Material dissolved in the ocean of Jupiter's moon, Europa, which lies beneath an icy shell estimated to be about 100 kilometers thick, nevertheless reaches
the surface, new research suggests. Astronomers have long been puzzled about the two faces of Europa: Its leading hemisphere, the one that perpetually
faces forward as it orbits Jupiter, has a yellowish appearance, whereas its trailing hemisphere is splattered and streaked with a reddish material. Sensors
on the Galileo probe, which orbited the planet from 1995 until 2003, looked at Europa in relatively wide wavelength bands, so researchers couldn't use that
data to identify specific chemicals tingeing the moon's surface. But now, using Earth-based telescopes that can observe the moon in wavelength bands
approximately one-fortieth as wide as Galileo's, researchers have spotted the spectral signature of magnesium sulfate on Europa's trailing hemisphere,
they report online today in The Astronomical Journal. A complex set of processes result in magnesium sulfate tainting only one hemisphere, the
scientists explain. A variety of dissolved elements—including sodium and potassium, which show up in trace amounts in Europa's atmosphere, and
magnesium, which does not—make their way from Europa's ocean to the moon's surface via geysers or through cracks in the ice (depicted in cross section,
above). Sulfur, spewed from volcanos on the jovian moon Io (small yellow orb at top center) and then ionized by Jupiter's magnetic field, slams into
Europa's surface at speeds of about 300,000 kilometers per hour. (The sulfur predominantly strikes the trailing hemisphere of Europa because Jupiter [upper
right] and its magnetic field rotate more quickly than Europa orbits the planet.) Once the elements are brought together, the chemical reactions resulting
in magnesium sulfate proceed. The new findings are exciting because they suggest that material that accumulates on Europa's surface might be making its way
down into the ocean, where sulfur-bearing compounds could serve as nutrients for microbial life if any exists there.
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