New Atheism

From Wikipedia, the free encyclopedia

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

New Atheism or Neo-Atheism, sometimes referred to as scientific atheism, is a 21st-century movement attributed to some atheist academics, writers, scientists, and philosophers opposed to superstition, religion, and irrationalism. New Atheists advocate the antitheist view that the various forms of theism should be examined, countered by rational and scientific arguments, and criticised, especially when they exert strong influence on the broader society, such as in government, education, and politics.

Major figures of New Atheism include Richard Dawkins, Daniel Dennett, Christopher Hitchens, and Sam Harris, sometimes collectively referred to as the "Four Horsemen" of the movement. Proponents of the New Atheist movement have experienced criticism from academics and other atheists.

Origins and terminology

The term New Atheism is usually traced to Gary Wolf's 2006 Wired article "The Church of the Non-Believers", in which he introduced the label in a critical context and portrayed writers such as Richard Dawkins, Sam Harris and Daniel Dennett as unusually strident and combative critics of religion. Scholars have often treated it as a label that emerged from media discussion rather than as the name of a clearly defined school of thought. James E. Taylor writes that the label arose from journalistic commentary on books by Sam Harris, Richard Dawkins, Daniel Dennett and Christopher Hitchens, while Christopher R. Cotter describes the early discussion of New Atheism as "scholarly, but largely journalistic". Thomas Zenk likewise argues that New Atheism is an umbrella term rooted in public discourse and cautions against using it uncritically as an analytical category; he also notes that the label does not necessarily correspond to the self-identification of those to whom it is applied. Steven Kettell argues that the label's popularisation was not merely descriptive, but also encouraged the view that a small group of atheist writers were both unusually militant and philosophically unoriginal.

Prominent figures

The "Four Horsemen"

The "Four Horsemen" of New Atheism (clockwise from top): Richard Dawkins (b. 1941), Daniel Dennett (1942–2024), Christopher Hitchens (1949–2011), and Sam Harris (b. 1967)

Key figures associated with New Atheism include evolutionary biologist Richard Dawkins, philosopher and neuroscientist Sam Harris, philosopher and cognitive scientist Daniel Dennett, and journalist Christopher Hitchens. The four are often collectively referred to as the "Four Horsemen", an allusion to the biblical Four Horsemen of the Apocalypse from the Book of Revelation. Before her 2023 conversion to Christianity, writer and politician Ayaan Hirsi Ali was sometimes referred to as the fifth "Horseman" or "Horsewoman" of New Atheism.

Harris's 2004 book The End of Faith: Religion, Terror, and the Future of Reason, a bestseller in the United States, was joined over the next couple of years by a series of bestsellers by atheist authors. Harris was motivated by the events of 11 September 2001, for which he blamed Islam, while also directly criticizing Christianity and Judaism. Two years later, Harris followed up with Letter to a Christian Nation, which was a severe criticism of Christianity. Later Harris wrote several bestselling non-fiction books including The Moral Landscape, and Waking Up, along with two shorter works (initially published as e-books) Free Will and Lying.

Dawkins is the founder of the Richard Dawkins Foundation for Reason and Science. In 2006, following his television documentary series The Root of All Evil?, he published The God Delusion, which was on the New York Times best-seller list for 51 weeks. Dawkins writes in the introduction to the 10th anniversary edition of the book: "I don't object to the horseman label, by the way. I'm less keen on 'new atheist': it isn't clear to me how we differ from old atheists."

On 30 September 2007, Dawkins, Harris, Hitchens, and Dennett met at Hitchens' residence in Washington, D.C., for a private two-hour unmoderated round table discussion. The event was videotaped and titled "The Four Horsemen". During "The God Debate" in 2010 with Hitchens versus Dinesh D'Souza, the group was collectively referred to as the "Four Horsemen of the Non-Apocalypse".

Hitchens, the author of God Is Not Great, was named among the "Top 100 Public Intellectuals" by Foreign Policy and Prospect magazines. He served on the advisory board of the Secular Coalition for America. In 2010, Hitchens published his memoir Hitch-22 (a nickname provided by close personal friend Salman Rushdie, whom Hitchens always supported during and following The Satanic Verses controversy). Shortly after its publication, he was diagnosed with esophageal cancer, which led to his death in December 2011. Before his death, Hitchens published a collection of essays and articles in his book Arguably; a short edition, Mortality, was published posthumously in 2012. These publications and numerous public appearances provided Hitchens with a platform to remain an astute atheist during his illness, even speaking specifically on the culture of deathbed conversions and condemning attempts to convert the terminally ill, which he opposed as "bad taste".

Dennett was the author of Darwin's Dangerous Idea and Breaking the Spell. He had been a vocal supporter of The Clergy Project, an organization that provides support for clergy in the US who no longer believe in God and cannot fully participate in their communities any longer. He was also a member of the Secular Coalition for America advisory board, and a member of the Committee for Skeptical Inquiry, as well as an outspoken supporter of the Brights movement. He did research into clerics who are secretly atheists and how they rationalize their works. He found what he called a "don't ask, don't tell" conspiracy because believers did not want to hear of loss of faith. This made unbelieving preachers feel isolated, but they did not want to lose their jobs and church-supplied lodgings. Generally, they consoled themselves with the belief that they were doing good in their pastoral roles by providing comfort and required ritual. The research, with Linda LaScola, was further extended to include other denominations and non-Christian clerics. The research and stories Dennett and LaScola accumulated during this project were published in their 2013 co-authored book, Caught in the Pulpit: Leaving Belief Behind.

The book The Four Horsemen: The Conversation That Sparked an Atheist Revolution was released in 2019.

Hirsi Ali is a Somali-born Dutch-American writer, politician, and well-known critic of Islam. She was a central figure of New Atheism until she announced her conversion to Christianity in November 2023. Originally scheduled to attend the 2007 meeting, she later appeared with Dawkins, Dennett, and Harris at the 2012 Global Atheist Convention, where she was referred to as the "one horse-woman" by Dawkins. Robyn Blumner, CEO of the Center for Inquiry, has described Hirsi Ali as the fifth "horseman". Hirsi Ali has been vocal in opposing Islamic ideology, especially concerning women, as exemplified by her books Infidel and The Caged Virgin.

Others

Others have either self-identified as or been classified by some commentators as new atheists:

• Dan Barker (b. 1949), author of Godless: How an Evangelical Preacher Became One of America's Leading Atheists
• Peter Boghossian (b. 1966), philosopher and author of A Manual for Creating Atheists
• Greta Christina (b. 1961), author of Why Are You Atheists So Angry?: 99 Things that Piss Off the Godless
• Jerry Coyne (b. 1949), author of Faith Versus Fact: Why Science and Religion Are Incompatible
• Rebecca Goldstein (b. 1950), philosopher and author of 36 Arguments for the Existence of God
• Michel Onfray (b. 1959), author of Atheist Manifesto: The Case Against Christianity, Judaism, and Islam
• Michael Schmidt-Salomon (b. 1967) author of Manifesto of Evolutionary Humanism and identified as Germany's "Chief Atheist"
• TJ Kirk (b. 1985), YouTube personality and podcast host known for his YouTube Channel Amazing Atheist
• Rebecca Watson (b. 1980), author of the blog Skepchick
• Victor J. Stenger (1935–2014), author of God: The Failed Hypothesis

Some writers sometimes classified as new atheists by others have explicitly distanced themselves from the label:

• A. C. Grayling (b. 1949), philosopher and author of The God Argument
• John W. Loftus (b. 1954), author of The Outsider Test For Faith
• P. Z. Myers (b. 1957), writer and biologist. Author of the blog Pharyngula

Perspective

The scarlet A, symbol of Out Campaign

Many contemporary atheists write from a scientific perspective. Unlike previous writers, many of whom thought that science was indifferent or even incapable of dealing with the "God" concept, Dawkins argues to the contrary, claiming the "God Hypothesis" is a valid scientific hypothesis, having effects in the physical universe, and like any other hypothesis can be tested and falsified. Victor Stenger proposed that the personal Abrahamic God is a scientific hypothesis that can be tested by standard methods of science. Both Dawkins and Stenger conclude that the hypothesis fails any such tests, and argue that naturalism is sufficient to explain everything we observe. They argue that it is unnecessary to introduce God or the supernatural to understand reality.

Scientific testing of religion

Non-believers (in religion and the supernatural) assert that many religious or supernatural claims (such as the virgin birth of Jesus and the afterlife) are scientific claims in nature. For instance, they argue, as do deists and Progressive Christians, that the issue of Jesus' supposed parentage is a question of scientific inquiry, rather than values or morals. Proponents believe science is capable of investigating at least some, if not all, supernatural claims. Institutions such as the Mayo Clinic and Duke University have conducted empirical studies to try to identify whether there is evidence for the healing power of intercessory prayer. According to Stenger, the experiments found no evidence that intercessory prayer worked.

Logical arguments

In his book God: The Failed Hypothesis, Victor Stenger argues that a God having omniscient, omnibenevolent, and omnipotent attributes, which he termed a 3O God, cannot logically exist. A similar series of alleged logical disproofs of the existence of a God with various attributes can be found in Michael Martin and Ricki Monnier's The Impossibility of God, or Theodore Drange's article, "Incompatible-Properties Arguments: A Survey".

Views on non-overlapping magisteria

Richard Dawkins has been particularly critical of the conciliatory view that science and religion are not in conflict, noting, for example, that the Abrahamic religions constantly dabble in scientific matters. In a 1998 article published in Free Inquiry magazine, and later in his 2006 book The God Delusion, Dawkins expresses disagreement with the view advocated by Stephen Jay Gould that science and religion are two non-overlapping magisteria (NOMA), each existing in a "domain where one form of teaching holds the appropriate tools for meaningful discourse and resolution".

In Gould's proposal, science and religion should be confined to distinct non-overlapping domains: science would be limited to the empirical realm, including theories developed to describe observations, while religion would deal with questions of ultimate meaning and moral value. Dawkins contends that NOMA does not describe empirical facts about the intersection of science and religion. He argued: "It is completely unrealistic to claim, as Gould and many others do, that religion keeps itself away from science's turf, restricting itself to morals and values. A universe with a supernatural presence would be a fundamentally and qualitatively different kind of universe from one without. The difference is, inescapably, a scientific difference. Religions make existence claims, and this means scientific claims."

Science and morality

Main article: Science of morality

Harris considers that the well-being of conscious creatures forms the basis of morality. In The Moral Landscape, he argues that science can in principle answer moral questions and help maximize well-being. Harris also criticizes cultural and moral relativism, arguing that it prevents people from making objective moral judgments about practices that clearly harm human well-being, such as female genital mutilation. Harris contends that we can make scientifically-based claims about the negative impacts of such practices on human welfare, and that withholding judgment in these cases is tantamount to claiming complete ignorance about what contributes to human well-being.

Politics

In the context of international politics, the principles of New Atheism establish no particular stance in and of themselves. P. Z. Myers stated that New Atheism's key proponents are "a madly disorganized mob, united only by [their] dislike of the god-thing". That said, the demographic that supports New Atheism is a markedly homogeneous one; in America and the Anglo-sphere more generally, this cohort is "more likely to be younger, male and single, to have higher than average levels of income and education, to be less authoritarian, less dogmatic, less prejudiced, less conformist and more tolerant and open-minded on religious issues." Because of this homogeneity among the group, there exists not a formal dynamic but a loose consensus on broad political "efforts, objectives, and strategies".

For example, one of the primary aims is to further reduce the entanglement of church and state, which derives from the "belief that religion is antithetical to liberal values, such as freedom of expression and the separation of public from private life". Additionally, new atheists have engaged in the campaign "to ensure legal and civic equality for atheists", in a world considerably unwelcoming to and distrustful of non-religious believers. Hitchens may be the atheist concerned most with religion's incompatibility with contemporary liberal principles, and particularly its imposed limitation on both freedom of speech and freedom of expression. Because New Atheism's proliferation is accredited partly to the 11 September attacks and the ubiquitous, visceral response, Richard Dawkins, among many in his cohort, believes that theism (in this case, Islam) jeopardizes political institutions and national security, and he warns of religion's potency in motivating "people to do terrible things" against international polities.

Criticism

See also: Criticism of atheism § New Atheism

According to Nature, "Critics of new atheism, as well as many new atheists themselves, contend that in philosophical terms it differs little from earlier historical forms of atheist thought."

General criticism and scientism

Critics of the movement described it as "militant atheism", "fundamentalist atheism", and "secular fundamentalists". Theologians Jeffrey Robbins and Christopher Rodkey take issue with what they regard as "the evangelical nature of the New Atheism, which assumes that it has a Good News to share, at all cost, for the ultimate future of humanity by the conversion of as many people as possible", and believe they have found similarities between New Atheism and evangelical Christianity and conclude that the all-consuming nature of both "encourages endless conflict without progress" between both extremities. Amarnath Amarasingam, an extremism researcher, argues that the New Atheists fall prey to cognitive biases such as the fundamental attribution error and the out-group homogeneity bias, among others. These biases pose a substantive problem for New Atheism’s claims to rationality and objectivity. Political philosopher John Gray asserts that New Atheism, humanism, and scientism are extensions of religion, particularly Christianity. Anthropologist and psychiatrist Simon Dein considers New Atheism to be a mirror image of religious fundamentalism, based on an analysis of characteristics identified by the Fundamentalism Project: reactivity, dualism, absolutism and inerrancy, and apocalypticism. In addition, he also notes a shared emphasis on evidentialism. Sociologist William Stahl has said, "What is striking about the current debate is the frequency with which the New Atheists are portrayed as mirror images of religious fundamentalists."

The philosopher of science Michael Ruse states that Richard Dawkins would fail "introductory" courses on the study of "philosophy or religion" (such as courses on the philosophy of religion), courses which are offered, for example, at many educational institutions such as colleges and universities around the world. Ruse also says that the movement of New Atheism—which is perceived by him to be "a bloody disaster"—makes him ashamed, as a professional philosopher of science, to be among those holding to an atheist position, particularly as New Atheism, as he sees it, does science a "grave disservice" and does a "disservice to scholarship" at a more general level. Paul Kurtz, editor in chief of Free Inquiry, founder of Prometheus Books, was critical of many of the new atheists. He said, "I consider them atheist fundamentalists... They're anti-religious, and they're mean-spirited, unfortunately. Now, there are very good atheists and very dedicated people who do not believe in God. But you have this aggressive and militant phase of atheism, and that does more damage than good." Rabbi Jonathan Sacks, author of The Great Partnership: Science, Religion, and the Search for Meaning, feels the new atheists miss the target by believing the "cure for bad religion is no religion, as opposed to good religion". He wrote:

Atheism deserves better than the new atheists whose methodology consists of criticizing religion without understanding it, quoting texts without contexts, taking exceptions as the rule, confusing folk belief with reflective theology, abusing, mocking, ridiculing, caricaturing, and demonizing religious faith and holding it responsible for the great crimes against humanity. Religion has done harm; I acknowledge that. But the cure for bad religion is good religion, not no religion, just as the cure for bad science is good science, not the abandonment of science.

The philosopher Massimo Pigliucci contends that the new atheist movement overlaps with scientism, which he finds to be philosophically unsound. He writes: "What I do object to is the tendency, found among many New Atheists, to expand the definition of science to pretty much encompassing anything that deals with 'facts', loosely conceived ... it seems clear to me that most of the New Atheists (except for the professional philosophers among them) pontificate about philosophy very likely without having read a single professional paper in that field ... I would actually go so far as to charge many of the leaders of the New Atheism movement (and, by implication, a good number of their followers) with anti-intellectualism, one mark of which is a lack of respect for the proper significance, value, and methods of another field of intellectual endeavor."

In The Evolution of Atheism, Stephen LeDrew wrote that New Atheism is fundamentalist and scientist; in contrast to atheism's tradition of social justice, it is right-wing and serves to defend "the position of the white middle-class western male". Professor Jacques Berlinerblau has criticised the new atheists' mocking of religion as being inimical to their goals and claims that they have not achieved anything politically. Roger Scruton has extensively criticized New Atheism on various occasions, generally on the grounds that they do not consider the social effects and impacts of religion in enough detail. He has said, "Look at the facts in the round and it seems likely that humans without a sense of the sacred would have died out long ago. For that same reason, the hope of the new atheists for a world without religion is probably as vain as the hope for a society without aggression or a world without death." He has also complained of the new atheists' idea that they must "set people free from religion", calling it "naive" because they "never consider that they might be taking something away from people".

Edward Feser has critiqued the new atheists' responses to arguments for the existence of God:

It can safely be said that if you haven't both understood Aquinas and answered him – not to mention Anselm, Duns Scotus, Leibniz, Samuel Clarke, and so on, but let that pass – then you have hardly "made your case" against religion. Yet Dawkins is the only "New Atheist" to offer anything even remotely like an attempt to answer him, feeble as it is.

— Edward Feser, The Last Superstition (2008)

Sexism

There have been criticisms of such movements perpetuating patriarchal beliefs and practices such as sexism, despite internal claims of gender equality. This has contributed to female atheists feeling shut out, trivialized, and silenced. The New Atheist movement was accused of sexism after "Elevatorgate", a controversy surrounding atheist blogger Rebecca Watson's criticism of unwanted sexual advances in the atheist community. In 2014, Sam Harris said that New Atheism was "to some degree intrinsically male".

Islamophobia

Some commentators have accused the New Atheist movement of Islamophobia. Wade Jacoby and Hakan Yavuz assert that "a group of 'new atheists' such as Richard Dawkins, Sam Harris, and Christopher Hitchens" have "invoked Samuel Huntington's 'clash of civilizations' theory to explain the current political contestation" and that this forms part of a trend toward "Islamophobia ... in the study of Muslim societies". William W. Emilson argues that "the 'new' in the new atheists' writings is not their aggressiveness, nor their extraordinary popularity, nor even their scientific approach to religion, rather it is their attack not only on militant Islamism but also on Islam itself under the cloak of its general critique of religion."

Far-right politics

In 2019, Steven Poole of The Guardian claimed: "For some, New Atheism was never about God at all, but just a topical subgenre of the rightwing backlash against the supposedly suffocating atmosphere of 'political correctness'." The same year, Scott Alexander, a blogger in the ideologically similar rationalist community, countered that New Atheism declined because it "seamlessly merged into the modern social justice movement", with many former New Atheists becoming more interested in fighting sexism, racism, and homophobia under the label "atheism-plus", leaving only right-wing atheists remaining in the original movement. In a June 2021 retrospective article, Émile P. Torres of Salon argued that prominent figures in the New Atheist movement had aligned themselves with the far-right.

Reception and legacy

Tom Flynn, editor of Free Inquiry, wrote that the only "new" thing about New Atheism was the wider publication of atheist material by big-name publishers, books that appeared on bestseller lists and were read by millions. In November 2015, The New Republic published an article entitled "Is the New Atheism dead?" In 2016, evolutionary biologist David Sloan Wilson wrote: "The world appears to be tiring of the New Atheism movement." In 2017, PZ Myers, who formerly considered himself a new atheist, publicly renounced the New Atheism movement.

In a June 2022 retrospective article, Sebastian Milbank of The Critic stated that, as a movement, "New Atheism has fractured and lost its original spirit", that "much of what New Atheism embodied has now migrated rightwards", and that "another portion has moved leftwards, embodied by the 'I Fucking Love Science' woke nerd of today." Following the conversion of writer Ayaan Hirsi Ali to Christianity in 2023, the columnist Sarah Jones wrote in New York magazine that the New Atheism movement was in "terminal decline".

Late Heavy Bombardment

From Wikipedia, the free encyclopedia

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

Timescale

Artist's impression of the Moon during the Late Heavy Bombardment (above) and today (below)

The Late Heavy Bombardment (LHB), or lunar cataclysm, is a hypothesized astronomical event thought to have occurred approximately 4.1 to 3.8 billion years (Ga) ago, at a time corresponding to the Neohadean and Eoarchean eras on Earth. According to the hypothesis, during this interval, a disproportionately large number of asteroids and comets collided into the terrestrial planets and their natural satellites in the inner Solar System, including Mercury, Venus, Earth (and the Moon) and Mars. These came from both post-accretion and planetary instability-driven populations of impactors. Although it has gained widespread credence, definitive evidence remains elusive.

Evidence for the LHB derives from moon rock samples of Lunar craters brought back by the Apollo program astronauts. Isotopic dating showed that the rocks were last molten during impact events in a rather narrow interval of time, suggesting that a large proportion of craters were formed during this period. Several hypotheses attempt to explain this apparent spike in the flux of impactors in the inner Solar System, but no consensus yet exists. The Nice model, popular among planetary scientists, postulates that the giant planets underwent orbital migration, scattering objects from the asteroid belt, Kuiper belt, or both, into eccentric orbits and into the path of the terrestrial planets.

Other researchers doubt the heavy bombardment, arguing for example that the apparent clustering of lunar impact-melt ages is a statistical artifact produced by sampling rocks scattered from a single large impact. A range of evidence suggests that there may instead have been a more extended period of lunar bombardment, lasting from approximately 4.2 billion years ago to 3.5 billion years ago.

Evidence for a cataclysm

The main piece of evidence for a lunar cataclysm comes from the radiometric ages of impact melt rocks that were collected during the Apollo missions. The majority of these impact melts are thought to have formed during the collision of asteroids or comets tens of kilometres across, forming impact craters hundreds of kilometres in diameter. The Apollo 15, 16, and 17 landing sites were chosen as a result of their proximity to the Imbrium, Nectaris, and Serenitatis basins, respectively.

The apparent clustering of ages of these impact melts, between about 3.8 and 4.1 Ga, led investigators to postulate that those ages record an intense bombardment of the Moon. They named it the "lunar cataclysm" and proposed that it represented a dramatic increase in the rate of bombardment of the Moon around 3.9 Ga. If these impact melts were derived from these three basins, then not only did these three prominent impact basins form within a short interval of time, but so did many others based on stratigraphic grounds. At the time, the hypothesis was considered controversial.

As more data have become available, particularly from lunar meteorites, this hypothesis, while still controversial, has become more popular. The lunar meteorites are thought to randomly sample the lunar surface, and at least some of these should have originated from regions far from the Apollo landing sites. Many of the feldspathic lunar meteorites probably originated from the lunar far side, and impact melts within these have recently been dated. Consistent with the cataclysm hypothesis, none of their ages was found to be older than about 3.9 Ga. Nevertheless, the ages do not "cluster" at this date, but span between 2.5 and 3.9 Ga.

Dating of howardite, eucrite and diogenite (HED) meteorites and H chondrite meteorites originating from the asteroid belt reveal numerous ages from 3.4–4.1 Ga and an earlier peak at 4.5 Ga. The 3.4–4.1 Ga ages has been interpreted as representing an increase in impact velocities as computer simulations using hydrocode reveal that the volume of impact melt increases 100–1,000 times as the impact velocity increases from the current asteroid belt average of 5 km/s to 10 km/s. Impact velocities above 10 km/s require very high inclinations or the large eccentricities of asteroids on planet-crossing orbits. Such objects are rare in the current asteroid belt but the population would be significantly increased by the sweeping of resonances due to giant planet migration.

Studies of the highland crater size distributions suggest that the same family of projectiles struck Mercury and the Moon during the Late Heavy Bombardment. If the history of decay of late heavy bombardment on Mercury also followed the history of late heavy bombardment on the Moon, the youngest large basin discovered, Caloris, is comparable in age to the youngest large lunar basins, Orientale and Imbrium, and all of the plains units are older than 3 billion years.

Criticisms of the cataclysm hypothesis

While the cataclysm hypothesis has recently become more popular (in the last fifty years), particularly among dynamicists who have identified possible causes for such a phenomenon, it is still controversial and based on debatable assumptions. Two criticisms are that (1) the "cluster" of impact ages could be an artifact of sampling a single basin's ejecta, and (2) that the lack of impact melt rocks older than about 4.1 Ga is related to all such samples having been pulverized, or their ages being reset.

The first criticism concerns the origin of the impact melt rocks that were sampled at the Apollo landing sites. While these impact melts have been commonly attributed to having been derived from the closest basin, it has been argued that a large portion of these might instead be derived from the Imbrium basin. The Imbrium impact basin is the youngest and largest of the multi-ring basins found on the central nearside of the Moon, and quantitative modeling shows that significant amounts of ejecta from this event should be present at all of the Apollo landing sites. According to this alternative hypothesis, the cluster of impact melt ages near 3.9 Ga simply reflects material being collected from a single impact event, and not several. Additional criticism also argues that the age spike at 3.9 Ga identified in ⁴⁰Ar/³⁹Ar dating could also be produced by an episodic early crust formation followed by partial ⁴⁰Ar losses as the impact rate declined.

A second criticism concerns the significance of the lack of impact melt rocks older than about 4.1 Ga. One hypothesis for this observation that does not involve a cataclysm is that old melt rocks did exist, but that their radiometric ages have all been reset by the continuous effects of impact cratering over the past 4 billion years. Furthermore, it is possible that these putative samples could all have been pulverized to such small sizes that it is impossible to obtain age determinations using standard radiometric methods. Scientists continue to study the bombardment history of the moon in an attempt to clarify the history of the inner solar system.

Geological consequences on Earth

Life timeline
−4500 — – — – −4000 — – — – −3500 — – — – −3000 — – — – −2500 — – — – −2000 — – — – −1500 — – — – −1000 — – — – −500 — – — – 0 —	Water   Single-celled life   Photosynthesis   Multicellular life   Plants   Arthropods Molluscs Flowers Dinosaurs   Mammals Birds Primates Hadean Archean Proterozoic Phanerozoic	← Earth formed ← Earliest water ← LUCA ← Earliest fossils ← Atmospheric oxygen ← Sexual reproduction ← Earliest fungi ← Neoproterozoic oxygenation event ← Ediacaran biota ← Cambrian explosion ← Earliest tetrapods ← Earliest hominoid
(million years ago)

If a cataclysmic cratering event truly occurred on the Moon, Earth would have been affected as well. Extrapolating lunar cratering rates to Earth at this time suggests that the following number of craters would have formed:

• 22,000 or more impact craters with diameters >20 km (12 mi),
• about 40 impact basins with diameters about 1,000 km (620 mi),
• several impact basins with diameters about 5,000 km (3,100 mi),

Before the formulation of the LHB hypothesis, geologists generally assumed that Earth remained molten until about 3.8 Ga. This date could be found in many of the oldest-known rocks from around the world, and appeared to represent a strong "cutoff point" beyond which older rocks could not be found. These dates remained fairly constant even across various dating methods, including the system considered the most accurate and least affected by environment, uranium–lead dating of zircons. As no older rocks could be found, it was generally assumed that Earth had remained molten until this date, which defined the boundary between the earlier Hadean and later Archean eons. Nonetheless, in 1999, the oldest known rock on Earth was dated to be 4.031 ± 0.003 billion years old, and is part of the Acasta Gneiss of the Slave Craton in northwestern Canada.

Older rocks could be found, however, in the form of asteroid fragments that fall to Earth as meteorites. Like the rocks on Earth, asteroids also show a strong cutoff point, at about 4.6 Ga, which is assumed to be the time when the first solids formed in the protoplanetary disk around the then-young Sun. The Hadean, then, was the period of time between the formation of these early rocks in space, and the eventual solidification of Earth's crust, some 700 million years later. This time would include the accretion of the planets from the disk and the slow cooling of Earth into a solid body as the gravitational potential energy of accretion was released.

Later calculations showed that the rate of collapse and cooling depends on the size of the rocky body. Scaling this rate to an object of Earth mass suggested very rapid cooling, requiring only 100 million years. The difference between measurement and theory presented a conundrum at the time.

The LHB offers a potential explanation for this anomaly. Under this model, the rocks dating to 3.8 Ga solidified only after much of the crust was destroyed by the LHB. Collectively, the Acasta Gneiss in the North American cratonic shield and the gneisses within the Jack Hills portion of the Narryer Gneiss Terrane in Western Australia are the oldest continental fragments on Earth, yet they appear to post-date the LHB. The oldest mineral yet dated on Earth, a 4.404 Ga zircon from Jack Hills, predates this event, but it is likely a fragment of crust left over from before the LHB, contained within a much younger (~3.8 Ga old) rock.

The Jack Hills zircon led to an evolution in understanding of the Hadean eon. Older references generally show that Hadean Earth had a molten surface with prominent volcanos. The name "Hadean" itself refers to the "hellish" conditions assumed on Earth for the time, from the Greek Hades. Zircon dating suggested, albeit controversially, that the Hadean surface was solid, temperate, and covered by acidic oceans. This picture derives from the presence of particular isotopic ratios that suggest the action of water-based chemistry at some time before the formation of the oldest rocks (see cool early Earth).

Of particular interest, Manfred Schidlowski argued in 1979 that the carbon isotopic ratios of some sedimentary rocks found in Greenland were a relic of organic matter: the ratio of carbon-12 to carbon-13 was unusually high, normally a sign of "processing" by life. There was much debate over the precise dating of the rocks, with Schidlowski suggesting they were about 3.8 Ga old, and others suggesting a more "modest" 3.6 Ga. In either case it was a very short time for abiogenesis to have taken place, and if Schidlowski was correct, arguably too short a time. The Late Heavy Bombardment and the "re-melting" of the crust that it suggests provides a timeline under which this would be possible: life either formed immediately after the Late Heavy Bombardment, or more likely survived it, having arisen earlier during the Hadean. A 2002 study suggest that the rocks Schidlowski found are indeed from the older end of the possible age range at about 3.85 Ga, suggesting the latter possibility is the most likely answer. Studies from 2005, 2006 and 2009 have found no evidence for the isotopically-light carbon ratios that were the basis for the original claims of early Hadean life. However, a similar study of Jack Hills rocks from 2008 shows traces of the same sort of potential organic indicators. Thorsten Geisler of the Institute for Mineralogy at the University of Münster studied traces of carbon trapped in small pieces of diamond and graphite within zircons dating to 4.25 Ga.

Three-dimensional computer models developed in May 2009 by a team at the University of Colorado at Boulder postulate that much of Earth's crust, and the microbes living in it, could have survived the bombardment. Their models suggest that although the surface of Earth would have been sterilized, hydrothermal vents below Earth's surface could have incubated life by providing a sanctuary for thermophile microbes. In April 2014, scientists reported finding evidence of the largest terrestrial meteor impact event to date near the Barberton Greenstone Belt. They estimated the impact occurred about 3.26 billion years ago and that the impactor was approximately 37 to 58 kilometres (23 to 36 miles) wide. The crater from this event, if it still exists, has not yet been found.

Possible causes

Giant-planet migration

Further information: Nice model

Simulation showing outer planets and planetesimal belt: (a) Early configuration, before Jupiter (green) and Saturn (orange) reach 2:1 resonance; (b) Scattering of planetesimals into the inner Solar System after the orbital shift of Neptune (dark blue) and Uranus (light blue); (c) After ejection of planetesimals by planets.

In the Nice model, the Late Heavy Bombardment is the result of a dynamical instability in the outer Solar System. The original Nice model simulations by Gomes et al. began with the Solar System's giant planets in a tight orbital configuration surrounded by a rich trans-Neptunian belt. Objects from this belt stray into planet-crossing orbits, causing the orbits of the planets to migrate over several hundred million years. Jupiter and Saturn's orbits drift apart slowly until they cross a 2:1 orbital resonance, causing the eccentricities of their orbits to increase. The orbits of the planets become unstable and Uranus and Neptune are scattered onto wider orbits that disrupt the outer belt, causing a bombardment of comets as they enter planet-crossing orbits. Interactions between the objects and the planets also drive a faster migration of Jupiter and Saturn's orbits. This migration causes resonances to sweep through the asteroid belt, increasing the eccentricities of many asteroids until they enter the inner Solar System and impact the terrestrial planets.

The Nice model has undergone some modification since its initial publication. The giant planets now begin in a multi-resonant configuration due to an early gas-driven migration through the protoplanetary disk. Interactions with the trans-Neptunian belt allow their escape from the resonances after several hundred million years. The encounters between planets that follow include one between an ice giant and Saturn that propels the ice giant onto a Jupiter-crossing orbit followed by an encounter with Jupiter that drives the ice giant outward. This jumping-Jupiter scenario quickly increases the separation of Jupiter and Saturn, limiting the effects of resonance sweeping on the asteroids and the terrestrial planets. While this is required to preserve the low eccentricities of the terrestrial planets and avoid leaving the asteroid belt with too many high-eccentricity asteroids, it also reduces the fraction of asteroids removed from the main asteroid belt, leaving a now-nearly-depleted inner band of asteroids as the primary source of the impactors of the LHB. The ice giant is often ejected following its encounter with Jupiter, leading some to propose that the Solar System began with five giant planets. Recent works, however, have found that impacts from this inner asteroid belt would be insufficient to explain the formation of ancient impact spherule beds and the lunar basins, and that the asteroid belt was probably not the source of the Late Heavy Bombardment.

Late formation of Uranus and Neptune

According to one planetesimal simulation of the establishment of the planetary system, the outermost planets Uranus and Neptune formed very slowly, over a period of several billion years. Harold Levison and his team have also suggested that the relatively low density of material in the outer Solar System during planet formation would have greatly slowed their accretion. The late formation of these planets has therefore been suggested as a different reason for the LHB. However, recent calculations of gas-flows combined with planetesimal runaway growth in the outer Solar System imply that Jovian planets formed extremely rapidly, on the order of 10 My, which does not support this explanation for the LHB.

Planet V hypothesis

Main article: Planet V

The Planet V hypothesis posits that a fifth terrestrial planet caused the Late Heavy Bombardment when its meta-stable orbit entered the inner asteroid belt. The hypothetical fifth terrestrial planet, Planet V, had a mass less than half of Mars and originally orbited between Mars and the asteroid belt. Planet V's orbit became unstable due to perturbations from the other inner planets causing it to intersect the inner asteroid belt. After close encounters with Planet V, many asteroids entered Earth-crossing orbits, causing the Late Heavy Bombardment. Planet V was ultimately lost, likely plunging into the Sun. In numerical simulations, an uneven distribution of asteroids, with the asteroids heavily concentrated toward the inner asteroid belt, has been shown to be necessary to produce the LHB via this mechanism. An alternate version of this hypothesis in which the lunar impactors are debris resulting from Planet V impacting Mars, forming the Borealis Basin, has been proposed to explain a low number of giant lunar basins relative to craters and a lack of evidence of cometary impactors.

Disruption of Mars-crossing asteroid

A hypothesis proposed by Matija Ćuk posits that the last few basin-forming impacts were the result of the collisional disruption of a large Mars-crossing asteroid. This Vesta-sized asteroid was a remnant of a population which initially was much larger than the current main asteroid belt. Most of the pre-Imbrium impacts would have been due to these Mars-crossing objects, with the early bombardment extending until 4.1 billion years ago. A period without many basin-forming impacts then followed, during which the lunar magnetic field decayed. Then, roughly 3.9 billion years ago, a catastrophic impact disrupted the Vesta-sized asteroid, significantly increasing the population of Mars-crossing objects. Many of these objects then evolved onto Earth-crossing orbits, producing a spike in the lunar impact rate during which the last few lunar impact basins are formed. Ćuk points to the weak or absent residual magnetism of the last few basins and a change in the size–frequency distribution of craters which formed during this late bombardment as evidence supporting this hypothesis. The timing and the cause of the change in the size–frequency distribution of craters is controversial.

Other potential sources

A number of other possible sources of the Late Heavy Bombardment have been investigated. Among these are additional Earth satellites orbiting independently or as lunar trojans, planetesimals left over from the formations of the terrestrial planets, Earth or Venus co-orbitals, and the breakup of a large main belt asteroid. Additional Earth satellites on independent orbits were shown to be quickly captured into resonances during the Moon's early tidally-driven orbital expansion and were lost or destroyed within a few million years. Lunar trojans were found to be destabilized within 100 million years by a solar resonance when the Moon reached 27 Earth radii. Planetesimals left over from the formation of the terrestrial planets were shown to be depleted too rapidly due to collisions and ejections to form the last lunar basins. The long-term stability of primordial Earth or Venus co-orbitals (trojans or objects with horseshoe orbits) in conjunction with the lack of current observations indicate that they were unlikely to have been common enough to contribute to the LHB. Producing the LHB from the collisional disruption of a main belt asteroid was found to require at minimum a 1,000–1,500 km parent body with the most favorable initial conditions. Debris produced by collisions among inner planets, now lost, has also been proposed as a source of the LHB.

Exosystem with possible Late Heavy Bombardment

Main article: Eta Corvi

Evidence has been found for Late Heavy Bombardment-like conditions around the star Eta Corvi.