Science - Astronomy

Crater-free Europa

By D.J. Batzer

ONE of the most unexpected findings of Nasa's probes, Voyagers 1 and 2, was the individuality of Jupiter's large moons.

For example, Europa, the second of the four large satellites from Jupiter, is devoid of impact craters. The four inner planets, including Earth, have lots of impact craters. Most satellites and those asteroids we have seen close up, are covered with craters, almost all of impact origin.

In fact, the size and density of impact craters can be used to estimate the age of a surface. On our Moon, for example, the dark "seas" or marea (singular mare, from the Latin for sea) are sparsely sprinkled with craters, indicating that they are features of more recent formation than the mountainous areas where the number of craters per unit area is far higher.

Io, the innermost of the Galilean moons, is devoid of impact craters but has lots of volcanic ones. There is so much volcanic activity on Io that its entire surface is covered with freshly deposited lava and the scars of any impacts are quickly hidden.

Yet there are no volcanoes on Europa. So how did it acquire its "young" surface?

Europa, with a radius of 1,570km, is slightly smaller than the Moon and has a density of three times that of water compared to the Moon's 3.3. Why then are the two bodies so different in appearance? The Moon is liberally sprinkled with impact craters, while Europa is almost as smooth as the proverbial billiard ball.

The average density of rock is about 2.7 times that of water. The fact that both the bodies in question have higher average densities is due to their constituent rocks being compressed into a compact crystalline form at the core. Since the densities of the two are not all that different, it follows that the reason for the different surfaces must lie outside the satellite.

When sunlight is reflected off a surface, the spectrum of the reflected light is characteristic of the surface composition. In the case of Europa, the reflected spectrum is that of frozen water.

Without doubt Europa has suffered its share of meteorite impacts. But over time these have been erased. We can see this in the ghostly remains of some collisions. They are called "palimpsests".

In the middle ages in Europe when materials for writing were scarce and expensive, old texts were erased to make way for new writing. But this always left some trace of the old writing. Hence the term applied to Europa.

Water ice, at the temperature of Europa's surface, should be as hard as rock and craters should persist. That they do not lies in the tidal effect of Io, the nearest large satellite to Jupiter. With a radius of 1,820km, Io is slighter larger than the Moon whose which has an orbital period of 1.76 days, just half of Europa's. The result is that these two moons are subject to regular periodic squeezing. This tidal energy finishes up as heat in both satellites.

In Europa this has produced a layer of water under the ice crust. This has been confirmed by Nasa's Galileo space probe in orbit around Jupiter. It repeatedly flies past Europa, on orbits as close as 200km to the satellite. At that distance the probe is able to identify features as small as 6m across. Looking at the debris pattern around the 26km wide palimpsest named Pwyll, astronomers concluded that the impact occurred between 10 million and 100 million years ago.

The ice crust is estimated to be between 10km and 30km thick. It is called the lithosphere. This crust is fractured and the slabs move relative to each other, allowing water to ooze up from below. This results in the gradual obliteration of craters and is analogous to the movement of "plates" in the Earth's mantle. Below the crust is thought to be a liquid water "ocean" some 70km deep, called the asthenosphere. Its base sits on rock.

The presence of liquid water means that life, as we know it, is a possibility. Scientists have pointed to the existence of simple organisms living in rock crevices in the so-called "dry" valleys in Antarctica, where temperatures rarely rise above freezing.

However, it is almost certain that the organisms did not originate in these adverse conditions. They probably adapted to the environment when conditions deteriorated to their present state. It is doubtful whether life could have originated in the inky darkness of Europa's "ocean."

The only way to find out is to dispatch a robot to land on the surface of Europa and drill down to the water to get a sample for examination back on Earth. This is certainly no easy task.

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