Near-Earth object

From Wikipedia, the free encyclopedia

Radar-imaging of 2006 DP14

VLT image of the very faint near-Earth asteroid 2009 FD.^[1]

Near-Earth asteroid 433 Eros visited by the space probe NEAR Shoemaker (December 2000)

A near-Earth object (NEO) is a Solar System object whose orbit brings it into proximity with Earth. All NEOs have a closest approach to the Sun (perihelion) of less than 1.3 AU.^[2] They include more than ten thousand near-Earth asteroids (NEAs), near-Earth comets, a number of solar-orbiting spacecraft, and meteoroids large enough to be tracked in space before striking the Earth. It is now widely accepted that collisions in the past have had a significant role in shaping the geological and biological history of the planet.^[3] NEOs have become of increased interest since the 1980s because of increased awareness of the potential danger some of the asteroids or comets pose to Earth, and active mitigations are being researched.^[4]

Those NEOs that are asteroids (NEA) have orbits that lie partly between 0.983 and 1.3 astronomical units away from the Sun.^[5] When an NEA is detected it is submitted to the IAU's Minor Planet Center (located at the Harvard–Smithsonian Center for Astrophysics) for cataloging. Some near-Earth asteroids' orbits intersect that of Earth's so they pose a collision danger.^[6] The United States, European Union, and other nations are currently scanning for NEOs^[7] in an effort called Spaceguard.

In the United States, NASA has a congressional mandate to catalogue all NEOs that are at least 1 kilometer wide, as the impact of such an object would be catastrophic. As of February 2015^[update], there have been 867 near-Earth asteroids larger than 1 km discovered, of which 153 are potentially hazardous asteroids (PHAs).^[8] It was estimated in 2006 that 20% of the mandated objects have not yet been found.^[7] As a result of NEOWISE in 2011, it is estimated that 93% of the NEAs larger than 1 km have been found and that only about 70 remain to be discovered.^[9] Our inventory is much less complete for smaller objects, which still have potential for large scale damage.

Potentially hazardous objects (PHOs) are currently defined based on parameters that measure the object's potential to make threatening close approaches to the Earth.^[10] Mostly objects with an Earth minimum orbit intersection distance (MOID) of 0.05 AU or less and an absolute magnitude (H) of 22.0 or brighter (a rough indicator of large size) are considered PHOs. Objects that cannot approach closer to the Earth (i.e. MOID) than 0.05 AU (7,500,000 km; 4,600,000 mi), or are smaller than about 150 m (500 ft) in diameter (i.e. H = 22.0 with assumed albedo of 13%), are not considered PHOs.^[2] The NASA Near Earth Object Catalog also includes the approach distances of asteroids and comets measured in lunar distances,^[11] and this usage has become a common unit of measure used by the news media in discussing these objects.

Some NEOs are of high interest because they can be physically explored with lower mission velocity even than the Moon, due to their combination of low velocity with respect to Earth (ΔV) and small gravity, so they may present interesting scientific opportunities both for direct geochemical and astronomical investigation, and as potentially economical sources of extraterrestrial materials for human exploitation.^[12] This makes them an attractive target for exploration.^[13] As of 2012, three near-Earth objects have been visited by spacecraft: 433 Eros, by NASA's Near Earth Asteroid Rendezvous probe,^[14] 25143 Itokawa, by the JAXA Hayabusa mission,^[15] and 4179 Toutatis, by CNSA's Chang'e 2 spacecraft.^[4][16]

History of human awareness of NEOs

Asteroid Toutatis from Paranal.

Human perception of near-Earth objects as benign objects of fascination or killer objects with high risk to human society have ebbed and flowed in the short period of human history that NEOS have been scientifically observed.^[17]

Risk

Asteroid 4179 Toutatis is a potentially hazardous object that has passed within 2.3 lunar distances.

More recently, a typical frame of reference for looking at NEOs has been through the scientific concept of risk. In this frame, the risk that any near-Earth object poses is typically seen through a lens that is a function of both the culture and the technology of human society. "NEOs have been understood differently throughout history." Each time an NEO is observed, "a different risk was posed, and throughout time that risk perception has evolved. It is not just a matter of scientific knowledge."^[18]

Such perception of risk is thus "a product of religious belief, philosophic principles, scientific understanding, technological capabilities, and even economical resourcefulness."^[18]

Risk scales

There are two schemes for the scientific classification of impact hazards from NEOs:

• the simple Torino Scale, and
• the more complex Palermo Technical Impact Hazard Scale

The annual background frequency used in the Palermo scale for impacts of energy greater than E megatonnes is estimated as:^[19]

$f_{B}=0.03E^{{-0.8}}\;$

For instance, this formula implies that the expected value of the time from now until the next impact greater than 1 megatonne is 33 years, and that when it occurs, there is a 50% chance that it will be above 2.4 megatonnes. This formula is only valid over a certain range of E.

However, another paper^[20] published in 2002 – the same year as the paper on which the Palermo scale is based – found a power law with different constants:

$f_{B}=0.00737E^{{-0.9}}\;$

This formula gives considerably lower rates for a given E. For instance, it gives the rate for bolides of 10 megatonnes or more (like the Tunguska explosion) as 1 per thousand years, rather than 1 per 210 years as in the Palermo formula. However, the authors give a rather large uncertainty (once in 400 to 1800 years for 10 megatonnes), due in part to uncertainties in determining the energies of the atmospheric impacts that they used in their determination.

Highly rated risks

Plot of orbits of known potentially hazardous asteroids (size over 460 feet (140 m) and passing within 4.7 million miles (7.6×10^⁶ km) of Earth's orbit) as of early 2013 (alternate image).

On 24 December 2004, minor planet 99942 Apophis (at the time known by its provisional designation 2004 MN₄) was assigned a 4 on the Torino scale, the highest rating ever achieved. There was a 2.7% chance of Earth impact on 13 April 2029. However, on 28 December 2004, the risk of impact dropped to zero for 2029, but future potential impact solutions were still rated 1 on the Torino scale. The 2036 risk was lowered to a Torino rating of 0 in August 2006. The Palermo rating is −3.2.^[21]

The only known NEO with a Palermo scale value currently greater than zero is (29075) 1950 DA, which may pass very close to or collide with the Earth (probability ≤ 0.003) in the year 2880. Depending on the unknown orientation of its axis of rotation, it will either miss the Earth by tens of millions of kilometers, or have a 1 in 300 chance of hitting the Earth. However, humanity has over 800 years to refine the orbit of (29075) 1950 DA, and to deflect it, if necessary.^[22]

List of current threats

NASA maintains a continuously updated Sentry Risk Table of the most significant NEO threats in the next 100 years.^[21] All or nearly all of the objects are highly likely to eventually drop off the list as more observations come in, reducing the uncertainties and enabling more accurate orbital predictions. (The list does not include (29075) 1950 DA, because that will not strike for at least 800 years.)^[4][22]

History of NEO science and exploratory mission proposals

Goldstone radar-image asteroid to be studied and then have samples returned to Earth by NASA's OSIRIS-REx spacecraft (launch planned for 2016)

In a 2013 article in Wired Science, David Portree provides an overview of NEO science and proposed asteroidal missions, with an emphasis on the outcome of two conferences held in the 1970s. The International Astronomical Union minor planets workshop was held in Tucson, Arizona in March 1971 and a consensus "emerged that launching spacecraft to asteroids would be 'premature'."^[17] "In January 1978, NASA’s Office of Space Science held a workshop at the University of Chicago to "assess the state of asteroid studies and consider options for the future."^[17]

Of all of the near-Earth asteroids (NEA) that had been discovered by mid-1977, it was estimated that spacecraft could rendezvous with and return from only about one in 10 using less propulsive energy than is necessary to reach Mars. "Because even the most massive NEA—35 kilometres (22 mi)-wide 1036 Ganymed, discovered in 1924, has a very low surface gravity—landing and takeoff would need very little energy. This meant that a single spacecraft could sample multiple sites on any given NEA."^[17] Overall, it was estimated that about one percent of all NEAs might provide opportunities for human-crewed missions, or no more than about ten known NEAs. Therefore, unless the NEA discovery rate were "immediately increased five-fold, no opportunity to launch 'astronaut-scientists' to an NEA was likely to occur within a decade of the Chicago workshop."^[17]

Number and classification of near-Earth objects

Near-Earth objects are classified as meteoroids, asteroids, or comets depending on size and composition. Asteroids can also be members of an asteroid family, and comets create meteoroid streams that can generate meteor showers.

As of February 2014^[update], 10,713 NEOs have been discovered:^[8] 94 near-Earth comets and 10,619 near-Earth asteroids. Of those there are 815 Aten asteroids, 4,016 Amor asteroids, and 5,775 Apollo asteroids. There are 1,458 NEOs that are classified as potentially hazardous asteroids (PHAs).
Currently, 154 PHAs and 867 NEAs have an absolute magnitude of 17.75 or brighter, which roughly corresponds to at least 1 km in size.^[8]

As of July 2014^[update], there are 499 NEAs on the Sentry impact risk page at the NASA website.^[21] A significant number of these NEAs – 215 as of May 2010^[update] – are equal to or smaller than 50 meters in diameter and none of the listed objects are placed even in the "green/yellow zone" (Torino Scale 1-2), meaning that none warrant the attention of general public.^[24] The JPL Small-Body Database lists 1,885 near Earth asteroids with an absolute magnitude (H) dimmer than 25 (roughly 50 meters in diameter).^[25]

Near-Earth asteroids smaller than ~1 meter are near-Earth meteoroids and are listed as asteroids on most asteroid tables. The smallest known near-Earth meteoroid is 2008 TS₂₆ with an absolute magnitude of 33^[25] and estimated size of only 1 meter.^[26]

Near-Earth asteroids

Near-Earth asteroids by size.

Near-Earth asteroids classification.

Cumulative discoveries of near-Earth asteroids known by size, 1980–2013.

Play media

There are significantly fewer near-Earth asteroids in the mid-size range than previously thought.

These are objects in a near-Earth orbit without the tail or coma of a comet. As of February 2015^[update], 12,113 near-Earth asteroids are known,^[8] ranging in size from 1 meter up to ~32 kilometers (1036 Ganymed). The number of near-Earth asteroids over one kilometer in diameter is estimated to be about 981.^[9][27][28] The composition of near-Earth asteroids is comparable to that of asteroids from the asteroid belt, reflecting a variety of asteroid spectral types.^[29]

NEAs survive in their orbits for just a few million years.^[5] They are eventually eliminated by planetary perturbations, causing ejection from the Solar System or a collision with the Sun or a planet. With orbital lifetimes short compared to the age of the Solar System, new asteroids must be constantly moved into near-Earth orbits to explain the observed asteroids. The accepted origin of these asteroids is that asteroid-belt asteroids are moved into the inner Solar System through orbital resonances with Jupiter. The interaction with Jupiter through the resonance perturbs the asteroid's orbit and it comes into the inner Solar System. The asteroid belt has gaps, known as Kirkwood gaps, where these resonances occur as the asteroids in these resonances have been moved onto other orbits. New asteroids migrate into these resonances, due to the Yarkovsky effect that provides a continuing supply of near-Earth asteroids.^[30] The asteroid with the greatest known chance of impacting Earth is 2010 RF12 with a 1 in 16 chance of impacting Earth on 5 September 2095.

A small number of NEOs are extinct comets that have lost their volatile surface materials, although having a faint or intermittent comet-like tail does not necessarily result in a classification as a near-Earth comet, making the boundaries somewhat fuzzy. The rest of the near-Earth asteroids are driven out of the asteroid belt by gravitational interactions with Jupiter.^[5][31]

Near-Earth asteroids are divided into groups based on their semi-major axis (a), perihelion distance (q), and aphelion distance (Q):^[2][5]

• The Atiras or Apohele asteroids have orbits strictly inside Earth's orbit: an Atira asteroid's aphelion distance (Q) is smaller than Earth's perihelion distance (0.983 AU). That is, Q < 0.983 AU. (This implies that the asteroid's semi-major axis is also less than 0.983 AU.)
• The Atens have a semi-major axis of less than 1 AU and cross Earth's orbit. Mathematically, a < 1.0 AU and Q > 0.983 AU.
• The Apollos have a semi-major axis of more than 1 AU and cross Earth's orbit. Mathematically, a > 1.0 AU and q > 1.017 AU. (1.017 AU is Earth's aphelion distance.)
• The Amors have orbits strictly outside Earth's orbit: an Amor asteroid's perihelion distance (q) is greater than Earth's aphelion distance (1.017 AU). Amor asteroids are also near-earth objects so q < 1.3 AU. In summary, 1.017 AU < q < 1.3 AU. (This implies that the asteroid's semi-major axis (a) is also larger than 1.017 AU.) Some Amor asteroid orbits cross the orbit of Mars.

(Note: Some authors define the Atens group differently: they define it as being all the asteroids with a semi-major axis of less than 1 AU. That is, they consider the Atiras to be part of the Atens.
Historically, until 1998, there were no known or suspected Atiras, so the distinction wasn't necessary.)

Because the Atens and all Apollos have orbits that cross Earth's orbit, they might impact the Earth. Atiras and Amors do not cross the Earth's orbit and are not immediate impact threats, but their orbits may change to become Earth-crossing orbits in the future.

Near-Earth comets

As of February 2015^[update], 96 near-Earth comets have been discovered.^[8] Although no impact of a comet in Earth's history has been conclusively confirmed, the Tunguska event may have been caused by a fragment of Comet Encke.^[32] Cometary fragmenting may also be responsible for some impacts from near-Earth objects. It is rare for a comet to pass within 0.1 AU (15,000,000 km; 9,300,000 mi) of Earth.^[33]

These near-Earth objects were probably derived from the Kuiper belt, beyond the orbit of Neptune.

Impact rate

Stony asteroids with a diameter of 4 meters (13 ft) impact Earth approximately once per year.^[34]
Asteroids with a diameter of roughly 7 meters enter Earth's atmosphere with as much energy as Little Boy (the atomic bomb dropped on Hiroshima, approximately 15 kilotonnes of TNT) about every 5 years.^[34] These ordinarily explode in the upper atmosphere, and most or all of the solids are vaporized.^[35] Every 2,000–3,000 years, objects produce explosions of 10 megatons comparable to the one observed at Tunguska in 1908.^[36] Objects with a diameter of one kilometer hit the Earth an average of twice every million year interval.^[5] Large collisions with five kilometer objects happen approximately once every twenty million years.^[34]

Assuming that these rates will continue for the next billion years, there exist at least 2,000 objects of diameter greater than 1 km that will eventually hit Earth. However, most of these are not yet considered potentially hazardous objects because they are currently orbiting between Mars and Jupiter. Eventually they will change orbits and become NEOs. Objects spend on average a few million years as NEOs before hitting the Sun, being ejected from the Solar System, or (for a small proportion) hitting a planet.^[5]

Frequency of small asteroids roughly 1 to 20 meters in diameter impacting Earth's atmosphere.

Close approaches

Flyby of asteroid 2004 FH (centre dot being followed by the sequence). The other object that flashes by is an artificial satellite.

On August 10, 1972, a meteor that became known as 1972 Great Daylight Fireball was witnessed by many people moving north over the Rocky Mountains from the U.S. Southwest to Canada. It was an Earth-grazing meteoroid that passed within 57 kilometers (about 34 miles) of the Earth's surface. It was filmed by a tourist at the Grand Teton National Park in Wyoming with an 8-millimeter color movie camera.^[37]
On March 23, 1989, the 300-meter (1,000-foot) diameter Apollo asteroid 4581 Asclepius (1989 FC) missed the Earth by 700,000 kilometers (430,000 mi) passing through the exact position where the Earth was only 6 hours before. If the asteroid had impacted it would have created the largest explosion in recorded history, 12 times as powerful as the Tsar Bomba, the most powerful nuclear bomb ever exploded. It attracted widespread attention as early calculations had its passage being as close as 64,000 km (40,000 mi) from the Earth, with large uncertainties that allowed for the possibility of it striking the Earth.^[38]

On March 18, 2004, LINEAR announced a 30-meter asteroid, 2004 FH, which would pass the Earth that day at only 42,600 km (26,500 mi), about one-tenth the distance to the Moon, and the closest miss ever noticed. They estimated that similar-sized asteroids come as close about every two years.^[39]

On March 31, 2004, two weeks after 2004 FH, meteoroid 2004 FU162 set a new record for closest recorded approach, passing Earth only 6,500 km (4,000 mi) away (about one-sixtieth of the distance to the Moon). Because it was very small (6 meters/20 feet), FU₁₆₂ was detected only hours before its closest approach. If it had collided with Earth, it probably would have harmlessly disintegrated in the atmosphere.

Asteroid 2011 MD (Spitzer Space Telescope; IRAC; February 2014).

On March 2, 2009, near-Earth asteroid 2009 DD45 flew by Earth at about 13:40 UT. The estimated distance from Earth was 72,000 km (45,000 mi), approximately twice the height of a geostationary communications satellite. The estimated size of the space rock was about 35 meters (115 feet) wide.^[40]

On January 13, 2010, at 12:46 UT, near-Earth asteroid 2010 AL30^[41] passed at about 122,000 km (76,000 mi). It was approximately 10–15 m (33–49 ft) wide. If 2010 AL₃₀ had entered the Earth's atmosphere, it would have created an air burst equivalent to between 50 kt and 100 kt (kilotons of TNT). The Hiroshima "Little Boy" atom bomb had a yield between 13-18 kt.^[42]

On June 28, 2011, an asteroid designated 2011 MD, estimated at 5–20 m (16–66 ft) in diameter, passed within 20,000 km (12,000 mi) of the Earth, passing over the Atlantic Ocean.^[43]

On November 8, 2011, (308635) 2005 YU55 (at about 400m diameter) passed within 324,600 km (201,700 mi) (0.85 lunar distances) of Earth. Ten weeks later, on January 27, 2012, the 10-metre wide asteroid 2012 BX34 passed a mere 60,000 km (37,000 mi) from Earth.^[44]

On February 15, 2013, 367943 Duende (2012 DA₁₄) passed approximately 27,700 km (17,200 mi) above the surface of Earth. This was closer than satellites in geosynchronous orbit. The asteroid was not visible to the unaided eye.

Future impacts

Radar image of asteroid 1950 DA.

Although there have been a few false alarms, a number of objects have been known to be threats to the Earth. (89959) 2002 NT7 was the first asteroid with a positive rating on the Palermo Technical Impact Hazard Scale, with approximately one in a million on a potential impact date of approximately February 1, 2019; it is now known that 2002 NT₇ will actually safely pass 0.4078 AU (61,010,000 km; 37,910,000 mi) from the Earth on January 13, 2019.^[45]

Asteroid (29075) 1950 DA was lost after its discovery in 1950, since its observations over just 17 days were insufficient to determine its orbit, and then rediscovered on December 31, 2000. It has a diameter of about a kilometer (0.6 miles). The chance that it could impact Earth during its March 16, 2880 close approach had been estimated as 1 in 300. This is roughly 50% greater than the combined chance of impact for all other similarly large objects until 2880.^[46] The next radar opportunity for 1950 DA is in 2032,^[47] and will pinpoint our knowledge of the orbit, but additional optical position measurements have already reduced the probability of a 2880 impact to 1 in 20 000.

Only the asteroids 99942 Apophis (provisionally known as 2004 MN4) and (144898) 2004 VD17 have briefly had above-normal rankings on the Torino Scale.

Projects to minimize the threat

Number of near Earth asteroids detected.

Number of near Earth asteroids detected, 1 km or more.

Several surveys have undertaken "Spaceguard" activities (an umbrella term), including Lincoln Near-Earth Asteroid Research (LINEAR), Spacewatch, Near-Earth Asteroid Tracking (NEAT), Lowell Observatory Near-Earth-Object Search (LONEOS), Catalina Sky Survey, Campo Imperatore Near-Earth Object Survey (CINEOS), Japanese Spaceguard Association, and Asiago-DLR Asteroid Survey. In 1998, the United States Congress mandated the Spaceguard Survey – detection of 90% of near-earth asteroids over 1 km diameter (which threaten global devastation) by 2008. In 2005, this was extended by the George E. Brown, Jr. Near-Earth Object Survey Act, which calls for NASA to detect 90% of NEOs with diameters of 140 meters or greater, by 2020.^[48]

As of 2011, 911 of the largest (>1 km diameter) near-Earth asteroids have been found, with an estimate of 70 yet to be found.^[9]

Marine biology

From Wikipedia, the free encyclopedia

Only 29 percent of the Earth's surface is land. The rest is ocean, home to marine life. The oceans average nearly four kilometres in depth and are fringed with coastlines that run for 360,000 kilometres.^[1][2]

Marine biology is the scientific study of organisms in the ocean or other marine or brackish bodies of water. Given that in biology many phyla, families and genera have some species that live in the sea and others that live on land, marine biology classifies species based on the environment rather than on taxonomy. Marine biology differs from marine ecology as marine ecology is focused on how organisms interact with each other and the environment, while biology is the study of the organisms themselves.

A large proportion of all life on Earth exists in the ocean. Exactly how large the proportion is unknown, since many ocean species are still to be discovered. The ocean is a complex three-dimensional world^[3] covering about 71% of the Earth's surface. The habitats studied in marine biology include everything from the tiny layers of surface water in which organisms and abiotic items may be trapped in surface tension between the ocean and atmosphere, to the depths of the oceanic trenches, sometimes 10,000 meters or more beneath the surface of the ocean. Specific habitats include coral reefs, kelp forests, seagrass meadows, the surrounds of seamounts and thermal vents, tidepools, muddy, sandy and rocky bottoms, and the open ocean (pelagic) zone, where solid objects are rare and the surface of the water is the only visible boundary. The organisms studied range from microscopic phytoplankton and zooplankton to huge cetaceans (whales) 30 meters (98 feet) in length.

Marine life is a vast resource, providing food, medicine, and raw materials, in addition to helping to support recreation and tourism all over the world. At a fundamental level, marine life helps determine the very nature of our planet. Marine organisms contribute significantly to the oxygen cycle, and are involved in the regulation of the Earth's climate.^[4] Shorelines are in part shaped and protected by marine life, and some marine organisms even help create new land.^[5]

Many species are economically important to humans, including food fish (both finfish and shellfish). It is also becoming understood that the well-being of marine organisms and other organisms are linked in very fundamental ways. The human body of knowledge regarding the relationship between life in the sea and important cycles is rapidly growing, with new discoveries being made nearly every day. These cycles include those of matter (such as the carbon cycle) and of air (such as Earth's respiration, and movement of energy through ecosystems including the ocean). Large areas beneath the ocean surface still remain effectively unexplored.

History

HMS Challenger during its pioneer expedition of 1872–76

Early instances of the study of marine biology trace back to Aristotle (384–322 BC) who made several contributions which laid the foundation for many future discoveries and were the first big step in the early exploration period of the ocean and marine life.^[6] In 1768, Samuel Gottlieb Gmelin (1744–1774) published the Historia Fucorum, the first work dedicated to marine algae and the first book on marine biology to use the then new binomial nomenclature of Linnaeus. It included elaborate illustrations of seaweed and marine algae on folded leaves.^[7][8] The British naturalist Edward Forbes (1815–1854) is generally regarded as the founder of the science of marine biology.^[9] The pace of oceanographic and marine biology studies quickly accelerated during the course of the 19th century.

The observations made in the first studies of marine biology fuelled the age of discovery and exploration that followed. During this time, a vast amount of knowledge was gained about the life that exists in the oceans of the world. Many voyages contributed significantly to this pool of knowledge. Among the most significant were the voyages of the HMS Beagle where Charles Darwin came up with his theories of evolution and on the formation of coral reefs.^[10] Another important expedition was undertaken by HMS Challenger, where findings were made of unexpectedly high species diversity among fauna stimulating much theorizing by population ecologists on how such varieties of life could be maintained in what was thought to be such a hostile environment.^[11] This era was important for the history of marine biology but naturalists were still limited in their studies because they lacked technology that would allow them to adequately examine species that lived in deep parts of the oceans.

The creation of marine laboratories was important because it allowed marine biologists to conduct research and process their specimens from expeditions. The oldest marine laboratory in the world, Station biologique de Roscoff, was established in France in 1872. In the United States, the Scripps Institution of Oceanography dates back to 1903, while the prominent Woods Hole Oceanographic Institute was founded in 1930.^[12] The development of technology such as sound navigation ranging, scuba diving gear, submersibles and remotely operated vehicles allowed marine biologists to discover and explore life in deep oceans that was once thought to not exist.^[13]

Subfields

Coral reefs form complex marine ecosystems with tremendous biodiversity.

The marine ecosystem is large, and thus there are many sub-fields of marine biology. Most involve studying specializations of particular animal groups, such as phycology, invertebrate zoology and ichthyology.

Other subfields study the physical effects of continual immersion in sea water and the ocean in general, adaptation to a salty environment, and the effects of changing various oceanic properties on marine life. A subfield of marine biology studies the relationships between oceans and ocean life, and global warming and environmental issues (such as carbon dioxide displacement).

Recent marine biotechnology has focused largely on marine biomolecules, especially proteins, that may have uses in medicine or engineering. Marine environments are the home to many exotic biological materials that may inspire biomimetic materials.

Related fields

Marine biology is a branch of biology and is closely linked to oceanography. It also encompasses many ideas from ecology. Fisheries science and marine conservation can be considered partial offshoots of marine biology (as well as environmental studies). Marine Chemistry, Physical oceanography and Atmospheric sciences are closely related to this field.

Animals

Birds

Birds adapted to living in the marine environment are often referred to as seabirds. Examples include albatross, penguins, gannets, and auks. Although they spend most of their lives in the ocean, species such as gulls can often be found thousands of miles inland.

Fish

Fish anatomy includes a two-chambered heart, operculum, swim bladder, scales, fins, lips, eyes and secretory cells that produce mucous. Fish breathe by extracting oxygen from water through their gills. Fins propel and stabilize the fish in the water. Many fish fall under two major categories - Elasmobranchii and Teleostei.

A reported 32,700 species of fish have been described (as of December 2013),^[14] more than the combined total of all other vertebrates. About 60% of fish species are saltwater fish.^[15]

A crown-of-thorns starfish.

Invertebrates

As on land, invertebrates make up a huge portion of all life in the sea. Invertebrate sea life includes Cnidaria such as jellyfish and sea anemones; Ctenophora; sea worms including the phyla Platyhelminthes, Nemertea, Annelida, Sipuncula, Echiura, Chaetognatha, and Phoronida; Mollusca including shellfish, squid, octopus; Arthropoda including Chelicerata and Crustacea; Porifera; Bryozoa; Echinodermata including starfish; and Urochordata including sea squirts or tunicates.

Sea otters.

Mammals

There are five main types of marine mammals.

• Cetaceans include toothed whales (Suborder Odontoceti), such as the Sperm Whale, dolphins, and porpoises such as the Dall's porpoise. Cetaceans also include baleen whales (Suborder Mysticeti), such as the Gray Whale, Humpback Whale, and Blue Whale.
• Sirenians include manatees, the Dugong, and the extinct Steller's Sea Cow.
• Seals (Family Phocidae), sea lions (Family Otariidae - which also include the fur seals), and the Walrus (Family Odobenidae) are all considered pinnipeds.
• The Sea Otter is a member of the Family Mustelidae, which includes weasels and badgers.
• The Polar Bear (Family Ursidae) is sometimes considered a marine mammal because of its dependence on the sea.

Green turtle.

Reptiles

Reptiles which inhabit or frequent the sea include sea turtles, sea snakes, terrapins, the marine iguana, and the saltwater crocodile. Most extant marine reptiles, except for some sea snakes, are oviparous and need to return to land to lay their eggs. Thus most species, excepting sea turtles, spend most of their lives on or near land rather than in the ocean. Despite their marine adaptations, most sea snakes prefer shallow waters nearby land, around islands, especially waters that are somewhat sheltered, as well as near estuaries.^[16][17] Some extinct marine reptiles, such as ichthyosaurs, evolved to be viviparous and had no requirement to return to land.

Fungi

Over 1500 species of fungi are known from marine environments.^[18] These parasitize marine algae or animals, or are saprobes on algae, corals, protozoan cysts, sea grasses, wood and other substrata, and can also be found in sea foam.^[19] Spores of many species have special appendages which facilitate attachment to the substratum.^[20] A very diverse range of unusual secondary metabolites is produced by marine fungi.^[21]

Plants and algae

Microscopic algae and plants provide important habitats for life, sometimes acting as hiding and foraging places for larval forms of larger fish and invertebrates.

Algal life is widespread and very diverse under the ocean. Microscopic photosynthetic algae contribute a larger proportion of the world's photosynthetic output than all the terrestrial forests combined. Most of the niche occupied by sub plants on land is actually occupied by macroscopic algae in the ocean, such as Sargassum and kelp, which are commonly known as seaweeds that creates kelp forests.

Plants that survive in the sea are often found in shallow waters, such as the seagrasses (examples of which are eelgrass, Zostera, and turtle grass, Thalassia). These plants have adapted to the high salinity of the ocean environment. The intertidal zone is also a good place to find plant life in the sea, where mangroves or cordgrass or beach grass might grow. Microscopic algae and plants provide important habitats for life, sometimes acting as hiding and foraging places for larval forms of larger fish and invertebrates.

Microscopic life

A copepod.

Microscopic life undersea is incredibly diverse and still poorly understood. For example, the role of viruses in marine ecosystems is barely being explored even in the beginning of the 21st century.^[22]
The role of phytoplankton is better understood due to their critical position as the most numerous primary producers on Earth. Phytoplankton are categorized into cyanobacteria (also called blue-green algae/bacteria), various types of algae (red, green, brown, and yellow-green), diatoms, dinoflagellates, euglenoids, coccolithophorids, cryptomonads, chrysophytes, chlorophytes, prasinophytes, and silicoflagellates.

Zooplankton tend to be somewhat larger, and not all are microscopic. Many Protozoa are zooplankton, including dinoflagellates, zooflagellates, foraminiferans, and radiolarians. Some of these (such as dinoflagellates) are also phytoplankton; the distinction between plants and animals often breaks down in very small organisms. Other zooplankton include cnidarians, ctenophores, chaetognaths, molluscs, arthropods, urochordates, and annelids such as polychaetes. Many larger animals begin their life as zooplankton before they become large enough to take their familiar forms. Two examples are fish larvae and sea stars (also called starfish).

Marine habitats

Marine habitats can be divided into coastal and open ocean habitats. Coastal habitats are found in the area that extends from the shoreline to the edge of the continental shelf. Most marine life is found in coastal habitats, even though the shelf area occupies only seven percent of the total ocean area. Open ocean habitats are found in the deep ocean beyond the edge of the continental shelf

Alternatively, marine habitats can be divided into pelagic and demersal habitats. Pelagic habitats are found near the surface or in the open water column, away from the bottom of the ocean. Demersal habitats are near or on the bottom of the ocean. An organism living in a pelagic habitat is said to be a pelagic organism, as in pelagic fish. Similarly, an organism living in a demersal habitat is said to be a demersal organism, as in demersal fish. Pelagic habitats are intrinsically shifting and ephemeral, depending on what ocean currents are doing.

Marine habitats can be modified by their inhabitants. Some marine organisms, like corals, kelp and seagrasses, are ecosystem engineers which reshape the marine environment to the point where they create further habitat for other organisms.

Intertidal and shore

Tide pools with sea stars and sea anemone in Santa Cruz, California

Intertidal zones, those areas close to shore, are constantly being exposed and covered by the ocean's tides. A huge array of life lives within this zone.

Shore habitats span from the upper intertidal zones to the area where land vegetation takes prominence. It can be underwater anywhere from daily to very infrequently. Many species here are scavengers, living off of sea life that is washed up on the shore. Many land animals also make much use of the shore and intertidal habitats. A subgroup of organisms in this habitat bores and grinds exposed rock through the process of bioerosion.

Reefs

Reefs comprise some of the densest and most diverse habitats in the world. The best-known types of reefs are tropical coral reefs which exist in most tropical waters; however, reefs can also exist in cold water. Reefs are built up by corals and other calcium-depositing animals, usually on top of a rocky outcrop on the ocean floor. Reefs can also grow on other surfaces, which has made it possible to create artificial reefs. Coral reefs also support a huge community of life, including the corals themselves, their symbiotic zooxanthellae, tropical fish and many other organisms.
Much attention in marine biology is focused on coral reefs and the El Niño weather phenomenon. In 1998, coral reefs experienced the most severe mass bleaching events on record, when vast expanses of reefs across the world died because sea surface temperatures rose well above normal.^[23][24] Some reefs are recovering, but scientists say that between 50% and 70% of the world's coral reefs are now endangered and predict that global warming could exacerbate this trend.^{[25][26][27][28]}

Open ocean

The open ocean is relatively unproductive because of a lack of nutrients, yet because it is so vast, in total it produces the most primary productivity. Much of the aphotic zone's energy is supplied by the open ocean in the form of detritus.

Deep sea and trenches

The deepest recorded oceanic trench measured to date is the Mariana Trench, near the Philippines, in the Pacific Ocean at 10,924 m (35,840 ft). At such depths, water pressure is extreme and there is no sunlight, but some life still exists. A white flatfish, a shrimp and a jellyfish were seen by the American crew of the bathyscaphe Trieste when it dove to the bottom in 1960.^[29]

Other notable oceanic trenches include Monterey Canyon, in the eastern Pacific, the Tonga Trench in the southwest at 10,882 m (35,702 ft), the Philippine Trench, the Puerto Rico Trench at 8,605 m (28,232 ft), the Romanche Trench at 7,760 m (25,460 ft), Fram Basin in the Arctic Ocean at 4,665 m (15,305 ft), the Java Trench at 7,450 m (24,440 ft), and the South Sandwich Trench at 7,235 m (23,737 ft).

In general, the deep sea is considered to start at the aphotic zone, the point where sunlight loses its power of transference through the water.^{[citation needed]} Many life forms that live at these depths have the ability to create their own light known as bio-luminescence.

Marine life also flourishes around seamounts that rise from the depths, where fish and other sea life congregate to spawn and feed. Hydrothermal vents along the mid-ocean ridge spreading centers act as oases, as do their opposites, cold seeps. Such places support unique biomes and many new microbes and other lifeforms have been discovered at these locations .^{[citation needed]}

Distribution factors

An active research topic in marine biology is to discover and map the life cycles of various species and where they spend their time. Technologies that aid in this discovery include pop-up satellite archival tags, acoustic tags, and a variety of other data loggers. Marine biologists study how the ocean currents, tides and many other oceanic factors affect ocean life forms, including their growth, distribution and well-being. This has only recently become technically feasible with advances in GPS and newer underwater visual devices.^{[citation needed]}

Most ocean life breeds in specific places, nests or not in others, spends time as juveniles in still others, and in maturity in yet others. Scientists know little about where many species spend different parts of their life cycles especially in the infant and juvenile years. For example, it is still largely unknown where juvenile sea turtles and some year-1 sharks travel. Recent advances in underwater tracking devices are illuminating what we know about marine organisms that live at great Ocean depths.^[30] The information that pop-up satellite archival tags give aids in certain time of the year fishing closures and development of a marine protected area. This data is important to both scientists and fishermen because they are discovering that by restricting commercial fishing in one small area they can have a large impact in maintaining a healthy fish population in a much larger area.