* Impact events have had major effects in the history of the Earth, most significantly in the ending of the age of dinosaurs. A future massive impact could destroy civilization, so work is underway to provide warning, and hopefully avert such an event.
* In 1980, a team of researchers led by Nobel-Prize-winning physicist Luis Alvarez; his son, geologist Walter Alvarez; and a group of colleagues published a paper in the scientific press. The paper pointed out that fossilized sedimentary layers found all over the world dating back to the end of the Cretaceous era, 65.7 million years ago, contain a high proportion of iridium, which is relatively common in asteroids.
The iridium concentration was almost two orders of magnitude greater than normal. The end of the Cretaceous coincided with the end of the dinosaurs and was in general a period of extraordinary mass extinction, leading to the Paleogene era, in which mammals began to predominate on the Earth. Incidentally, back in the 1980s geologists had a different scheme for naming the post-Cretaceous eras, with the first era being called the "Tertiary".
In any case, the paper suggested that the dinosaurs had been killed off by the impact of a ten-kilometer-wide asteroid on the Earth. The resulting blast would have been hundreds of millions of times more devastating than the most powerful nuclear weapon ever detonated, possibly created a hurricane of unimaginable fury, and would have certainly thrown massive amounts of dust and vapor into the upper atmosphere or even into space. The worldwide cloud would have choked off sunlight for years, resulting in a "long winter" that wiped out many existing species, as well as creating "acid rains" that would have inflicted further hardship on the environment.
Although further studies of the "Cretaceous-Paleogene" or "K-Pg" layer -- "K" is used by geologists to refer to the Cretaceous era, because "C" is taken by the "Cambrian" era -- consistently showed the excess of iridium, the idea that the dinosaurs had been exterminated by an asteroid remained a matter of controversy among geologists for over a decade. There was the particular problem that no crater was known that matched the event. That was not a lethal blow to the theory; although the crater resulting from the impact would have been 150 to 200 kilometers in diameter, as mentioned in the previous section the Earth's geological processes tend to hide craters over time. Still, finding a crater would obviously have buttressed the "Alvarez hypothesis", as it came to be known.
In early 1990, Alan K. Hildebrand, a graduate student at the University of Arizona, visited a small mountain village named Beloc in Haiti. He was investigating certain K-Pg deposits that include thick, jumbled deposits of coarse rock fragments, which were apparently scoured up from one location and deposited elsewhere by kilometers-high "tsunamis", giant sea waves, that most likely resulted from an Earth impact. Such deposits occur in many locations, but seem to be concentrated in the Caribbean basin.
Hildebrand found a greenish brown clay with an excess of iridium, and containing shocked quartz grains and small beads of weathered glass that appeared to be tektites. He and his faculty adviser William V. Boynton published the results of the research in the scientific press, suggesting not only that the deposits were the result of an Earth impact, but that the impact couldn't have been more than 1,000 kilometers away. This was particularly puzzling, because no crater of any size was known to exist in the Caribbean basin. Hildebrand and Boynton also reported their findings to an international geological conference, sparking substantial interest.
Evidence pointed to possible crater sites off the north coast of Colombia or near the western tip of Cuba. Then Carlos Byars, a reporter for the HOUSTON CHRONICLE, contacted Hildebrand and told him that a geophysicist named Glen Penfield had discovered what might be the impact crater in 1978, buried under the northern Yucatan Peninsula.
In that year, Penfield had been working for Petroleos Mexicanos (PEMEX, the Mexican state-owned oil company), as a staff member for an airborne magnetic survey of the Yucatan Peninsula. When Penfield examined the survey data, he found buried in the noisy data a huge underground "arc", with its ends pointing south, in the Caribbean off the Yucatan that was inconsistent with what he knew about the region's geology. Penfield was intrigued, and managed to obtain a gravity map of the Yucatan that had been made in the 1960s and was gathering dust in PEMEX's archives. He found another arc, but this one was on the Yucatan itself, and its ends pointed north. He matched up the two maps and found that the two arcs joined up in a neat circle, 180 kilometers wide, with its center at the village of Puerto Chicxulub.
Penfield was an amateur astronomer and had a good idea of what he was looking at. Although PEMEX would not allow him to release specific data, the company did allow him and a PEMEX official named Antonio Camargo to present their results at a geological conference in 1981. Unfortunately, that particular conference was under-attended in that year, ironically because most geologists were attending a workshop on Earth impacts. The report attracted very little attention, though it did get back to Byars.
Penfield didn't give up. He knew that PEMEX had drilled exploratory wells in the region in 1951. One of the wells had bored into a thick layer of igneous rock known as "andesite" about 1.3 kilometers down. Such a structure could have resulted from the intense heat and pressures of an Earth impact, but at the time of the borings it had been written off as a "volcanic dome", even though such a feature was out of place in the geology of the region. Further studies of the archived well cores would have resolved the issue, but unfortunately most of them had been destroyed in a warehouse fire in 1979. Penfield then flew down to the Yucatan to see if he could find anything out from the "tailings" left by the wellheads. This idea didn't pan out, and in one case Penfield found himself digging through a communal pigsty that had been set up on a wellhead site, a task he described as "unpleasant and unrewarding."
After Hildebrand got in touch with Penfield, however, the two men were able to locate two separate samples from the wells drilled by PEMEX in 1951. Analysis of the samples clearly revealed shock-metamorphic materials. Studies by other geologists of the debris found in Haiti at Beloc also showed it to be clearly the result of an impact. This research was persuasive, and received a major boost when a team of California researchers, including Kevin O. Pope, Adriana C. Ocampo, and Charles E. Duller, conducted a survey of satellite images of the region. They found that there was a nearly perfect ring of sinkholes centered on Puerto Chicxulub that matched the ring Penfield had found in his data. The sinkholes were likely caused by subsidence of the crater's wall. This evidence was enough to persuade the geological community there was really something going on worth paying attention to.
The K-Pg impact event has been described as a disaster of Biblical proportions. Along with the simple explosive force of the collision and the tidal waves that it generated, it tossed up hot debris to an altitude almost a quarter of the way to the Moon. When the debris came back down over the next few days, it started fires almost everywhere it landed, with only the northern latitudes escaping complete destruction. The marker of this global firestorm is a layer of soot along with the iridium.
Between the material thrown up by the impact itself and the smoke generated by the fires, the Earth was shrouded in dark clouds that cut off sunlight for months or years. Once the clouds faded away, there was so much carbon dioxide in the atmosphere that global temperatures skyrocketed. It took anywhere from centuries to millennia for the global ecosystem to stabilize again, and when it did stabilize it was a different ecosystem from the one that had existed before the day of fire.
* However, although the K-Pg impact theory has acquired a considerable following, skepticism over the theory, at least as it was proposed, persists. Nobody seriously doubts that the Chicxulub impact occurred and that it was cataclysmic, but there is doubt that it really killed off the dinosaurs.
Paleontologists were skeptical, since their reading of the fossil record suggested that the mass extinctions did not take place over a period as short as a few years, instead occurring gradually over about ten million years. There were also odd patterns to the extinctions: some species that would have been expected to die out under the conditions postulated for the impact cataclysm survived, while others that might have been expected to survive died out. Luis Alvarez, who died in 1988, did acknowledge that other factors, such as a drop in sea level and massive volcanic eruptions, might have contributed to the mass extinction, but he maintained that the impact was of primary importance and that paleontologists were being misled by sparse data.
One interesting proposal is that there was a much bigger impact a few hundred thousand years earlier, its remnant being a large undersea depression off the west coast of India. Advocates of the "Shiva impact", as it has been named in honor of the Hindu god of destruction, claim the object was four times bigger than the object that struck the Yucatan. They claim that the single iridium layer observed in strata actually consists of two closely-spaced layers. Skeptics suggest the "crater" is a figment of imagination.
Researchers have found more convincing evidence of a second major impact around the time of the Chicxulub impact: a previously undiscovered crater 400 kilometers off the west coast of Africa. Uisdean Nicholson, an assistant professor at Heriot-Watt University in Edinburgh, found the crater by accident while reviewing seismic survey data for another project on Atlantic plate tectonics. He said: "While interpreting the data, I [came] across this very unusual crater-like feature, unlike anything I had ever seen before. It had all the characteristics of an impact crater." It had the right ratio of crater width to depth, the height of the rims, and the height of the central uplift.
The crater is near the Nadir Seamount, and so was named the "Nadir Crater". It is 8 kilometers (5 miles) wide, and Nicholson believes it was likely caused by an asteroid more than 400 meters wide smashing into the Earth's crust. It was not at all in the same league as the Chicxulub impact -- but it was still monstrous. Nicholson said:
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The [Nadir] impact would have had severe consequences locally and regionally -- across the Atlantic Ocean at least. There would have been a large earthquake [magnitude 6.5 to 7], so significant ground shaking locally. The air blast would have been heard across the globe, and would have itself caused severe local damage across the region.
END_QUOTE
It would have generated a tsunami wave up to a kilometer high at the source, fading to a few meters by the time it reached South America. Inspection of microfossils in nearby exploration wells show that the crater was formed around 66 million years ago, at the end of the Cretaceous period. However, there's still uncertainty of about a million years of its exact age. There may or may not be any linkage between this impact and the Chicxulub impact, but it is possible that the two were due to the breakup of a single large asteroid, whose pieces then struck the Earth.
* It is tempting to link impact events to other mass extinctions in the history of life on Earth, but the evidence for such is even more ambiguous than for the K-Pg event. The biggest known mass extinction was at the end of the Permian period, 250 million years ago, with about 90% of species wiped out, including the well-known "sailfin" or "sailback" reptiles. This mass extinction was long thought to be due to slow climate change and not some global cataclysm, but more recently evidence of huge volcanic eruptions was uncovered, suggesting that some catastrophe was involved. Recent studies have suggested that an impact event might have occurred and identified craters as possibly being created by the event.
To further confuse matters, remnants have been found of other major impacts that are not linked to any mass extinctions. Nobody doubts the impacts of course, and few deny the possibility that extinctions due to such impacts may have occurred during the history of the Earth. Indeed, in the early history of the Earth, about four billion years ago, they were almost certainly common since the skies were far more full of "junk" than at present. Such impacts could have included strikes by asteroids hundreds of kilometers in diameter, with explosions so powerful that they vaporized all the Earth's oceans. It was not until this "hard rain" began to slacken, so it seems, that life could have emerged on the surface of the Earth -- though there are speculations that it could have been brewing underground before the end of the impact era.
BACK_TO_TOP* The publicity generated by the Alvarez hypothesis raised awareness of the possibility of future Earth impacts with asteroids that cross the Earth's orbit. A few hundred such "near-Earth objects (NEOs)" are known, ranging in size up to four kilometers. Tens of thousands probably exist, with estimates placing the number of NEOs larger than one kilometer in diameter at up to 1,000. Earlier estimates placed the number of such large NEOs at about 2,000, but closer examination of NEOs showed that their reflectivity had been underestimated slightly, making them seem bigger than they really were.
Astronomers believe that NEOs only survive in their orbits for 10 million to 100 million years. They are eventually eliminated either by collisions with the inner planets, or by being ejected from the solar system by near misses with the planets. Such processes should have eliminated them all long ago, but it appears they are resupplied on a regular basis.
Some of the NEOs with highly eccentric orbits appear to actually be extinct "short period" comets that have lost all their volatiles, and in fact a few NEOs still show faint comet-like tails. These NEOs were likely derived from the "Kuiper Belt", a repository of comets residing beyond the orbit of Neptune. The rest of the NEOs appear to be true asteroids, driven out of the asteroid belt by gravitational interactions with Jupiter.
There is also a threat of impacts by comets falling into the inner Solar System after having been disturbed from their orbits in the "Oort Cloud", a huge, tenuous sphere of comets surrounding the Solar System. Such "long period" comets are only infrequent visitors into the inner Solar System and they do not generally fall in orbits in the same plane as that of Earth, but there is nothing to rule out the possibility that one might collide with the Earth. The impact velocity of a long-period comet would likely be several times greater than that of an NEO, making it much more destructive.
Although there have been a few false alarms, a number of asteroids are definitely known to be threats to the Earth. Asteroid 1950 DA was lost after its discovery in 1950 since not enough observations were made to allow plotting its orbit, and then rediscovered on 31 December 2000. Proper calculation of its orbit then demonstrated that it has a 1 in 300 chance of hitting the Earth on 16 March 2880. This probability is a thousand times greater than any other known asteroid threat, and 50% greater than all other known asteroid threats combined. 1950 DA has a diameter of a kilometer.
It is difficult to determine the chances of its impact better than that. The uncertainty is due to minor irregularities in the Sun's shape, and so its gravitational field; weakening of the Sun's gravity through mass loss from the solar wind of particles that streams out from its atmosphere; uncertainties in the masses and so the gravitational pull of the planets; variations in the tidal pull of the surrounding galaxy; the subtle pressure of sunlight; and, in particular, a phenomenon known as the "Yarkovsky effect".
This effect was discovered by a Russian engineer named I.O. Yarkovsky a century ago. It is a subtle process: the heating of the asteroid's surface causes it to emit thermal radiation, which creates a slight amount of thrust. It is somewhat unpredictable, since an asteroid's ability to soak up heat from the Sun depends on its terrain, and the effect is also influenced by the asteroid's spin orientation and rotation rate -- obviously an asteroid that is spinning rapidly along an axis at a right angle to the Sun will tend to radiate heat more or less evenly in all directions, while one that is spinning slowly or along an axis pointing to the Sun will emit heat towards the Sun.
* Astronomers have been conducting surveys to locate the NEOs. One of the first to go into operation was the "Spacewatch" project, which used an old 90-centimeter telescope sited at the Kitt Peak Observatory in Arizona, updated with automatic pointing, imaging, and analysis gear to search the skies for intruders. The project was set up in 1980 by Tom Gehrels and Dr. Robert S. McMillan of the Lunar & Planetary Laboratory of the University of Arizona in Tucson, and is now under the direction of Dr. Mellissa J. Brucker.
The Spacewatch project later acquired a 1.8-meter telescope, also at Kitt Peak, to hunt for NEOs, and has provided the old 90-centimeter telescope with an improved electronic imaging system with much greater resolution, improving its search capability. Another asteroid survey project, the "Lincoln Near-Earth Asteroid Research (LINEAR) Project", was initiated in 1998 with NASA and USAF funding, with telescopes at White Sands Missile Range in New Mexico. LINEAR has cataloged large numbers of NEOs.
There are now at least half a dozen loosely-affiliated NEO search efforts going on around the world. The most ambitious survey effort is the University of Hawaii's "Panoramic Survey Telescope And Rapid Response System (PAN-STARRS)", which went online in 2010 on Mauna Kea; it also consists of a 1.8-meter wide-field telescope. A second telescope, "PAN-STARRS 2", has been set up nearby, with two more planned, though they haven't been funded.
The four PAN-STARRS telescopes will be independent optical systems, but they will all be steered together, with each telescope slightly skewed from the common boresight to provide a field of view of 3 degrees, six times greater than the full Moon. In completion, PAN-STARRS 4 will, in the course of conducting a general all-sky survey, map all NEOs larger than 140 meters that can be observed from Hawaii.
Satellite-based surveys have been flown. The first, the Canadian Space Agency's "Near Earth Object Surveillance Satellite (NEOSSat)" microsatellite, was launched as one of a set of payloads by an Indian Polar Satellite Launch Vehicle on 25 February 2013. NEOSSat was a space observatory with a launch mass of about 72 kilograms; it carried a 15-centimeter telescope to scan for asteroids crossing Earth's orbital path, with a precise pointing system permitting long exposures. However, almost nothing was said about the satellite after its launch, and it appears the satellite accomplished little or nothing.
Another space NEO search has been conducted by NASA as a secondary mission for the agency's "Widefield Infrared Survey Explorer (WISE)" satellite, launched on 14 December 2009. WISE used a cryogenically cooled telescope to perform an infrared sky survey, that survey including the observation of tens of thousands of asteroids. After its hydrogen coolant ran out in October 2010, WISE was still able to make observations with degraded sensitivity, and spent several months scanning the sky for NEOs, the mission being renamed "NEOWISE". The spacecraft was put into hibernation in early 2011, to be revived in late 2013 for a follow-on NEOWISE survey. At last notice, it was still conducting its survey.
NASA is now planning a dedicated NEO-hunting satellite, under the "NEO Surveyor" mission -- originally "NEO Camera (NEOCAM) -- which will involve launching an asteroid-hunting satellite in 2028. NEO Surveyor will have a launch mass of about 1,300 kilograms, and will have a 50-centimeter (20-inch) infrared telescope; instead of using expendable helium, the detectors will be cooled to cryogenic temperatures using passive cooling. The satellite will be placed at the Earth-Sun Lagrange point, where it will be balanced between the gravitational pulls of Earth and Sun. That will allow it to make observations not interrupted by orbiting the Earth, and also more easily spot NEOs within Earth orbit.
* The search for threatening asteroids implies some means of diverting those objects likely to strike the Earth. Detonating a nuclear weapon above the surface of an NEO would be one option, with the blast vaporizing part of the surface of the object and nudging it off course with the reaction. However, it is becoming increasingly obvious that many asteroids are "flying rubble piles" that are loosely glued together, and a nuclear detonation might just break up the object without adjusting its course. This has led to a variety of less drastic ideas for dealing with the threat:
Efforts on the matter continues, with an international collaboration named "NEOShield" initiated in 2012. The NEOShield research effort is intended to perform technical studies and provide recommendations for policy-makers on the subject.
As a practical experiment in redirection, on 24 November 2021 NASA launched the "Double Asteroid Redirection Test (DART)" spacecraft to the double asteroid Didymos / Dimorphous. Didymos is about 780 meters in diameter, while Dimorphous is about 160 meters in diameter.
DART weighed about 500 kilograms, and was propelled by a xenon-ion thruster; it did not carry an instrument suite beyond a navigation system and imager. It was accompanied by a 6-unit (6U) CubeSat from the Italian Space Agency named the "Light Italian CubeSat for Imaging of Asteroids (LICIACube)", which carried a narrow-angle and wide-angle imager; it was to be released 15 days before impact to observe the results. On 26 September 2022, DART slammed into Dimorphous, with later observations showing that the trajectory of the asteroid had been significantly altered.
The ESA is partnering with NASA on the DART effort, by launching a follow-on mission named "Hera" in 2024, with the collaboration named the "Asteroid Impact & Deflection Assessment (AIDA)" project. It will reach Didymos in 2027, its primary mission being to determine how much Didymos was deflected. It will carry an instrument suite for observations, including an optical imager, thermal infrared imager, and lidar. It will also carry two CubeSats -- "Juventus" and "Milani" -- for additional observations.
BACK_TO_TOP* As a footnote to the subject of Earth impacts, of course impacts have occurred on other worlds of our Solar System, with the obviously cratered face of the Earth's Moon being visible to the naked eye. Over 9,000 lunar craters have been named. The biggest is the "South Pole / Aitken Basin", on the lunar farside, which is about 2,500 kilometers in diameter. As with many of the lunar craters, it dates from the early era of planetary bombardment, being about four billion years old.
Although not one of the biggest craters, the crater "Tycho" is likely the most visible. It has a diameter of only about 85 kilometers, but it is surrounded by a bright "star" of ejecta material that makes it prominent. The ejecta is visible because Tycho is relatively young, only about 108 million years; over billions of years, continuing meteorite impacts would have degraded it. The prominent lunar maria, the dark "seas" easily visible to Earth, are not exactly craters. They were produced by impacts, but the impacts led to floods of basaltic lavas that created the plains.
Lunar impacts have been recorded in human history. A great streak of fire crossing the Earth's sky and terminating in the Moon was recorded by the monk Gervase of Canterbury in June 1178. Since the 1970s, this was suspected as evidence of a major lunar impact event on the time, resulting in the creation of the young crater "Giordano Bruno", which is about 22 kilometers across.
Recent studies have suggested that Gervase had probably observed a large fireball passing through the Earth's atmosphere that happened to cross his line-of-sight to the Moon. There are no other records of such an event at the time, suggesting the local nature of the event; the crater Giordano Bruno appears to be substantially older than 800 years; and a major impact on the Moon would have showered the Earth with fragments, resulting in spectacular meteor showers, for which there is no record.
One impact on the Moon has been more or less verified. On 15 February 1953, an amateur astronomer in Oklahoma, Dr. Leon Stuart, photographed a flash in the center of the face of the Moon that he believed to be due to an impact. There was a great deal of skepticism over "Stuart's Event", as it came to be known, with some suggesting that he had actually photographed a meteor strike in the Earth's atmosphere that just happened to be in the line-of-sight to the Moon. The matter was unresolved when Stuart died in 1968.
NASA researchers decided to inspect images returned by lunar orbiting probes to see if they could find a fresh crater that might have been caused by such an event. They estimated from the size of the flash on Stuart's photograph that the meteor had to have been about 20 meters across, and would have left a crater a kilometer or two across. The lunar surface is subjected to a form of "space weathering" over time, and a fresh crater would be brighter than neighboring craters.
Examination of images taken by the NASA Lunar Orbiter probes in the 1960s turned up nothing particularly interesting, but inspection of better images returned by the NASA Clementine probe, launched in 1994, revealed a bright crater about 1.5 kilometers in diameter right in the target area of Stuart's photograph. The NASA researchers calculated that the impact that produced the crater had an explosive yield of 500 kilotons. Absolute proof would probably require a close-up inspection of the crater, but the finding did make Stuart's belief that he had witnessed an impact much more plausible. The lunar surface is kept under continuous observation to spot impacts. One was observed on 17 March 2023; it was a relatively small impact, the meteorite being estimate to weigh about 40 kilograms.
It was something of a surprise in the 1960s when space probes showed the planet Mars to have a heavily cratered surface, telescopic observations from Earth before that time not having been clear enough to provide useful details. Mars has about a thousand craters, the biggest being "Huygens", with an average diameter of about 465 kilometers. Mars orbiter probes have recorded a number of impacts on the planet over the past few decades, for example two 130-meter craters that appeared in 2021 -- both of which created seismic waves that were picked up by Mars lander probes.
Mars has two little moons, "Deimos" and "Phobos". They are more or less typical of the small bodies of the Solar System, being irregular lumps pocked by craters, Phobos being well more cratered than Deimos. In fact, a chunk was taken out of one end of Phobos, leaving the crater "Stickney".
The rocky inner planets Mercury and Venus also have heavily cratered surfaces. The four "gas giant" outer planets -- Jupiter, Saturn, Uranus, and Neptune -- do not have solid surfaces and so have no craters, but all four have substantial sets of moons, which generally have impact features. However, these moons are to varying degrees icy worlds and support "cryo-geological" activities, such as ice volcanoes or drifting plates, that can modify the surface and, over time, erase craters.
Of course, if the four gas giants don't have craters, they are big and of course have impacts -- with Jupiter, well bigger than all the other planets put together, having an impact rate thousands of times greater than Earth. Over a dozen major impacts have been observed in the last half-century, the most spectacular being the impact of fragmented Comet Shoemaker-Levy 9 in 1994 -- with almost two dozen impacts observed, the biggest impact sites being the diameter of the Earth. There are few, if any, reports of impacts on the other gas giant planets.
BACK_TO_TOP* Incidentally, in the early 1970s I heard a word-of-mouth report of the 1972 Teton fireball -- the tale making it out to be a UFO that had run up the Rockies, gone in circles near the Canadian border, and then flown back out into space. I didn't actually learn about the actual incident for years, and then this story came back to me, illustrating just how urban myths get rolling.
* Sources include:
* Illustration credits:
* Revision history:
v1.0 / 01 jun 01 v1.0.1 / 01 dec 01 / Review & polish. v1.0.2 / 01 jul 02 / Added comments on SD 1950, cleanup. v1.0.3 / 01 jul 04 / Added Stuart's Event. v1.0.4 / 01 mar 06 / Review & polish. v1.0.5 / 01 apr 06 / A few tweaky changes. v1.0.6 / 01 feb 08 / Minor updates. v1.0.7 / 01 jan 10 / Comments on Shiva. v1.0.8 / 01 dec 11 / Review & polish. v1.1.0 / 01 nov 13 / Chelyabinsk fireball, NEOSSAT, NEOWISE. v1.1.1 / 01 may 15 / Review & polish. v1.1.2 / 01 mar 17 / Review & polish. v1.2.0 / 01 feb 19 / Added Hiawatha crater. v1.3.0 / 01 jan 21 / NeoSM, AIDA, new illustrations. v2.0.0 / 01 sep 23 / Went to two chapters, reorganized.BACK_TO_TOP
