High Tide on Europa

By Cynthia Phillips
from the SETI Institute’s Center for the Study of Life in the Universe
posted: 07:00 am ET
03 October 2002

High Tide on Eruopa

In a previous article we discussed Jupiter’s moon Europa as part of a mini-solar system consisting of the four large Galilean satellites orbiting Jupiter. Europa is worthy of special consideration because of the possibility that the right conditions exist on this small icy moon, either in the past or at the present, for life. One of the requirements for life is liquid water. But how could liquid water exist on such a small world, so far from the Sun? The answer is tidal heating, which could provide sufficient energy to maintain an ocean of liquid water beneath Europa’s icy surface that is greater in volume than all the oceans of Earth combined!
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This image shows the markedly different appearances of the surfaces of the four Galilean satellites, from left: Io, Europa, Ganymede, and Callisto. Io’s volcanic surface and Europa’s bright cracked icy exterior contrast with the dark surfaces of tectonic Ganymede and old, cratered Callisto.

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Europa is locked in a tidal resonance with its neighboring satellites, volcanic Io on the inside and large Ganymede on the outside. Like each of the Galilean moons, Europa is spin-locked to Jupiter, rotating about its pole with a period identical to its orbital period of 3.6 Earth days. Europa is in a 1:2:4 resonance with Ganymede and Io – for every two times that Europa orbits Jupiter, Ganymede orbits once, and Io orbits four times. This resonance keeps the satellites from perfect circular orbits, causing a forced eccentricity that varies their distances from Jupiter. Jupiter’s strong gravity raises a bulge on the portion of the surface facing towards the planet, but since the distance between the moons and Jupiter varies, the height of the tidal bulge also varies throughout the satellite’s orbit. The variation in tidal amplitude results in tidal flexing as the bulge goes up and down, producing an internal dissipation of energy called tidal heating.

The tidal heating is most intense at Io, due to its proximity to the giant planet Jupiter, and results in constant volcanic activity on the small moon. Io is, in fact, the most volcanically active body in the solar system, surpassing even Earth. If the current rate of volcanic activity on Io is typical, then there has been sufficient volcanic processing to recycle the entire crust of Io through volcanoes multiple times in its history. The volcanic activity on Io could have also have driven off most volatiles over time. If any water were present on Io from its formation early in its history, it is now long gone.

Ganymede is also subject to tidal heating, although to a lesser extent than Io and Europa given its larger distance from Jupiter (and thus Jupiter’s reduced gravitational pull). Still, Ganymede seems to have had sufficient tidal heating to result in some geologic activity in this past, which allowed Ganymede to become internally differentiated into an icy mantle and a rocky/metal core. Ganymede’s surface consists of old, cratered terrains, but also of younger grooved and ridged areas that are the result of more recent tectonic activity. An interesting comparison can be made between Ganymede and its neighboring satellite Callisto. Callisto does not participate in the resonance with the other three Galilean satellites, and its surface resembles Earth’s Moon - it is an old, cratered body with few, if any, signs of geologic activity. Callisto also is thought not to be fully differentiated, although recent measurements suggest that it could have a small core.

A substantial amount of heat is dissipated within Europa’s core, mantle, and ice shell as well. Debate is still taking place as to exactly how much heat is tidally dissipated within Europa, and where exactly this heating takes place – is it near the surface, at the ice/water interface (if there is one), or at the water / rock or ice/rock interface at the bottom of the ice layer?

Thus, the theoretical possibility exists for the presence of liquid water on Europa. Still, though, do we have any evidence? In fact we do, and the next article in this occasional series about life on Europa will consider the lines of evidence for liquid water.

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