First you would die, either from asphyxiation or hypothermia. Mars's carbon dioxide atmosphere is 100 times less dense than Earth's and the average surface temperature is -60 deg C. The exact cause of death would depend on the season, the time of day (Martian temperatures fluctuate as much as 100 degrees from dawn to dusk), and the latitude of your surprise landing site.

Next you would begin to dry out. There is no liquid water on the surface of Mars and little, if any, water vapor in the atmosphere. Your lifeless body would become dessicated like an Egyptian mummy.

Finally, not that it would matter terribly, you would contract a very nasty sunburn. The Red Planet's rarefied atmosphere does a poor job blocking UV rays from the sun (there is no protective ozone layer in the atmosphere). Radiation levels are so intense that they probably sterilize the uppermost layers of Martian soil.

The next time you visit Mars, take a space suit.

Undoubtedly present-day Mars is not a congenial place for life as we know it, but it may have been friendlier in the distant past. There is growing evidence to support a view of ancient Mars as a remarkably Earth-like planet: Between 3 and 4 billion years ago liquid water flowed in channels and collected in lakes and ponds all over the Red Planet. There may have even been an ocean. The surface temperature was a balmy 0o C or above to allow liquid water. To allow that to happen, the atmosphere must have been a lot denser than it is today. A dynamic molten core gave rise to a global magnetic field that protected Mars from the ravages of the solar wind and powered tectonic activity in the Martian crust. Hot springs were likely commonplace. Billowing volcanoes resupplied a dense Martian atmosphere with greenhouse gases needed to sustain a warm and wet climate.

The reality of this picture is somewhat controversial, but if it is true, it seems likely to many scientists that early Mars could have teemed with simple forms of life.

"Microbial communities developed on early Earth in less than a billion years, so it's plausible that simple organisms also developed on an early wet and warm Mars," says Dr Jack Farmer, a geobiologist at Arizona State University. "Current conditions on the martian surface are hostile to life, but there might be a fossil record of ancient microorganisms if we look in the right places."

Farmer (formerly of NASA/Ames) along with his collaborators at ASU, is a pioneer in the new scientific discipline called exopaleontology -- the search for signs of primeval life on other planets.

"Mars may harbor the best preserved rocks in the solar system," he continued. "For example, the Allan Hills meteorite [an ancient potato-sized rock from Mars that crashed into Antarctica 13,000 years ago] is nearly 4.6 billion years old. The fossil record on Mars might go all the way back to the earliest history of the planet."

Farmer says he wouldn't mind visiting Mars to prospect for fossils in person, but an unmanned probe is likely to be the first exopalentologist on the Red Planet. Where should a Mars lander set down to seek out the elusive fossil record? The answer to that question may be found here on Earth in an other-worldly place called Mono Lake.

Mono Lake in California is nearly 700,000 years old, making it one of the oldest lakes in North America. Throughout its long existence, salts and minerals have washed into the lake from Eastern Sierra streams, but there is no outlet. Fresh water evaporating leaves behind salts and minerals so that now Mono Lake is about 2 1/2 times as salty and 80 times as alkaline as the ocean. Swimmers in the lake find that they literally cannot sink (dissolved carbonates, chlorides and sulfates make floating easy) but their skin does tend to bleach and burn in the alkaline water.

Although Mono Lake is an extreme environment for life, it hosts a thriving ecosystem. There are no fish, but the lake supports trillions of brine shrimp (which feed vast numbers of nesting and migrating birds) and a bizarre variety of scuba-diving alkaline flies. It is also brimming with microorganisms such as diatoms, cyanobacteria and filamentious algae.

"The geology of the Mono Basin reminds me of many old Martian lake beds," says Farmer. "Take Gusev Crater for example. It's a basin on Mars formed by an impact more than 3.5 billion years ago. Water flowed in through channels in a huge canyon called Ma'adim Vallis, but there was no outlet. It was an evaporative lake site."

There is almost certainly no life in places like Gusev Crater today. All the ancient ponds and lakes on Mars are now bone dry and scorched by solar UV radiation. Nevertheless, there could be fossils of life forms that thrived billions of years ago, and a curious geological feature of Mono Lake may be telling us where and how to look for them.

At first glance the most striking aspect of Mono Lake are the weird mineral spires called tufa, a type of freshwater limestone. They are formed when calcium-rich spring water bubbles up through the alkaline lake, which is rich in bicarbonate. The calcium and bicarbonate combine, precipitating out as limestone. Tufa towers only grow while underwater, but at Mono Lake they can be seen towering as much as 12 feet above the surface. That's because the lake level has been lowered in recent years to supply water to Los Angeles, 360 miles to the south.

"Whenever you have minerals that precipitate rapidly as they do around the springs in Mono Lake, microorganisms become entombed," says Farmer. "The fossils of soft-bodied microbes formed by this process could be preserved for billions of years."

Farmer has spent many years studying the tufa at Mono Lake as an analog of carbonate deposits that might one day be discovered on Mars.

"There are lots of microfossils here and there in the tufa, formed where the rapid precipitation of carbonates captured microorganisms," continued Farmer. "I've seen larval casings of alkaline flies and cyanobacteria fossils, also things that look like algae (simple multicellular plants). I haven't yet found any fossils of brine shrimp, but I'm still looking."

"In thin sections of tufa I've also found clumps of decayed organic material called kerogen, which may contain chemofossil signatures. Chemofossils are the chemicals produced by the breakdown of cell walls. For example, Mono Lake diatoms have a hard shell with an organic coating that protects them from the alkaline water. When they die, the coating dissolves and so does the diatom. All that's left of this organic material is trace chemicals. It is possible to relate such products to specific organisms like diatoms or algae, but its not always easy. You have to become a Sherlock Holmes and piece together what the community must have been like from clues (both chemical and fossil) that are preserved."

"In evaporative basins, there's a lot of variation in chemistry from basin to basin, and throughout the history of the lake," Farmer continued. "What's beautiful about Mono Lake is that we have an active system of tufa-formation and mineral precipitation. Other paleo lake basins in Western North America are now dry because the climate has changed and evaporation now dominates inflow."

In Search of Mono Lake -- on Mars?

Finding microfossils on Mars won't be easy, even if life once existed there. After all Mars is a big planet and fossils are not likely to be found just anywhere. No one knows for sure, but Farmer and collaborators think that a good starting point might be evaporative basins with carbonate deposits where microbial fossils could be entombed, in other words, places that were once like Mono Lake.

The chemical mixture in an evaporative basin depends on what kinds of rocks are in the vicinity. When water flows into a lake, it flows over rocks and dissolves minerals and ions such as sodium, chloride ions, pottasium, calcium -- all the salts commonly found in the Western Salt Lakes. In an evaporative basin the salts and minerals become concentrated, and the lake naturally becomes alkaline with ph > 9. The detailed chemical balance depends on the details of the terrain. This general picture is true on both Mars and Earth.

"Compared to Earth, Mars has a much different set of source rocks," explains Farmer. "On Mars the crust is more like the ocean floor on Earth, featuring basalts, iron, magnesium, and silicate-poor rocks. Rocks in the Mono Basin are enriched in silica, sodium and potassium. Because water was less abundant, it took longer to build up briney water on Mars through evaporation. But the waters there would be richer in calcium, magnesium, and iron. In spite of these chemical differences, the basic picture is still the same: rapid precipitation of minerals would have been an important process in these ancient martian basins, and if microorganisms were there, their fossils would have been entombed."

Phil Christensen, one of Farmer's collaborators, is using the Thermal Emission Spectrograph (TES) on Mars Global Surveyor to search out places on Mars with tufa-like carbonate formations in evaporated lake beds. Carbonates have specific kinds of absorption features in mid-infrared spectra that should be easy to identify.

Unfortunately, the resolution of the TES is only 3 km/pixel, which would make smaller carbonate deposits like those at Mono Lake difficult to detect. In March 2001 an Arizona State University instrument called THEMIS (Thermal Emission Imaging System) is scheduled for launch on NASA's Mars Surveyor 2001 orbiter. With a spatial resolution of 100m per pixel, the ASU spectrometer could easily detect the signature of carbonate deposits at the scale of the Mono Lake tufas.

"I'm optimistic," concludes Farmer. "Eventually I believe we're going to find carbonate deposits on Mars -- places that remind us of Mono Lake -- and when we do we'll have strong arguments for a landing site for exobiology. It's just a matter of time."