatibility) may have arisen as a result of anisogamy (gamete dimorphism), or the male and female sexes may have evolved before anisogamy. It is also unknown why most sexual organisms use a binary mating system, and why some organisms have gamete dimorphism. Charles Darwin was the first to suggest that sexual selection drives speciation; without it, complex life would probably not have evolved.

The right time in evolution

Timeline of evolution; human writings exists for only 0.000218% of Earth's history.

While life on Earth is regarded to have spawned relatively early in the planet's history, the evolution from multicellular to intelligent organisms took around 800 million years. Civilizations on Earth have existed for about 12,000 years and radio communication reaching space has existed for less than 100 years. Relative to the age of the Solar System (~4.57 Ga) this is a short time, in which extreme climatic variations, super volcanoes, and large meteorite impacts were absent. These events would severely harm intelligent life, as well as life in general. For example, the Permian-Triassic mass extinction, caused by widespread and continuous volcanic eruptions in an area the size of Western Europe, led to the extinction of 95% of known species around 251.2 Ma ago. About 65 million years ago, the Chicxulub impact at the Cretaceous–Paleogene boundary (~65.5 Ma) on the Yucatán peninsula in Mexico led to a mass extinction of the most advanced species at that time.

Rare Earth equation

The following discussion is adapted from Cramer. The Rare Earth equation is Ward and Brownlee's riposte to the Drake equation. It calculates ${\displaystyle N}$, the number of Earth-like planets in the Milky Way having complex life forms, as:

According to Rare Earth, the Cambrian explosion that saw extreme diversification of chordata from simple forms like Pikaia (pictured) was an improbable event

${\displaystyle N=N^{*}\cdot n_{e}\cdot f_{g}\cdot f_{p}\cdot f_{pm}\cdot f_{i}\cdot f_{c}\cdot f_{l}\cdot f_{m}\cdot f_{j}\cdot f_{me}}$

where:

• N* is the number of stars in the Milky Way. This number is not well-estimated, because the Milky Way's mass is not well estimated, with little information about the number of small stars. N* is at least 100 billion, and may be as high as 500 billion, if there are many low visibility stars.
• ${\displaystyle n_{e}}$ is the average number of planets in a star's habitable zone. This zone is fairly narrow, because constrained by the requirement that the average planetary temperature be consistent with water remaining liquid throughout the time required for complex life to evolve. Thus ${\displaystyle n_{e}}$=1 is a likely upper bound.

We assume ${\displaystyle N^{*}\cdot n_{e}=5\cdot 10^{11}}$. The Rare Earth hypothesis can then be viewed as asserting that the product of the other nine Rare Earth equation factors listed below, which are all fractions, is no greater than 10⁻¹⁰ and could plausibly be as small as 10⁻¹². In the latter case, ${\displaystyle N}$ could be as small as 0 or 1. Ward and Brownlee do not actually calculate the value of ${\displaystyle N}$, because the numerical values of quite a few of the factors below can only be conjectured. They cannot be estimated simply because we have but one data point: the Earth, a rocky planet orbiting a G2 star in a quiet suburb of a large barred spiral galaxy, and the home of the only intelligent species we know; namely, ourselves.

• ${\displaystyle f_{g}}$ is the fraction of stars in the galactic habitable zone (Ward, Brownlee, and Gonzalez estimate this factor as 0.1).
• ${\displaystyle f_{p}}$ is the fraction of stars in the Milky Way with planets.
• ${\displaystyle f_{pm}}$ is the fraction of planets that are rocky ("metallic") rather than gaseous.
• ${\displaystyle f_{i}}$ is the fraction of habitable planets where microbial life arises. Ward and Brownlee believe this fraction is unlikely to be small.
• ${\displaystyle f_{c}}$ is the fraction of planets where complex life evolves. For 80% of the time since microbial life first appeared on the Earth, there was only bacterial life. Hence Ward and Brownlee argue that this fraction may be small.
• ${\displaystyle f_{l}}$ is the fraction of the total lifespan of a planet during which complex life is present. Complex life cannot endure indefinitely, because the energy put out by the sort of star that allows complex life to emerge gradually rises, and the central star eventually becomes a red giant, engulfing all planets in the planetary habitable zone. Also, given enough time, a catastrophic extinction of all complex life becomes ever more likely.
• ${\displaystyle f_{m}}$ is the fraction of habitable planets with a large moon. If the giant impact theory of the Moon's origin is correct, this fraction is small.
• ${\displaystyle f_{j}}$ is the fraction of planetary systems with large Jovian planets. This fraction could be large.
• ${\displaystyle f_{me}}$ is the fraction of planets with a sufficiently low number of extinction events. Ward and Brownlee argue that the low number of such events the Earth has experienced since the Cambrian explosion may be unusual, in which case this fraction would be small.

The Rare Earth equation, unlike the Drake equation, does not factor the probability that complex life evolves into intelligent life that discovers technology. Barrow and Tipler review the consensus among such biologists that the evolutionary path from primitive Cambrian chordates, e.g., Pikaia to Homo sapiens, was a highly improbable event. For example, the large brains of humans have marked adaptive disadvantages, requiring as they do an expensive metabolism, a long gestation period, and a childhood lasting more than 25% of the average total life span. Other improbable features of humans include:

• Being one of a handful of extant bipedal land (non-avian) vertebrate. Combined with an unusual eye–hand coordination, this permits dextrous manipulations of the physical environment with the hands;
• A vocal apparatus far more expressive than that of any other mammal, enabling speech. Speech makes it possible for humans to interact cooperatively, to share knowledge, and to acquire a culture;
• The capability of formulating abstractions to a degree permitting the invention of mathematics, and the discovery of science and technology. Only recently did humans acquire anything like their current scientific and technological sophistication.

Advocates

Writers who support the Rare Earth hypothesis:

• Stuart Ross Taylor, a specialist on the Solar System, firmly believes in the hypothesis. Taylor concludes that the Solar System is probably unusual, because it resulted from so many chance factors and events.
• Stephen Webb, a physicist, mainly presents and rejects candidate solutions for the Fermi paradox. The Rare Earth hypothesis emerges as one of the few solutions left standing by the end of the book
• Simon Conway Morris, a paleontologist, endorses the Rare Earth hypothesis in chapter 5 of his Life's Solution: Inevitable Humans in a Lonely Universe, and cites Ward and Brownlee's book with approval.
• John D. Barrow and Frank J. Tipler (1986. 3.2, 8.7, 9), cosmologists, vigorously defend the hypothesis that humans are likely to be the only intelligent life in the Milky Way, and perhaps the entire universe. But this hypothesis is not central to their book The Anthropic Cosmological Principle, a thorough study of the anthropic principle and of how the laws of physics are peculiarly suited to enable the emergence of complexity in nature.
• Ray Kurzweil, a computer pioneer and self-proclaimed Singularitarian, argues in The Singularity Is Near that the coming Singularity requires that Earth be the first planet on which sapient, technology-using life evolved. Although other Earth-like planets could exist, Earth must be the most evolutionarily advanced, because otherwise we would have seen evidence that another culture had experienced the Singularity and expanded to harness the full computational capacity of the physical universe.
• John Gribbin, a prolific science writer, defends the hypothesis in Alone in the Universe: Why our planet is unique.
• Guillermo Gonzalez, astrophysicist who supports the concept of galactic habitable zone uses the hypothesis in his book The Privileged Planet to promote the concept of intelligent design.
• Michael H. Hart, astrophysicist who proposed a narrow habitable zone based on climate studies, edited the influential book Extraterrestrials: Where are They and authored one of its chapters "Atmospheric Evolution, the Drake Equation and DNA: Sparse Life in an Infinite Universe".
• Howard Alan Smith, astrophysicist and author of 'Let there be light: modern cosmology and Kabbalah: a new conversation between science and religion'.
• Marc J. Defant, professor of geochemistry and volcanology, elaborated on several aspects of the rare earth hypothesis in his TEDx talk entitled: Why We are Alone in the Galaxy.
• Brian Cox, physicist and popular science celebrity confesses his support for the hypothesis in his BBC production of the Human Universe.

Criticism

Cases against the Rare Earth hypothesis take various forms.

The hypothesis appears anthropocentric

The hypothesis concludes, more or less, that complex life is rare because it can evolve only on the surface of an Earth-like planet or on a suitable satellite of a planet. Some biologists, such as Jack Cohen, believe this assumption too restrictive and unimaginative; they see it as a form of circular reasoning.

According to David Darling, the Rare Earth hypothesis is neither hypothesis nor prediction, but merely a description of how life arose on Earth. In his view, Ward and Brownlee have done nothing more than select the factors that best suit their case.

What matters is not whether there's anything unusual about the Earth; there's going to be something idiosyncratic about every planet in space. What matters is whether any of Earth's circumstances are not only unusual but also essential for complex life. So far we've seen nothing to suggest there is.

Critics also argue that there is a link between the Rare Earth hypothesis and the unscientific idea of intelligent design.

Exoplanets around main sequence stars are being discovered in large numbers

An increasing number of extrasolar planet discoveries are being made with 4,704 planets in 3,478 planetary systems known as of 1 April 2021. Rare Earth proponents argue life cannot arise outside Sun-like systems, due to tidal locking and ionizing radiation outside the F7–K1 range. However, some exobiologists have suggested that stars outside this range may give rise to life under the right circumstances; this possibility is a central point of contention to the theory because these late-K and M category stars make up about 82% of all hydrogen-burning stars.

Current technology limits the testing of important Rare Earth criteria: surface water, tectonic plates, a large moon and biosignatures are currently undetectable. Though planets the size of Earth are difficult to detect and classify, scientists now think that rocky planets are common around Sun-like stars. The Earth Similarity Index (ESI) of mass, radius and temperature provides a means of measurement, but falls short of the full Rare Earth criteria.

Rocky planets orbiting within habitable zones may not be rare

Planets similar to Earth in size are being found in relatively large number in the habitable zones of similar stars. The 2015 infographic depicts Kepler-62e, Kepler-62f, Kepler-186f, Kepler-296e, Kepler-296f, Kepler-438b, Kepler-440b, Kepler-442b, Kepler-452b.

Some argue that Rare Earth's estimates of rocky planets in habitable zones (${\displaystyle n_{e}}$ in the Rare Earth equation) are too restrictive. James Kasting cites the Titius-Bode law to contend that it is a misnomer to describe habitable zones as narrow when there is a 50% chance of at least one planet orbiting within one. In 2013, astronomers using the Kepler space telescope's data estimated that about one-fifth of G-type and K-type stars (sun-like stars and orange dwarves) are expected to have an Earth-sized or super-Earth-sized planet (1–2 Earths wide) close to an Earth-like orbit (0.25–4 F_⊕), yielding about 8.8 billion of them for the entire Milky Way Galaxy.

Uncertainty over Jupiter's role

The requirement for a system to have a Jovian planet as protector (Rare Earth equation factor ${\displaystyle f_{j}}$) has been challenged, affecting the number of proposed extinction events (Rare Earth equation factor ${\displaystyle f_{me}}$). Kasting's 2001 review of Rare Earth questions whether a Jupiter protector has any bearing on the incidence of complex life. Computer modelling including the 2005 Nice model and 2007 Nice 2 model yield inconclusive results in relation to Jupiter's gravitational influence and impacts on the inner planets. A study by Horner and Jones (2008) using computer simulation found that while the total effect on all orbital bodies within the Solar System is unclear, Jupiter has caused more impacts on Earth than it has prevented. Lexell's Comet, a 1770 near miss that passed closer to Earth than any other comet in recorded history, was known to be caused by the gravitational influence of Jupiter. Grazier (2017) claims that the idea of Jupiter as a shield is a misinterpretation of a 1996 study by George Wetherill, and using computer models Grazier was able to demonstrate that Saturn protects Earth from more asteroids and comets than does Jupiter.

Plate tectonics may not be unique to Earth or a requirement for complex life

Geological discoveries like the active features of Pluto's Tombaugh Regio appear to contradict the argument that geologically active worlds like Earth are rare.

Ward and Brownlee argue that for complex life to evolve (Rare Earth equation factor ${\displaystyle f_{c}}$), tectonics must be present to generate biogeochemical cycles, and predicted that such geological features would not be found outside of Earth, pointing to a lack of observable mountain ranges and subduction. There is, however, no scientific consensus on the evolution of plate tectonics on Earth. Though it is believed that tectonic motion first began around three billion years ago, by this time photosynthesis and oxygenation had already begun. Furthermore, recent studies point to plate tectonics as an episodic planetary phenomenon, and that life may evolve during periods of "stagnant-lid" rather than plate tectonic states.

Recent evidence also points to similar activity either having occurred or continuing to occur elsewhere. The geology of Pluto, for example, described by Ward and Brownlee as "without mountains or volcanoes ... devoid of volcanic activity", has since been found to be quite the contrary, with a geologically active surface possessing organic molecules and mountain ranges like Tenzing Montes and Hillary Montes comparable in relative size to those of Earth, and observations suggest the involvement of endogenic processes. Plate tectonics has been suggested as a hypothesis for the Martian dichotomy, and in 2012 geologist An Yin put forward evidence for active plate tectonics on Mars. Europa has long been suspected to have plate tectonics and in 2014 NASA announced evidence of active subduction. In 2017, scientists studying the geology of Charon confirmed that icy plate tectonics also operated on Pluto's largest moon.

Kasting suggests that there is nothing unusual about the occurrence of plate tectonics in large rocky planets and liquid water on the surface as most should generate internal heat even without the assistance of radioactive elements. Studies by Valencia and Cowan suggest that plate tectonics may be inevitable for terrestrial planets Earth sized or larger, that is, Super-Earths, which are now known to be more common in planetary systems.

Free oxygen may be neither rare nor a prerequisite for multicellular life

Animals in the genus Spinoloricus are thought to defy the paradigm that all animal life on earth needs oxygen

The hypothesis that molecular oxygen, necessary for animal life, is rare and that a Great Oxygenation Event (Rare Earth equation factor ${\displaystyle f_{c}}$) could only have been triggered and sustained by tectonics, appears to have been invalidated by more recent discoveries.

Ward and Brownlee ask "whether oxygenation, and hence the rise of animals, would ever have occurred on a world where there were no continents to erode". Extraterrestrial free oxygen has recently been detected around other solid objects, including Mercury, Venus, Mars, Jupiter's four Galilean moons, Saturn's moons Enceladus, Dione and Rhea and even the atmosphere of a comet. This has led scientists to speculate whether processes other than photosynthesis could be capable of generating an environment rich in free oxygen. Wordsworth (2014) concludes that oxygen generated other than through photodissociation may be likely on Earth-like exoplanets, and could actually lead to false positive detections of life. Narita (2015) suggests photocatalysis by titanium dioxide as a geochemical mechanism for producing oxygen atmospheres.

Since Ward & Brownlee's assertion that "there is irrefutable evidence that oxygen is a necessary ingredient for animal life", anaerobic metazoa have been found that indeed do metabolise without oxygen. Spinoloricus cinziae, for example, a species discovered in the hypersaline anoxic L'Atalante basin at the bottom of the Mediterranean Sea in 2010, appears to metabolise with hydrogen, lacking mitochondria and instead using hydrogenosomes. Studies since 2015 of the eukaryotic genus Monocercomonoides that lack mitochondrial organelles are also significant as there are no detectable signs that mitochondria were ever part of the organism. Since then further eukaryotes, particularly parasites, have been identified to be completely absent of mitochondrial genome, such as the 2020 discovery in Henneguya zschokkei. Further investigation into alternative metabolic pathways used by these organisms appear to present further problems for the premise.

Stevenson (2015) has proposed other membrane alternatives for complex life in worlds without oxygen. In 2017, scientists from the NASA Astrobiology Institute discovered the necessary chemical preconditions for the formation of azotosomes on Saturn's moon Titan, a world that lacks atmospheric oxygen. Independent studies by Schirrmeister and by Mills concluded that Earth's multicellular life existed prior to the Great Oxygenation Event, not as a consequence of it.

NASA scientists Hartman and McKay argue that plate tectonics may in fact slow the rise of oxygenation (and thus stymie complex life rather than promote it). Computer modelling by Tilman Spohn in 2014 found that plate tectonics on Earth may have arisen from the effects of complex life's emergence, rather than the other way around as the Rare Earth might suggest. The action of lichens on rock may have contributed to the formation of subduction zones in the presence of water. Kasting argues that if oxygenation caused the Cambrian explosion then any planet with oxygen producing photosynthesis should have complex life.

A magnetic field may not be a requirement

The importance of Earth's magnetic field to the development of complex life has been disputed. Kasting argues that the atmosphere provides sufficient protection against cosmic rays even during times of magnetic pole reversal and atmosphere loss by sputtering. Kasting also dismisses the role of the magnetic field in the evolution of eukaryotes, citing the age of the oldest known magnetofossils.

A large moon may be neither rare nor necessary

The requirement of a large moon (Rare Earth equation factor ${\displaystyle f_{m}}$) has also been challenged. Even if it were required, such an occurrence may not be as unique as predicted by the Rare Earth Hypothesis. Recent work by Edward Belbruno and J. Richard Gott of Princeton University suggests that giant impactors such as those that may have formed the Moon can indeed form in planetary trojan points (L₄ or L₅ Lagrangian point) which means that similar circumstances may occur in other planetary systems.

Collision between two planetary bodies (artist concept).

The assertion that the Moon's stabilization of Earth's obliquity and spin is a requirement for complex life has been questioned. Kasting argues that a moonless Earth would still possess habitats with climates suitable for complex life and questions whether the spin rate of a moonless Earth can be predicted. Although the giant impact theory posits that the impact forming the Moon increased Earth's rotational speed to make a day about 5 hours long, the Moon has slowly "stolen" much of this speed to reduce Earth's solar day since then to about 24 hours and continues to do so: in 100 million years Earth's solar day will be roughly 24 hours 38 minutes (the same as Mars's solar day); in 1 billion years, 30 hours 23 minutes. Larger secondary bodies would exert proportionally larger tidal forces that would in turn decelerate their primaries faster and potentially increase the solar day of a planet in all other respects like Earth to over 120 hours within a few billion years. This long solar day would make effective heat dissipation for organisms in the tropics and subtropics extremely difficult in a similar manner to tidal locking to a red dwarf star. Short days (high rotation speed) cause high wind speeds at ground level. Long days (slow rotation speed) cause the day and night temperatures to be too extreme.

Many Rare Earth proponents argue that the Earth's plate tectonics would probably not exist if not for the tidal forces of the Moon. The hypothesis that the Moon's tidal influence initiated or sustained Earth's plate tectonics remains unproven, though at least one study implies a temporal correlation to the formation of the Moon. Evidence for the past existence of plate tectonics on planets like Mars which may never have had a large moon would counter this argument. Kasting argues that a large moon is not required to initiate plate tectonics.

Complex life may arise in alternative habitats

Complex life may exist in environments similar to black smokers on Earth.

Rare Earth proponents argue that simple life may be common, though complex life requires specific environmental conditions to arise. Critics consider life could arise on a moon of a gas giant, though this is less likely if life requires volcanicity. The moon must have stresses to induce tidal heating, but not so dramatic as seen on Jupiter's Io. However, the moon is within the gas giant's intense radiation belts, sterilizing any biodiversity before it can get established. Dirk Schulze-Makuch disputes this, hypothesizing alternative biochemistries for alien life. While Rare Earth proponents argue that only microbial extremophiles could exist in subsurface habitats beyond Earth, some argue that complex life can also arise in these environments. Examples of extremophile animals such as the Hesiocaeca methanicola, an animal that inhabits ocean floor methane clathrates substances more commonly found in the outer Solar System, the tardigrades which can survive in the vacuum of space or Halicephalobus mephisto which exists in crushing pressure, scorching temperatures and extremely low oxygen levels 3.6 kilometres deep in the Earth's crust, are sometimes cited by critics as complex life capable of thriving in "alien" environments. Jill Tarter counters the classic counterargument that these species adapted to these environments rather than arose in them, by suggesting that we cannot assume conditions for life to emerge which are not actually known. There are suggestions that complex life could arise in sub-surface conditions which may be similar to those where life may have arisen on Earth, such as the tidally heated subsurfaces of Europa or Enceladus. Ancient circumvental ecosystems such as these support complex life on Earth such as Riftia pachyptila that exist completely independent of the surface biosphere.