The idea startled and amazed many scientists, because it contradicted conventional wisdom about Mars. It is thought that the curtain came down on Martian geologic activity 2 to 3 billion years ago. While the planet was certainly once warm and wet, it is believed to be hydrologically dead at present, with nothing but thin wind acting to shape its red dusty surface.

"They (the new images) provide essentially a corroboration of things we've been seeing increasingly from Mars," said Victor Baker, a hydrologist and planetary scientist at the University of Arizona. More than a decade ago Baker and a group of colleagues discovered signs of recent glacial activity on Mars.

"There are very young land forms that are associated with active water or ice processes on the surface of the planet. And these pictures just whet our appetites to spend more time documenting numerous such features and try to document their relationships so we can understand them."

Baker and a group of colleagues have been active in investigating apparently young features on the surface of Mars features that are fraction of the 2 billion to 3 billion-year-old age commonly accepted as the age of the planets surface. They have identified large flood channels, telltale signs of recent glacial activity, and even signs of near-contemporary volcanism.

But this group is not the only team finding young features on the Red Planet.

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William Hartmann is a researcher at the Planetary Science Institute in Tucson, Arizona. He has analyzed many of what look to be Mars youngest lava flows. Based on crater counts, Hartmann estimates that lava has flowed on the surface of Mars within the past 10 million to 100 million years or less. That time period represents the last 1 percent of Martian history.

If active volcanoes erupted that recently, there is almost certainly still volcanic activity, Hartmann said.

"It's unlikely that volcanism has continued through 99 percent of the history of the planet and then just shut off just before we got there. So its almost the same as saying we have contemporary volcanism on Mars," Hartmann said.

Meanwhile, a team of scientists at NASAs Ames Research Center has been examining craters on Mars in search of lakebeds. In 1999, Nathalie Cabrol and Edmond Grin published a study that catalogued nearly 200 craters in which lakes appeared to have collected. These flat crater bottoms seem to exhibit shorelines and feed channels that look like they flow down crater walls. Some of these channels form deltas of deposited material at their inlets.

This image of the Medusa Paleolake on Mars shows evidence that a lake filled this crater relatively recently. From Catalog of Martian Impact Crater Lakes, (Cabrol and Grin 1999).

Importantly, many of these craters appear to be young -- less than a million years old -- according to crater-density calculations.

Documenting these so-called young features has been the easy part. Understanding their relationships and making sense of them is a much more difficult project, but one that has already offered some tantalizing ideas about the climate on Mars.

The big problem for anybody who argues for the existence of liquid water on Mars is finding a heat source. The planet is so cold that any water within several hundred yards (meters) of the surface should be frozen locked up in a deep layer of permafrost.

Moreover, Mars carbon-dioxide atmosphere is so thin (the atmospheric pressure is less than 1 percent that of Earths), that water boils away as soon as it melts.

But Baker, Hartmann and others believe that volcanic activity in the Martian crust could heat the ground enough in local spots around the globe to release water, and create some of the water-related features discovered recently.

"The underlying bottom line is that the interior of Mars must be holding more heat and have a more active interior than previously thought," Hartmann said.

"My inference immediately is that you've got geothermal activity that's producing heat from the mantle, or from down below the crust of Mars, bringing up molten rock. Well, for every time that you get the subsurface hot enough to melt rock, there must be a hundred times that it gets just warm enough to melt ice," he said.

This volcanic heating in the crust could create underground rivers and hydrothermal springs. These could flow underground, and might at times intersect hillsides and cliffs. When this happens, water might break out of a ridge, pour down slope and create the type of channels revealed in the Mars Global Surveyor images.

The kind of water-related land features that appear on Mars may not require a long time to form. They could be shaped in a matter of minutes or hours, so wouldnt necessarily require a radically different climate. Instead, dramatic change may occur with volcanic events on very quick time scales. Volcanic outbursts could even release water and other gases into the atmosphere, changing the climate and atmospheric pressure for brief periods, he suggested.

This could be enough to drive enough water vapor into the atmosphere to cause snowfall in certain areas snowfall that could cause the buildup of glaciers of which Baker has seen signs.