Is the occurrence of multi-kiloton impact events on Earth truly random?

In 1984 palaeontologists David Raup and Jack Sepkowski of the Department of Geophysical Sciences at the University of Chicago analysed the occurrence of mass extinction events on Earth and came to the conclusion that these events appeared to be cyclic in nature. Since no known Earthly phenomena is cyclic on a scale of tens of millions of years, he suggested an extra-terrestrial cause for such events, leading to a brief period when it became fashionable to try to explain such extinction events by means of large impact events. This hypothesis lives on in the Alvarez Impact Event Theory, which postulates that the End Cretaceous Extinction was caused by a large body impacting into the Yucatan Peninsula in Mexico, but has otherwise fallen from favour. Our ability to place dates in ancient events has dramatically improved since the 1960s, and has led scientists to conclude that extinction events were not cyclic in occurrence, and we have come up with robust explanations for most of these events which do not need to invoke extra-terrestrial causes. At the same time our understanding of Solar System dynamics has improved to the point where we can rule out cyclic events on a tens of millions of years scale, and we have found numerous large impact craters on Earth that do not seem associated with even the smallest extinction events.

While major impact events occurring regularly on a tens of millions of years scale has now been ruled out, at the opposite end of the scale we do expect small events to occur regularly on an annual scale, for example we can state with confidence the days on which meteor showers will fall, as the Earth passes through the orbital trail of comets or other bodies which leave trails of dusty or sand-size grains in their paths, resulting in harmless but visible showers of micrometeorites falling on Earth.

In a paper published on the arXiv database at Cornell University Library on 1 September 2014 and accepted for publication in the Monthly Notesof the Royal Astronomical Society, Carlos and Raul de la Fuente Marcos of the UniversidadComplutense de Madrid describe the results of a study in which they analysed the distribution of multi-kiloton impacts on Earth since 2000 in order to determine whether the distribution of these events was truly random.

De la Fuente Marcos and de la Fuente Marcos started by taking data collected by the Comprehensive Nuclear-Test-Ban Treaty Organization, which uses infrasound sensors to look for clandestine nuclear tests, but which have also recorded a 26 events attributed to objects with masses between one and 500 kilotons impacting the Earth’s atmosphere since 2000. This was adjusted for known inaccuracies, such as the 22 September 2007 event which was recorded as occurring over the Indian Ocean, but which was observed over the South Pacific, and other events not detected by the network, such as the 18 kiloton object which impacted the atmosphere over Botswana on 21 November 2009, resulting in a list of 33 events between 2000 and 2013 that were included in the study. Only one of these events, the 7 October 2008 impact of Apollo asteroid 2008 TC3 near Almahatta Sitta in Sudan was detected in advance, and then only by a matter of hours.

Geographical distributions of the events included in the study. De la Fuente Marcos and de la Fuente Marcos (2014).

The list included four calendar days on which more than on object hit the Earth on the same day, albeit in different years (including one set of three objects on 8 October 2004, 2008 and 2009). In addition there were 9 events that occurred within one day in different years, 14 events that occurred within two days on different years and three events that occurred within three days. De la Fuente Marcos and de la Fuente Marcos calculate that there is a 35.7% chance of two such events falling on the same day, but only a 4.05% chance of four such pairings occurring. They further calculate that there is only a 1.9% chance of 9 events occurring within one day in different years, a  0.93% chance of 14 events occurring within two days and a 2.16% chance of 16 events occurring within three days in different years.

Calendar-day distributions of the events in the study. Green points correspond to impacts in the northern hemisphere and blue points correspond to impacts in the southern hemisphere. There are 17 impacts in the northern hemisphere and 16 in the southern one. The calendar day distribution for impacts in the northern hemisphere is almost uniform. In contrast, the equivalent distribution for the southern hemisphere is very asymmetric. The overall impact risk for the southern hemisphere is 50 per cent higher than that of the northern hemisphere from September to December.De la Fuente Marcos and de la Fuente Marcos (2014).

This strongly suggests that the distribution of impact events throughout the year is non-random, and that impacts are preferentially likely to happen on certain days of the year, presumably as the Earth’s orbit intersects other orbital pathways where more than one object follows the same path. This can occur for one of two reasons. Firstly a single object can break up for some reason, and the resultant bodies continue to follow the same path. This most notably occurred in 1994, when comet Shoemaker-Levy 9 broke up into several component bodies after a close encounter with Jupiter, each of these daughter bodies going on to impact the planet. Alternatively different bodies can be shepherded into similar orbital paths by repeated encounters with planets. This occurs because when two bodies pass close to one-another they exchange some momentum energy, so that one body speeds up slightly and the other slows down slightly. When this happens repeatedly between a very large body, such as a planet, and a very small body, such as an asteroid, the smaller body is herded into a resonant orbit with the larger body, and over time many objects can be herded into such orbital paths.

Images of the impacts on Jupiter caused by the breakup of comet Shoemaker-Levy 9 in 1994 at (left) visible and (right) ultraviolet wavelengths. NASA/Hubble Space Telescope.

The 15 February 2013 Chelyabinsk impact event has been studied particularly extensively, due to the large and conspicuous nature of this event, and the impacting body has tentatively been linked to a family of resonant asteroids. The Chelyabinsk object is believed to have been on an orbital pathway intersecting the Earth’s orbit at an angle of 146.5˚. The  South China Sea impact event of 18 February 2000 occurred within three calendar days of the Chelyabinsk event, and this object has also been suggested to have been travelling at 146.5˚ to the Earth’s orbital trajectory, strongly supporting the idea that these bodies hailed from a single population.

The fireball caused by the Chelyabinsk Bolide. Astro Bob.

The North Pacific object of 23 April 2001 fell between Hawaii and California. The Sutter’s Mill Meteor of 22 April 2012 fell in California and may be related to this. The Sutter’s Mill Meteor if thought to have been on a trajectory at an angle of ~212.5˚ to the orbit of the Earth, falling towards the Sun at the time of the impact, possibly similar to the trajectory of a Jupiter Family Comet (a comet with a period of less than 20 years, and an inclination of less than 30˚ to the plain of the Solar System). The Santiago del Estero object fell in Argentina in 21 April 2013, but appears to have had quite different properties to the other two impacts, and is probably unrelated.

Sutter’s Mill Meteor seen from Reno, Nevada. Lisa Warren/NASA.

The Crete Bolide of 6 June 2002 and the Reisadalen Event of 7 June 2007, which occurred close to the Norway/Finland border, both involved objects travelling at an angle of 255.5˚ to the Earth’s orbital trajectory, and falling towards the Sun at the time of the impact.  A bright fireball observed over Washington State on 3 June 2004 may also be related to these objects.

 

The Southern Ocean event of 3 September 2004, which fell near Antarctica, the Indian Ocean Event of 2 September 2006 and the South Pacific Event of 3 September 2010 all appear to be related. All were apparently travelling at an angle of 340˚ to the orbital path of the Earth, and falling towards the Sun at the time of the impact. It has been suggested that the parent body of the Southern Ocean event was a small Aten Family Asteroid (an asteroid which orbits mostly inside the orbit of the Earth, but which is further away some of the time), with an orbital period of 293 days. Aten Family Asteroids are thought to have particularly short lifetimes, as the planets of the Inner Solar system are quite close together, so that orbits are easily perturbed by repeated encounters with these planets, resulting in objects breaking up, colliding with something or being thrown out of the system.

The South Pacific event of 22 September 2007 and the Vitim event over Siberia on 24 September 2002 appear to be related, both objects having been travelling at an angle of 359˚ to the Earth’s orbit prior to their impacts.

The Indian Ocean event of 7 October 2004, the Almahatta Sitta impact in Sudan on 7 October 2008 also appear to be connected, both having involved objects travelling at 14˚ to the Earth’s orbit. The Almahatta Sitta impact involves the only asteroid in the study observed prior to its impact on the Earth, 2008 TC3, which in turn has been linked to the Nysa-Polana Asteroid Family (expand). The South Sulawesi event of 8 October 2009 appears to be unrelated.

A recovered piece of the Almahatta Sitta meteorite. Jon Taylor/Wikimedia Commons.

The 10 November 2002 North Pacific impact and 9 November 2005 New South Wales impact also appear to be related. Both caused by objects travelling at almost 47˚ to the Earth’s orbital trajectory.

The 26 December 2007 South Pacific event and 25 December 2010 North Pacific event also appear related, both having impacted at an angle of ~93˚.

De la Fuente Marcos and de la Fuente Marcos are confident that there is less than a 5% chance that all of the objects occurring on similar trajectories on similar dates could be coincidences. They note that streams of asteroids containing large bodies that intersect the Earth’s orbit are unlikely to persist for very long periods of time, as such bodies are likely to either impact the Earth or Moon or be disrupted into different orbits by their gravity or those of nearby planets. However such streams of asteroids could still potentially persist for several decades, leading to a series of connected impacts.

See also…

The Chelyabinsk Meteorite and its implications for the origin of the Baptistina Asteroid Family.

The Chelyabinsk Meteorite detonated in the atmosphere over the southern Russia on 15 February 2013 with an equivalent energy to 500 kilotons of TNT. From the size of the explosion it is estimated to have...

The nature of the Chicxulub impactor.

65 million years ago, at the end of the Cretaceous, the Earth underwent the last of the five great mass extinctions recorded in the fossil record. While this is by no means the largest of these events, it is the most familiar to the general public, as it was responsible for the extinction of, amongst other things, the non-Avian Dinosaurs and the large marine Reptiles of the...

The origin of the Bosumtwi Impactor.

Lake Bosumtwi is an 8 km diameter roughly circular lake about 30 km to the southeast of Kumasi in the Ashanti Province of Ghana. It is thought to have been created by a large meteorite impacting the Earth roughly 1.07 million years ago during the Late...

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Evidence for a Younger Dryas impact event?

The last Pleistocene Ice Age started to end about 20 000 years ago; after this temperatures rose more-or-less steadily till the end of the Pleistocene. This warming trend was interrupted by three reversals, in which the temperatures fell sharply, known as the Oldest, Older and Younger Dryas (so named because they can be identified in drilled core samples by increases in the abundance of the pollen of the alpine plant Dryas octopetala). The Oldest Dryas occurred between about 18 000 and 15 000 years ago, the Older Dryas between about 14 000 and 13 500 years ago (though this is not well calibrated & other dates are often quoted), and the Younger Dryas between about 12 800 and 11 500 years ago (the end of the Younger Dryas is generally also considered to be the end of the Pleistocene and the beginning of the Holocene).

Ice Age fauna such as Wooly Mammoths either had their last flourish in the Younger Dryas, or were wiped out by it, depending on which people you talk to. BBC.

The causes of these temperature reversals are not entirely clear and are therefore the subject of considerable scientific debate, the most popular theories being that they are the result of melting ice-sheets dumping large amounts of cool fresh water into the oceans, both cooling the oceans and altering the flow of ocean currents, and therefore causing periods of cooling and flooding as part of a general warming trend (this is a predicted outcome of modern anthropogenic global warming, and was used to good effect by science fiction writer John Wyndham in his 1953 novel The Kraken Wakes, in which alien invaders deliberately melt the Earth's ice caps, cooling the climate and flooding our cities), or that these cooler periods are the result of Milankovitch Cycles, long term variations in the Earth's tilt which effect the climate.

How Milankovitch Cycles work. Lyndon State College of Vermont.

In 2007 a group of scientists at a meeting of the American Geophysical Union proposed that the Younger Dryas event was caused by an impact event, with one or more large objects either slamming into the Earth, or exploding in the upper atmosphere and causing widespread devastation and a cooling in the Earth's climate, based upon samples from sites across North America that they interpreted as containing impact related material. This was not well received, as such cooling events can clearly occur without impact events, and over the next few years much of the evidence was debunked; minerals were shown to have been misidentified, and in one case 'shock-related carbon spherules' were shown to be fungus spores.

Artist's impression of a possible impact event at the start of the Younger Dryas. Florida Frontiers/Neily Trappman Studios.

In a paper published in the Proceedings of the National Academy of Sciences, a team of scientists lead by Isabel Israde-Alcántara of the Departamento de Geología y Mineralogía at the Instituto de Investigaciones Metalúrgicas at Universidad Michoacana de San Nicólas de Hidalgo announce the results of a study of deposits in Lake Cuitzeo in central Mexico which they interpret as evidence supporting the Younger Dryas impact theory.

The location and topography of Lake Cuitzeo. From Israde-Alcántara et al. (2012) supplementary material.

Israde-Alcántara et al. report the discovery of a horizon in cores from Lake Cuitzeo that they interpret as the start of the Younger Dryas, at which they find extensive charcoal, magnetic grains, framboidal spherules and weekly magnetic volcanic glass, material they interpret as indicative of an impact event.

Scanning Electron Microscope images of possible impact grains from Lake Cuitzeo. (A, B) Magnetic spherules with dendritic (branching) surface patterns. (C) Framboidal (raspberry-patterned) pyrite spherules. (D) Collisional magnetic impact spherules (?). (E) Light micrograph of (D). (F) Teardrop-shaped particle with dentritic surface pattern. (G) Light micrograph of (F). Scale for (D-G) not given in original. From Israde-Alcántara et al. (2012).

However there are a number of problems with this diagnosis.

Firstly there is the dating of the horizon to the beginning of the Younger Dryas. Israde-Alcántara et al. attempted to gain carbon-dates for the sequence (itself not a perfect method), but were unable to do so, apparently because the sediments had been overturned by turbulence, which should have set alarm-bells ringing. Instead they dated the onset of the Younger Dryas as a peak in organic carbon content in the sediments, above which the amount of oak pollen was dramatically reduced, and the amount of grass pollen dramatically increased.

This is widely seen at the onset of the Younger Dryas, but is not indicative of it. It probably shows the death of an oak forrest and its replacement by grasslands, which might be caused by global climatic breakdown, but could also be caused by a local forrest fire; on its own it is not evidence of anything.

Oak pollen became much less abundant at the start of the Younger Dryas, as climatic breakdown killed off many oak forests. University of Arizona.

Then there are the particles themselves. Alcántara et al. observe that similar particles can be generated by volcanic eruptions, but that such eruptions do not typically spread them over a wide area, and therefore reject a volcanic origin for the particles. However the particles were not gathered over a wide area, but from a single lake, surrounded by volcanoes. This does not enable a volcanic origin to be ruled out (indeed taken with a local die-off of oak forests, this is quite a good explanation). Even if the particles were scattered over a wide area, Alcántara et al. would need to establish that an unusual volcanic event, in which such particles were scattered over a wider area, was a less likely explanation than a major impact event.

Alcántara et al. are clearly enthusiastic supporters of impact events to explain terrestrial phenomena. They are keen to stress the similarities between the putative Younger Dryas impact event, and the impact event at the end of the Cretaceous. Unfortunately this event has been repeatedly shown to have occurred before the end of the period, implying that it cannot have been responsible for the extinction event.

Most scientist have moved on from this theory, and those that have not are forced to deal with the survival of multiple groups of Cretaceous animals into the Palaeocene (for example a forthcoming paper by Landman et al. in the journal Acta Palaeonotologica Polonica, proposing the survival of Ammonites beyond the end of the Cretaceous, as defined by impact stratigraphy). This is nonsensical; the geological periods were defined by their fossils, as this has been the easiest way to date rocks for much of the history of geology. To start redefining geological periods to fit in with impact events that didn't cause mass extinction events would be close to insanity.

Scientists need to be enthusiastic about their work to succeed, but when this enthusiasm clouds judgement in favor of pet theories, then the science produced will inevitably be poor. The best science is often produced by scientists able to reject their own preferred theories (which also has the advantage of saving them the embarrassment of somebody else doing it).

Were a string of volcanic eruptions responsible for the Little Ice Age, See also Did the first land plants cause an Ordovician glaciation? Asteroid 2012 BX34 to pass within 59 000 km of Earth, Possible impact crater in Palm Valley, Northern Territory, Australia and The End of the Cretaceous.

Asteroid 2011MD

At about 5.00 p.m. Greenwich mean time today (27th June 2011) Asteroid 2011 passed over Australia, Southern Africa and Central America at a distance of about 12 000 km or 32 times as close as the moon, which sounds pretty close, but, to give a sense of proportion, is also 36 times as far from the Earth as the International Space Station.

The asteroid was discovered five days previously (on the 22nd June) by the Lincoln Near-Earth Asteroid Research (LINEAR) project in New Mexico and given the designation 2011 MD, which means the fourth asteroid discovered in the period 16-30 June 2011. For the purpose of naming asteroids the year is split into 24 half month periods, numbered A to Y (I is not used).

An image of 2011MD, a faster moving streak against the background of stars.

2011MD is thought to be a house sized chunk of rock, between 10 and 45 meters across. This is small enough that it would break up in out atmosphere; small chunks might reach the ground but it would be unlikely to do any significant damage. Nevertheless reports of the near miss in the popular press have been peppered with stories about dinosaur killing asteroids. These step from the 1980 theory proposed by physicist Luis Walter Alvarez that the mass extinction 65 million years ago at the end of the Cretaceous Period, most noted for the loss of all non-avian dinosaurs, was caused by a massive meteorite impact, and the subsequent discovery of a global iridium layer at the same stratigraphic level by his son Walter Alvarez. This theory was popular throughout the 80s and 90s, particularly after the discovery of an impact crater off the Yucatan Peninsula in Mexico which seemed to fit the bill. Subsequent study by geologists has suggested that this crater is in fact to early to mark the end of the Cretaceous, and the impact theory in general is not as widely supported in the geological community as might seem to be the case to an outsider. Many geologists feel that volcanic activity associated with the collision of India and Asia, particularly the massive Deccan Traps flood basalts, may have caused a catastrophic climatic breakdown which caused the extinction. However the impact theory has rather more 'Hollywood' appeal than the volcanic one so it remains the theory most likely to be seen in the popular press, and it is still widely supported by astronomers and astrophysicists, partly out of tribal loyalty, and partly because they do not tend to follow geological journals closely.

Almost certainly not how the dinosaurs died out.

This is not to say that a sufficiently large asteroid impact couldn't cause devastation on such a large scale, or indeed far, far worse. The moon is thought to have been produced as the result of a collision between the Earth and a Mars-sized planet over four billion years ago, but as the solar system has aged the number of large objects floating about has steadily decreased; the odds of such a large impact happening now, let alone with an object we had not yet seen, are vanishingly small.

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