Caves on Mars?

The BBC News website is reporting that 'cave entrances' have been spotted on Mars. This seems to come from a talk given at the Lunar and Planetary Science Conference (LPSC) held last week in Houston. The LPSC have the paper available on their website as a PDF. Below I've included a screenshot of Figure 1 from that paper which shows the features of interest.

Possible Martian Caves
Seven proposed cave skylights. Clockwise from upper-left: Dena, Chloë, Wendy, Annie, Abbey, Nikki and Jeanne. Arrows signify direction of solar illumination (I) and direction of North (N) CREDIT: G. E. Cushing, T. N. Titus, J. J. Wynne and P. R. Christensen
In the centre of each of the six regions you can see dark spots. These were initially found by looking at red light (654 nm) observations taken with the THEMIS instrument onboard Mars Odyssey. Using the THEMIS resolution of 18 m per pixel, they calculated that the features have diameters between 100 and 250 metres - equivalent to between 10 and 25 London buses parked end-to-end.

What are these features? Well, from the images above they certainly do look like holes in the surface. However, you have to rule out other possibilities first. The most obvious thing for them to be would be dark sand/rocks or impact craters. The dark rock idea is ruled out by infrared observations that allowed the authors to monitor the temperature changes of the features during the day; they didn't change in the way that you would expect rock/sand illuminated by the Sun to. So the second possibility is that they are impact craters. However, comparing them with neighbouring craters in the images above, you'll notice that they are very dark and sunlight isn't reaching the bottom of them. Knowing the time of day at each point on the surface you can work out the angle of the Sun at that time. If none of the bottom is illuminated, you can say that the hole must be deeper than a certain amount. The minimum depths calculated for these seven are between 73 and 96 m, but of course they could be deeper than this. These seem too deep and have sides much too steep to be craters. Having ruled out our two alternatives, the authors suggest that these are "sky-light openings into subsurface cavernous spaces" and the temperature measurements seem to back this up. However, the authors caution that more observations taken at different times of day (and therefore illumination angles) would help to confirm this.

The idea of caves on Mars is quite exciting because it may mean areas more conducive to the existence of life i.e. the temperature doesn't fluctuate too much during the day/year and the metres of rock/regolith above protect against harmful UV and high energy particles present at the surface. That being said, these particular caves seem quite unlikely as residences for subterranean lifeforms as they are located high on the flanks of Arsia Mons where the atmospheric pressure is even lower than normal for Mars - cold and lacking in atmosphere. Still, they might be interesting to future Martian speleological societies.

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Posted in astro blog by Stuart on Monday 19th Mar 2007 (19:19 GMT) | Permalink
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