FROM THE MOON TO THE OORT CLOUD Part Two - Part Three - Part Four
by Bruce Moomaw
Cameron Park - April 4, 2000 - While Mars was the star of this year's Lunar and Planetary Science Conference at Houston, other solar system targets got their share of attention.
Among these was Jupiter's volcanic moon Io, thanks to two, mostly-successful, close flybys which the "Galileo" spacecraft made last October and November. The results from the third successful flyby in February were not yet available at the time of the conference.
However, the multiple reports on the latest Io results included few really spectacular revelations which had not already been announced by JPL previously.
Of note though was how "Galileo's" close-up thermal maps made clear -- as pointed out by Diana L. Blaney and Rosaly Lopes-Gautier in their reports -- was that Io has many more small volcanic eruptions going on than had been thought.
Indeed, Lopes-Gautier reported that "Most dark deposits seen in visual images are sites of thermal emission", and Blaney concluded that the photo it took during the November flyby of a brilliantly glowing curtain of lava spurting upwards from the floor of the Tvashtar caldera may not have been such a long-shot stroke of luck as had been thought:
"The chances of observing some level of active volcanism in any given area [of] at least 135 square km is quite reasonable, and the [February flyby] may well provide us with more high-resolution samples of such phenomena" -- especially since during that flyby [unlike the previous two], there were no spacecraft malfunctions that caused Galileo to loose some of its planned photos. One of its upcoming photos, by the way, will be a full-color view of the Tvashtar eruption.
Color is very important in understanding Io's geology, since it is covered with a dazzling multitude of flows and layers of different erupted materials ranging in color from yellow to red to white to black.
Sulfur, by itself, accounts for many of these different shades -- and Galileo's photos of the currently-dormant Emakong caldera (some of which were newly released by D.A. Williams during his talk) confirm that the black caldera is surrounded by great yellowish flows which seem to be deposits of pure sulfur.
Paul Geissler reported that active Ionian calderas are surrounded by red deposits which seem to be still-warm sulfur, while Lopes-Gautier said that Galileo had proven that the white areas which coat much of Io -- and thought to have been fields of sulfur dioxide snow -- are apparently yet another form of sulfur.
And while the white areas may not be SO2 snow, there is unquestionably a lot of the latter all over Io's surface -- one of Galileo's major new discoveries has been that some of the smaller volcanic plumes, such as Prometheus, are not direct eruptions but actually great clouds of SO2 snow being boiled into vapor as fresh lava flows creep slowly across them.
The February flyby, by the way, also obtained both detailed color views and black-and-white extreme close ups of the Prometheus area, which should soon be available.
But while Io's volcanoes are spectacular, the biggest mystery about the moon so far lies in the fact that its tallest mountains by far are not volcanoes -- but are rather scattered mountains as much as 16 km high, which seem to be "tilted blocks that are bounded by steep scarps".
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