Scientists say Martian spots worth a close look
EUROPEAN SPACE AGENCY SCIENCE RELEASE
Posted: March 14, 2002
Are dark spots that appear near the south pole of Mars in early spring, a sign of life on the Red Planet? No-one can say for sure, according to a group of scientists who met at ESTEC, ESA's technical centre in the Netherlands. But the spots are certainly fascinating, the meeting agreed, and well worth a detailed look by Mars Express, the European Space Agency's Mars mission, when it goes into orbit around the Red Planet in late 2003.
The controversy 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, had previously suggested an explanation involving evaporation and re-freezing of predominantly carbon dioxide ice. The meeting considered these and other possibilities.
Spots appear in Spring, disappear in Summer
The spots are colonies of photosynthetic Martian microorganisms, they hypothesise, which over-winter beneath the ice cap. As the Sun returns to the pole during early spring, light penetrates the ice, 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 grey. When it has completely melted, they rapidly desiccate and turn black. This explains why many dark dune spots have a black centre surrounded by a grey 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 colour of the spots on Mars, Marcello Coradini, ESA's Solar System Missions coordinator, told the meeting. "Images taken by the Giotto spacecraft showed that the black colour of cometary nuclei is formed when a mixture of carbon and water ice is exposed to ultraviolet radiation," he said. Experiments on board Mars Express could help to determine whether the same had happened on Mars.
Life at the extreme on Earth
Nobody knows just how tolerant life can be to these and other environmental stresses. On Earth, cyanobacteria, for example, come with a very efficient UV filter, said Wynn-Williams. "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 said.
A dearth of water, however, 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, doubted whether there is any ice at all over the dunes in question. "The dunes appear very distinct in the images," he said. "If there is a covering of ice, it must be very thin."
John Bridges from the Natural History Museum in London argued for an investigation of similar spots found in the northern polar region, pointing out that wind blown dust could have a role in their formation. Rock weathering, though, was dismissed as a cause "because the spots turn from black to white to black again - and you can't reverse weathering," said Ori.
The meeting agreed, however, that other geological explanations could not be ruled out. "The biological explanation is by far the most complex and is much less likely than a physical or chemical explanation," according to Wynn-Williams. "What we need to settle this is ground truth. We have to go there," said Ori.
Mars Express could take a look
OMEGA, the infrared mapping spectrometer, for example, could determine the mineral composition of the spots, allowing some hypotheses to be eliminated. PFS, the planetary Fourier spectrometer, could measure the amount of carbon dioxide and water ice present, the temperature of the spots compared with their surroundings and the pressure of the local atmosphere. MARSIS, the radar sounder, could determine the thickness of the ice and the HRSC, the camera, could take high resolution, 3D, full-colour images of the spots.
Images and data from orbit may eliminate some hypotheses, but proof of life on Mars will require landers and possibly humans to see the evidence firsthand. A future Mars lander could carry a Raman spectrometer capable of detecting the sorts of pigments used by microbes on Earth to harness solar energy for photosynthesis and to protect them from UV, Wynn-Williams told the meeting. Opportunities to fly this and other innovative instruments to Mars could be provided by Aurora, ESA's programme of planetary exploration currently under discussion.
Malcolm Fridlund, project scientist for Darwin, an ESA mission to search for life on extrasolar planets, however, ended the meeting on a philosophical note which expressed an understandable sentiment: "I find it hard to believe," he said "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."
Several papers on dark dune spots on Mars will be presented at the European Geophysical Society annual conference in Nice, France, 22-26 April 2002