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Negative mass anomaly at Enceladus’s south pole

THIS CARTOON DEPICTS THE POSSIBLE INTERIOR OF ENCELADUS BASED ON CASSINI’S GRAVITY INVESTIGATION. AT HIGH SOUTHERN LATITUDES, A REGIONAL WATER OCEAN IS SHOWN SANDWICHED BETWEEN AN ICY OUTER SHELL AND A LOW DENSITY, ROCKY CORE. CASSINI IMAGING SCIENCE SUBSYSTEM IMAGES WERE USED TO DEPICT THE SURFACE GEOLOGY AND THE PLUMES. Credit: NASA/JPL-Caltech
Nel 2005, la sonda Cassini ci ha inviato una serie di immagini spettacolari di Encelado, la sesta luna più grande di Saturno, in cui si potevano ammirare sulla superficie ghiacciata le caratteristiche striature, note come “tiger stripes”. Fu una grande notizia per un satellite del diametro di appena 500 chilometri scoprire una tale attività. Da allora, gli scienziati hanno ipotizzato che possa esistere una enorme riserva di acqua al di sotto dello strato di ghiaccio superficiale che, molto probabilmente, rifornisce i pennacchi formando getti d’acqua come una sorta di “geyser spaziali”. Oggi, nuovi dati raccolti dalla sonda Cassini utilizzando un metodo geofisico implicano che esista effettivamente un enorme oceano d’acqua in prossimità del polo sud di Encelado che sarebbe così responsabile della formazione delle strutture superficiali.

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 gushing out of these fractures we see at the south pole“. During three flybys of Enceladus, between April 2010 and May 2012, the scientists collected extremely precise measurements of Cassini‘s trajectory by tracking the spacecraft’s microwave carrier signal with NASA’s Deep Space Network. The gravitational tug of a planetary body, such as Enceladus, alters a spacecraft’s flight path ever so slightly.

By measuring the effect of such deflections on the frequency of Cassini’s signal as the orbiter traveled past Enceladus, the scientists were able to learn about the moon’s 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 Enceladus’s 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 at Enceladus’s 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 spherical body. Since there is a known depression in the surface of Enceladus’s 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.

More at CALTECH: Gravity Measurements Confirm Subsurface Ocean on Enceladus

Science Magazine: The Gravity Field and Interior Structure of Enceladus