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Bruce Yardley appointed Chief Geologist

Bruce Yardley (Leeds University) has been appointed Chief Geologist by The Radioactive Waste Management Directorate (RWMD) of the Nuclear Decommissioning Authority (NDA).

Chartership news

Chartership Officer Bill Gaskarth reports on a projected new logo for use by CGeols, advice on applications and company training schemes

Climate Change Statement Addendum

The Society has published an addendum to 'Climate Change: Evidence from the Geological Record' (November 2010) taking account of new research

Cracking up in Lincolnshire

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Critical metals

Kathryn Goodenough* on a Society-sponsored hunt for the rare metals that underpin new technologies

Déja vu all over again

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Done proud

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Earth Science Week 2014

Fellows - renew, vote for Council, and volunteer for Earth Science Week 2014!  Also - who is honoured in the Society's Awards and Medals 2014.

Fookes celebrated

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Geology - poor relation?

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Nancy Tupholme

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Power, splendour and high camp

Ted Nield reviews the refurbishment of the Council Room, Burlington House

The Sir Archibald Geikie Archive at Haslemere Educational Museum

You can help the Haslemere Educational Museum to identify subjects in Sir Archibald Geikie's amazing field notebook sketches, writes John Betterton.

Top bananas

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Stony meteorites of Mars

McCall 1 SantaCatarina surface.jpg

Joe McCall extends his survey of iron meteorites discovered on the Martian surface to include the stones.

Geoscientist Online Special Issue 22.01 February 2012

Since writing my earlier on-line note about iron meteorites on Mars1, I have been in contact with Phil Bland, who agrees that the possibility of meteorites being buried shallowly on impact on Mars and being exposed by wind deflation of the cover is a real one. However he directed me to the work of Chappelow and this adds some valuable information and opinions on the subject.

Figure 1. Santa Catarina on the Mars surface.

Block Island


Chappelow and Golombek2 discuss the event and conditions that produced the iron meteorite Block Island: four iron masses found by Opportunity vary from 50-240 kg and are dispersed over 10 km of Meridium Planum (this was before two more were encountered 1), their surface hollows like regmaglypts on terrestrial irons indicate atmospheric ablation, and the largest, Block Island, must have landed at a speed below hypervelocity (<2 km sec-1) to survive. Low entry angles of 10-13o are suggested: these authors suggest that such conditions are very rare: involving 0.007% of incoming iron meteorites. The dynamic pressures involved on Mars are probably too low to break up an iron meteorite, leaving doubt as to whether the multiple iron finds by Opportunity1 are paired: they are more likely to have resulted from separate falls.

McCall 2 SantaCatarina.gif

Four stony meteorites?


Figure 2. Santa Catarina close-up (merged MI mosaic). This shows discrete reflective specks of nickel iron, but they do not form an enclosing matrix to the silicates as is typical of mesosiderites and there is no evidence of the irregularly distributed larger metal patches and metal poor clots that characterise mesosiderites.

Chappelow was also among the authors of an article by Schröder et al.3 describing how rover Opportunity also ‘serendipitously’ investigated four rocks (informally named “Barberton, Santa Catarina, Santorini, Kasos”) (Figures 1,above; 2 right,3,4 below), which are similar in composition to the howardite, eucrite, diogenite achondritic (differentiated) stony meteorites, but with more iron content, and thus thought to be possibly mesosiderites on account of the increased nickel iron content . They cite my and W H Cleverly’s 1966 article describing the Mount Padbury, Western Australia, mesosiderite find 4.


McCall 3 Kasos.jpg The almost identical composition of the four suggests that they are paired, but they were found kilometres apart (Figure 5, below). Small amounts of ferric iron as well as native and ferrous iron indicate some weathering. No fusion crusts were observed. They lie on a surface of basaltic sand and hematite lag.
Strewn field or scatter after impact?

Figure 3. Kasos close-up: this does not resemble mesosiderite texture at all: it is very even and finely granular: mesosiderites have a very uneven texture and are dominated by metal.

It seems possible that Opportunity is driving across a strewn field, but there is some doubt whether entry though the thin present Mars atmosphere could produce as strewn field due to fragmentation and differential drag in the current thin atmosphere, and the alternative possibility is that they dispersed after impact . Descent at shallow entry angles and wide dispersal after atmospheric break-up is favoured, but they might alternatively result from spallation after impact.

McCall 4 Barberton-cobble.gif

Possible relation to Victoria Crater?


Santa Catarina and a large accumulation of similar ‘cobbles’ (not studied or analysed, of unknown composition) were found on the rim of Victoria Crater (Figure 4), and this impact may have formed the crater, but the association may be coincidental, particularly as they are only on one side of the crater.

Figure 4. Barberton. This again is finely even textured and does not resemble a mesosiderite.

Relation to the six irons found by Opportunity?

A genetic link between these stony irons and the six irons found by Opportunity1 is unlikely, and the weathering states suggest that these may be from a later fall.

Possibility of long term survival of meteorites on Mars’s surface, from a time of a denser atmosphere, or more recent descent through the current thin atmosphere

The possibility has been suggested that these meteorites arrived long ago, at a time when the Mars atmosphere was denser and more resistant to incoming objects, but the accepted thinking is surely that this was millions of years ago, and descent through the present thin atmosphere is also possible? Bland and Smith5 suggested that masses of up to 0.1kg could come through the current Mars atmosphere and survive. The masses are estimated as Barberton, 20-25g; Santa Caterina, 2.5- 3.1kg; Santorini, 427-528g; Kasos, 373-463g.

Tests on meteoroids up to 10kg have been inconclusive whether such bodies could fragment and disperse through atmospheric drag, coming down through the current thin Mars atmosphere. Rare incidence at very shallow angles cannot be ruled out? Yet with very slow weathering processes at the surface and beneath it, and possible burial and exposure at the surface by deflation of cover, even more than once, it seems just credible that iron and stony iron/stony meteorites such as are described here could have fallen to the surface even millions or billions of years ago and still be preserved on the surface?


McCall 5 Opportunity Route.jpg

Differentiated stony meteorite or mesosiderite (stony-iron)?


Figure 5. The path of Opportunity to Sol 2063, showing where the four stony meteorites were encountered (1 Barberton, 2 Santa Catarina, 3 Santorini, 4 Kasos) and Victoria Crater (diameter ~ 750 m).

Are these stony meteorites or mesosiderites? The latter has been suggested on account of high nickel iron content, higher than usual in howardites, eucrites or diogenites, and usually pyroxene is dominant in these over olivine, whereas here olivine is dominant over pyroxene. There is considerable olivine in Mt Padbury 4, as these authors note, though it occurs in discrete relatively metal-free clots, as do the eucrite areas in Mt Padbury. The absence of fusion crust is striking – it is suggested that it might be due to the lack of free oxygen in the current Martian atmosphere – it certainly does militate against fragmentation on atmospheric passage, and make impact dispersion more likely?

I have seen many mesosiderites, as well as Mt Padbury (of which still I have some small pieces in a teaching collection) and the pictures (Figs 3.4) do not resemble mesosiderites, which on terrestrial descent through the atmosphere produce no significant fusion crust, and show more metal than these objects appear to. In particular, the even textured and granular character of Kasos and Barberton do not look like mesosiderite textures, though they could be achondrite inclusions, more or less free of nickel iron, like those in Mt Padbury. My opinion is that these are, as these authors suggest, alternatively, differentiated stones close to howardite composition, but with slightly anomalously high metal content and also anomalously high olivine content compared to pyroxene, and constitute a new group of differentiated stony meteorites not so far sampled on Earth.

McCallMundra1145kg_.jpg

Wider inference


Opportunity has covered only about 10,000 metres (Figure 4). There is a strong indication from the evidence of distribution and the slow weathering of the meteorites so far encountered, that the Mars surface is littered with meteorites. It may be a super-analogue of the arid Nullarbor Plain, Australia, where the Mundrabilla iron meteorite is believed to have sat for a million years on isotopic evidence6. One of the Mars irons, Block island, was on a pedestal 4cm high of S-rich outcrop’ 3, and resembles the case of the Mundrabilla mass, which rested on centimetres of iron shale, above the limestone plain (Figure 6). It is not certain, but it now seems likely, that on Mars meteorites could reside on the surface for millions of years.

Figure 6. The Mundrabilla, Australia, second mass (5-tonne) in the foyer of the South Australian Museum. The Nullarbor Plain where it was collected is an arid limestone desertand weathering is minimal. It is believed to have fallen about a million years ago.  More information and picture provenance.

Opportunity has opened up a vast new field of research, quite unsuspected. There is undoubtedly a wealth of meteorites, likely including some of types unknown to us on Earth, littering the planet’s surface, but we are like prisoners in a cage, and have no way of exploring this wealth further. Possibly improved technology will permit such exploration the future, but I doubt whether the funds for such an enterprise will ever be available?

References

  1. McCall, G J H 2011 Many old irons  Geoscientist Online, August
  2. Chappelow, J E and Golombek, M P 2010 Event and conditions that produced the iron meteorite Block Island on Mars Journal of Geophysical Research, 115 E00F07, 11 ppSchröder, C, Herkenhoff, K E, Farrand, W H et al 2010 Properties and distribution of paired candidate stony meteorites on Meridium Planium, Mars Journal of Geophysical Research 115, EOOFO9, 14 pp , 2010 doi 10 1029/2010JE003616, 14pp
  3. McCall, G J H, and Cleverly, W H , 1965 A newly discovered mesosiderite containing achondrite fragments - the Mt Padbury Meteorite Nature, 207, 851-852
  4. Bland, P A and Smith, T B 2000 Meteorite accumulations on Mars Icarus 144, 21-26
  5. McCall, G J H 1998 The Mundrabilla iron meteorite: an update In: Moore, P (ed) 1999 Yearbook of Astronomy, 156-168