Geoscientist Online

The rover ‘Opportunity’ has encountered six iron meteorites on Mars: a major enigma, suggests Joe McCall.

Geoscientist Online Special 31 August 2011

I first mentioned the encounter by the rover Opportunity of a large iron meteorite on Mars¹ (‘Heat Shield Rock’, but officially renamed ‘Meridium Planum’). I pointed out that because of the lack of friction to impede or break up such a large mass arriving on Mars, for such a mass to break up due to atmospheric friction in approach or brake and land gently on the cold Martian surface was quite impossible.

• Image: The first iron to be encountered by 'Opportunity'

I and others later discussed this² and suggested either it was product of a much earlier fall, when the atmosphere was warmer and wetter; or, it fell into a lake or sea then, and that drained away or evaporated; or that it hit a sand dune which absorbed the shock, was buried and re-excavated by deflation, leaving the meteorite exposed; or finally, that it could have survived cycles of building up and erosion of the plain, originally being buried but now being exposed.

I am not alone in remarking on the impossibility of such a large mass plunging through Mars’s thin atmosphere and hitting the ground and ‘sitting on it’. I quote³: “A meteorite the size of a huge water melon on Mars is revealing new clues to the planet’s environment. It weighs at least half a ton, making it much too large to have plunged through Mars’s thin atmosphere and hit the ground without being obliterated on impact”, rover scientists said. “Either the atmosphere was thicker than expected at some time in the past……” Analysis showed it to be made of iron and nickel.

All this would be old hat now - but Opportunity has since encountered a multiplicity of similar irons since that first encounter.

An excellent poster in the aborted (because of nearby riots) 9 August poster session at the 74th Meteoritical Society Meeting at Greenwich⁴ illustrated such masses and their location on the path of Opportunity. The first three new ones have been named, ‘Block Island’, ‘Shelter Island’ and ‘Mackinac Island’. The abstract reported that in fact six had been encountered in all. The rover spotted number five at the end of its 81-metre drive on 16 September 2009. And there is a sixth now being examined close up, and analysed⁵. The fifth was named ‘Oiléan Ruaidh’ and the sixth ‘Ireland’.

The thrust of this abstract and poster was to describe the contrasting degrees of weathering due to limited oxidation and hydration, observed in cavities where troilite has been lost. The meteorites have been subjected to analysis by Pancam & Navcam Imaging, Alpha Particle X-Ray Spectrometer, Mössbauer Spectrometer and Microscopic Imager.

‘Block Island’ suggests partial burial and exposure and a large pit lined with delicate iron protrusions suggests inclusion removal by a gentle process⁴. There is an expression of Widmanstätten pattern in four of the meteorites⁴ – suggestive of differential abrasion - and spatially discontinuous iron oxide–rich coating elevated in Mn and Zn relative to the substrate, and large-scale cavernous weathering in two⁴. Several of the meteorites have regmaglypts, suggesting that some areas altered more than others since their entry into the atmosphere of Mars. The possibility of acidic exposure to water, rather than wind abrasion, is discussed⁴. Mechanical abrasion was also present⁴. The coating is definitely the result of a post-fall process and is not a fusion crust⁴.

ANOMALY

These authors do not consider the presence of such large iron meteorite masses on the surface of Mars as anomalous. The atmosphere on Mars consists of 95.3% CO₂, 2.7% NO₂, 0.13% O₂ and 0.035% H₂O. The atmospheric density on Mars is less than 7mb or less than 1% of that on Earth - enough to slow some meteorites down, but surely not such large iron masses as ‘Opportunity’ is encountering (pers. comm. Peter Cadogan).

• Image: Opportunity close-up of the fifth meteorite found by Mars rover ‘Opportunity, given the name ‘Oilean Ruaidh’. It is described as a toaster-‘sized’ rock.

The Martian atmosphere is only 11km deep and meteorites enter it at velocities of at least 5.1 lm/sec. Bland and Smith⁶ investigated this subject and say in their abstract:


It is clear from their conclusions which were written before ‘Heat Shield Rock’ was encountered and a large iron meteorite was recognised, that they were contemplating survival of small stony meteorites, and they did not entertain break up on atmospheric entry. They did not consider the possibility of the find of large masses of iron meteorites.

They concluded:


What is apparent is that the six iron meteorite bodies found by Opportunity are completely beyond the rational expectations outlined by Bland and Smith. Do these excellent NASA scientists⁴ fully realise the extraordinary nature of what they have discovered? It is on a par with Birger Schmitz’s remarkable find in Sweden of repeated meteorite rains during the Ordovician⁷, over millions of years, quite beyond the normal expectation of meteoritics. I do not propose to offer here an explanation for this Mars enigma, but it seems obvious that these large irons could not have entered the present thin Martian atmosphere and landed pat on the surface.

The question whether the six represent one fall, broken up on entry, is being investigated: the differences in weathering observed are being studied and suggest that they are from different falls; though such differences are possible in a single fall, if the fragments had complex histories on and within the Martian surface rocks after arrival. However atmospheric fragmentation should not have occurred at all during entry into the present thin Martian atmosphere. The fact that six meteorites have been discovered during only 32km of traverse (by July 2011) suggests possible statistics that boggle the mind, if the entire Martian surface is littered with iron meteorites (which are rare on Earth) like this; but this may be a fortuitous encounter with an unrepresentative local concentration on Meridium Planum.

What ‘Opportunity’ has revealed is ostensibly recording the impossible: and to invoke a past different climate on Mars ignores the fact that, though there is evidence of an earlier quite different climate there, such is only hypothesised many millions of years ago, and could not surely have a bearing on the presence of large iron meteorites in numbers littering the surface.

My correspondent Peter Cadogan (pers. comm.) asked whether they could possibly come from within Mars itself. I had thought of this – the possibility of diatremes ejecting Martian core material onto the surface – but I discount this as being extremely unlikely, especially as Widmanstätten patterns are evident. These have all the signatures of being asteroidal meteorites. However, it is going to be extremely difficult to find a rational explanation for this enigma, and the fact that we cannot do more than obtain results from a rover working under remote control at a huge distance will not help.

Shall we ever know the truth?

References

1. McCall, G.J.H. 2005. An iron meteorite on Mars. Geoscientist 15 (7), 14.
2. McCall, G.J.H., Bowden, A.J. , Wood, A.J., Marvin, U.B. 2005. Epilogue. In: McCall, G.J.H., Bowden, A.J., Howarth, R.J. The History of Meteoritics and Key Meteorite Collections: Fireballs, falls and finds. Geological Society of London Special Publication No. 256 495-504.
3. World News 2010. NASA rover finds another possible meteorite on Mars.
4. Ashley, J.W., Golombek, M.P., Christensen, P.R. et al. 2011. Post-fall surface modification features of iron meteorites found by Opportunity Rover and their implications for Martian weathering processes. 74th Meeting of the Meteoritical Society August 8-12 2011, University of Greenwich, London, UK: Meetjng Programme Abstracts p22 (#5480).
5. Wall, M. 2011. Opportunity rover finds 6th Mars meteorite. http://www.msnbc.msn.co
6. Bland, P.A., Smith, T.B. 2000. Meteorite accumulations on Mars. Icarus 144, 21-26.
7. Schmitz, B. 2003. Shot stars. Geoscientist 13(5),4-7.