Meteorites on Mars




Abstract– The article by Fairén et al. (2011) is very interesting to me, as not only have I published, albeit very cursorily, on the occurrence of meteorites on Mars (McCall 2005, 2011, 2012; McCall et al. 2006), but I was the author (McCall 1965) of the description of the Mount Padbury mesosiderite cited by Schröder et al. (2010) in their description of the four stony meteorites also found by the Opportunity rover. I have given my reasons elsewhere for thinking these are not mesosiderites (McCall 2012), but are likely differentiated stony meteorites of a hitherto unknown type.

In my first brief note on the first iron meteorite discovery, I suggested, on account of the apparent impossibility of such a large iron landing pat on the surface after descent through Mars’s thin atmosphere, that it was buried and re-exposed by deflation, and that burial could have occurred more than once. This is very much the process suggested by Fairén et al. What I find difficult to accept is that the fall of the six irons now discovered could be so early in Mars’s history as they suggest, when the climate was wet, that is, millions and millions of years ago. The problem is that with the wet climate, weathering would have been rapid, and these six irons do show evidence of weathering, but relatively slight—as is apparent in illustrations presented as a poster at 74th Annual Meeting of the Meteoritical Society (Ashley et al. 2011). Both Bland (personal communication) and I agree that meteorites have likely survived on the Martian surface for a very long period, even millions of years. But to take the falls so far back in time as Fairén et al. suggest seems to be self-defeating because of the weathering problem. It is very difficult to credit that iron meteorites, which are very susceptible to weathering under hydrous conditions, could survive so long buried or exposed under such hydrous conditions, without fairly rapid total decomposition. I think the irons must have fallen while the present arid/cold conditions ruled, although burial and exposure is still likely, although is not the only answer.

Finally, Schröder et al. (2010) mentioned that one of the stony meteorites was on a “pedestal.” This was also the case when the 12 and 5 ton Mundrabilla irons were discovered on shale “pedestals” above limestone surface of the arid Nullarbor Plain, Western Australia (see Figure 3 in McCall 1998): cosmic-ray evidence showed that they had been there for about 1 Myr.

Editorial Handling— A. J. Timothy Jull