Sending one-way spacecraft to learn if life exists or has ever been resident on the red planet is a tall order its been that way for decades.

"Searching for life elsewhere is hard," emphasized David Des Marais, a Mars Exploration Rover scientist from NASAs Ames Research Center, Moffett Field, California.

The Spirit and Opportunity robots were designed from the start to find and assess past environmental conditions on Mars, Des Marais said, with an emphasis on "following the water." 

The big science payoff so far has come via Opportunity at the Meridiani Planum site.

"Opportunity has shown that, sometime in the past, conditions could have sustained life in at least one location and for a period of time sufficiently long for water and rocks to interact extensively," Des Marais told SPACE.com. "Because life itself is a kind of aqueous chemistry, the demonstration that extensive water-rock reactions indeed occurred on Mars is a monumental step along the path in the search for life," he added.

Additionally, Opportunity data shows that clement conditions seemingly existed across a region at least as extensive as Meridiani Planum, Des Marais said. Opportunity will explore this prospect when the robot soon wheels out of its small crater home onto the surrounding geography.

The fact that the Opportunity rover has yielded strong evidence that water hung around at least one spot on Mars has reinforced the belief that -- at this setting the planet could have been friendly to biology.

Lots of standing liquid water, slowly evaporating -- perhaps going through many cycles of wetting and evaporating is good news for life, said Benton Clark of the Lockheed Martin Corporation in Littleton, Colorado. He is also a Mars Exploration Rover team scientist, expert in chemical alteration processes on Mars.

The robotic field geologist has additionally detected very high concentrations of sulfur in a rock outcrop inside the small crater in which it is exploring. The chemical form of this sulfur appears to be in magnesium, iron or other sulfate salts. Elements that can form chloride or even bromide salts were also revealed by Opportunitys science instruments.

There are organisms that like sulfates, Clark said, actually using them as an energy source in combination with hydrogen gas in the atmosphere, Clark said. "That gives organisms on Mars a way to live without having to use the Sun and be exposed to the ultraviolet light. Thats really good too," he explained.

Lastly, Clark said that the evaporation that seems to have occurred at Meridiani Planum was a slow process. "So that gives the organisms time to adapt."

While there is no evidence for past microbial life at the Opportunity site, Clark said that the environmental conditions found there "are not incompatible with some bacteria we know about."

One such candidate, a Clark favorite, is Desulfotomaculum. "Ive been a fan of these organisms ever since I found sulfate in the martian soil. And this one forms a spore. It can tolerate high salt and it likes sulfate."

Still to be sorted out is whether the aqueous mineralogy seen at the Opportunity rock outcrop site is the result of a lake or inland sea sitting atop Meridiani Planum. Subsurface groundwater possibly hydrothermal activity -- might be responsible too.

"The next step -- and this is one that any field geologist would endorse I think -- is to get a sense of how things change both vertically and laterally away from the outcrop we are on," said Jack Farmer, Mars Exploration Rover scientist from Arizona State University in Tempe.

"That means moving from our present location, up and over the crater rim. In doing so, well also be getting a look at any other outcrops along the wayespecially those that represent a higher, and younger, stratigraphic level."

From there, Farmer said, is to steer Opportunity toward a larger crater on the horizon. That crater may have excavated deeper into the underlying rocks. That affords a chance to see deeper into Mars history at Meridiani Planum. "If this was a large lake, you might expect the ordered units and basic stratigraphic patterns to persist for some distance," he said.

As for life that may have found a home in the past at Meridiani Planum, Farmer agreed with Lockheed Martins Clark that there are many known sulfur/sulfate utilizing organisms.

"What we may be looking for here are acid-tolerant, if not acid-loving, organisms that utilize sulfate. If the hydrothermal hypothesis proves correct, then also throw in high temperatures," Farmer said.

And from the standpoint of studying fossil organisms and related remains on Mars, Farmer said that looking for biomarkers clear signs that life existed on the planet is vital. Biomarker assemblages might be important to target for future on-site investigations and Mars-to-Earth sample return missions, he said.

As talented as the Spirit and Opportunity rovers have shown themselves to be, they are ill-equipped by design for sniffing out life on Mars.

"I do not believe that the rovers and their gear will be effective at demonstrating the past or present existence of life on Mars," NASAs Des Marais pointed out.

Des Marais' reasoning: First, they face the same challenges that confront studies of ancient (Archaean) rocks on Earth. Even if life existed, it is very hard to find in such old rocks. Second, the very rover design that performed so well as a "water detector" within budget constraints is necessarily also a design that is not very effective as a life detector.

"The next step involves sending another rover that can identify large suites of minerals definitively and that can analyze for the compounds of the life-forming elements carbon, nitrogen, hydrogen and sulfur," Des Marais advised. That rover should be able to drill into rocks more deeply than can Spirit and Opportunity.

Furthermore, the next rover should visit a site where orbital observations indicate that aqueous chemistry might have occurred. As was the case for Opportunity, high-flying sensors were crucial in spotting Meridiani Planums bounty of hematite, Des Marais said, and the hematite was a sign of possible past water.

There is no doubt. Looking for life in all the right places is by no means easy.

"One of the hardest parts is convincing key constituencies that this search necessarily requires a patient, iterative, stepping-stone approach," Des Marais said. "It might indeed require that, ultimately, intelligently selected samples must be returned to Earth-based laboratories for state-of-the-art scrutiny. We are still at the stage of learning where to find and how to select those crucial samples intelligently."

Jennifer Heldmann, a Mars scientist at the NASA Ames Research Center, has her eyes set on Martian gullies. Those intriguing features were first announced in 2000, by scientists using imagery from the orbiting Mars Global Surveyor.

Gullies on Mars may prove to be sites of present day, near surface, liquid water. And that holds out the tantalizing possibility of a sweet spot for martian life.

Heldmann suggests that gullies have been formed from the release of relatively pure liquid water from a subsurface aquifer under current Martian environmental conditions. But she added it seems as though there are several different types of water activity that have acted on Mars. 

The new findings from Opportunity suggest standing water -- similar to lakes -- for an unknown amount of time at the Meridiani site, Heldmann said. The presence of salty water on Mars has long been hypothesized because salty water can result in vapor-pressure suppression and a lower freezing point.

Translation: Salty water can be stable on the surface even with lower atmospheric pressure and colder conditions. Pure water on Mars is currently unstable with respect to temperature and pressure, meaning that the water will boil and freeze when exposed to current Martian surface conditions. 

An obvious next step in Mars exploration -- to help understand the history of water on the red planet -- would be to visit the sites of gully occurrence, Heldmann said. Findings there could then be contrasted with data gleaned by the grand successes of the Mars Exploration Rovers, she said.

Meridiani shows evidence of past surface water. The gullies suggest present and/or past subsurface water. Such a comparative analysis could provide tremendous insights into the hydrologic cycle on Mars, Heldmann stated.

The gully features appear to have been episodic releases of smaller quantities of water, Heldmann said. Detailed looks at the gullies indicate the relative youth of the features. The gullies may have formed within the past few million years, she said. 

Heldmann said that gullies can indeed form from pure liquid water which is simultaneously boiling and freezing. Salts are not required, although their presence can't be entirely ruled out, she said. 

Analysis of information from the orbiting Mars Global Surveyor tends to support the notion of an underground liquid water aquifer feeding the martian gullies, Heldmann said. Subsurface temperatures on present-day Mars at the depths of these postulated aquifers are above freezing. Also, conditions fall within the liquid water temperature-pressure stability regime which indicates that underground liquid water aquifers can be sustained at these locales. 

"The presence of liquid aquifers on Mars would be a tremendous target for astrobiological research and the search for life," Heldmann observed. Such reserves could also serve a crucial role for future human expeditions to Mars which will require the use of on-the-spot resources, such as liquid water, she said.   

To hunt for Mars life, head for the planets north polar ice cap. Thats the personal opinion of Frank Carsey, a leading scientist in polar oceanography as well as developing ice-cutting "cryobots" at the Jet Propulsion Laboratory (JPL) in Pasadena, California.

Carsey is leading a team of experts in scoping out "Palmer Quest" a proposal to NASA that would search for life below the ice caps of Mars.

"Ours is a mission study for a very aggressive mission in which a small nuclear reactor -- the smallest in the U.S. collection of designs -- will provide heat and power for a thermal probe that melts its way to the bed of the north polar ice cap of Mars," Carsey told SPACE.com. "The objective is search for life at the bed where there may be nutrients and suitable long-term protection from the surface chemistry and radiation for life to exist."

Carsey said Palmer Quest is a step in the "follow the water" strategy adopted by NASA, leading to exploration of the largest known reservoir of water on Mars. That is, exploring the deep subsurface of a major ice sheet on Mars.

Digging deep on Mars translates into spacecraft-deployed hardware cutting through roughly 1.2 miles (2 kilometers) of dusty ice. A range of sensor-laden robotic technologies to drill this distance -- acquiring data on the way down and at the bed -- are now in development.

For a community of Mars organisms to survive, Carsey said, there are "must have" requirements: Long-term liquid water, carbon, energy (including heat), and protection from the present hostile surface environment of Mars.

At Mars north polar ice cap, the proposed spacecraft landing site is relatively warm, has had liquid water outflow in the recent past, and has the promise of chemical diversity suited to supporting life. The lower layers of the cap appear to offer accessible locations for liquid water. They should also provide a nifty protective niche for life against the harsh climes of Mars.

"Consequently, ice deposits on Mars offer some of the most attractive and compelling regions for astrobiological exploration," Carsey notes in a report summary of Palmer Quest. Both present-day and fossil life may be found there.

The mission Carsey and his colleagues have put forward is named for Nathaniel B. Palmer, a 21-year-old American sea captain. In 1820, Palmer was the first person to sight the landmass of Antarctica on a search for seals. That area is now called Palmer Land.

"We plan to follow his example to fearlessly search for life on Mars," Carsey explained. "We do not know what we will discover."

• NASA to Announce 'Major' Mars Discovery Monday, March 23
• Mars Rovers: Complete Coverage

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