The evidence for an “ocean” of water under the surface of Saturn’s moon Enceladus appears overwhelming – yes an ocean on another planet. The little world has excited scientists ever since jets of icy material were seen squirting into space from a striped region at its south pole.
Now, exquisite measurements using Nasa’s Cassini probe as it flew over the moon have allowed researchers to detect the water’s gravitational signal.
“The measurements that we have done are consistent with the existence of a large water reservoir about the size (volume) of Lake Superior in North America,” Prof Luciano Iess told BBC News.
The findings of Prof Iess and his team will boost the view that the 500km-wide moon would be one of the best places beyond Earth to go look for the existence of microbial life. Cassini’s data suggests the liquid volume lies about 40km under Enceladus’s ice crust. This would put it directly on top of the moon’s layered, rocky interior.
The case for a subglacial ocean has been growing ever since Cassini first sensed a diffuse atmosphere at the moon in 2005. Subsequent observations pinned the source of this atmosphere to mineral-rich streams of water vapour flowing away from surface fractures dubbed “tiger stripes” for their resemblance to the markings on a big cat.
Cassini even flew through the plumes to “taste” their load of salts and organic (carbon-rich) molecules.
Enceladus’s orbit around Saturn is eccentric – it is non circular. The giant planet’s gravity should therefore be expected to squeeze and stretch the little moon as it travels this path, heating some of its ices and melting them.
Science Magazine provides both an abstract and Editor’s Summary here: “The Gravity Field and Interior Structure of Enceladus.”
Abstract: The small and active Saturnian moon Enceladus is one of the primary targets of the Cassini mission. We determined the quadrupole gravity field of Enceladus and its hemispherical asymmetry using Doppler data from three spacecraft flybys. Our results indicate the presence of a negative mass anomaly in the south-polar region, largely compensated by a positive subsurface anomaly compatible with the presence of a regional subsurface sea at depths of 30 to 40 kilometers and extending up to south latitudes of about 50°. The estimated values for the largest quadrupole harmonic coefficients (106J2 = 5435.2 ± 34.9, 106C22 = 1549.8 ± 15.6, 1σ) and their ratio (J2/C22 = 3.51 ± 0.05) indicate that the body deviates mildly from hydrostatic equilibrium. The moment of inertia is around 0.335MR2, where M is the mass and R is the radius, suggesting a differentiated body with a low-density core.
Editor’s Summary: Inside Enceladus
Saturn’s moon Enceladus has often been the focus of flybys of the Cassini spacecraft. Although small—Enceladus is roughly 10 times smaller than Saturn’s largest moon, Titan—Enceladus has shown hints of having a complex internal structure rich in liquid water. Iess et al. (p. 78) used long-range data collected by the Cassini spacecraft to construct a gravity model of Enceladus. The resulting gravity field indicates the presence of a large mass anomaly at its south pole. Calculations of the moment of inertia and hydrostatic equilibrium from the gravity data suggest the presence of a large, regional subsurface ocean 30 to 40 km deep.