Earth impacts
Earth belongs to the solar system, a collection of planets, asteroids, comets and other objects. In the course of its orbit, our planet often crosses paths with some of these object, some as large as several kilometers in diameter, and every now and then one of those chunks of rocks or ice slams into the Earth with spectacular results.
Recent pre-historic impact events
The Tunguska fireball was the only major impact event in history to be witnessed and recorded, but there is plenty of evidence for other recent, by geological standards at least, major impact events.
In hindsight, the Barringer Crater looks so much like the result of an impact that it is hard to imagine anyone could think it was anything else, but not all remnants of impacts events are so obvious.
See also the Rio Cuarto craters.
Another major impact apparently occurred much more recently; see the Wabar craters.
The geology of earth impacts
Centuries ago, the Western vision of the past saw an Earth that had been created a few thousand years ago, and had been shaped since that time by a number of global cataclysms. This view gradually gave way to the consensus that the Earth was several billion years old, and that its features reflected the slow processes of gradual change.
Since 1970, this view has gradually expanded to accommodate the fact that the Earth has in fact gone through periods of abrupt and catastrophic change due to the impact of large asteroids and comets on the planet. A few of these impacts may have caused massive climate change and the extermination of large numbers of plants and animals.
The fact that this modified view of the Earth's history did not emerge until recently seems surprising.
Impacts & mass extinctions
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 an article in the scientific press. The article 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 Tertiary era, in which mammals began to predominate on the Earth. 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-Tertiary" or "K-T" 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 problem that no crater was known that matched the event. This 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-T 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 tektikes. 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 Columbia 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 a 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 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, and their 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 presently 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 showed 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 get most of the geological community on the bandwagon, and further studies have reinforced the consensus. Indeed, some evidence has accumulated that the actual crater is 300 kilometers wide, and the 180 kilometer ring is just an inner wall.
However, paleontologists remained skeptical, as 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 was also a certain general distrust of a group of physicists intruding into their domain of expertise.
Luis Alvarez, who died in 1988, replied that that paleontologists were being misled by sparse data. His assertion did not go over well at first, but later intensive field studies of fossil beds lent weight to his claim. Eventually, most paleontologists began to come around to the idea that the mass extinctions at the end of the Cretaceous were largely due to a massive Earth impact. However, even Walter Alvarez has acknowledged that there were other major changes on Earth even before the impact, such as drop in sea level and massive volcanic eruptions in India, that may have also contributed to the extinctions.
Skeptics remain. Although there is now general agreement that there was a huge impact at the end of the Cretaceous that led to the iridium enrichment of the K-T boundary layer, remnants have been found of other impacts of the same order of magnitude that did not result in any mass extinctions, and in fact there is no clear linkage between an impact and any other incident of mass extinction.
Nonetheless it is now widely believed, if a little on faith, that mass extinctions due to impacts are an occasional event in 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 begun to evolve on Earth.
The NEA threat
The general acceptance of the Alvarez hypothesis has raised the awareness of the possibility of future Earth impacts with asteroids that cross the Earth's orbit. A few hundred such "near-Earth asteroids (NEAs)" are known, ranging in size up to four kilometers. Tens of thousands probably exist, with estimates placing the number of NEAs larger than one kilometer in diameter at up to 2,000.
There are three "families" of NEAs:
- The "Atens", which have average orbital diameters closer than one "astronomical unit" (AU, the distance from the Earth to the Sun), placing them inside the orbit of Earth.
- The "Apollos", which have average orbital diameters greater than that of the Earth. Notice the important condition of "average" orbital diameters. Some Atens and Apollos have eccentric orbits that cross the orbit of the Earth, making them a potential threat to our planet.
- The "Amors", which have average orbital diameters in between the orbits of Earth and Mars. Amors often cross the orbit of Mars, but they do not cross the orbit of Earth. The two moons of Mars, Deimos and Phobos, appear to be Amor asteroids that were captured by the Red Planet.
Astronomers believe that NEAs 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 NEAs with highly eccentric orbits appear to actually be extinct "short period" comets that have lost all their volatiles, and in fact a few NEAs still show faint comet-like tails. These NEAs were likely derived from the "Kuiper Belt", a repository of comets residing beyond the orbit of Neptune. The rest of the NEAs 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 NEA, making it much more destructive.
The threat of an Earth impact was emphasized by the collision of the comet Shoemaker-Levy 9 with Jupiter on 16 July 1994, resulting in explosive impacts that would have been catastrophic on Earth. To be sure, Jupiter is far larger and more massive than the Earth and so undergoes far more impacts, but the event still provided an illustration that such things do happen and can be unimaginably 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.
Astronomers have been conducting surveys to locate the NEAs. One of the best-known is the "Spacewatch" project, which uses 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 being operated by Dr. McMillan.
The Spacewatch project has acquired a 1.8 meter telescope, also at Kitt Peak, to hunt for NEAs, and has provided the old 90 centimeter telescope with an improved electronic imaging system with much greater resolution, improving its search capability. These new resources promise to increase the rate of NEA discoveries by Spacewatch from 20 to 30 a year to 200 or more.
NASA has considered a more extensive network of six 2.5-meter automated telescopes named "Spaceguard" to watch the skies for such intruders, but at present astronomers are still assessing the actual level of the threat posed by NEOs, and moving to such a "defensive" network is not generally regarded as justified at present.
Nonetheless, the fact that an impact of an NEA a kilometer or more in size would be a catastrophe unparalleled in human history has kept the idea of a defensive network alive, as well as led to speculations on how to divert objects that might be a threat. Detonating a nuclear weapon above the surface of an NEA 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 other ideas for dealing with the threat:
- Setting up "mass drivers" on the object to scoop up dusty material and shoot it away, giving the object a slow, steady nudge.
- Flying a big sheet of reflective mylar to wrap itself around the asteroid, acting as a "solar sail" to use the pressure of sunlight to shift the object's orbit.
- Dusting the object with powdered chalk or soot to perform a similar adjustment, using the Yarkovsky effect.
Thinking on the matter continues - see Asteroid deflection strategies - and if there is no prospect of immediate action, the issue isn't going away, either.