In 2005, NASAs Cassini spacecraft sent pictures back to Earth depicting1 an icy Saturnian moon spewing water vapor2 and ice from fractures, known as tiger stripes, in its frozen surface. It was big news that tiny Enceladus -- a mere3 500 kilometers in diameter -- was such an active place. Since then, scientists have hypothesized that a large reservoir of water lies beneath that icy surface, possibly fueling the plumes4. Now, using gravity measurements collected by Cassini, scientists have confirmed that Enceladus does in fact harbor a large subsurface ocean near its south pole, beneath those tiger stripes. For the first time, we have used a geophysical method to determine the internal structure of Enceladus, and the data suggest that indeed there is a large, possibly regional ocean about 50 kilometers below the surface of the south pole, says David Stevenson, the Marvin L. Goldberger Professor of Planetary Science at Caltech and an expert in studies of the interior of planetary bodies. This then provides one possible story to explain why water is gushing5 out of these fractures we see at the south pole.Stevenson is one of the authors on a paper that describes the finding in the current issue of the journal Science. Luciano Iess of Sapienza University of Rome is the papers lead author.During three flybys of Enceladus, between April 2010 and May 2012, the scientists collected extremely precise measurements of Cassinis trajectory6 by tracking the spacecrafts microwave carrier signal with NASAs Deep Space Network. The gravitational tug7 of a planetary body, such as Enceladus, alters a spacecrafts flight path ever so slightly. By measuring the effect of such deflections on the frequency of Cassinis signal as the orbiter traveled past Enceladus, the scientists were able to learn about the moons gravitational field. This, in turn, revealed details about the distribution of mass within the moon.This is really the only way to learn about internal structure from remote sensing, Stevenson says. In fact, more precise measurements would require the placement of seismometers on Enceladuss surface -- something that is certainly not going to happen anytime soon.The key feature in the gravity data was a so-called negative mass anomaly（异常，不规则） at Enceladuss south pole. Put simply, such an anomaly exists when there is less mass in a particular location than would be expected in the case of a uniform spherical8 body. Since there is a known depression in the surface of Enceladuss south pole, the scientists expected to find a negative mass anomaly. However, the anomaly was quite a bit smaller than would be predicted by the depression alone.So, you say, Aha! This is compensated9 at depth, Stevenson says.
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1 depicting 描绘，描画； 描述 参考例句： a painting depicting the Virgin and Child 一幅描绘童贞马利亚和圣子耶稣的画 The movie depicting the battles and bloodshed is bound to strike home. 这部描写战斗和流血牺牲的影片必然会获得预期成效。
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