Saturday, December 22, 2007

Asteroid impacts might be beneficial (in the long term)

The image on the left is an artists impression (from of the first moments of the K-T impact, the asteroid impact that probably caused the mass extinction event at the end of the Cretaceous period about 65 million years ago.

Although mass extinctions wipe out many of the species that are present on Earth at the time of the extinction event, the number and diversity of animal and plant species ultimately increases after mass extinctions. Thus, although mass extinctions are in the short term (several million years) destructive, on the longer terms of hundreds of millions of years they may actually beneficial. Carl Zimmer discusses this, more specifically asteroid-related extinctions, in an article at Wired (NB, only a few of the documented mass extinctions have been plausibly shown to be due to asteroid impact).

The ambiguous and poorly-understood long-term effect of destructive astronomical events (such as asteroid impacts, supernovae, gamma-ray bursts, AGN, etc) on biological life is something that renders assessing the size of the galactic habitable zone difficult, if not impossible, at the present and with our current understanding of these astronomical events and the fossil record.

In order to arrive at a conception of a very small Galactic Habitable Zone (visible in the images from the 2001 Gonzalez et al Scientific American article) , i.e. that the Solar system and the Earth was very unusual (a "privileged planet" that was then evidence of divine favor [or in code: Intelligent Design]), Guillermo Gonzalez assumed that anything that increases the chances of asteroid impacts, or nearby supernova, or high UV or cosmic ray fluxes, was negative and harmful to life.

But those assumptions are by no means robustly justified by existing data, nor are they unique (as the opposite effect, as discussed in Zimmer's article, could plausibly be true), as other astrobiologists pointed out the him at the time. Indeed, it could be that external events are ultimately responsible for driving greater evolutionary diversity and hence for increasing the chances of complex multi-cellular life evolving.

While on the subject of asteroid impacts, has two asteroid-related articles.

The first article, by Charles Choi, discusses new simulations by researchers at Sandia National Labs that suggest that the 1908 Tunguska explosion could have been caused by meteorite only 20 meters in diameter, smaller that previously thought. As there the number of asteroids of a given size is a strongly decreasing function of the their size., this implies that Tunguska-level events might be more common than previously thought. However, the actual destruction caused by the Tunguska explosion is also probably less than previously estimated, so rest easy!

The second article, by Alicia Chang, discusses an asteroid that has a 1 in 75 chance of hitting Mars (not us) this coming January 30th. This asteroid, 2007 WD5, would also cause a Tunguska-level explosion, equivalent to an explosion of about 15 Megatons of TNT.

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