NASAās Cassini spacecraft made its deepest plunge yet into the plumes of Saturnās moon Enceladus on Monday. The dive might reveal complex organic molecules that could hint at life.
Researchers have been fascinated with Enceladus since July 2005, when Cassini revealed plumes of ice particles and water vapour shooting out from the moonās south pole.
The origin of the plumes is still being debated. But evidence is mounting that the moon may have liquid water beneath its surface, a potential habitat for life.
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Until now, Cassini has maintained a cautious distance from the densest part of the plumes, flying at distances of 260 kilometres or more from the surface. But mission members changed that policy after they determined that ice grains would not pose a threat to the spacecraft if it made a slow, close approach of the plumeās central regions.
Hunting for the unexpected
They used the tug of Saturnās biggest moon, Titan, to steer Cassini onto a path that on Monday took it into the plumes just 100 km above Enceladusās south pole. There, the plumes emanate from long fissures called tiger stripes.
What will the spacecraft find? āWe wonāt know until weāve got the data,ā says planetary scientist of the Southwest Research Institute in Boulder, Colorado. āWe may find something completely unexpected. Weāre going somewhere we have never been before.ā
Because the plume is denser closer to the moonās surface, the dive could reveal molecules that have so far evaded detection because they are fairly rare. These could include complex organic molecules that might hint at life.
Moonās birthplace
Researchers also hope to catch evidence of krypton and other noble gases, unreactive atoms that will have remained stable over the course of Enceladusās lifetime. The abundance of these elements changes depending on how they are trapped ā for example, whether they are trapped in ice ā so this measurement could shed more light on how and where in the solar system Enceladus formed.
Recent measurements of the ratio of two isotopes of hydrogen suggest Enceladusās composition is strangely comet-like. That hints the icy moon might not have formed near Saturn but instead farther out, closer to the birthplaces of Uranus and Neptune.
Cassini is slated to make another deep plunge into the plumes in April 2010. But unlike the current fly-through, at that time it will not use its thrusters. This will enable precise measurements of the moonās gravitational tug, which could help reveal whether Enceladus harbours a large internal ocean.
The probe could make other close flybys if Cassiniās mission is extended beyond September 2010, a decision that might be revealed in February, when the US budget for 2011 is proposed.
