Alex wrote:
the general rule of thumb is that the volatile (in this case assumed to be water or ice) must lie at a depth not greater than one half the thickness of the lava flow for water vapor pressure to exceed the lithostatic pressure for phreatic explosions on Mars.

LJ writes:
Thanks, Alex - that makes it clearer, and would seem to me to mostly eliminate CO2 as a possible source of volatiles for producing these formations... if it's deep enough to be there in the first place, it's too deep to be heated enough by the lava. Sounds like this area is a prime target for Mars Odyssey.

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Unlike its recent predecessor, NASA's latest Mars-bound spacecraft appears to be on track for the Red Planet. The Mars Odyssey, which launched earlier this year and is set to arrive on October 23, currently is about a third of a way to its destination.

"The Odyssey is working very, very well," said spacecraft manager Roger Gibbs. "Hopefully that will continue, and we’re preparing for the next phase of the mission."
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The Gamma Ray Spectrometer, or GRS, will look at Mars in the Gamma ray spectrum to find how basic elements are distributed on the surface. In particular, it will search for hydrogen. Plaut said that hydrogen would most likely indicate the presence of water ice on, or just below, the surface.
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above from:
http://www.space.com/missionlaunches/missions/mars_odyssey_ready_010115.html

(A little unfair to the "recent predecessors" who were also on track at this point in their missions)