Until a couple years ago, Mars has always been the top candidate for a planet other than Earth where life might be found. What if Mars disappoints us? Are there other candidates in the solar system?
The answer is yes, and the next-best shot at finding extra-terrestrial life forms is on Europa, a moon of the planet Jupiter. Right now, there is no other body in the solar system that attracts as much scientific attention as this bright strange-looking moon, the smallest of Jupiter's four large satellites. Shown below, top to bottom (and as you would approach them moving outward from Jupiter) are Io, Europa, Ganymede and Callisto.
The Galilean moons of Jupiter: Io, Europa, Ganymede and Callisto. (Courtesy: NASA)
Europa seems like a good bet. Life might be a bit strange, but perhaps not a lot stranger than the life forms recently found around hot vents in the abyssal ocean. Liquid water and sources of energy are the essential prerequisites for Life. Europa might well have them. Strong tidal heat could keep the inside of Europa warm enough to have liquid water beneath a layer of ice. Therefore, any organic compounds would be mobile, in water. They could interact.
Let's have a closer look at Europa as a possible host of Life. Europa is the fourth largest moon of Jupiter and the sixth largest moon in the solar system. Europa's surface is covered with ice. From the pictures taken by Galileo (the space craft, not the astronomer), its surface looks like broken glass that is repaired by icy glue oozing up from below. Low ridges, straight and curved, crisscross the surface. Flows and fractures, pits and frozen "puddles" - all hint at a unique geologic history. Due to its smooth surface, it is the brightest moon is the solar system.
It is quite unlike its fellow moons Callisto and Ganymede with their heavily cratered crusts. Europa has almost a complete absence of craters as well as almost no vertical relief. This interesting feature makes it difficult to find out the age of this moon. Geologists determine the relative age of a surface by counting the number of impact craters. We all know our Moon has many craters. It suggests that our moon has been geologically inactive for about 4000 million years. In contrast, there is very little evidence of impacts on the surface of Europa. Unless Europa is fortunate enough to somehow avoid the impacts suffered throughout the rest of the solar system, it must have gone through some kind of geological process that erased the evidence of almost all of those impacts.
Europa has an icy crust that has been severely fractured, as indicated by the dark linear, curved, and wedged-shaped bands seen in images of the surface. These fractures have broken the crust into plates as large as 30 kilometers (18.5 miles) across. Areas between the plates are filled with material that was probably icy slush contaminated with rocky debris. Some individual plates were separated and rotated into new positions.
Surface of Europa showing fractured crust and absence of any cratering. (Courtesy: NASA)
By studying Europa's density we can tell that it has a shell of water ice, parts of which could be liquid. Models of Europa's interior show that beneath a thin 5 km (3 miles) crust of water ice, Europa may have oceans as deep as 50 km (30 miles) or more. There is some evidence indicating that Europa may be slushy just beneath the icy crust and possibly even warmer at greater depths. The evidence includes a strangely shallow impact crater, chunky textured surfaces like icebergs, and gaps where new icy crust seems to have formed between continent-sized plates of ice.
Thus, much of the evidence points to a mobile surface provided by liquid water. Liquid water, scientists believe, could exist below the surface because of internal tidal heating from gravitational interactions with Jupiter and the other Galilean moons. (The Galilean moons are the four shown above, so named because they were first observed by Galileo.)
Another planetoid mentioned in connection with Life in the solar system is Titan, the largest of the moons of Saturn. Titan has an atmosphere largely made up of nitrogen, with less than one percent of methane. The surface pressure of Titan's atmosphere is higher than that of Earth, but the temperature is extremely cold. A rich assortment of organic molecules is found in Titan's atmosphere, as products of the ammonia/methane chemistry. The mixture includes hydrogen cyanide, a compound in the path to the synthesis of amino acids. This finding has led to speculations that primitive life forms might be present. The low temperatures on this satellite make it unlikely.