Press Release: Cassini Finds an Active, Watery World at Saturn's Enceladus

JPL has put out a press release describing the results from July 14 Enceladus flyby. In addition to the CIRS, MAG, and UVIS results mentioned here in other posts from today, INMS and CDA results are described. INMS, which normally looks at Titan in order to measure the composition and density of its upper atmosphere, found that Enceladus' atmosphere is composed of 65% Water vapor and 20% molecular hydrogen, with trace amounts of methane, carbon dioxide, and a combination of nitrogen and Carbon Monoxide (both molecules have the same mass so INMS can't distinguish between the two). The presence of Water vapor in its molecular form and the absense of atomic oxygen (also seen by UVIS), suggests that Enceladus' atmosphere is created from outgassing or evaporation at areas of elevated temperatures, and not from sputtering or impacts, like Europa's atmosphere. INMS also found spatial variations, both between regions near the south polar region and away from, and in layers, suggesting that there is a localized source of the atmosphere in the south pole, results that jive well with the MAG and UVIS data.

The CIRS data is the kicker. CIRS made temperature measurements both in dedicated scans and as ride-along observations with ISS. In both measurements, elevated temperatures were found in the south polar region near the tiger stripes and particularly within the tiger stripes. Color temperatures suggest that some areas in the tiger stripes could be as warm as 140K, indicating that the tiger stripes are regions of escaping internal heat. Such observations are indications that Enceladus is one of only three moons in the outer solar system with active volcanism (though the jury is still out with Titan, no elevated surface temperatures have been found there).

The CDA instrument found that dust kicked off by micrometeorite impacts are the source for the E-ring. Now, I should clear up some confusion brought up when comparing the atmospheric and dust results. Gas particles that make up the atmosphere appear to be generated by evaporation of warm ice or outgassing. The impact-generated cloud that forms the source of the E-ring consists of dust-sized particles, which are different from the atmosphere discussed earlier. The gases produced during a micrometorite must not be generated at a fast enough rate to be a significant contributor to Enceladus' atmosphere.