Enceladus

Although Voyager 1 did not approach Enceladus closely, it confirmed the telescopic inference that the moon has a very highly reflective surface. In fact, with a geometric albedo of 95 per cent, it is more reflective than a field of fresh snow. Voyager 2 was able to provide imagery of the northern portion of the trailing hemisphere at a resolution of several kilometres per pixel.

At 500 kilometres in diameter, Enceladus is only slightly larger than Mimas, and it had been thought that as they were both deep in the Saturnian system they would be physically similar. However, Enceladus was found to have undergone extensive resurfacing. The absence of large craters indicates that none of the surface dates back to the post-accretional bombardment.25 A physiographic analysis identified cratered, smooth and ridged plains. The cratered plains have an abundance of craters in the 10- to 20-kilometre size range. However, in some areas these craters are sharply defined and in other areas they are softened, possibly due to viscous flow. The degree of relaxation of the craters differs markedly on the various terrains, suggesting differences in viscosity due to their composition and varying heat flow. Studies of the crater forms concluded that the moon's lithosphere is a mixture of ammonia ice and water ice.26,27

In large areas the early craters have been 'replaced' by plains whose sparsity of impacts implies either a very low cratering rate or the relative youth of the surface. In some places, these smooth plains are criss-crossed by rectilinear fractures which appeared to be grabens formed as subsurface ice froze, expanded and cracked the brittle outer shell. There are also ridges rising a kilometre or so, with smooth plains lying between. These ridged plains, which predominate on the trailing hemisphere, could be fluids that oozed from fractures and solidified in place. In fact, the extent of the resurfacing and tectonic activity on this small moon is surprising. One analysis concluded that Enceladus would rapidly have lost its accretional heat (due to its high ratio of surface area to volume) and it is probably too small to have facilitated significant radiogenic heating. It must therefore have endured an exogenic mode of heating at a later stage.28 Indeed, it appears to have undergone several episodes of resurfacing over an extended period by lava consisting of a water-ammonia eutectic

The remote Voyager 1 imagery suggested that Enceladus had been resurfaced, but the variety of activity evident in this Voyager 2 view was astonishing.

that would melt at around 173K and, being of a lower density, would tend to rise through the water ice to flood the surface. The most plausible cause of heating is tidal stress.29 The moon's free eccentricity is very small, indicating that it must have suffered significant tidal dissipation in the past. Although the eccentricities of the current orbits of the moons are too small to induce significant tidal stress, they were probably greater in the past, as the satellites settled into their present locations. One suggestion is that since the co-orbital pair of Janus and Epimetheus are in an almost 2:1 resonance with Enceladus, if these bodies are pieces of a progenitor that was in the same orbit prior to being shattered by a catastrophic impact, the resonance with that object might have driven much of Enceladus's activity.30 However, if the substantial cratering observed on these moonlets indicates that they date back to the end of the post-accretional bombardment,31 then this could not have driven the most recent phase of activity, which would appear to have occurred at some time within the last billion years. As an alternative, it has been suggested that the most intense activity was driven by a temporary resonance with Mimas as that moon migrated outwards due to its interaction with the ring system.32 Enceladus is currently in a 2:1 resonance with Dione, farther out, and this will provide an ongoing source of heat.33 While this may be just sufficient to melt the interior if it still contains a significant fraction of either methane clathrate or ammonia monohydrate, it was not clear whether this would be able to drive cryovolcanism.

The densest part of the tenuous 'E' ring is coincident with Enceladus's orbit.34 It is thought that the moon's surface is highly reflective because it is coated with a layer of fresh ice crystals. The 'E' ring would appear to be composed of very fine ice particles.35 The sparse cratering on Enceladus argues against this material having

An enlargement of Samarkand Sulcus cutting across the cratered terrain on Enceladus. Note that the resurfacing process breached the craters Ahmad and Peri-Banu at its margin.

Tethys 151

Tethys 151

Surface Depicting Space
An artist's impression of the surface of Enceladus, depicting a hypothetical geyser venting water vapour into space, where it replenishes the ice crystals in the 'E' ring. (Courtesy of David Seal.)

been ejected by large impacts. It may be being ejected by micrometeoroid impacts, but if so why do the other icy moons not form rings? The most intriguing possibility is that water is being explosively vented by cryogenic geysers to freeze into a myriad of ice crystals, many of which fall back to coat the surface, but some of which are able to escape to space.36 Without replenishment, the 'E' ring would long since have faded away. Its continued presence is therefore strong evidence that Enceladus is undergoing geyser activity.

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