Humanity is engaged in the tireless effort to search for life on other worlds in our solar system. Or maybe not? The truth is that this idea doesn’t correspond at all with reality. In the history of space exploration there has been only one mission dedicated to the search for extra-terrestrial life: Viking, which landed two twin probes on Mars in 1976.
Even so, space exploration has given us enough data to figure out on which planets or celestial bodies there exist the best conditions for life. Let’s take a tour of the five places in our solar system (not including Earth, of course) that scientists think are the most habitable.
The eternal mystery of whether microbial life exists on Mars couldn’t be solved in 1976 with the Viking probes, which produced confusing results. Since then, other missions have tried to determine the presence of the necessary components for life as we understand it, one of which is carbon. In recent years, the varying presence of methane, the simplest organic molecule, has been discovered in the Martian atmosphere. The majority of the terrestrial methane is of biological origin, but the source of the Martian gas is still a mystery. On the other hand, the presence of more complex organic molecules could be confirmed this year by the Curiosity rover.
Another requirement for life is the existence of liquid water. In 2011, what looked like seasonal flow of brine trickling down the Martian slopes was detected, but later studies have concluded that it is probably simply dry sand. However, in July 2018, the likely presence of a large liquid lake under the ice of the south pole of Mars was revealed. In short, although the chemistry of the Martian soil does not seem optimal for life, the bets are still open.
For decades, the smallest of the four great moons of Jupiter—slightly smaller than Earth’s Moon—has been occupying a preferential position in the speculations about the existence of life in other places of the solar system. The main reason is the large ocean of liquid water that is assumed to exist under Europa’s icy crust, a hypothesis that matches the observations but that has not yet been confirmed. The mass of water, which can reach a depth of 100 kilometres under an ice surface of between 10 and 30 kilometres, could be more than double the volume of the terrestrial oceans. Europa’s ocean would remain liquid thanks to the heat generated by the tidal friction due to Jupiter’s gravity.
A new recent analysis of the data obtained by the NASA Galileo probe in 1997 seems to confirm that the ocean of Europa expels geysers through the ice, which would facilitate the analysis of its composition without the need to land a probe on the surface. At least two missions will fly over Europa in the next decade, Europa Clipper from NASA and Jupiter Icy Moons Explorer (JUICE) from the European Space Agency (ESA).
The case of Enceladus is similar to that of Europa. However, on this moon of Saturn just 500 kilometres in diameter, the presence of water vapour geysers was confirmed in 2005 and 2008 by the Cassini probe, which has also detected other components such as methane, nitrogen, carbon dioxide, ammonium and complex organic molecules, all of them compounds of interest in biology. There are also solid data that point to the presence of a salty ocean under its ice sheet, which was initially thought to be limited to the southern hemisphere, but is now believed to occupy the full extent of the moon. In addition to the propitious chemical composition of the ocean, the signs that suggest the existence of hydrothermal activity under the surface have turned Enceladus into a firm candidate for hosting life.
The largest of Saturn’s moons is the only world in the outer solar system where a terrestrial instrument has landed—the Huygens probe from ESA in 2005. For 90 minutes, Huygens sent back hundreds of images of Titan’s surface, populated by ice and rocks.
Titan lives up to its name as the second largest moon in the solar system, 50% larger than Earth’s satellite and possessing two unique qualities: it is the only moon with a dense atmosphere and the only known world besides Earth with liquid masses on its surface; they are not water, but rather methane, and in these lakes and seas could live microbes capable of consuming hydrogen instead of oxygen and producing methane instead of carbon dioxide. In addition, the possible ocean of liquid water beneath Titan’s surface could also accommodate life forms more similar to terrestrial ones or to those that could be found on other moons such as Europa or Enceladus.
Due to its size, Ganymede is an almost planetary entity. This satellite of Jupiter is the ninth largest object in the solar system, behind Mars and ahead of Titan and Mercury, as well as being the only moon with its own magnetic field. In 2015, the Hubble Space Telescope data on the auroral movement caused by this magnetic field confirmed the presence of a 100-kilometre-thick salt ocean hidden under 150 kilometres of ice.
Currently, scientists speculate that Ganymede could be the object in the solar system with the most water, perhaps up to six times more than on the Earth’s surface, and that its interior could form a sandwich with several layers of ice and liquid water. The possibility that the innermost layer of water is in contact with the inner rock mantle supports the possibility of life, since this interface provides elements necessary for biology. Ganymede will be the main objective of ESA’s JUICE mission, whose launch is scheduled for 2022.
According to experts, designing experiments that can travel aboard a robotic probe to confirm the presence of life on other planets or moons is not an easy task. And to this is added another obstacle that is gaining increasing prominence in recent years: planetary protection, a set of guidelines that follows the provisions of Article IX of the Outer Space Treaty of the United Nations and advises against sending probes to the alien enclaves most likely to harbour life, because of the risk of contaminating them with terrestrial microbes. This, added to the high cost of the possible missions, means that the possibility of verifying the existence of life in any one of these five special spots in the solar system is still far away.