PLUMES ACROSS THE SOLAR SYSTEM

The seasonal CO2 ices at the polar caps of Mars display tens of thousands of plumes every spring. These plumes are started when the sunlight and some regional heat meet at the bottom of the seasonal ice layer (1m deep or less), and cause the seasonal ice to sublimate. The trapped gas finds a vent, then escapes, jetting out surface products along with the gas. The plumes go 10s of meters into the ambient air, then fall back to the surface and make blotches / fans, directed by the winds.

In contrast the plumes of water and ice jetted away from the south pole of Enceladus are clearly internally driven. The interior of Enceladus hosts a sub-surface sea under its cracked icy surface. The tidally-driven motion pushes and pulls the southern surface and has caused the stresses to crack into 4 fissures. From each fissure we get different amounts of dust and water vapor. Much of the surface material falls back and with Enceladus' tiny size it may go practically to the northern pole. The water vapor is disconnected at the surface and goes into space forming Enceladus' environment.

Triton is clearly the outlier: are the nitrogen plumes observed at Triton internally or externally driven? When the plumes were first discovered the consensus opinion was that they were externally driven by solar forces. They were all located in the southern terrain, mostly southward of the sun's orbit. There were problems with that consensus, and it took encountering Enceladus and finding Enceladus' plumes at the south pole that made us look again. Why shouldn't the plumes have a special place? In particular the mass ejection was much more Enceladus-like than Mars-like. More recent data by Hofgarten (2022) suggests otherwise, but we don't know. Pluto was a good test case - it had some of the attributes that Triton had. But it had no plumes.

Our search now turns to Europa. Many of the Europa Clipper instruments are prepared to look for plumes. The ones purportedly observed are water vapor, not water crystals. The Europa Imaging System will take looks at distances beginning with the observatory. At fairly great distances we will look for emanations of bright material over the limb and along the terminator. When we reach the prime mission we will add observations of the surface to look for color changes and changes in surface properties. Europa is a likely candidate for plumes - what will we see?