Webb Space Telescope Maps Surprisingly Large Plume Jetting From Saturn’s Moon Enceladus

By Keith Cowing
Press Release
May 30, 2023
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Webb Space Telescope Maps Surprisingly Large Plume Jetting From Saturn’s Moon Enceladus
NASA/ESA/CSA’s James Webb Space Telescope’s exquisite sensitivity and highly specialised instruments are revealing details into how one of Saturn’s moon’s feeds the water supply for the entire system of the ringed planet. Larger image — ESA/NASA

A water vapour plume from Saturn’s moon Enceladus spanning more than 9600 kilometres — long enough to stretch across the Eurasian continent from Ireland to Japan — has been detected by researchers using the NASA/ESA/CSA James Webb Space Telescope. Not only is this the first time such water ejection has been seen over such an expansive distance, but Webb is also giving scientists a direct look, for the first time, at how this emission feeds the water supply for the entire system of Saturn and its rings.

Enceladus, an ocean world about four percent the size of Earth at just 505 kilometres across, is one of the most exciting scientific targets in our Solar System in the search for life beyond Earth. Sandwiched between the moon’s icy outer crust and its rocky core is a global reservoir of salty water. Geyser-like volcanoes spew jets of ice particles, water vapour, and organic chemicals out of crevices in the moon’s surface informally called ‘tiger stripes’.

Images from the NASA/ESA/CSA James Webb Space Telescope’s NIRCam (Near-Infrared Camera) show a water vapour plume jetting from the south pole of Saturn’s moon Enceladus, extending out 40 times the size of the moon itself. The inset, an image from the Cassini orbiter, emphasises how small Enceladus appears in the Webb image compared to the water plume. — ESA/NASA

Previously, observatories have mapped jets hundreds of kilometres long from the moon’s surface, but Webb’s exquisite sensitivity reveals a new story.

The length of the plume was not the only characteristic that intrigued researchers. The rate at which the water vapour is gushing out, about 300 litres per second, is also particularly impressive. At this rate, you could fill an Olympic-sized swimming pool in just a couple of hours. In comparison, doing so with a garden hose on Earth would take more than 2 weeks.

The NASA/ESA/ASI Cassini mission spent over a decade exploring the Saturnian system, and not only imaged the plumes of Enceladus for the first time but flew directly through them and sampled what they were made of. While Cassini’s position within the Saturnian system provided invaluable insights into this distant moon, Webb’s unique view from the Sun-Earth Lagrange Point 2, 1.5 million kilometres from Earth, along with the remarkable sensitivity of its Integral Field Unit aboard the NIRSpec (Near-Infrared Spectrograph) Instrument, is offering new context.

The Webb observations directly demonstrate how the moon’s water vapour plumes feed the torus, a fuzzy doughnut of water that is co-located with Saturn’s E-ring. By analysing the Webb data, astronomers have determined that roughly 30 percent of the water stays within this torus, and the other 70 percent escapes to supply the rest of the Saturnian system with water.

Enceladus’ surface geometric albedo and detected water vapor emissions. a) Surface geometric albedo of the trailing hemisphere normalized with respect to a reflected solar model30. The spectrum shows several strong signatures of H2O ice, while no absorptions are observed at the expected wavelengths for CO2, CO or NH3 ice. b) Model of the observed water outgassing, in which 4 distinct regions are identified: the center region (orange circle) within 7 Enceladus radii (RE); the inner plume region between 7 and 30 RE; the extended plume region (blue contour) towards the South and between 30 and 38 RE; and the torus background region (pink contour) towards the North and between 30 and 38 RE. c) Data (black lines) and model (colored lines) of the H2O fluorescence emissions within the four regions of panel b, shifted vertically for clarity. The retrieved number of molecules for each region is also indicated. All models are consistent with a rotational temperature of 25 ±3K.

In the coming years Webb will serve as the primary tool for observing the ocean moon Enceladus, and discoveries from Webb will help inform future Solar System satellite missions that will look to explore the depth of the subsurface ocean, how thick the ice crust is, and more.

Building on discoveries made by Webb, as well as those made by ESA’s Jupiter Icy Moons Explorer (Juice) mission, ESA is planning to get even closer to the icy moons of Jupiter and Saturn with future missions, to search for possible biosignatures.

Webb’s observations of Enceladus were completed under Guaranteed Time Observation (GTO) programme 1250. The initial goal of this programme is to demonstrate the capabilities of Webb in a particular area of science and set the stage for future studies.

Water emission is observed across the whole field of view, revealing an immense water plume emanating from Enceladus which is feeding an extended background torus around Saturn. a) The observations sample the trailing hemisphere of Enceladus and the edge of the torus, where RS refers to the mean radius of Saturn. b) At each spaxel (0.1″×0.1″), the H2O column density was retrieved from the observed molecular fluorescence emissions in the 2.62 to 2.72 μm range. Enceladus is 0.07” in diameter (smaller than a spaxel), and the continuum image of the Point-Spread-Function (PSF) is shown in the inset box. Some residual diagonal striping is observed, which we suspect originates from detector effects. c) A model30 with two components as shown in Figure 1b, consisting of a plume and a torus background emission, reproduce the observations well. d) A residual image was computed by subtracting the outgassing model from the observations, revealing a close fit to the data.

The team’s results were recently accepted for publication on 17 May in Nature Astronomy. A JWST molecular mapping and characterization of Enceladus’ water plume feeding its torus is here

More information

Webb is the largest, most powerful telescope ever launched into space. Under an international collaboration agreement, ESA provided the telescope’s launch service, using the Ariane 5 launch vehicle. Working with partners, ESA was responsible for the development and qualification of Ariane 5 adaptations for the Webb mission and for the procurement of the launch service by Arianespace. ESA also provided the workhorse spectrograph NIRSpec and 50% of the mid-infrared instrument MIRI, which was designed and built by a consortium of nationally funded European Institutes (The MIRI European Consortium) in partnership with JPL and the University of Arizona.

Webb is an international partnership between NASA, ESA and the Canadian Space Agency (CSA).


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