SETI [ASTRO] Unearthing Clues To Martian Fossils


Larry Klaes (lklaes@bbn.com)
Mon, 14 Jun 1999 08:26:49 -0400


>X-Authentication-Warning: brickbat12.mindspring.com: majordom set sender to owner-astro using -f >Date: Fri, 11 Jun 1999 22:17:51 GMT >From: Ron Baalke <BAALKE@kelvin.jpl.nasa.gov> >To: astro@lists.mindspring.com >Subject: [ASTRO] Unearthing Clues To Martian Fossils >Sender: owner-astro@brickbat12.mindspring.com >Reply-To: Ron Baalke <BAALKE@kelvin.jpl.nasa.gov> > > >http://science.nasa.gov/newhome/headlines/ast11jun99_1.htm > >Unearthing clues to Martian fossils >Marshall Space Flight Center >June 11, 1999 > >The hunt for signs of ancient life on Mars leads scientists to Mono Lake, CA > >Feature Story: NASA Space Science News presents "Feature Stories", where you >can sit back, relax, and enjoy an in-depth look at ongoing research (or >sometimes a story that's just plain > >June 11, 1999: If, while innocently enjoying this article, you were >unexpectedly transported to the surface of Mars, three things would happen >before you could finish reading: > >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 > > "Mono Lake lies in a lifeless, hideous desert ... This solemn, > silent, sailless sea - this lonely tenant of the loneliest spot on > earth - is little graced with the picturesque." > Mark Twain, Roughing It, 1875 > >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." >



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