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News in Brief - December 2008

Volcanism on Mercury

Geoscientist 18.12 December 2008

The 1974-5 Mariner-10 fly-by of Mercury yielded only low resolution images: Mercury looked very like the Moon, heavily cratered, but with less reflectance contrast between the cratered areas and the plains. No obvious volcanic features were revealed, though some crater patterns suggested causes other than impact and I concluded in 2005 that volcanic features were likely to be revealed by later probes capable of greater optical definition1. MESSENGER has yielded much better resolution. I reported recently my interpretation of the ‘Spider Structure’ revealed by its first fly-by within the huge Caloris Basin as a caldera volcano2. Since then, I have had correspondence with Michael Carr, an expert on Mars, about it: he did accept that there could be evidence of volcanic modification but thought the central crater was an impact crater. I still believe the entire Spider structure is a caldera volcano.

However, since then Head et al.3 have recognised numerous volcanic features in MESSENGER images. They identify volcanic vents around the inner margin of the the Caloris Basin and evidence that the lava plains were emplaced sequentially inside and adjacent to large impact craters, to thicknesses of several kilometres. Volcanic origin is now favoured for several regions of the plains and it is clear that volcanic eruption and magmatic processes played an important part in the development of the surface of Mercury. There can be little argument about this.

Particularly interesting are the lava-flooded circular structures, which Head et al.3 interpret as lava flooded impact craters (Fig 1 if used). They are obviously lava flooded craters, but attribution to impact for the crater may be being overplayed here. There is rigid thinking in NASA circles1,2 about the certainty of identification of impact craters on extraterrestrial bodies: such identifications depend on highly subjective arguments, based on excellent research on terrestrial structures, much less certain comparison-based identification on the Moon and Mars (and especially Venus). Could these not conceivably be lava-flooded volcanic craters? I believe the alternative should not be discounted.

No mention is made of the ‘Spider’. Ah, well, it takes time for pennies to drop!


  1. McCall, G.J.H. 2005. Mercury. In; Selley, R.C., Cocks, L.R.M., Plimer, I.R., ‘Encyclopedia of Geology’, Elsevier, Amsterdam etc., Vol.5, 238-244.
  2. McCall, G.J.H. 2008. Spider is a caldera volcano. Geoscientist 18(5); 6.
  3. Head, J.W., Murchie, S.L., Prockter, L.M. et al. Volcanism on Mercury: evidence from the First MESSENGER fly-by. 2008 Science 321 (5885); 69-72.

An even larger biosphere?

Geoscientist has drawn attention before to the increasing extent of the known biosphere, indicated by discoveries of microbial habitats hitherto unguessed at 1. A letter to Nature by by Santelli and other authors, mainly from Woods Hole, concerns new and surprising findings of microbial life in the ocean crust at great depths 2. The alteration of basalt on the ocean floor could theoretically support growth of chemolithoautotrophic microbes, by supplying the necessary energy by iron and sulphide oxidation 3.

These authors, utilising the Alvin and Pisces submersibles on the East Pacific Rise at 9 degrees North and near Hawaii, have demonstrated basalts below deep sea waters more than 1.5 km down harbour prokaryotes, in populations 3-4 orders greater than in the overlying deep sea waters. The epilithic and endolithic communities exist on a variety of rock systems – pillow lavas, fresh glassy lavas and oxide-coated lavas. The communities were extraordinarily diverse, more so than in hydrothermal vent communities. Each gram of rock contained up to 1 trillion microbes! More than a third are the ancient organisms called Archaea. Ordinary sea-water contains about 90,000 microbes per gram 4 . The East Pacific Rise is a 60,000 km long volcanic Mid-Ocean Ridge, a figure which indicates the staggering amount of microbes that must exist there.

Two questions come to mind as a result of this important statement The microbes are in or on rock cracks, cavities and surfaces and have obviously been collected from the top of the volcanic pile. When the lava flows are buried, successively deeper in the pile, do these microbes continue to thrive or die off? One would think that, as they do not need contact with the oxygenated sea water, that they could continue to thrive in depth on the alteration products. The mind boggles at the implications of this in terms of numbers of populations in a single mid-Ocean ridge and on the Ocean floor generally, which is floored by basaltic lavas.

Are they really chemolithoautotrophs? 4. There is as yet no clear evidence of this: and another microbial oceanographer at Woods Hole, Julie Huber, suggests that microbes love surfaces and rather than living off the altered rock, they may be congregating there, because ‘it’s a nice place to hang out’.

Does this discovery relate to the great unsolved problem of the origins of the first unicellular life in the early Archaean? Here one comes up against the controversy concerning the rise of oxygen is the Early Earth: would the same alteration processes that we observe today on the sea floor basalts, have been active then: I doubt it.

The future results of ongoing research into these microorganisms and their habitat will be important and most interesting.


  1. McCall, J. 2003. The deep biosphere. Geoscientist 13(3); 11
  2. Santelli, C.M., Orcutt, B., Banning, E., Bach, W., Moyer, C.L., Sogin, M.L., Staudigel, H., Edwards, K.J. 2008. Abundance and diversity of microbial life in ocean crust. Nature 453, 29 May 2008; 653 -656.
  3. Bach, W., Edwards, K.J. 2003. Iron and sulphide oxidation within basaltic ocean crust: implications for chemolithoautotrophic microbial mass production. Geochimica et Cosmochimica Acta 67; 3871-3887.
  4. Willyard, C. 2008. Life at Rock Bottom. Earth 53(9); 17.