Most of us know about the 8 planets in our solar system (formerly 9 planets…poor Pluto). But you may not know too much about these planet’s moons. Some of these moons are strikingly similar to our own moon, but there are many that are vastly different and incredibly diverse. One such moon is Europa.
Europa is one of the four Galilean moons originally discovered by Galileo Galilei in the 17th century. Europa is not quite as large as the Earth’s moon but is still pretty big, with a diameter of roughly 3,100 km (1,926 miles). It orbits around Jupiter approximately 780 million kilometers (some 484 million miles) from the Sun. This is a long way by Earthly standards (as Earth is just 150million km [93million miles] from the Sun), but astronomically it is just a stone’s throw away. It has a whole plethora of moon brothers and sisters that orbit along with it, with the current estimate at 66.
So what makes Europa so interesting, you may ask? Well, when the Pioneer spacecraft (1973-74) did its first fly-by of the solar system’s giant (Jupiter), it snapped a fuzzy and uninspiring image of Europa. The image showed what appeared to be a small unremarkable moon that looked like a giant white marble, but that was about it. It wasn’t until the Voyager space crafts (1979) did a survey of the Jupiter moon system on a fly-by that astronomers got their first significant image of the ghostly moon. What the images showed was a small icy moon, which led astronomers to theorize that Europa may possibly have a liquid ocean. Obviously, technology has advanced quite considerably since the 70’s, and we now have many amazing high definition images of Europa that further support the theory that Europa has a warm salt water ocean considerably larger than Earth’s.
In short, Europa is interesting because it is incredibly likely to have a very deep salt water ocean underneath its thin icy crust.
Why is that special? Well, it is possibly the most likely place that we may find other forms of life in our own solar system. What evidence is there for this? First, we have the observed age of Europa’s surface. It is very young, with no impact craters. There is no way that a body in the solar system could have no craters. Ultimately, what this suggests is that there has to be some form of geological mechanism at work that is constantly replacing the surface material to wipe the impact record. Usually the culprit is volcanism, like here on Earth, but on Europa it appears to be the movement of the surface ice in constant turmoil, like a slushy machine of ice water constantly being blended up. This movement replaces the old surfaces and keeps it appearing young. This is one of the greatest indicators to a liquid ocean under Europa’s icy surface.
You may be wondering how is it that Europa is even able to have a liquid ocean when it is so far from the sun, well beyond where water can exist as a liquid. Well it is because of a physical process called tidal heating. It can be explained with a simple analogy as follows: Image you are a baker rolling and kneading some dough. The constant stretching and compression heats the dough up and makes it feel warm to touch. The more you knead and roll it, the warmer it gets. A similar process is at work in Europa. Think of Jupiter as the baker and Europa as the dough. As Europa orbits, Jupiter stretches and compresses it gravitationally. This constant interaction between Jupiter and Europa increases Europa’s internal temperature and raises it to a level at which the majority of water existing on Europa remains as a liquid. That is why Europa is so interesting, because it has a liquid ocean under its surface and the possibility of finding life as we know it there is far greater than anywhere else in the solar system.
It was once thought that life could not survive at the bottom of the Earth’s ocean, but expeditions found that to be, not only wrong, but shockingly wrong. Life is abundant at the bottom of the ocean. It thrives around geothermal vents in temperatures that would boil a normal lobster in seconds. The life around these vents is so different from other life forms on Earth that it is almost alien. This is what makes Europa so exciting—we may find that, under its icy surface, there is an ocean thriving with unusual forms of alien life having evolved completely separately from Earth.
While this is all speculation, if we look to Earth as an example, it is not much of a stretch to imagine that life can arise in another vast deep ocean. But obviously the only way to find out is to go there, or at least send some sort of robot to go in our place. There are several missions currently in the planning stages that hope to send rovers to melt through the ice to get to the ocean, and all sorts of other fascinating proposals. Only time will tell if the plans come into fruition, but here’s hoping they do. We may very well discover that we are not alone in the universe and we have some fishy friends of sorts.
With the risk of sounding super nerdy… The truth is out there (we just need to find it).
This article is part of a series on the moons of Jupiter.