Europa, Water, and Life – [An Astrobiological Perspective]
Without a doubt, Europa is one of the prime candidates in our search for alien life in the Solar System. Europa is one of Jupiter’s satellites, and you can think of it as big ball of ice filled with saltwater with some surprising estimates showing that Europa could quite possibly have a greater overall amount of water than does Earth. It’s currently unknown how thick the ice shell is, with estimates ranging all the way from 3 km to 170 km thick! (I’ll explain this in further detail later.) But, we know life requires more than just a liquid like water to survive, it needs energy, too! Let’s dig in!
[For those interested, here are the basics: Europa has a radius of 1,561 km and a mass 65% that of our Moon. It’s composed of mostly silicates and water ice and has a density of 3.01 g/cm^3. It has an orbital period of 3.55 Earth-days, and interestingly, it causes Io to have an especially eccentric orbit due to a resonant interaction. Fun fact: For every orbit of Ganymede, Europa makes two orbits and Io makes four! Moving on…]
The first thing scientists noticed about Europa back when Voyager 2 took some close up photos in the early 80s was that Europa was incredibly smooth. In fact, Europa probably has the smoothest surface of any planet or moon in the Solar System. Europa has but a few craters, and its topography doesn’t change much more than a few hundred meters overall.
Next, the Galileo orbiter’s magnetometer found that Europa has a magnetic field, and this magnetic field is, intriguingly, perfectly opposed to Jupiter’s much stronger field. The fact that Europa has a magnetic field is what led us to reason that Europa had a big body of saltwater to begin with, as opposing magnetic fields can be caused by eddy currents. Naturally, it was assumed that this large body of water is kept liquid through tidal heating, and simulations have shown that this is a very likely scenario.
Now, about those ice shell measurements in the introduction… I realize that 3 to 170 km in thickness is similar to saying, “Well, it could be 10 or 20 or 97, etc.,” or more like, “Well, we really have no idea whatsoever.” Well, we do have an idea, so let me try to explain:
First, and we come back to the Galileo orbiter, the Galileo took gravity measurements of Europa and determined that the COMBINED ice and water shell can be no more than 70 km to 170 km thick. Unfortunately, the Galileo could not tell the difference between solid ice and liquid water, though. Second, computer simulations based on the Galileo’s detailed photographs of what few craters exist on Europa show that the ice shell is as little as 3 to 4 km thick up to 25 km thick. Personally, I hope for something under 10 km so that we may one day send robotic explorers to swim in Europa’s oceans.
Well, I don’t know if/how many of you have realized it yet, but we are missing a very, very important piece of the “life puzzle,” and that is a source of ENERGY, as all living things require energy to survive! This is a much more complicated matter, and we aren’t really sure what type of energy source(s) are potentially driving metabolism on Europa. One theory suggests that the Sun is driving photosynthesis, while another more likely theory suggests that the same geothermal energy which keeps the ocean liquid could have also created some source of energy at the bottom, similar to hydrothermal vents on the ocean floor on Earth. However, the most appealing theory comes from SETI’s Chris Chyba, and it goes something like this:
“Europa, Chyba notes, orbits deep within Jupiter’s intense radiation fields, and this radiation breaks down the ice chemically on the moon’s surface. That is, when the high-energy protons and electrons in the radiation impact the ice, they tear its molecules apart and create highly reactive species. This ‘radiolysis’ has, in fact, been observed: spectroscopic studies of Europa from the Hubble Space Telescope indicate that radiolysis provides Europa with a tenuous oxygen atmosphere. Other important products of this reaction probably include the simple organic compound formaldehyde and the oxidizing compound hydrogen peroxide. From estimates of the rate with which these species are formed on Europa, Chyba has calculated that this energy source could support up to 500 tons of microorganisms!”
So, what’s your take? Does Europa support life? Why or why not?
Please write your thoughts in the comments below!
Plaxco K, Gross M. Astrobiology: a brief introduction. Baltimore: The John Hopkins University Press; 2006, 2011. 330 p.
Pasachoff J, Filippenko A. The cosmos: astronomy in the new millenium. third edition. Belmont, CA. Brooks/Cole. 2007. 500 p.