Mars is a wanted world -- dead or alive.

Scientists find themselves awash in a range of intriguing findings regarding the distinctive landscapes of the red planet. The onslaught of sensor data from trailblazing Mars orbiters -- along with the ongoing Spirit and Opportunity rover missions -- are setting the stage for more refined inquiries into the planet's past and its present status.

The observation by Europe's Mars Express orbiter that methane (CH4) has been found on Mars is new data in the form of old news. However, this information could well put scientists on the pathway for seeing a different Mars.

Very good sign

Mars Express has detected methane on Mars in the parts-per-billion range that is consistent with Earth-based telescope observations, said James Garvin, NASA Lead Scientist for Mars Exploration and the Moon at the space agency's headquarters in Washington, D.C.

The Mars Express finding of methane "is a very good sign that there is a side of Mars we do not understand very well," Garvin said.

"What makes methane interesting, of course, is its most likely mode of origin," Garvin said. Today, based on current knowledge about Mars, he added, methane is most readily manufactured, photochemically, via recently active volcanism…or via biogenic processes associated with classical metabolism -- typically at microbial levels. 

First, methane is readily destroyed by ultraviolet photochemical reactions in the modern-day martian atmosphere. Current wisdom suggests its dwell time in the atmosphere is roughly 300 years or so, Garvin noted.  

Therefore, given the amount of methane detected now, Garvin said, it must either have formed from some sort of recent volcanic exhalations -- either eruptions or more subtle near-surface hydrothermal activity -- or some as-yet-unknown biogenic processes. "Either would be fantastic," he said.

Active volcanism of "any sort" would be a very positive indication of the possibility of modern day "potential habitats", Garvin stated. That finding would result in targeting future Mars exploration spacecraft to go to the source of volcanic activity. However, the methane could be the byproduct scientists find eking from fumaroles and other localized geothermal "hot spots", he said.

More sensitive instruments orbiting or on Mars' surface are required to discern if the methane is from biogenic sources, presumably microbial. "Even shallow subsurface microbial systems could process carbon dioxide and nutrients, giving off methane as a metabolic byproduct," Garvin said. 

The trick is to measure CH4 at a parts-per-trillion level, Garvin said. That's a tough assignment from Mars orbit, although ways to do so have been proposed, he added, drawing upon methods in use by Earth remote sensing satellites.

The next wave of Mars explorers -- orbiters, landers, perhaps even airborne vehicles slipping through martian skies -- will help scientists write a much more solid methane-on-Mars story, Garvin said.

For instance, NASA's Phoenix lander to be launched in 2007 will carry hardware to examine near-polar martian atmospheric gases and ices. Detecting levels of methane entombed in ices extracted from the soil "would be a most interesting finding," Garvin said.

Then there's NASA's Mars Science Laboratory, slated for a 2009 launch. It too could haul to the red planet highly-precise sensors to understand the source of any detectable methane.

Furthermore, NASA's robotic Mars Exploration Program now calls for two Mars low-cost Scout missions for 2011. "I would hazard a guess that innovative ways of measuring methane from orbit, from the air, and on the ground will be proposed," Garvin concluded.

Given the wealth of surprises already relayed by Opportunity at Meridiani Planum, the robot is on a roll to help piece together the story of whether or not life took hold there.

"It has been very satisfying that Meridiani has turned out to be water related," said Phil Christensen, a Mars Rover Exploration team scientist from Arizona State University (ASU) in Tempe. 

"The more we can learn about the environment in this lake and the subsequent groundwater, the better we can estimate its potential for life. One of the key questions is the duration of the lake. The more evidence we find for minerals deposited from this lake, the longer that duration must have been," Christensen explained.

Joy Crisp, Mars Exploration Rover Project Scientist at the Jet Propulsion Laboratory (JPL), said that Spirit and Opportunity remain on duty to snoop out telltale "water marks" on the red planet.

"Our focus is still on using the rovers as robotic field geologists, to look for clues to past environmental conditions when water was around, by examining and interpreting the mineral, chemical, and textural clues," Crisp said.

While the Mars rovers aren't well equipped to search out fossil life, JPL's Crisp said the team "will continue to carefully examine all of the images we get back." The strength of the instrument payload toted by each rover, she said, is in carrying out geologic studies, and our investigation approach is the same as when we started the mission."

As for the potential for astrobiology on Mars, "it is hard to imagine a more promising site," than Meridiani, ASU's Christensen continued.

Opportunity's instruments have found mineral evidence for water, not just morphologic evidence. Morphologic features -- such as channels -- can form very rapidly, within weeks, and may not have provided sites for life to form or survive.  

Christensen said it is important to remember that minerals take time to form.

"The presence of minerals formed in water implies significant duration for this water. Sediments like we have found at Meridiani also preserve evidence for life, again making this an extremely attractive site for future exploration," Christensen said.

Early martian life?

William Hartmann, a Mars scientist based in Tucson, Arizona said the results from Opportunity at Terra Meridiani are spectacular. "It's a major step forward in the quest to learn whether life ever started on Mars, and fully support the NASA mantra of 'follow the water'", he said.

Hartmann said that the Opportunity rover results suggest rocks at that locale contain 20 percent to 40 percent sulfates, and observable concentrations of salts. Then there are the images taken by the robot showing surface ripple marks.

"It's a strong indication that this area was heavily exposed to water…perhaps under water at some point," Hartmann emphasized, "and that the rock strata may even have been laid down as lakebed sediments." 

This would be the remnants of exactly the kind of environment that could have hosted early Martian life, Hartmann said. The question now is: How long did that water and that environment last?

For Hartmann, that stirs up two key questions: "Was this an area that saw water for 100 million years -- long enough, perhaps, to create single celled algae-like organisms?  Or does it reflect just a one-week catastrophic flood due to suddenly melted ice?"

The French Connection

There is evidence to suggest that the major Martian riverbeds were formed in short term catastrophic local water release; for example, from ice melted in a volcanic outburst, Hartmann noted. On the other hand, Hartmann added, some of the fine-scale, dentritic, "valley networks" suggest more widespread sources of water runoff -- perhaps by snow melt.

One of the most interesting Mars discoveries received little notice because it wasn't part of a spacecraft mission. Computational work in France shows that the most common axial tilt of Mars was not today's 25 degrees, Hartmann said. Rather, the French studies indicate the tilt of Mars in the past was 46 degrees.

Climate models for Mars, done by other scientists, suggest that at 46 degrees frost deposits and snow falls would have take place at very low latitudes on the planet. These actions take place in the martian summer whereby ice burns off the pole and bumps up the water vapor content in the atmosphere. In short, Mars may have had a lot more water transport and surface frosts and perhaps melting snows, Hartmann said.

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