Astrobiology is all about finding alien life, and many astrobiologists believe life could be found in our Solar System, on Mars, on moons like Titan, Europa or Enceladus, and potentially on other bodies, like Kuiper Belt Objects.
In this post I’ll look at why some scientists think life may exist on Europa, and maybe even Ganymede and Callisto too, three of Jupiter’s largest moons.
So what’s so special about theses ice worlds?
Europa, Ganymede and Callisto are moons of Jupiter. Jupiter has 63 known moons, and potentially more undiscovered. Most are small and irregular shaped objects, likely captured bodies like asteroids and comets. However Jupiter also has four large moons, comparable in size to the planet Mercury called Io, Europa, Ganymede and Callisto (in fact Ganymede is bigger than Mercury). They’re collectively known as the Galilean Moons as Galileo discovered them all in 1609 and 1610, as he improved his telescopes.
The Galilean Moons are outside of our Solar System’s habitable zone, the region of space around our Sun that orbiting planets would be at the right temperature to have surface liquid water, a vital condition for life. Obviously Earth is in the habitable zone; we have surface liquid water and life. Venus is just about in it, but doesn’t have liquid water as it has an enormous greenhouse effect due to the huge amounts of CO2 in it’s atmosphere and is far too hot (you could melt lead on its surface). Mars is on the outer edge of the habitable zone, but has no atmosphere, and no greenhouse effect, so water on its surface is mostly cold and frozen, and sublimes directly to water vapour when its heated, due to the low atmospheric pressure.
The Galilean Moons are way outside of our Sun’s habitable zone though, even with an optimistic interpretation of our habitable zone they’re around two to three times too far out. So why would we expect to find life on them? Surely they’re so far away from the Sun that any water they have should be frozen solid?
This was the scientific consensus until 1979, when the NASA spacecrafts Voyager 1 and 2 arrived at Jupiter, and made an exciting discovery on Io. Io is the other Galilean moon, and the one that orbits closest to Jupiter, but it’s not a likely home for life as the Voyager probes revealed that it’s the most volcanically active body in our Solar System. Voyager 1 discovered that Io’s surface was covered in fresh lava flows, spotted with large volcanic craters and even saw volcanic plumes from ongoing eruptions. Although one team of scientists had predicted that Io would be volcanically active, most planetary scientists were surprised, as a planetary body as small as Io should have lost its internal heat by now, and should be geologically dead.
And this is true; Io has lost its original internal heat, but is heated instead by Jupiter’s gravity. When our Moon orbits the Earth its gravity has an effect on the Earth, causing the Earth’s surface to bulge slightly in the direction of the Moon, partly causing the tides on Earth, and the same happens on the Moon, it bulges slightly towards the Earth. The same happens on Io, but it’s subject to a much greater degree of bulging as Jupiter is such a massive planet and Io orbits relatively close to it. Jupiter’s attraction causes Io’s rocky mantle and crust to bulge, and as Io orbits around Jupiter this bulge moves as the moon rotates, causing Io’s crust and mantle to be constantly flexed and stretched, creating large amounts of internal heat due to friction as the layers of rock scrape past each other, this is called tidal heating, and provides the heat that drives Io’s volcanoes.
Scientists realised that if Io is heated internally by tidal heating then so must the other Galilean Moons, although to a lesser extent, as they orbit further away from Jupiter than Io. But on these moons we don’t see volcanoes, the tidal heating has a different effect.
Europa, Ganymede and Callisto all have large amounts of water, primarily frozen as ice. Europa likely has a small metallic core, an inner mantle of silicate rock, and an outer mantle and crust of ice, likely a 100km thick. Ganymede has a similar internal structure, but with a much thicker layer of ice, up to 800 to 1000km thick, and Callisto doesn’t appear to have differentiated internal layers, is likely made of a mixture of ice and rock, but with more rock towards the centre and more ice towards the surface.
And here’s where the fun starts, many scientists believe that each of these moons may have a layer of liquid water beneath the surface, in which life could possibly be found. Theoretical models suggest that Jupiter should supply enough tidal heat energy to melt regions of ice within each of the three moons, particularly if ammonia is present in the water, as this will lower its freezing point, and some observational data appears to provide evidence for this, as the bodies each have interesting magnetic effects that could be caused by a layer of liquid salty water. Europa is thought best bet for a sub-surface ocean, as its closer to Jupiter than Ganymede and Callisto, and thus experiences more tidal heating, but Ganymede and Callisto could also have sub-surface water bodies, beneath many kilometres of ice.
However this sounds like a pretty strange place for life, as the sub-surface oceans would be cut-off from the Sun, as the Sun would be much weaker at Jupiter’s distance and sunlight cannot penetrate through more than a few meters of ice. Yet most life on Earth is supported by the Sun, energy is introduced into our ecosystems primarily through plants converting sunlight into chemical energy through photosynthesis. This energy is then passed up the food chain as herbivores eat the plants and carnivores eat the herbivores. So could we really expect to find alien life in these cold and dark oceans?
Maybe. Because life has been found on Earth in similar conditions. In 1977 the famous submarine Alvin was conducting a survey of a volcanically active region of the Pacific Ocean floor and make a surprise discovery when it came across an unknown ecosystem on the seabed. The geologists controlling Alvin found shrimp, crabs, clams, giant tubeworms and many other forms of life all on the ocean floor at a depth at which no sunlight could penetrate. Scientists quickly realised that this ecosystem wasn’t powered by sunlight, but by hot water and chemical reactions, as the organisms were found clustered around volcanic vents on the seabed.
Volcanic vents, or hydrothermal vents, are created when hot magma beneath the sea floor heats water in the crust, causing it to expand and rush towards the surface of the seabed where it streams out of vents, like a geyser does on land. This hot water can carry a rich mix of dissolved minerals and elements in it as hot water can dissolve many substances pretty well (think about sugar in hot tea), but when the water reaches the surface of the sea-floor it quickly cools and the minerals and elements solidify and drop out of the water around the vent. Sometimes this creates a kind of hydrothermal vent called a black smoker, when large amounts of minerals solidify out of the streaming water in clouds of black particles.
The ecosystem discovered in 1977 was clustered around black smokers, with the food chain based on bacteria that created energy by reacting the solidified minerals and elements with oxygen, rather than plants capturing sunlight. Larger organisms then exploited and fed off these bacteria.
So similar life could also exist on Europa, and maybe even on Ganymede and Callisto, with silicate rock melted by tidal heating driving hydrothermal vents on the floor of the sub-surface oceans, around which ecosystems could thrive. Its likely that such environments wouldn’t be able to support ecosystems as large and as complex as those on Earth, so we may never talk to intelligent creatures from Europa, but still, it would be mind-blowing to find life on a different planetary body.
This hydrothermal model is the ‘traditional view’ for life on Europa, but there are alternatives. The planetary scientist Richard Greenberg has proposed that life could live nearer Europa’s surface, and even swim in its oceans.
Europa’s icy crust shows evidence that it’s regularly cracked open by tidal forces, and many cracks may be repeatedly pulled open and pushed together as Europa orbits Jupiter, pumping water from the sub-surface ocean up into the cracks. Greenberg proposed that ecosystems of bacteria and even multicellular life could live in these surface cracks, having just enough shelter from UV radiation and being supplied with heat and nutrients in the water pumped into the cracks. He even proposed that ultraviolet radiation striking Europa’s surface could free oxygen atoms from the ice, which could be absorbed into the sub-surface ocean via these cracks, and could oxygenate Europa’s ocean, allowing more complex life-forms to be supported, that could feed off the simpler life; there may even be something similar to fish on Europa.
So when are we going to the Galilean Moons to find out?
NASA and ESA (the European Space Agency) are currently planning a joint mission to Europa in 2020, called the Europa Jupiter System Mission, but the project has yet to secure funding and may well be cancelled, as have other previously planned missions to Europa. If it does go ahead, then the plan is to place at least one orbiter in the Jupiter system, to analyse Europa and the other Galilean moons from space, not to actually land on Europa.
Scientists have speculated about sending a lander craft to Europa that would be able to drill through the outer ice layer and explore the sub-surface ocean. Yet there are two big hurdles to overcome before we can do this. One is that it would have to drill through tens of kilometres of ice, something that we can’t do on Earth yet, let alone on a distant moon, and the other is that the probe would have to be sterilized to an extreme degree to stop us accidentally infecting Europa with life from Earth, that could potentially wipe-out any existing indigenous life (I’ve talked more about this in a previous post here).
So in summary, Europa offers the best chance of life on Jupiter’s moons due to the possible presence of liquid water, and a number of different models of life have been proposed, with the most popular being ecosystems clustered around hydrothermal vents on the ocean floor. And hopefully within our lifetime we’ll explore Europa, although it’ll be a tough nut to crack, particularly for us to safely explore its possible ocean.
In the next post we’ll jump to Jupiter itself and explore the possibility that it could harbour life of its own.