Mercury is not generally considered to be likely to be a planet with life. It is too close to the Sun, extremely hot and dry, and has very little atmosphere. Venus is equally inhospitable which is surprising given that it would seem to have a lot in common with Earth. They are similar sizes and masses and Venus is only 28% closer to the Sun than Earth. However, Venus’ atmosphere has almost no water vapor in it and is 96.5% carbon dioxide. Its atmosphere is 100 times as massive as Earth’s, which means the atmospheric pressure on the surface of Venus is 90 times that of Earth. The carbon dioxide atmosphere on Venus acts as a greenhouse gas and as a result, the surface of Venus has an average temperature of 460°C. This is hot enough to melt lead, tin and zinc. Several Soviet spacecraft landed on Venus and sent data back. The heat and pressure on the surface of Venus meant that each one survived at most a couple of hours before being disabled by the extreme conditions. Scientists believe that Venus probably had water at one time in the past, but conditions in the atmosphere caused the planet to become too hot and all of the water has evaporated into space.
Scientists have been curious for centuries about whether there is life on Mars. In 1877, the Italian astronomer Giovanni Schiaparelli observed Mars through his telescope and observed what he thought were channels. The American astronomer, Percival Lowell, translated the word “canali” as canals, instead of channels, and took them as evidence that there was intelligent life on Mars, capable of constructing large canals.
Other astronomers observed that Mars’ polar ice caps change in size with the seasons, and that the entire color of the planets seems to vary. At first it was thought that these seasons must be like seasons on Earth, and the color changes meant there were plants changing color. We now know that these changes in color are due to Mars’ periodic dust storms, which are large enough to cover larger areas of the planet.
Many missions including Mariners 4,6,7 and 9, the Viking orbiters and landers 1 and 2, Mars Pathfinder, Mars Global Surveyor, Mars Odyssey, Mars Express, Spirit, Opportunity, Mars Reconnaissance Orbiter, and Phoenix have orbited and actually landed on the surface of Mars in search of life. While Mars might have been able to support life in the past, so far no evidence of past or present life has been discovered. Mars’ atmosphere is only about 1% as dense as Earth’s, so is too small to maintain liquid water on its surface for long before it evaporates.
Future missions including orbiters and landers are scheduled to go to Mars and continue searching for life. It is clear that water ice exists on Mars, both in the ice caps and in glaciers at the equator under layers of dust and debris. Liquid water, either on or below the surface has not been discovered.
Perspective view of Korolev crater on Mars filled with water ice.
Some missions have searched for microbes in the dusty surface of Mars by collecting samples of soil, adding water and other nutrients and measuring the gas output. So far such experiments and others have not yielded evidence of any organisms.
Jupiter has at least 66 moons, most of which are relatively small. It has four large moons, the Galilean moons, which are very exciting for astrobiologists because some of them might have environments that could support life.
Io is the nearest Galilean moon to Jupiter. It is in a resonant orbit with Ganymede and Europa. For every one orbit of Jupiter that Ganymede makes, Europa makes two and Io makes four. The gravitational interactions between these moons and Jupiter cause huge tidal forces to act on Io’s crust and as a result, Io is very active geologically. The number of active volcanoes has turned Io into a completely dry planet, and it is very unlikely that there would be any life there.
Europa, the next Galilean moon out from Jupiter, is very different from Io. Its surface seems to be a thick layer of ice. Because of the way Europa’s magnetic field seems to behave, astronomers suspect that beneath the ice on Europa is a global ocean of saltwater. The water probably remains liquid because of tidal heating from Europa’s interaction with Jupiter and the other large moons. It is not clear how thick the ice is or how deep the ocean is. The ice and water combined are probably a shell between 70 and 170 km thick on top of a rocky surface. The ice is most likely at least 3-4 km thick, and may be as thick as 25 km. This environment might be similar to some of the underground lakes in Antarctica that are being explored and likely contain life.
Ganymede, which is the largest moon in the solar system also seems to have a saltwater ocean beneath an ice crust. The energy from the tidal interactions between Ganymede and the other Galilean moons should not generate enough energy to keep saltwater liquid on this moon, and astronomers think the water is probably kept liquid by a liquid iron core inside the moon, and the insulation of a the ice which is hundreds of kilometers thick. Ganymede also seems to have a very thin atmosphere.
Callisto is the fourth Galilean moon out from Jupiter, and although it is not in a resonant orbit with Jupiter in a way that could cause the tidal energy the other moons experience, it also seems to have a liquid ocean beneath a thick ice crust. The liquid water is likely to have ammonia and salts dissolved in it which lower the freezing point of water. Some heat is probably generated by radioactive elements in the rocks on Callisto and this seems to be enough to maintain the liquid water.
Saturn has at least 62 moons, ranging in size from only a kilometer or less across, to the largest moon in the solar system, Titan, which is larger than the planet Mercury! Several of these moons have features that make them of interest to astrobiologists.
Titan is the only moon in the solar system that has a significant atmosphere. The atmosphere is about 1.5 times the atmospheric pressure on Earth and is composed mainly of nitrogen and a small amount of methane. Titan’s surface is very cold, about -180°C, and is covered in lakes or perhaps even oceans of liquid methane. Methane may exist as a solid liquid and gas on Titan, and may behave much like water does on Earth, regulating climate and causing weather. Lightning has not been detected in the atmosphere, but over the course of billions of years, amino acids and other complex molecules might have had time to develop.
Rhea also has an incredibly thin carbon dioxide and oxygen atmosphere, but is only a trillionth the density of the atmosphere on Earth.
Enceladus has some very surprising features that make it very interesting to astrobiologists. It is white and reflects almost 100% of the light that strikes it. It also has geysers on its surface that shoot out jets of water with ammonia and organic compounds. The temperatures near these geysers are likely very high, while the rest of the moon is very cold with an average temperature of about 220°C.