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Cassini reveals puzzling red streaks on Saturn's moon Tethys

Morgan Rehnberg, Correspondent
Jul 30, 2015, 17:17 UTC

Sen—Not to be outdone by stunning images of the Pluto system returned by New Horizons, NASA’s other outer Solar System explorer released its own baffling image of an icy world this week. In images captured by the Cassini spacecraft back in April, but only now made public, the surface of Saturn’s moon Tethys is seen marked by long red streaks.

Tethys, the planet’s fifth largest moon, orbits beyond the tenuous E ring. With a radius of about a thousand kilometers, it has a mass just under one per cent that of our Moon.

Saturn’s icy moons have proven to be among the planet’s most surprising features. Geysers on active Enceladus are believed to be the source of Saturn’s E ring, while distant Iapetus possesses perhaps the Solar System’s most striking color dichotomy, and an enormous crater gives Mimas a Death Star-like appearance.

We can now add Tethys to the list. Few objects are known to sport similar streaks of color on their surfaces. The image above, however, is not how the human eye would perceive it. Instead, Cassini scientists combined images of Tethys captured through filters in the infrared, visible, and ultraviolet parts of the electromagnetic spectrum that makes up light. When combined in a pseudo red-green-blue combination, the resulting image can reveal subtle details on the moon’s surface.

So what are these mysterious red arcs? Scientists are still puzzled. Their position relative to other surface features indicates that they must be a recent development and at least two possibilities are being investigated. One hypothesis is that freshly-exposed ice contains red-inducing chemical impurities. Another suggests that perhaps gas leaking from within the moon is discoloring the surface as it escapes to space.

Cassini scientists plan to perform follow-up observations when the spacecraft encounters Tethys again later this year.