Science & Environment

Cassini spacecraft captures Saturn moon geyser images

Enceladus
Image caption The jets emanate from hot fissures known as "tiger stripes" at the south pole

The Cassini spacecraft has captured striking images from flying by three moons of Saturn, including new pictures of Enceladus's gushing geysers.

Cassini made its lowest pass yet over the south pole of Enceladus, at at an altitude of 74km (46 miles).

This allowed it to "taste" the jets of water vapour and ice that the moon spews forth into space.

The Nasa probe also made relatively close flypasts of two other Saturnian satellites: Dione and Janus.

The observationswere made over 27 and 28 March.

The encounter was primarily designed for Cassini's ion and neutral mass spectrometer instrument, which sampled the composition of Enceladus's south polar plume.

Image caption Abundant evidence of geological activity criss-crosses Enceladus's surface

Other instruments, including the Cassini plasma spectrometer and composite infrared spectrometer, also took measurements.

Before the closest approach to Enceladus, Cassini's onboard cameras captured images of the geysers, which contain organic compounds along with the ice and vapour.

The jets erupt from cracks, or hot fissures, at the south pole known as "tiger stripes".

Image caption Dione's surface has hints of past or present activity, but at a lower level than on Enceladus

Several lines of evidence suggest the jets are fed by a liquid water ocean beneath Enceladus' outer icy shell.

Scientists have previously detected salts in these jets, which suggests the ocean is probably in contact with the moon's rocky core.

"Cassini has flown several times now through this spray and has tasted it. And we have found that aside from water and organic material, there is salt in the icy particles. The salinity is the same as that of Earth's oceans," said Dr Carolyn Porco, head of the imaging team on Cassini.

As the spacecraft passed Enceladus, the cameras made a nine-frame mosaic of the surface of Enceladus's leading hemisphere.

Cassini then flew by the small moon Janus with a closest approach distance of 44,000km. The planet was in the background in some of these views.

On 28 March, the spacecraft passed Dione at roughly the same distance and captured, among other observations, a nine-frame mosaic depicting the side of the moon that faces away from Saturn.

Scientists recently presented evidence that Dione has features resembling tiger stripes and a cryovolcano, which erupts water-ammonia or methane instead of molten rock.

It is unclear whether there is current geological activity at Dione, but, if so, it is almost certainly at a lower level than on Enceladus.

The discovery that Enceladus probably harbours an ocean in contact with the rocky core makes this moon an even more important target in the search for life elsewhere in the Solar System. The rocks could furnish the ocean with the chemical ingredients thought essential for life.

"The kind of ecologies Enceladus might harbour could be like those deep within our own planet," Dr Porco said in aninterview with Nasa's science website.

Image caption Janus is one of the inner satellites of Saturn, measuring some 200km across

The habitable zone on Enceladus might be comparatively easy to access by future robotic space missions. Dr Porco added: "It's erupting out into space where we can sample it. It sounds crazy but it could be snowing microbes on the surface of this little world.

"In the end, it's the most promising place I know of for an astrobiology search. We don't even need to go scratching around on the surface. We can fly through the plume and sample it. Or we can land on the surface, look up and stick our tongues out."

The source of Enceladus's heat appears to be Saturn itself. The moon moves around Saturn in a distorted, oval-shaped orbit rather than a circular one.

This causes it to be pulled and squeezed by Saturn's gravity, inducing the heat that enables geological activity on the icy moon.

Paul.Rincon-INTERNET@bbc.co.uk and follow me onTwitter

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