TCS Daily


The Beagle 2 Is Landing

By Sallie Baliunas - August 27, 2003 12:00 AM

Start with the premise that life is possible where energy and water are available. Now think of Mars.

 

Mars is approximately one-tenth the mass of Earth, so the interior heat of the planet escaped to space soon after the formation of the solar system, around 4.5 billion years ago. Spacecraft flybys, orbiters and landers have sent back information that reveals an early Mars environment of volcanic activity, warmth, rivers and possibly shallow seas around four billion years ago. Mars today is dry and cold.

 

The southern portion of Mars is anciently and heavily cratered, and the northern lowlands, sunken about five kilometers below the southern highlands, are much less cratered. The difference in cratering between the two hemispheres might be explained by old shallow oceans in the north, where the ejecta of somewhat younger craters seem to have been formed in mud rather than dry soil. A shallow sea may have left an old, faintly outlined shoreline, now eroded, in the northern hemisphere's Utopia Basin in the imagery sent from NASA's Mars Global Surveyor spacecraft that began mapping in 1997.

 

Old gullies seem also to have been cut by flowing water. Liquid surface water suggests a thicker and warmer atmosphere around four billion years ago than today. Did life form on Mars then?

 

The oldest specimens of solar system rock come from the moon -- they were brought back by the Apollo astronauts -- and Mars. That Mars rock has controversial evidence suggesting that life existed on Mars approximately four billion years ago.

 

The Mars rock was found in Antarctica in 1984, near the Alan Hills mountains, and was labeled as meteorite specimen ALH84001. Meteorites from Mars can be identified by trapped traces of gas that have the same, unique chemical markers as Mars' atmosphere that were measured by the Viking landers beginning in 1976.

 

Of more than 20,000 meteorites known on the earth, only 28 so far have been identified as originating on Mars. Among these, ALH84001 merits special attention because of its great age. The rock crystallized on Mars about 4.5 billion years ago. It contains orange-yellow spherules, carbonate globules that contain organic matter and seem to have formed approximately four billion years ago during Mars' period of warmth and liquid water. Approximately 15 million years ago, a meteor struck the dried, ancient southern highlands and excavated the rock that traveled through space and landed in Antarctica some 13,000 years ago.

 

NASA scientists announced in 1996 that the carbonate globules contained four lines of circumstantial evidence that, taken together, suggested early life existed on Mars. One line of evidence came from microphotographs of tiny features, called Biologically Shaped Objects (BSOs) that might be construed as fossilized remnants of microbes. The BSOs, it was subsequently argued, are much smaller than bacteria on earth, so the BSOs in the Mars rock may be too small to hold the minimum amount of RNA and protein necessary for life as currently known. The BSOs and two other of the four lines of evidence have alternate explanations that are non-biological, hence do not convincingly indicate that Mars once incubated life.

 

Most intriguing -- and still standing after intense scientific debate -- is the presence of clipped chains of magnetite crystal (such as iron oxide, Fe3O4) built from hexagonal polyhedrons inside the carbonate globules. Some terrestrial bacteria contain very similar chains of magnetite crystals encased in a membrane -- a structure called a magnetosome -- that aids the organism in navigating with respect to Earth's magnetic field, for example, in a rapidly-changing tidal environment. The only known explanation of the tiny magnetite crystal chains is biological. Because early Mars had a planetary-scale magnetic field, if bacteria were to have existed on early Mars some might also have used magnetotaxis for efficiently moving through changing water environments.

 

The European Space Agency's Mars Express is speeding toward Mars to continue the search for early life there. Its Beagle 2 lander (named in honor of the Beagle on which Charles Darwin sailed to collect observations that formed the basis of his Origin of the Species) is expected to touch down December 25, 2003, and would conduct the first experiments to test for life since those of the NASA Viking landers did, beginning in 1976. Beagle 2 will look for chemical signatures of life ever having existed on Mars, as suggested by magnetite crystals in the Mars meteorite ALH84001.

 

As for current conditions, water remains on Mars in the form of ice -- the northern polar cap is mostly frozen water. Mars Odyssey orbiter began mapping in 2002, and sent information suggesting subterranean ice with varying concentration in the soil, for example, at higher densities in Utopia Basin.

 

Thus, life may still exist on Mars today, even under the severe conditions there. Extreme and apparently intolerable conditions on the earth support microbial life, a class of life called extremophiles. Such microbes exist in steam vents in the deep Atlantic, in cold and dry Antarctica or in environments with high concentrations of poisons like arsenic.

 

Extremophiles demonstrate that life is opportunistic and may be present in many locations, and even today on Mars, whose environment seems not as alien as that of some earthly extremophiles.

 

Beagle 2 will also look for present-day microbial activity in the subterranean ice-threaded soils of Mars. Experiments onboard Mars Express just may confirm and explain the positive signals for life detection that emanated from the Labeled Release experiments on the Viking landers nearly three decades ago. If life were not only opportunistic, but also tenacious, then ice may be opportunity enough for modern Martian microbes. They may have precariously clung, in evolving forms, to a harsh existence for over four billion years.
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