europa_bloom_000606 Take one look at Jupiters moon Europa, and youll see right away that its no ordinary satellite. In fact, with its icy surface crisscrossed by myriad cracks and ridges, Europa looks more like a giant ball of string than a planetary moon.
NASA's Galileo spacecraft has given planetary geologists an abundance of images with which to study Europas surface. In the last few years in particular, these pictures have given scientists a great deal of evidence for Europan tectonics, or crustal movement, which most likely continues today.
In this weeks issue of Nature, scientists add to this evidence a new model for how Europas crust may heat up, causing liquid water to ooze to the surface and then re-freeze as surface ridges narrow, raised strips of crust. Such heating, they say, may also provide a potential habitat for living organisms.
"Were looking at the generation of local heating of the crust," said Eric Gaidos, a planetary geologist at Caltech. He and Francis Nimmo, a planetary scientist at the University of Cambridge, argue that huge slabs of surface ice passing each other on this frigid moon cause frictional heating of small pockets of Europas icy crust.
Europan Tectonics
This motion described by Gaidos and Nimmo is known as "strike-slip" motion. It is similar to strike-slip movement on Earth, where tectonic plates pass each other laterally, often creating earthquakes. Of course, Europa is made mostly of ice rather than rock, so the comparison is not exactly the same. But in both cases, friction between two slabs moving in opposite directions generates heat. According to Gaidos and Nimmos model, such heat on Europa is substantial enough to create local pockets or blooms of melted ice, which later get displaced to the surface and re-freeze in the form of linear ridges.
In this false-color image taken by the Galileo spacecraft, reddish brown ridges and terrain indicate the presence of contaminants in the icy Europan surface.
The motion of Europas crust is thought to be caused by tidal forces gravitational interactions between Europa and its neighbors, including the massive planet Jupiter. Such forces squeeze Europa like a tennis ball. The resulting heat from this frictional squeezing, say scientists, is what keeps Europa from freezing solid.
In fact, Gaidos and Nimmo's model assumes that Europa has a subsurface liquid ocean, a theory to which many planetary geologists currently also subscribe. Such an ocean is assumed because in order for Europas icy crust to be able to move, there must be some fluid intermediate layer between the moons hard ice crust and its dense metal core.
"It doesnt have to be water, and it can be quite deep, but without it, the ice crust is too mechanically rigid to flex under the jovian tides," explained Gaidos.
Where theres water
The idea that blooms of local melt are occurring near the Europan surface brings about the question of whether life may exist on this small moon. Tiny organisms such as single-celled bacteria are known to exist on Earth at frigid temperatures. So the presence of liquid water on Europa would suggest a great place to look for life in the solar system.
Though the instruments on board Galileo have produced evidence for the existence of simple molecules including sulfates and peroxides on Europa, they are not sensitive enough to detect tiny organisms that may be lurking on or just beneath the jovian moon's surface. However, future missions to Europa could very well turn up signs for life.
"Unambiguous identification of any bio-markers would have to wait for a lander that could analyze surface samples," said Gaidos. "But it is certainly reasonable that a future orbiting spacecraft could look at the spectra signature of complex molecules produced by organisms."
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