BOULDER, COLORADO -- Over the decades, a flotilla of Mars spacecraft have relayed back to Earth freeze-frame portraits of a cold, dry, dusty and desolate planet.

But Mars experts are becoming progressively more surprised as they observe a world in constant change.

Evidence is mounting that the red planet bares witness to very young, water-related features. Mars has undergone episodic climate cycles that have caused dramatic changes to its surface. Some of these climate swings may have been fairly recent as measured in geologic time.

Mars researchers here at the University of Colorado are looking hard into what is shaping Martian gullies. New work by Jennifer Heldmann and Michael Mellon take into account the wide variety of proposed theories for gully formation.

Gushers of liquid carbon dioxide, melting ground ice in the upper regions of the soil, liquid water from dissipating snowpacks, shallow or deep aquifers, as well as dry landslides - these are among a gaggle of theories to explain the gully features.

No proposed idea can explain all of the observed gully characteristics, Heldmann and Mellon suggest. Their studies point to subsurface aquifers and geothermal heating within Mars as the most viable explanation for the puzzling gullies.

Pascal Lee of the Mars Institute and the SETI Institute in Mountain View, California is a leading Mars gully expert. He first reported that the features could be carved by liquid water from dissolving snowpacks, based upon observations and study carried out on Devon Island, in the Canadian arctic.

In February, Philip Christensen of Arizona State University in Tempe released compelling evidence using Mars Global Surveyor images that trickling water from melting snowpacks on Mars gives rise to ground-cutting gullies. He is principal investigator for Mars Odyssey's camera system, also now orbiting the red planet.

Christensen added that the snow acts as a greenhouse, protecting the water and allowing it to melt and flow, and not instantly evaporate in Mars' low-pressure atmosphere. That snow makes for an attractive abode for life, he pointed out.

Lee told SPACE.com that further work on Earth and Mars gully formation is required. "This particular explanation of gullies as a result of snowmelt needs to be explored in more detail," he said during the 34^th Lunar and Planetary Science Conference held March 17-21 in Houston, Texas.

"We need to understand the range of gully types and, therefore, the formation processes that we're dealing with," Lee said. Moreover, the tie between forms of microbial life and cold places that experience transient liquid water are deserving of further study, he said.

Lee's point is well taken.

There is amble evidence that organisms on Earth can thrive in ultra-severe settings. Biology has been discovered still having the time of its life, and may offer clues as to possible microbes on Mars.

Lynn Rothschild, a scientist at the Ecosystem Science and Technology Branch at NASA's Ames Research Center near San Francisco, California suggests that minuscule amounts of water could support Martian organisms.

Here on Earth, she points out, a hardy snow alga has been found, surviving in extreme environmental conditions. This microbe has the heady, scientific name of Chlamydomonas nivalis. Found in melting snow across our planet, the algae can display itself in the form of dense blooms, tinting the snow pink. That color has led to the common name of "watermelon snow".

This fuchsia-colored algae is seen thriving in harsh mountain environments, creating the watermelon snow in winter and then going dormant in summer, Rothschild said.

So goes it on Earth…why not on Mars?

Along with mystifying gullies, there are also baffling dark streaks.

They are the youngest of all Martian surface features. That is, these strange markings have unexpectedly popped up between Mars Global Surveyor photo shoots that stretch out over several months of time. Some Mars investigators think that small volumes of brine-saturated water might be dribbling across the planet's surface.

Others reason that the markings are a by-product from "dust avalanches".

Scientists anxiously await the 2005 launch of NASA's Mars Reconnaissance Orbiter (MRO) and its High Resolution Imaging Science Experiment (HiRISE). Using its zoom lens, MRO is built to yield exquisite detail of gullies, dark streaks, even surface colors that might be indicators of biology.

Another orbital aide headed for Mars is ground penetrating radar - hardware able to probe for shallow aquifers. That kind of gear is onboard the European Space Agency's Mars Express set for launch in a few months, followed by a more powerful system on the MRO.

Whichever way water may be gurgling around on Mars, its occurrence bolsters the case for present-day life on Mars.

You don't have to convince one scientist of that possibility. He's certain of the fact and has decades-old data to back up the claim.

Gilbert Levin was an investigator on the dual Viking landers that studied Mars in the late 1970s. The signal obtained from his Viking Labeled Release experiment was of biological origin. Viking detected microbial organisms in the Martian soil, he asserts.

The former Viking scientist is now chief executive officer for Spherix in Beltsville, Maryland.

Last month, Levin presented his unwavering life-on-Mars mantra at a symposium on the topic held at Johns Hopkins University in Baltimore, Maryland. "Since Viking, relevant discoveries have been made of organisms living under extreme environments. Many terrestrial microbial forms are now known that populate environmental extremes thought inimical [unfavorable] to life," he argued.

Life has been found on our home planet existing in radical conditions of temperature, pressure, atmosphere composition, and salinity…including those emulating the environment of Mars, Levin reported. Moreover, recent revelations by the Mars Odyssey suggest the red planet is awash with huge reservoirs of underground water ice. Odyssey data "make a strong case for a flux of water vapor rising from and through the near-surface to the surface of vast regions of Mars," he suggested.

"The Odyssey measurements indicate water ice close to the surface, possibly making liquid water available in biologically significant quantities, including at the 1976 Viking mission landing sites," Levin said.

Levin wants to get his Viking experiment on Mars once again, but with modification. It would detect "chiral specificity" in the soil - a property shown by biology but not chemistry. The scientist has devised the experiment and a miniaturized instrument that "could resolve the issue beyond reasonable doubt," he concluded.