The Earth’s biosphere contains all the known ingredients necessary for life as we know it. In a broad sense, these are: liquid water, at least one source of energy and a list of biologically useful elements and molecules.
But the recent discovery of a nutrient in the clouds of Venus reminds us that some of these ingredients exist elsewhere in the solar system. So where are the other most promising locations for extraterrestrial life?
Mars is one of the most Earth-like worlds in the solar system. It has a 24.5-hour day, polar ice caps that expand and contract with the seasons, and a large number of surfaces that have been created by water throughout the planet’s history.
The discovery of a lake under the south polar ice cap and methane in the Martian atmosphere (which changes with the season and even the time of day) makes Mars a very interesting candidate for life. Methane is important because it can be produced by biological processes. But the real source of methane on Mars is not yet known.
It is possible that there is life here, given the evidence that the planet once had a much more favorable environment. Today Mars has a very thin, dry atmosphere, almost entirely composed of carbon dioxide. This provides poor protection against solar and cosmic radiation. If Mars has managed to preserve some of its water reserves beneath its surface, it is possible that life could still exist.
Europa was discovered by Galileo Galilei in 1610, along with three other larger moons of Jupiter. It is slightly smaller than the Moon and orbits the gas giant at a distance of about 670,000 km every 3.5 days. Europa is constantly contracting and stretching by the competing gravitational fields of Jupiter and other Galilean moons. This process is known as tidal bending.
The moon is believed to be a geologically active world, like the Earth, because strong tidal curves heat its rocky, metallic interior and partially melt.
Europa’s surface is a vast expanse of water ice. Many scientists think that under the frozen surface there is a layer of liquid water – a global ocean that cannot freeze due to heat and which can be more than 100 km deep.
Evidence for this ocean includes geysers erupting through cracks in the surface ice, a weak magnetic field, and chaotic surface relief that could have been deformed by ocean currents circling below. This ice sheet insulates the underground ocean from the extreme cold and vacuum of space, as well as from Jupiter’s fierce radiation belts.
At the bottom of this oceanic world, we can find hydrothermal vents and volcanoes. On Earth, such features often support very rich and diverse ecosystems.
Like Europa, Enceladus is an ice-covered moon with a subsurface ocean of liquid water. Enceladus revolves around Saturn and first came to the attention of scientists as a potentially habitable world after the unexpected discovery of huge geysers near the moon’s south pole.
These jets of water emerge from large cracks in the surface and, given the weak gravitational field of Enceladus, are sprayed into space. This is clear evidence of the existence of underground storage of liquid water.
Not only water was found in these geysers, but also many organic molecules and, most importantly, tiny grains of solid silicate particles, which can only be present if the ocean’s subsurface water is in physical contact with the rocky bottom at a temperature of at least 90 ? C. This is very compelling evidence for the existence of hydrothermal vents at the bottom, providing the chemical composition necessary for life and localized energy sources.
Titan is Saturn’s largest moon and the only moon in the solar system with a solid atmosphere. It contains a thick orange haze of complex organic molecules and a methane meteorological system instead of water with seasonal rains, dry spells, and surface sand dunes created by wind.
The atmosphere is composed primarily of nitrogen, an essential chemical used to build proteins in all known life forms. Radar observations revealed the presence of rivers and lakes of liquid methane and ethane and, possibly, the presence of cryovolcanoes – volcano-like formations that spew liquid water rather than lava. This suggests that Titan, like Europa and Enceladus, has a supply of liquid water below the surface.
At such a huge distance from the Sun, Titan’s surface temperature is -180 Celsius, which is too cold for liquid water. But the abundance of chemicals available on Titan has given rise to speculation about the possible existence of life forms – potentially with a fundamentally different chemical composition from terrestrial organisms.