elsewhere. If deep biospheres are possible, even in the face of harsh surface conditions, then the prospects for subsurface life on Mars, Europa, or elsewhere seem greater. But we must remember that the requirements for habitability are not necessarily the same as the requirements for the origin of life. On Mars, it is at least possible that life originated at the surface, where it could take advantage of the tremendous available energy from the Sun, and then migrated to the subsurface as the surface became a freeze-dried desert. In the case of Jupiter’s moon Europa, which likely harbors a subsurface ocean of liquid water, it seems unlikely that there were hospitable surface conditions for more than a fleeting moment, if that, early in solar system history. For there to be life in Europa’s ocean, it would likely have to have originated in the subsurface. We do not understand the origin of life well enough to assess the plausibility of this scenario.
In both of these cases—Mars and Europa—life seems at least possible because of the likelihood of the presence of subsurface liquid water. It is fair to ask: must life depend on liquid water? How many of the apparently universal characteristics of life on Earth are requirements for life everywhere? Life on Earth is carbon-based; is this a general requirement or simply one of many possible alternatives?
Of course, we can not answer this question with confidence until we know more and have explored farther. But we are already getting some hints to the answer. Consider alternatives to carbon. Speculation has often focused on silicon-based life as an alternative to the carbon-based life we know. The theoretical reason for this can be seen by glancing at the periodic table of elements; silicon sits directly beneath carbon in this table, which is a short-hand way of saying that its chemical properties are similar. Since silicon, like carbon, is also an abundant element in the universe, it might seem to provide a good alternative. But in fact, silicon’s chemistry is more limited; except under extraordinary laboratory conditions, silicon atoms will not form double bonds with themselves, as carbon atoms do, so silicon chemistry is substantially more restricted than carbon chemistry. This is a consequence of the fact that the silicon atoms are simply bigger than carbon atoms, making double bonds much more difficult.
On top of this theoretical caution, there is an empirical discovery that comes from radio-wavelength investigations of the space between the stars, the so-called interstellar medium (ISM). Probing the ISM at radio frequencies reveals that there is a rich carbon chemistry throughout our galaxy; to date there are nearly a hundred carbon-based molecules observed in the ISM. There is no comparable suite of silicon-based molecules seen. Now, the ISM was not investigated primarily to test the hypothesis of silicon-based life. Rather, scientists simply wanted to learn what was out there—this was largely exploratory science, not hypothesis-testing science. But as a result of exploration, it seems more likely that carbon will be the basis for chemical life elsewhere in the universe, should any exist. Of course, this is at most an implication, not a strong conclusion.
All life we know on Earth is carbon-based, but it shares many more commonalities as well. Its basic biochem-