Life in the rest of the Solar System

If the development of life on Earth did not require a special set of circumstances, then we expect life to have started elsewhere in the galaxy. It is therefore of interest to search for life elsewhere, and the obvious starting place is our Solar System. Finding even primitive life elsewhere in the Solar System would indicate that the Earth is not just one lucky case, and would give us hope of finding it widespread in the galaxy. Also, finding certain types of life elsewhere in the Solar System would give us insights into how life actually formed on the Earth. When we talk about searches for life, we generally mean "life as we know it". That is, life based on carbon bearing (organic) molecules. (Science fiction writers have speculated on other forms of life, such as silicon based, but there is no current evidence to suggest that searching for such life forms would be fruitful.) Development of carbon based life generally requires water, so in choosing places to search for life, we would look at places with evidence for water, at least in the past. Such life would also require the presence of an atmosphere.

Let us think about how we might look for life elsewhere in the Solar System. Some used to suggest that "canals" on Mars were evidence for intelligent life, or that changing colors with seasons suggested vegetation. We now know that those canals don't really exist. They are artifacts of visually connecting unconnected features. The color changes are real, but are due to dust storms, which vary with season. So, we must look for life in much more subtle ways. The life we find may be microscopic, or it may be extinct, having left fossils. So we use remote sensing to select likely sites of current or former microscopic life, and then inspect those sites with various landing equipment. That landing equipment must have instruments capable of detecting the life or some by-product of its existence. For example, we may look for the results of respiration. Such searches may be limited in the sense that they are looking for some particular organism or by-product.

Lunar soil samples, returned to Earth by Apollo astronauts, have been extensively studied in the laboratory, with no evidence for extraterrestrial life. For the first few missions, astronauts were kept in a quarantine for an extended period of time, because of the fear that they might carry some form of previously unknown contagion. That practice was limited when the first few missions revealed no signs of microbial life. Not finding life on the Moon should not be surprising, as the Moon lacks both an atmosphere and water.

Mars is potentially an interesting place to look for life, either current or fossil. That is because we think that prior to 3.5 Gyr ago, when life was emerging on Earth, conditions on Mars were similar to those on Earth. There is evidence for abundant liquid water on Mars, in the form of rivers, lakes and possibly larger bodies, like oceans. We might ask how far the early, prebiotic, chemistry proceeded on Mars. Did such a chemistry develop so far as to lead to life - replicating molecules? If such early life started, how did it evolve? Is it still present or did it die off? If it is still present, we can look for it directly. If it died off, we can still look for fossil evidence.

The first attempts to answer these questions were made by the Viking landers. In designing experiments to look for chemical signs of life, e.g. respiration or photosynthesis, you have to make a decision about what chemicals you will look for. This requires making assumptions about the kind of life we are looking for. So, as a starting point, the Viking experiments were designed to look for microbial life with a chemistry similar to that on Earth. These experiments did not yield evidence for existing life "as we know it" at either site. A more extensive analysis of this data suggests that there is some evidence for organic chemical activity, but Martian life is not the only possible explanation. This shows some of the difficulties in designing and interpreting remote experiments to answer such subtle questions.

We are now early in the next phase of this search on Mars. From orbital mapping, we look for places that show evidence for an abundance of water in the past, as well as temperate weather conditions to promote chemical activity. (It has even been suggested that Mars had sources of heated water, like geothermal springs on Earth.) From these orbital missions landing sites have been chosen for current and future landers. These landers will use increasingly sophisticated experiments to explore the chemistry of the terrain surrounding the landing sites. Eventually, material from promising sites will be returned to Earth on unmanned spacecraft. These samples will be studied extensively in terrestrial laboratories.

There is already a small source of Mars surface material on Earth. These are rocks that were thrown off the surface of Mars by meteoritic impact, and then happened to strike the Earth, like other meteors that the Earth encounters. The hard part is to distinguish rocks from Mars from those that come from normal meteor showers. If we can study them in the laboratory, we find that their chemistry is generally like that at the Viking lander site. This strongly suggests that they are from Mars. One meteor "observatory" on Earth is in Antarctica, as it is easy to pick out rocks against the white snow/ice background. One object found in 1984 was not classified as Martian until 1993. It was studied extensively for the next two years, and the researchers found microscopic fossils, which they concluded may have come from Mars. However, other groups studying this meteor suggested that these fossils may have been contamination from Earth. This shows how difficult these experiments can be.

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