8. Mars: the red planet
Search for life on Mars
Viking 1 and 2 search for life
One of humanity’s most daring and imaginative experiments involved landing spacecraft on the surface of Mars, and searching for evidence of life there. The 3 billion-dollar gamble, in today’s dollars, began on 20 July 1976, when the Viking 1 lander came to rest on the western slopes of Chryse Planitia, the Plain of Gold, region of Mars. It appeared to have once been inundated by a great flood and was thus a promising place for life to have arisen. Six weeks later, the Viking 2 lander settled down in the Utopia Planitia region on the opposite side of the planet, near the maximum extent of the north polar cap, again a favorable site for water and possible life.
How did the Viking landers test for life on Mars? The first, most obvious, moving creature test consisted of looking to see if any creatures were frolicking on the Martian surface. The cameras could detect anything down to a few millimeters in size if it came within 1.5 meters of the landers. Pictures were taken of all the visible landscape, from the stubby lander-legs to the horizon, for two complete Martian years, but the view was always one of a desolate, rock-strewn, wind swept terrain. A careful inspection of all these pictures failed to reveal any motions or shapes that could suggest life, not a single wiggle or a twitch, or an insect or worm. Unless they look like rocks, there are probably no forms of life on Mars larger than a few millimeters in size.
Of course, no one really expected that the Viking eyes would see living things, and each of the landers carried a $50 million biology laboratory designed to detect tiny, invisible microbes. Computerized devices inside the landers measured samples of the Martian soil for organic molecules and for signs of growth that might signal the presence of living microorganisms. The presence of microbes could be inferred if the Martian soil contained organic molecules with carbon in them. Such a test for organic molecules might be called a dead-body test, for soil would be expected to contain a higher proportion of organic molecules derived from dead bodies than from living ones. But not a single carbon compound was detected, even though the instruments could have spotted organic molecules at a concentration of one in a billion.
The other experiments on board the Viking landers searched for the vital signs of living microbes. They did this by exposing the soil to various nutrients, and sniffing the atmosphere to see if microbes ate the food and released gas. Something did emit carbon dioxide and oxygen gas, but it wasn’t alive.
After further experiments, scientists concluded that the biological tests failed to detect any unambiguous evidence for life on Mars. Instead of being produced by organisms of any kind, all of the results were attributed to non-biological, chemical interactions. Highly oxidized minerals in the Martian soil were reacting with the nutrients, breaking them up and liberating some oxygen gas and even more carbon dioxide.
Mars Pathfinder and Sojourner Rover
Although the Pathfinder mission was not designed to look for direct signs of life, it did search for very indirect evidence of a formerly warm, wet Mars that might have supported life. The landing site in Chryse Planitia, the Plain of Gold, was chosen because it lies at the mouth of a large outflow channel, called Ares Vallis, apparently carved by catastrophic floods in the distant past. It is thought that running water flowed down the Ares Vallis and flooded the plain at the landing site between 3.6 and 4.5 billion years ago.
One of the major unexpected results of Mars Pathfinder was the inability to detect chemical diversity in the rocks. The data are consistent with all measured rocks being chemically the same and covered by different amounts of the same dust. This was a surprise, because the landing site was expected to contain a wide variety of ancient rocks washed down by the flooding waters.
The distant, streamlined hills, known as Twin Peaks, appear to have been smoothed by water, and evidence of layered sedimentation shows up in both the nearby rocks and the Twin Peaks. Even the presence of sand, as opposed to smaller dust particles, suggests the widespread action of flowing water. The sand detected from Pathfinder was light in color, just like beach sand on Earth; and sand on Earth is formed by running water. In addition, magnets on Pathfinder found that the airborne dust is very magnetic, which can be explained if liquid water helped embed magnetic minerals in the dust.
Thus, there is abundant evidence that liquid water once flowed across Mars, and that the climate must have once been warmer and wetter than at present. Perhaps the planet had a thicker atmosphere in its early history, and conditions were then conducive to the survival of life.
The immediate vicinity of the Pathfinder landing site, however, appears to have been dry and unchanged for eons. The region seems to have remained almost unaltered since catastrophic floods sent rocks tumbling across the plain more than two billion years ago. It has apparently been untouched by water ever since the ancient deluge. Only the winds remained to erode and shape the surface.
The small rover, called Sojourner, added an important scientific component to the Pathfinder mission, along with elements of drama and excitement that captivated the public. The tiny vehicle weighed just 10.6 kilograms, about the same as a house pet, and had the overall size and rectangular shape of a small microwave oven. Equipped with six-wheel drive, with each wheel driven separately, Sojourner explored about 250 square meters of the Martian surface, measuring the chemical makeup of the rocks and soil. In contrast, each of the two Viking landers were shackled to one location, unable to roam across the surrounding terrain, somewhat like getting sick in bed on vacation.
The rover examined an array of Martian rocks of different shapes, sizes and texture, provided with colorful names like Barnacle Bill, Yogi, Scooby Doo, Casper, Wedge, Shark and Half Dome. It also analyzed the Martian soil in the vicinity, showing that it is very similar to the soil at the Viking 1 and 2 landing sites.
Possible life in a rock from Mars
Interest in the possibility of Martian life was heightened when scientists found possible signs of ancient, primitive bacteria-like structures inside just one of these rocks from Mars. This meteorite crash landed on the blue ice near the South Pole toward the end of the last ice age, resting there in frigid isolation for millennia, most likely compressed in the snow and later exposed. Then in 1984 a geologist spotted it in the Allan Hills region of Antarctica, bagged it, and sent it to the United States for analysis. Since it was the first meteorite to be processed from the 1984 expedition, the unusual rock has been designated ALH (for Allan Hills) 84001.
The controversy and excitement centers on suggestions that microbial life took refuge in cracks within ALH 84001 a very long time ago. The microscopic and chemical evidence includes globules of calcium carbonate; the first organic molecules thought to be of Martian origin; several mineral features characteristic of biological activity; and what looks like fossils of extremely small bacteria-like organisms that lived on Mars billions of years ago. Some scientists argue that these observations collectively provide strong circumstantial evidence for past, primitive life on Mars; others reason that the evidence is not conclusive and that there are non-biological explanations for all of it.
The continuing hunt for extraterrestrial life
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Copyright 2010, Professor Kenneth R. Lang, Tufts University