Wednesday, October 28, 2015. The Saturn-orbiting Cassini spacecraft has now made a daring dive through the icy plume spewing from the South Pole of Saturn’s moon Enceladus. The spacecraft is near the end of its mission and will never again get so close to the moon’s surface. Cassini came within 30 miles (45 km) of Enceladus’ surface, blasting past the moon at 19,000 miles per hour (31,000 kph). Cassini used this plume dive to sample the gases in the plume. Since the source of the plume is believed to be an underground global ocean – and since the plume contains ice, water vapor and organic molecules – the goal is to judge whether the hidden ocean on Enceladus is suitable for life. NASA has issued seven key facts about Cassini’s plume dive of Enceladus. Here they are:
1. Early in its mission, Cassini discovered Enceladus has remarkable geologic activity, including a towering plume of ice, water vapor and organic molecules spraying from its south polar region. Cassini later determined the moon has a global ocean and likely hydrothermal activity, meaning it could have the ingredients needed to support simple life.
2. The October 28 flyby is Cassini’s deepest-ever dive through the Enceladus plume, which is thought to come from the ocean below. The spacecraft has flown close to the surface of Enceladus before, but never this low directly through the active plume.
3. The flyby is not intended to detect life, but it will provide powerful new insights about how habitable the ocean environment is within Enceladus.
4. Cassini scientists are hopeful the flyby will provide insights about how much hydrothermal activity – that is, chemistry involving rock and hot water – is occurring within Enceladus. This activity could have important implications for the potential habitability of the ocean for simple forms of life. The critical measurement for these questions is the detection of molecular hydrogen by the spacecraft.
5. Scientists also expect to better understand the chemistry of the plume as a result of the flyby. The low altitude of the encounter is, in part, intended to afford Cassini greater sensitivity to heavier, more massive molecules, including organics, than the spacecraft has observed during previous, higher-altitude passes through the plume.
6. The flyby will help solve the mystery of whether the plume is composed of column-like, individual jets, or sinuous, icy curtain eruptions — or a combination of both. The answer would make clearer how material is getting to the surface from the ocean below.
7. Researchers are not sure how much icy material the plumes are actually spraying into space. The amount of activity has major implications for how long Enceladus might have been active.
The October 28 Enceladus flyby is part of a series of “lasts” for Cassini, which has been orbiting Saturn, flying in and among its moons, since 2004. After the close moon flybys of Enceladus and other Saturn moons this year, the spacecraft will depart Saturn’s equatorial plane – where moon flybys occur most frequently – to begin a year-long setup of the mission’s daring final year. For its grand finale, Cassini will repeatedly dive through the space between Saturn and its rings.
Bottom line: Seven key facts about the Wednesday – October 28, 2015 – close flyby of Enceladus by the Cassini spacecraft.