Free Astronomy Magazine

SMALL BODIES means minor, as regards the shergottites’ age which turns out to be much youn- ger than that of the terrains from which they broke away. The Xanthe Terra re- gion is in fact 4.3 billion years old, while the radio- carbon dating performed on the available shergottites samples (all volcanic rocks) sets their age at between 150 and 600 million years ago. How can this huge age difference be explained? One of the main opponents to the hypothesis put for- ward by Werner and her col- leagues is Carl Agee (Institute of Meteoritics, University of New Mexico), who argues that such discrepancy is sufficient evidence to invalidate the hy- pothesis itself. Of the same opinion is also Alfred McEwen (University of Arizona), Princi- pal Investigator for the HiRISE instrument, the leading edge camera aboard the Mars Re- connaissance Orbiter. But Werner’s team contends that some of the elements con- stituting the shergottites may have undergone some alter- ations caused by the strong heat generated by previous small asteroids im- pacts, or as a result of the action of flowing water. Susceptible to this type of alterations would be for example an iron sulfide mineral called pyrrhotite, which was one of the most used for dating the shergottites. The isotopic clock could therefore be turned back, but the doubts remain. According to Mc- Ewen there are other craters which are more appropriate to be linked to shergottites, but it remains to evaluate whether the impacts that created them were able to transfer to the sur- face rocks enough energy to accelerate them to the escape velocity fromMars, i.e. more than 5 km/s. Some age-dating doubts have also emerged as regards the Mojave Crater, which S hergottites found in the Dar al Gani re- gion, Libya. The typical reddish- brown colour of the surface is due to the oxidized iron contained in the combination of minerals that characterises them. The several dark looking in- clusions scattered on the surface are chondrules. [Utas collection]