Cosmic Cataclysms and the Martian Hemispheric Dichotomy
The 26 June 2008 issue of Nature is devoted to cosmic cataclysms, specifically the impacts of asteroids with planets and moons. The issue contains articles about the still-mysterious Tunguska blast, the South Pole–Aitken basin on the Moon, and research into near-Earth asteroids. These are followed by a photo gallery of meteor craters on Earth, Mars, Phobos, Ganymede, Callisto, Mimas, and the Moon.
The issue continues with a commentary about Spaceguard, a survey of near-Earth asteroids that concluded that the chance of one of these striking the Earth is extremely small. Despite this, the Los Angeles Times is running a story this week that declares that the United States is unprepared for an impact by an asteroid. This press release from the Planetary Society contains additional information. (I believe it would be a waste of money to pursue this further.)
Finally, the issue contains three papers proposing explanations for the martian hemispheric dichotomy, which is the fact that the northern lowlands, the Borealis Basin, are on average four kilometers lower than the southern highlands. Using different approaches, the three papers provide evidence that the hemispheric dichotomy was caused by the impact of a dense asteroid with a diameter 25% to 40% of the diameter of Mars. Since the basin that exists today has an elliptical shape, it is proposed that the blow was not a vertical strike.
The evidence provided by these three papers strengthens the hypothesis of an impact origin, but Walter S. Kiefer points out in a News & Views article that an alternative explanation is that the Borealis Basin was formed by convection of the mantle. It is likely that the issue will not be resolved until we can directly examine the composition of the rocks from different parts of Mars.
References:
Andrews-Hanna JC, Zuber MT, Banerdt WB. 2008. The Borealis basin and the origin of the martian crustal dichotomy. Nature 453:1212–1215. DOI: 10.1038/nature07011.
Marinova MM, Aharonson O, Asphaug E. 2008. Mega-impact formation of the Mars hemispheric dichotomy. Nature 453:1216-1219. DOI: 10.1038/nature07070.
Nimmo F, Hart SD, Korycansky DG, Agnor CB. 2008. Implications of an impact origin for the martian hemispheric dichotomy. Nature 453:1220–1223. DOI: 10.1038/nature07025.
July 05 2008 10:40 am | Astronomy
