One question that NASA seeks to answer is, is there life somewhere else? That raises the question, how can you tell? The simplest answer is, find something that only life can make. The problem with that is that life uses chemistry, and chemistry occurs anyway, so it is sometimes possible that what you find might be due to life, or it might be due to geophysical or geochemical action. Another problem is, some of the molecules that life makes more readily than simple geology does, say, may be difficult to find. When looking for minor traces, it is possible to find “signals” but misinterpret them. In an earlier post, I suggested the so-called signals for phosphine on Venus fell into that class, and what I have seen since reinforces that view. Many now think it was one of the signals from sulphur dioxide, which is known to be there.
One of the strongest indications we could find would be to find a number of homochiral chemicals. Thus when sugars are made chemically, say by condensing formaldehyde, they are either in the D or L configuration. Chirality can be thought of as “handedness”; your left hand is different from your right hand, and the same thing happens for chemicals used by life – life only makes one sort, the reason being that reproduction from nucleic acids only work if they can make a double helix, and that only works if there is a constant pitch, which in turn requires the linking group, the ribose, to be in one form – left or right handed. Amino acids are similar because enzymes only work in specific configurations, as do many of the other properties of proteins. The problem with chirality as evidence of life is that it is hard to measure. The usual method is to isolate the compound in a pure form in solution, pass polarised light through it, and measure the rotation of the polarization. But that really needs a chemist on the spot. Remote sensing is not really suitable. Forget that for exoplanets.
One approach has been to find a gas in the atmosphere typical of life. If you found an atmosphere with as much oxygen as Earth’s, it would almost certainly have life because oxygen cannot be accreted directly by a planet in the habitable zone. The bulk of Earth’s oxygen has probably come from photosynthesis, or the photolysis of water. The latter occurs in the absence of life, but when it does in the atmosphere, it forms ozone, which stops the reaction because water will be below the ozone. On Mars, some water has been photolysed on the surface, but it formed peroxides or superoxides with iron oxide, or perchlorates with chlorides. So a lot of oxygen is indicative. Another gas is methane. Methane is given of by anaerobic bacteria, but it is also made geologically by reacting carbon compounds, such as the dioxide, with water and ferrous ions, which are common in the olivine-type minerals, which in turn are very common. Almost any basalt will react, in time. So methane is ambiguous.
Perhaps, we should look for more complicated molecules. There are still traps. Recent work has shown that the chemicals that are part of the Krebs cycle, which is rather fundamental to life, actually can be made from carbon dioxide, iron, and some metal ions such as zinc. Even these are not characteristic of life, although the work may give further clues as to how life got underway, and why the chemicals used in the Krebs cycle “got involved”.
When NASA sent its Viking rovers to Mars, their approach was to treat soil samples with water and nutrients that microbes could metabolise, and then they looked to see if there were any products. One experiment detected radio-labelled gases from samples treated with carbon-14-labelled nutrients, and the idea was if the 14C got into the gas phase, where its radioactivity could be detected, it would mean life. Maybe not. If the nutrients landed on a superoxide, they would have been converted to gas. It is not easy doing this remotely.The one difference that characterises Earth when seen from space is its colour. However, the blue merely means oceans. It is possible that planets with oceans will also have what is required for life, but we could not guarantee that. If we recognised spectral signals from chlorophyll, that would be a strong indication, but whether such signals can be observed, even if there are plants there, is unclear. Again, this is not easy.