Search for Life Within the Solar System

The Moon, Mars and Venus are the first places where people looked for signs of other civilizations. Until spaceprobes were able to approach, and in some cases land on, the other planets of the Solar System, our knowledge of them was sketchy. Mars, one thought to have had a colossal "alien"-made irrigation system, as the result of exploration of Mariner and Pioneer probes, was found to have a very thin atmosphere of mostly carbon dioxide and no liquid water. The canals turned out to be optical illusions, although the large valleys scouring the surface of Mars are likely to be the product of wear of flowing water. Mars still has water, but not in liquid form, and it is mostly confined in the polar caps and in the subsurface layers. The atmosphere that Mars once had has been dispersed away, since Mars is much less massive than the Earth and incapaple of exerting a gravitational pull strong enough to keep gases above its solid surface. Furthermore, Martian volcanism is absent now; thus, differently from the Earth, Mars is lacking the means to vent into the atmosphere gases generated in its interior

The discovery of possible past life forms on Mars, see section Hot Topics , has renewed the interest on Mars. Numerous space missions are planned, and there is talk of sending a human mission to Mars within the next twenty years. Venus was thought too to have inhabitants; until the Russian and then NASA missions visited the planet, little was known about it, since it is always seen covered by thick clouds. It turns out that the conditions on Venus are quite prohibitive; there is a dense atmosphere of corrosive gases and a high surface temperature. Venus is now considered as an example where a runaway greenhouse effect occured. In such scenario, gases that let visible rays from the Sun to penetrate but block the dispersion of infrared radiation (heat) into space, make the planet to become hotter and hotter. Under current conditions, to send unmanned probes to land on Venus is considered a challenging project, although a Russian probe survived on the surface of Venus for a biref periof of time. The probability of finding primitive organisms on Venus is considered quite remote. In recent years, the spaceprobe Galileo has returned tentalizing data on some of the moons of Jupiter. Specifically, it was found that Europa, a satellite of Jupiter of the dimension of the Moon, has probably thick sheets of ice on its surface, and possibly liquid water below it. In the next years we should know more about it, as scientists will refine their instrumentation to obtain more definite answers about the phsyical and chemical conditions of the surface of Europa and Io, another of the Jupiter's moons. By the way, the outer planets of the Solar System are poor candidates as sites for the develpment of life forms, since they are "gas giants", that is, they are composed primarely of gases of hydrogen and helium at high pressure. They are, therefore, quite different from the "terrestrial or inner planets", from Mercury to Mars, which have a solid crust and are composed of non-volatile materials.

Finally, a word about comets and meteoroids. These objects, which range in size from a fraction of a millimeter to a few Km, and wander through interplanetary spaces, can tell us about the composition of the original solar nebula from which the solar system formed. Some of these objects might contain material that predates the Solar System, i.e., material that has been picked up during interstellar travel before the Solar System was formed 4.5 billion years ago. Some scientists, beginning with Arrhenius a century ago, postulated that the seed from which life started on Earth, i.e., the DNA code, was delivered on Earth by a comet or meteorite. This hypothesis, called panspermia, has won some important converts, such as Crick, the co-discoverer of DNA. Carl Sagan calculated the odds of a piece of biological material coming from the neighborhood of another star and surviving the harsh conditions of interstellar travel. It is well known that biological material suffers damage from receiving doses of ultraviolet radiation. He found that survival rate for a piece of biological material during an interstellar travel was very low.
Detailed information about  Mars, Venus and Jupiter and space missions to those planets can be be obtained at the following NASA Web sites.