An artist's illustration of a large asteroid headed for Earth.
Asteroids may leave impact craters and occasionally wipe out entire species, but planetary scientist Elisabetta "Betty" Pierazzo also wants to know what happens to Earth's seas and skies. Her work at the Planetary Science Institute in Tucson, Ariz., has revealed not only how deep impacts altered Earth's past, but also how they might shape humanity's future.
Pierazzo's latest computer simulations showed how an ocean impact could destroy much of the ozone layer that protects life on Earth from harmful ultraviolet rays. Looking at such atmospheric effects of impacts is tough, but has provided a better understanding of past hits such as the one that wiped out the dinosaurs and left the Chicxulub crater.
Q: Do you think the case is now closed about the Chicxulub impact triggering the Cretaceous-Tertiary (K/T) extinction and the end of the dinosaurs?
A: One of the beauties of science is that no scientific conclusion is ever final. Any scientific theory is always valid as long as accumulated evidence – and I should say, not interpretation of data, but good solid data; unfortunately, it is easy to take bad data or interpretations of data, and use them in favor or against a theory; that is not good science – supports it.
I believe that the majority of scientists are pretty much convinced that the connection between the K/T mass extinction and the Chicxulub impact is very strong and compelling. A review of the evidence can be found in a recently published Science paper (Schulte et al., Science 327, p. 1214-1218). However, some scientists still believe that the evidence is not convincing enough and keep fighting this theory.
One of their objections is that besides the Chicxulub impact there is no other case in Earth's history in which a mass extinction has been linked to an impact event, and vice versa. This is a strange assertion, as nobody disputes the point that not all impacts can cause mass extinctions.
The Chicxulub impact was a case of the worst possible scenario: a large impact occurring in the wrong place and very possibly at the wrong time. The wrong place is the location of the impact, on a continental platform with a thick sedimentary layer that caused the release of large quantities of climatically active gases, on top of other catastrophic effects of a large impact. The wrong time is related to the fact that towards the end of the Cretaceous there was a slow but constant decline of the climate. That may have changed optimal living conditions for various species locally.
Q: How often have asteroids or comets triggered such mass extinction events on Earth?
A: The only event that shows a clear link between a mass extinction and an impact event is the K/T boundary. Keep in mind that the Chicxulub crater is the largest crater so far discovered on the Earth's surface in the Phanerozoic (the last 540 million years), when complex life evolved. At the same time, no other mass extinction has the same characteristics of the K/T extinction, especially with respect to the abruptness with which it occurred.
It is dangerous to try to find a unique cause for all known mass extinctions. They were all unique in some ways, and it is possible that every single one was triggered by different causes. Just because the K/T mass extinction is linked to the Chicxulub impact does not mean that all large mass extinctions are linked to an impact event, and vice versa, it takes very large impacts and particular impact conditions to trigger a large mass extinction.
Q: What are the big unknowns that remain about how past impacts affected Earth?
A: There is still so much we don't know very well about the effects of medium to large asteroid impacts. We do have an idea of the possible effects associated with impact events, but our understanding is mostly qualitative.
We know that large impacts can cause strong earthquakes or widespread tsunamis (if the impact is in the ocean). They can also eject large amounts of material around the globe, possibly enhanced by decomposition of sediments (if present) that may inject large amounts of climatically active gases in the upper atmosphere.
Material ejected from the impact way above the bulk of the Earth's atmosphere would re-enter the atmosphere from above, and due to friction with the atmosphere, generate a short-lasting global pulse of increased thermal radiation at the surface. Estimates of the level of radiation suggest strong thermal damage to the biosphere, even if high radiation levels lasted for only a few tens of minutes.
There has been a remarkable improvement in climate models that now allow us to combine models of the atmosphere, ocean, land, ice and biology into sophisticated single Earth-system models. There is hope that in the future we may make progress in applying these new models to the huge disturbances in the normal environment associated with large impact events. A big limiting factor has been the magnitude of the disturbance associated with large impacts, which has been too large for these complex models to handle.
Q: How do you think the public and policymakers should think about the ongoing or future risks of asteroid impacts?
A: We all live with various natural risks: the risk of earthquakes, volcanic eruptions, landslides, tsunamis, tornadoes, etc. Humans have an amazing capability of adapting and accepting risks while continuing with their lives. It is good practice to have plans for dealing with natural disasters, and this is something that should be thought of also in the case of the impact risk.
Overall, the probability of being hit by a medium asteroid is smaller than the probability of various natural disasters. However, the potential for loss of human lives and extent of the damage in the long term is much greater for a medium-size impact than for any natural hazard. This point has been expressed very well by David Morrison (from the NASA Ames Research Center): "The impact hazard represents the extreme case of a calamity of low probability but high consequences – consequences that include the possible end of civilization as we know it." I would say this is threatening enough that we should at least try to think about possible survival or recovery plans.
Q: Given NASA's limited budget, how would you prioritize detecting near Earth objects (NEOs) and figuring out ways to deflect them?
A: I believe that first of all we should try to quantify better what are the actual effects of different size asteroid impacts, especially in the medium range. Our understanding of what size asteroid may generate global effects is still limited; currently, the threshold for a civilization-destroying impact event is generally put somewhere between a 1 and 3 kilometer (0.62 and 1.86 mile) diameter object. In the case of smaller impactors, the intensity and extension of the environmental effects are not well established.
Scientists have leveraged data from explosion tests to get an idea of the intensity of some effects, but such data can assess only a limited number of effects. They also used data from natural events, such as volcanic explosions or wildfires, to try to understand the effect of large amounts of dust and various climatically active gases on the climate. However, the size of the disturbances associated with natural events and explosion tests is much smaller than that associated with medium to large impacts, and that makes for a very qualitative understanding of the effects associated with impacts.
The detection of NEOs is crucial to assess the risk of impacts in the near future. It also helps characterize the population of NEOs and validate theoretical models of the expected population, and that allows us to make estimates of potential NEOs which have not been discovered yet.
The Spaceguard program has been in place for over a decade now, and its success is clear. General estimates of NEO populations suggest that about 95 percent of asteroids are larger than 1 kilometer (0.62 miles) in diameter (the civilization destroyers). But more work is still necessary to detect slightly smaller objects. That may require access to more powerful telescopes that can detect smaller objects.
However, it is important to continue the assessment of possible ways to deflect asteroids in case we find one on a collision path with Earth. I believe this becomes quickly a political discussion, as the threat is global and not just for one country. Assessment and action on developing techniques for deflection may require international cooperation.
Q: Despite the destruction related to asteroid or comet impacts, are there any long-term benefits of such impacts for life on Earth?
A: When the Earth was young and life was starting to emerge, very large impacts could have had serious effects on its survival and evolution. At the same time, large impacts may have delivered elements and molecules that are important for the development of life. Today, the Earth's neighborhood is populated by smaller objects that could not destroy life on Earth. At the same time, life has allowed the development and buildup of all the important organic molecules, so life on Earth does not depend anymore on delivery of crucial building blocks for life to the Earth's surface.
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