Penny Boston is one of the leaders of the SLIME team - that's Subsurface Life in Mineral Environments. She studies bizarre microorganisms that live, often under extreme conditions, in subterranean caves.

At the recent NASA symposium "Risk and Exploration: Earth, Sea and the Stars," in Monterey, California, Boston talked about the relevance of her work below ground on Earth to the search for life on other worlds.

"The type of caves that we absolutely know exist elsewhere in the solar system," noted Boston, "are what are known as lava tubes. These are natural outgrowths of flood-basalt type, quiet, flowing lava eruptions. They are essentially rivulets that freeze on the outside. The rock on the outside freezes and forms a very good insulator that then allows the interior to remain molten and to continue to flow through."

"Eventually, when the eruption stops, they empty out and you have these very beautiful tubes. That's a very different class of cave from the kinds of dissolution-dominated caves that we often think of. "

In volcanic areas, channelized lava flows can form roofs which insulate the flowing lava. These features are termed lava tubes on Earth and are common features in basaltic flows. After the lava has drained, parts of the roof of the tube will collapse under its own weight. These collapse pits will only be as deep as the bottom of the original lava tube.

"It was known and recognized by Ron Greeley and other colleagues, even in the Apollo era, that a lot of structures that they were seeing on the moon were lava tubes, or unroofed sinuous rills (lava tubes without their tops)," said Boston.

"As we have gotten ever-better imaging of the planet Mars, we have seen that there are lava tubes scattered widely over the planet. They are quite easy to pick out."

Another type of collapse feature associated with volcanic areas arises when very large eruptions completely evacuate the magma chamber beneath the volcano. The weight of the volcano will cause the entire ediface to subside into the void space below it. Structural features including fractures and graben will form during the subsidence.

Many times collapse pits will form within the graben. In addition to volcanic collapse pits, Mars has many collapse pits formed when volatiles (such as subsurface ice) are released from the surface layers. As the volatiles leave, the weight of the surrounding rock causes collapse pits to form.

University of Buffalo volcanologist, Tracy Gregg, shares an interest in the unusual heating and cooling patterns from martian volcanoes: "Higher latitudes are interesting to me because that's where the large volcanoes are, and there's more opportunities for magma/water interactions. Those interactions are probably extremely important for the origin and evolution of life. "

"The gravity on Mars is much lower so lava tubes there scale accordingly," said Boston. "Not only does Mars have enormous examples of volcanism, but it has big whompin' lava tubes. The biggest lava tube on Earth is about 90 kilometers (56 miles) long, in Hawaii."

"That's the record-holder on Earth, but typically when you look at these features on Mars they're hundreds of kilometers long. And the diameters are equally great. On the average they're 3 to 10 times the size of the average diameter on Earth. They are truly enormous."

"I'd like to see us land ON a volcano," said Gregg. "Right on the flanks. Often the best place to look for evidence of life on any planet is near volcanoes..Since we think that the necessary ingredients for life on earth were water and heat, we are looking for the same things on Mars, and while we definitely have evidence of water there, we still are looking for a source of heat."

Volcanologists are not entirely convinced something isn't going on geothermally on Mars today. "If you'd asked me [if there were not active surface volcanoes] 10 years ago - or even 5 - I might've said yes," said Gregg. "Now I'm not so sure."

On Mars, "where would I look for recent volcanic activity? Depends on how you want to define it on Mars," said Gregg. "I strongly suspect there are still molten (or at least mushy) magma bodies beneath the huge Tharsis volcanoes , and beneath Elysium Mons ."

"But the youngest surficial activity discovered to date (and it's probably 1 million years old, which would be considered quite young, and possibly 'active' on Mars) is in a region that contains no large volcanic structures of any kind," said Gregg.

"Instead, there are cracks in the ground, and a few low-lying volcanoes that can't even be seen except in the high-resolution topography (they are too subtle for imagery to reveal). This area is called Cerberus Fossae ."

"But these are not only fabulous features," said Boston, "they're also places, at least on the moon and Mars - I wouldn't recommend astronauts going to Io or Venus - that can actually be exploited as human habitat."

"We just finished a study for NIAC (NASA Institute for Advanced Concepts), looking at enabling technologies that we would need to make these usable as structures for astronauts, for future bases on the moon and Mars."

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