When humans finally create a reliable, safe and speedy method of travel, which is quite inevitable; what will be our first destination? With 7 planets (other than Earth) that orbit the Sun, and many more moons that orbit these planets, there are almost too many options. Clearly, if our goal is to find evidence of extraterrestrial life, some of these bodies have significantly better prospects than others. So which planets/moons in our solar system are habitable enough for it to be worth our time to do some research there?
The most obvious answer is Mars. For hundreds of years, humans have pondered the possibility of us having Martian neighbors, and some (namely Percival Lowell) even became convinced that advanced civilizations inhabited the cold, barren planet. By now, technology has advanced to a point where we can easily disprove this; we have even put multiple landers on the surface of Mars, and none of them have observed any evidence of advanced organisms. However, life does not only exist in the form of super-evolved organisms capable of advanced intelligence. Instead, the best chance we have for searching for life on other planetary bodies lies in finding microbial life. Since microbes can survive in a much wider range of conditions than lifeforms like humans can, they could potentially survive in conditions that seem extremely hostile to us… Like on Mars. While Mars is extremely cold, it does actually have water (which is, of course, one of the essentials for life), just not in liquid form. Mars has nearly no atmosphere as a result of its relatively small size/gravity and solar stripping, so its atmospheric pressure is extremely low. Because of this, on Mars water goes directly from its water ice phase to its gaseous state, and never has an intermediate liquid phase. While this does diminish the likelihood of finding Martian life, Mars’ internal heat probably keeps water liquid at certain altitudes, all of which are below the surface. On top of this, it is theorized that Mars was warm enough 2-3 billion years ago to have had abundant flowing water on its surface, and as the Sun brightens with age, Mars will again enter the habitable zone (pictured below) in the future.
After several hundred million years, Mars will enter our conservative prediction of where the habitable zone will be
As you can see, both Mars’ proximity to Earth and much of our scientific analysis of the planet makes our neighbor one of the leading candidates for extraterrestrial life in our solar system.
The other place we would search for life, although it is much further away, is Europa, the second moon of Jupiter. Because liquid water is one of the main requirements for life in the universe, planetary bodies with oceans offer great potential locations to search for life. It is this reason that makes Europa such an exciting place to look for life. Europa’s surface consists almost entirely of solid water ice, and as a result is extremely cold. While prospects for life on Europa’s icy, desert-like surface are small, it is thought that this Galilean moon harbors a salty subsurface ocean many kilometers under its icy surface. There are a few reasons why astronomers theorize the existence of this ocean, the most important of which is the way that cracks on Europa’s icy surface seem to fit together. There are many rifts in the ice of the surface, which can be seen in the picture below, and each of these rifts seems like it might form a sort of jigsaw puzzle with the pieces around it.
The cracks in the ice likely represent places where masses of ice have broken apart, shifted as a result of the water they temporarily float in, and finally re-frozen
It is for this reason that astronomers believe that liquid water occasionally swells up from the ocean below to both repave the icy surface and to cause masses of ice to shift apart from each other, then re-freeze in place. This seems like fairly convincing evidence of a subsurface ocean, but there is one more detail that makes the existence of the ocean even more likely. The Galileo orbiter discovered that Europa is able to interact slightly with Jupiter’s magnetic field. This is only possible if there is some conductive material under the surface of Europa, and a salty subsurface ocean would offer a perfect example of this. These two lines of evidence go together to make a fairly convincing argument that Europa at least has a subsurface ocean. There is not yet any evidence of extraterrestrial life on this moon of Jupiter (which, if it exists, would probably be in the form of microbial/primordial life because of the moon’s distance from the Sun), but the potential existence of an ocean makes Europa one of the leading candidates for places to look for extraterrestrial life in our solar system.
It may be hundreds, even thousands of years before we get to thoroughly investigate other planetary bodies in our solar system for life. However, when our technologies have advanced to a point where interplanetary travel is fairly easy, Mars and Europa will likely be two of the locations where we begin our search for aliens.
