PLUMES ACROSS THE SOLAR SYSTEM

Twenty-five years ago plumes were discovered jetting from the surface of Neptune’s moon Triton [1, 2]. Since then water vapor and ice particles have been detected spewing from the south polar region of Saturn’s moon Enceladus [3, 4, 5]. A tentative identification of water vapor erupting from Jupiter’s moon Europa has come from Hubble Space Telescope ultraviolet images [6]. Solar-driven gas jets remain the best explanation for the seasonal activity that takes place every year associated with Mars’ seasonal CO₂ polar cap [7].

The Enceladus plume is clearly endogenic. Recent Cassini radio science data show that there is a subsurface gravity anomaly consistent with a body of liquid water 30 to 40 km below the south pole [8]. This body of water supplies the ice particles and water vapor propelled from Enceladus’ interior out to the vacuum of space through fissures across the south pole. Europa’s plume is likely similar, but at this time data is too limited to be definitive, in contrast to the plume at Enceladus, extensively studied by all the instruments on the Cassini spacecraft.

In contrast the jets at the poles of Mars are a surficial phenomena, driven by spring sublimation of the seasonal polar cap. Transmission of sunlight through a semi-translucent layer of impermeable ice heats the ground below the ice, causing basal sublimation of the ice [7]. The trapped gas ruptures the ice, and erodes the surface, entraining loose material as it escapes to the top of the ice layer.

An interesting question is what drives Triton’s plumes? Are they solar-driven or endogenic? The original assessment was that they were solar-driven [8], and the Mars model was largely inspired by Triton. But the circumstantial evidence that drove that conclusion was based on the fact that the subsolar latitude was in the southern hemisphere, as were the plumes. A regional endogenic expression of eruptive activity like Enceladus should be considered.

[1] Smith, B. A. et al., Science 246:1422 (1989).

[2] Soderblom, L. et al., Science 250:410 (1990).

[3] Dougherty, M. et al., Science 311:1406 (2006).

[4] Hansen, C. J. et al., Science 311:1422 (2006).

[5] Porco, C. et al., Science 311:1393 (2006).

[6] Roth, L. et al., Science 343:200 (2014).

[7] Kieffer, H. et al., Nature 442:793 (2006).

[8] Iess, L. et al., Science 344:78 (2014).

[9] Kirk, R. L. et al., Science 250:424 (1990).