Although the search for extraterrestrial life typically focuses on the potential habitability of newly discovered exoplanets, humanity's first contact might actually occur much closer to home. Unfortunately, while none of the Solar System's other planets have climates suitable for hosting life, there is compelling evidence that points to the moons of Jupiter and Saturn instead. Combined, the gas giants Jupiter and Saturn have almost 150 confirmed moons, ranging from asteroidal, rocky objects to planet-sized, geologically complex worlds. Hidden amongst the immense variety of lunar shapes, features, and environments, here are the best places to search for alien life in the Solar System:
Europa
The smallest of Jupiter's Galilean moons and slightly smaller than Earth's moon, Europa's plain, icy crust might be overlooked in favor of the more dramatic moons on this list. However, this unassuming moon actually boasts the smoothest solid surface of any known object in the entire Solar System, and this smoothness hints at the existence of a vast, subsurface ocean of liquid water. Because Europa's orbit is slightly eccentric, its distance from Jupiter changes throughout the orbit, and this variation in distance means the gravitational force Jupiter exerts on Europa also changes. Additionally, just like Earth's moon, Europa is tidally locked, as one hemisphere always faces Jupiter. Together with the changes in gravity, this means that Europa becomes elongated as it gets closer to Jupiter during its orbit and then returns to a spherical shape as it moves away from Jupiter. This constant stretching and relaxing heats Europa's interior, resulting in both a subsurface ocean and a polished surface.
While debate still continues about how thick Europa's ice crust is, a recent discovery in May revealed the existence of plumes, which offer scientists an incredible opportunity to study the ocean's composition without needing to penetrate the crust. And yet, not only does Europa have a vast reservoir of liquid water, it also has some of the building blocks for life on its surface and the geological mechanisms for transporting those compounds into the ocean below. Along with a tenuous atmosphere of molecular oxygen, these features indicate Europa is one of the most alluring places to study in the search for extraterrestrial life.
Titan
In contrast, Saturn's largest moon, Titan, immediately stands out from all other moons in the Solar System, as it not only is the only moon to have a dense atmosphere, but it is also the only place in the Solar System - other than Earth - where stable bodies of liquid can be found on the surface. In numerous ways, Titan challenges conventional wisdom about the characteristics and composition of moons. Larger than the planet Mercury, Titan has more in common with Earth than other moons; it has a seasonal climate, weather, and familiar surface features like rivers, dunes, and deltas. However, Titan's atmosphere is nitrogen, not oxygen, and the compelling seas and rivers on its surface are liquid methane and ethane, not water.
Titan is also the most distant place that a robotic probe has landed, the only place in the outer Solar System to host a landing, and it remains the only moon other than Earth's that humanity has visited. On January 14, 2005, the atmospheric entry probe, Huygens, landed on the surface of Titan in a frigid methane haze of −290 degrees Fahrenheit. The brief data it gathered revealed a complex weather patterns, dim sunlight, and a surface of icy grains and rocks at the landing site. Surprisingly, Titan is still a strong candidate for habitability despite the harsh conditions on its surface. Some studies have proposed the existence of subsurface liquid water and simulations have shown that amino acids and nucleotides, the building blocks of RNA and DNA, can be formed without liquid water by using the chemicals available in Titan's atmosphere. Unfortunately, Titan's chemistry and environment still lack many of the characteristic requirements for terrestrial life, so, if life does exist on Titan, it would have to be unlike any organism that exists on Earth.
Enceladus
Having only a thin, scant atmosphere and no Earth-like surface features to suggest viability, Enceladus couldn't be more different from its fellow Saturnian moon, Titan. Its smooth surface is the result of gravitational interactions similar to Europa's, and, also like Europa, its icy crust hides a global, subsurface ocean of liquid saltwater. On Enceladus, though, those interactions drive tectonic activity as well, causing a maze of rifts, grooves, and ridges across the surface. Most notably, they cause the powerful geysers in Enceladus' southern polar region that actually create one of the rings of Saturn. So, although Enceladus is tiny compared to the other moons on this list, it has quite a sizable impact on its surroundings.
In terms of habitability, Enceladus stands out due to a recent discovery in its jets: complex macromolecular organics. Scientists postulate that the organic material is driven from the ocean floor after being formed in deep, hydrothermal vents. On Earth, these vents and their complex chemistry are the likeliest birthplace for terrestrial life. Similarly, Enceladus' ocean is replete with hydrogen, which could be utilized as fuel by methane-producing organisms in a process that, on Earth, is a fundamental support for life. So, Enceladus not only has liquid water and an internal heat source but also hosts a variety of organic compounds whose analogous origins on Earth are known to host life.
From a smoggy world of chemical lakes to smooth, icy exteriors that hide vast underground oceans, the moons of the Solar System offer an unexpected diversity of environments in which astrobiologists can pursue the discovery of extraterrestrial life. Not only will such an unprecedented achievement alter humanity's perception of the rest of the galaxy, but it just might offer answers about life here on Earth: where it originated and how it first emerged over 3.8 billion years ago.
