Geoscientist Online

Joe McCall begs to differ - the "Spider", he says, is a caldera volcano...

Geoscientist Online.  Rec'd 17 March, Pub'd 18 March 2008 

Dr Joe McCall, a Senior Fellow of the Geological Society, has been comparing images of the mysterious 'Spider' structure, revealed in a recent MESSENGER image on planet Mercury. Unconvinced by the explanations advanced, and having lately reviewed Michael Carr's new book on Mars, he made visual comparisons. "The structure shares the characteristics of three giant caldera volcanoes on Mars, though it is even larger: these are Tyrrhenia Patera, Ceraunius Tholus and Uranius Tholus" Dr McCall told Geoscientist.   "Making these comparisons, there is no doubt at all in my mind that the 'Spider' is a giant caldera volcano, extinct, with an irregularly shaped central caldera." 

If Dr McCall is correct, the Spider is the first giant volcano to be recognised on Mercury. 

Joe McCall writes: I have had an interest in Mercury and regretted its long neglect² ; and have also written on the giant caldera volcano of Olympus Mons on Mars³. Fortuitously , I have just reviewed Michael Carr’s excellent reissue of his book on Mars, with its superb images of Martian caldera volcanoes⁴ – this review will be appearing shortly in Geoscientist. In NASA circles, impact origin is unfortunately the ‘default’ setting, and structures are only interpreted as volcanic if they are clearly undeniably so – for example, Olympus Mons. Mercury’s newly revealed ‘Spider’ in MESSENGER image exemplifies this. I quote from Science Daily   describing this new and surprising image (Figure 1):

"MESSENGER obtained high-resolution images of the floor of the Caloris basin on January 14 2008. Near the center of this basin, this remarkable feature - named ‘ the spider ‘ by the science team – was revealed. A set of troughs radiating outward are interpreted to be the result of the breaking apart of the floor materials that filled the Caloris basin after its formation. Other troughs near the center form a polygonal pattern. An impact crater 40 km (25 miles) in diameter appears to be centered on ‘the spider’ (Credit NASA / St Johns Hopkins University Applied Physics Laboratory / Carnegie Institution of Washington)."

"With respect, I beg to differ as to the interpretation. I do this having cut my geological teeth on mapping Menengai, Suswa, Kilombe, Silali caldera volcanoes in the Kenya Rift Valley, and Ambrym caldera in Vanuatu. The NASA interpreters have neglected to study images of Mars for comparison" McCall told Geoscientist.

First, look at Ceraunius Tholus and Uranius Tholus (Figure 2). These are ideal cases of caldera volcanoes ~60 and ~130km across, with summit calderas ~12 and ~25km across. The original volcano has produced voluminous material in both cases and built up massive cones with well defined circumferential margins. Both have radial fissures which I suggest were formed in the tumescent stage. Both have fewer and wider sinuous grooves or troughs slightly later and, lastly they have summit calderas, formed in the relaxation stage. Uranius has two calderas, nested; Ceraunius has a single caldera.

Now we have to look at the image of Tyrrhenia Patera, which is a less ideal case, the volcano ~200 km across and the caldera 40km across. The volcanic pile is flatter and there is less contrast with the surrounds, because the output was different and had less colour contrast, Here there is a radial pattern of fissures but they are wider grooves or troughs. There are two later, sinuous grooves or troughs, both wider and one passes smoothly into the caldera floor; the caldera is complex: there is also a partial concentric narrow ring groove which is clearly cut off by the complex caldera.. There is an outer shorter narrow groove parallel with this. In this case the relaxation stage was complicated and the first two ‘attempts’ to founder on a ring fractures were abortive.

Now look at ‘The Spider’ on Mercury. Again, as in the case of Tyrrhenia, the circumferential boundary with the surrounds is faintly visible and interrupted, but can be delineated. The volcano, and such as I believe it is, is has a diameter of ~330 km. Surprisingly, it is larger than the three Martian volcanoes, though not the pile of Olympus Mons on Mars3 ; which has a diameter well above 500 km: surprising, because Mercury is only 2/3 the diameter of Mars. Both these smaller-than-Earth planets display volcanic giantism?

‘The Spider’ displays almost straight radial fissures, of the tumescence stage; in this it resembles Ceraunius and Uranius on Mars. There are wider, more irregular grooves later than these, and one, as in the case of Tyrrhenia, appears to run smoothly into the caldera floor; the wall of the caldera has the form of an irregular circle – this irregular, subpolygonal form is like that of may terrestrial calderas – only Ambrym of the calderas that I have mapped is a neat circular basin. The contrast of the material forming the volcanic pile with surrounds is poor, but there is a darker, irregular area in the middle of the slopes. Flatter volcanic piles on Earth are produced by shield volcanoes made up of lava flows, but this might not apply to Mercury: The detail revealed by Messenger is far less than that revealed on the Martian images, and this structure will surely in the future reveal much more interesting detail, like Ceraunius, Uranius and Tyrrhenia, when an Orbiter can get close enough to reveal comparable images.

My case rests. I believe that ‘The Spider’ represents a huge caldera volcano of a (relatively) late stage in the moulding of Mercury’s crust. It is the first such giant volcano to be detected on this innermost planet?.

References

1. Baldwin, E. 2008. Messages from Mercury. Geoscientist 18(4), 4.
2. McCall,G.J.H. 2006. Mercury. In; Selley, RC, Cocks, L.R., Pilmer, I.R.; Encyclopedia of Geology, 5, 235-244.
3. McCall, G.J.H. 2006. A caldera volcano of Brobdingnagian scale: Olympus Mons. Geoscientist 16(4); 29-30..
4. Carr, M.H.. 2006. The surface of Mars. Cambridge University Press; 307 pp
5. Anon 2008. ‘The Spider’ on Mercury: MESSENGER spacecraft streams back surprises. Science Daily (http://sciencedaily.com/releases/2008/02/08201093149.htm)