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Europa geysers point to subsurface ocean.

FOR DECADES astrobiologists have pondered whether primitive life might exist in a global ocean presumed to lie beneath the smooth, icy crust of Jupiter's moon Europa. But this putative ocean must be at least several kilometers down--a formidable barrier to exploring it firsthand.

Now it appears that sampling this briny deep might be easier than once thought. Hints that Europa sports water-powered geysers first came to light in 2012, when a team led by Lorenz Roth (Southwest Research Institute) used the Hubble Space Telescope (HST) to spectroscopically detect localized clouds of hydrogen and oxygen atoms--presumably derived from water --in Europa's vicinity.

More recently, William Sparks (Space Telescope Science Institute) and others used HST to record images of Europa as it crossed in front of Jupiter. They wanted to see if the moon has a thin atmosphere, which would show up as a dark aura around Europa when viewed in silhouette against Jupiter.

Sparks' team tracked 10 transits of Europa from December 2013 to March 2015, as well as non-transit sessions to model the appearance of Europa itself. They used a far-ultraviolet channel centered at 150 nm, because that wavelength is scattered by Jupiter's high-altitude hazes and makes the planet's disk look featureless. Hubble's resolution is also best at ultraviolet wavelengths.

Three of the resulting image sets show what could be plumes. In two cases the putative eruptions appear to come from the moon's south polar region--the same general locale implicated by Roth's team--and the third was nearer the equator. The vague puffs seem to extend at least 200 km (125 miles) above Europa's limb. Details appear September 29th in the Astrophysical Journal.

The researchers are hopeful but cautious about their results. In one image, for example, the base of a possible plume doesn't coincide with the limb of Europa itself, as it should. "We are really working at the limits of Hubble's unique capabilities," Sparks says.

If the geysers are real, future spacecraft might be able to assess the ocean's composition and life-hosting qualities merely by landing on the surface near an eruptive vent or flying through one of the plumes at close range.

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Please note: Some tables or figures were omitted from this article.

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Title Annotation:SOLAR SYSTEM
Author:Beatty, J. Kelly
Publication:Sky & Telescope
Date:Feb 1, 2017
Words:365
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