The dark spots that appear near the south pole of Mars in early spring may be a sign of life. We may be able to find out for sure when Mars Express, the European Space Agency?s Mars mission, goes into orbit around Mars in late 2003.

Agustin Chicarro, European Space Agency project scientist for Mars Express, says, ?As a geologist, I found the spots quite perplexing and very exciting.?

A controversy about the Mars spots began when Andras Horvath, Tibor Ganti and Eors Szathmary from the Planetarium and the Institute for Advanced Study, Budapest, suggested that the spots could be colonies of Martian microbes which wax and wane with the season.

Michael Malin and Kenneth Edgett, designers of the Mars Orbital Camera on board NASA?s Mars Global Surveyor spacecraft, which recorded the images of the spots, suggested an explanation involving evaporation and re-freezing of carbon dioxide ice.

The spots appear on dunes found on the floors of craters in the south and north polar regions. The Hungarian team has examined the southern spots in detail. ?They appear in late winter and by summer they have disappeared. They appear first at the margins of the dune fields and rarely appear on the ridges of dunes,? Szathmary says. Their location (which is independent of the elevation of the land) and shape (which is circular on flat surfaces but elongated on slopes) seem to be at odds with a physical explanation and indicate a biological explanation instead.

If the spots are colonies of photosynthetic Martian microorganisms, they over-winter beneath the ice cap. As the Sun returns to the pole during early spring, light penetrates the ice and the microorganisms photosynthesise and heat their immediate surroundings. A pocket of water, which would normally evaporate instantly in the thin Martian atmosphere, is trapped around them by the overlying ice. As this ice layer thins, the microorganisms show through as a gray tint. When it has completely melted, they rapidly decay and turn black. This would explain why many dark dune spots have a black center surrounded by a gray aureole, say the Hungarian scientists.

Although there are several examples of black microorganisms growing in hostile environments on Earth, there could be a non-biological explanation for the dark color of the spots on Mars, says Marcello Coradini, ESA?s Solar System Missions coordinator. ?Images taken by the Giotto spacecraft showed that the black color of cometary nuclei [the centers of comets] is formed when a mixture of carbon and water ice is exposed to ultraviolet radiation,? he says. Experiments on board the Mars Express could help to determine whether the same thing happens on Mars.

Microorganisms have been found living in water pockets trapped within the ice cover of very salty lakes in Antarctic dry valleys. However, this environment, although hostile, is far more benign than that found in the Martian dune spots region, David Wynn-Williams from the British Antarctic Survey says. Temperatures at the Martian south pole reach much lower temperatures and the thin Martian atmosphere also transmits far more damaging UV radiation.

However, on Earth cyanobacteria have a very efficient UV filter, says Wynn-Williams, and Martian vegetation may have one too. ?We are going to put some Antarctic microbes into space on the International Space Station to find out just how much UV they can take,? he says.

A lack of water could be the biggest problem for Martian microorganisms. Martian south polar ice is thought to consist mainly of carbon dioxide and there may be insufficient water ice to sustain even the hardiest of microbes. GianGabriele Ori from the Gabriele D’Annunzio University in Pescara, Italy, doubts whether there is any ice at all over the dunes in question. ?The dunes appear very distinct in the images,? he says. ?If there is a covering of ice, it must be very thin.?

However, Ori doesn?t rule out a biological explanation for the spots. ?There could be a geological process which is supporting biological activity,? he says. One possibility is the gradual release of gas including water vapor from below the Martian crust. ?Such gas release could be responsible for the spots without biological activity. But the gas could also fuel biological activity.?

Scientists agree, however, that other geological explanations for the spots cannot be ruled out. ?The biological explanation is by far the most complex and is much less likely than a physical or chemical explanation,? says Wynn-Williams.

No spacecraft is presently planned to land near the dark dune spot areas. The Mars Express lander, Beagle 2, will touch down on Isidis Planitia, a large plain just north of the equator, at the end of 2003. However, several instruments on the Mars Express orbiter can observe selected areas of the Martian surface at very high resolution. ?If the dark dune spots are selected as targets for analysis, many outstanding questions about the spots could be answered,? says Chicarro.

Malcolm Fridlund, project scientist for Darwin, an ESA mission to search for life on other planets in our solar system, says, ?I find it hard to believe that Martian life, the last vestiges of a fertile time 3.5 billion years ago, has hung on by a thread for all this time until humans have developed the technology to find it.?

Images and data from orbit may eliminate some hypotheses, but proof of life on Mars will require landers and possibly human beings to see the evidence firsthand. In the U.S., we should be thankful that the European Space Agency is still interested in exploring life on Mars, now that NASA has had its budget cut and been told to concentrate on military applications. Ori speaks for all of the scientists when he says, ?What we need to settle this is ground truth. We have to go there.?

See news story, NASA Budget Promotes Space Nukes, click here.

To see photos of some of these provocative Mars images, read ?Dark matter, Missing Planets & New Comets? by Tom Van Flandern,click here.

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