NASA's James Webb Space Telescope (Webb) has spotted a huge water vapour plume erupting from Enceladus, one of Saturn's moons. Enceladus is an ocean world about four per cent the size of Earth. The plume spans more than 9650 kilometres, which is nearly equivalent to the distance between Los Angeles, California, and Buenos Aires, Argentia. This is the first time researchers have observed such a water emission over such a huge distance.
With the help of Webb's observation, scientists have obtained, for the first time, a direct look at how the water emission feeds the water supply for the entire system of Saturn and its rings, NASA said on its website.
Enceladus and its water vapour plume
Enceladus is one of the most studied celestial bodies because scientists hope to find signs of life on the moon. A global reservoir of salty water is present between the icy outer crust of the moon, and its rocky core. The moon's surface has crevices, through which ice particles, water vapour and organic chemicals come out. These materials, spewed out by geyser-like volcanoes, are called "tiger stripes".
Other observatories have spotted jets hundreds of kilometres from the surface of Enceladus, but Webb's sensitivity has helped unravel interesting mysteries.
The findings appear in an online pre-print, and have been accepted for publication in the journal Nature Astronomy.
How the active plume was observed
According to the study, the active plume, connected to the large liquid water subsurface ocean, was characterised using the sensitive Integral Field Unit aboard the Near Infrared Spectrograph (NIRSpec) on Webb. Scientists not only characterised the plume's composition and structure, but also searched for organic compounds.
Temperature and rate of eruption of the water plume
They observed fluorescence emissions of water, and found that the extraordinarily extensive plume has cryogenic temperatures of 25 K, or minus 248.15 degrees Celsius.
The rate at which the water vapour came out was 300 kilograms per second. This is similar to the rate derived from observations made by Cassini 15 years ago, suggesting that the vigour of gas eruption from Enceladus has been relatively stable for a long time.
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Were non-water gases observed?
The researchers searched for several non-water gases such as carbon dioxide, carbon monoxide, methane, ethane and methanol, but identified none in the spectra, the study said. They observed strong water ice features on the surface of the trailing hemisphere, including the crystalline form of water ice.
No ice signatures of carbon dioxide, carbon monoxide and ammonia were observed.
In a NASA statement, Geronimo Villanueva, the lead author on the paper, said he was shocked to detect a water plume more than 20 times the size of the moon, and that the water plume extends far beyond its release region at the southern pole.
According to NASA, the rate of water vapour eruption, which was 300 kilograms per second, is equivalent to about 79 gallons per second, and is a rate enough to fill an Olympic-sized swimming pool in just a couple of hours.
Since Webb is located at the Sun-Earth Lagrange Point 2, the telescope obtained an incredible view of Enceladus.
The orbit of Enceladus
Villanueva said that the orbit of Enceladus around Saturn is relatively quick, just 33 hours, and as it rotates around its host planet, the moon and its jets are splitting off water, leaving a halo, almost like a donut, in its wake. Therefore, Webb not only observed a large plume, but saw water almost everywhere.
What is Enceladus’s torus? What is its significance?
The scientists have described the fuzzy donut of water appearing "everywhere" as a torus. The torus is co-located with Saturn's outermost and widest ring, the dense "E-ring".
According to the study, an outgassing rate of 300 kilograms per second is sufficient to maintain a derived column density of 4.5×10^17 per square metres for the equatorial Torus, and establishes Enceladus as the prime source of water across the Saturnian system.
In astronomy, column density is a measure of the amount of intervening matter between an observer and the object being observed, and is measured as the number of atoms or molecules per unit area along a particular line of sight, according to the Swinburne University of Technology.
Therefore, Webb's observations suggest that the Saturnian system is fed by Enceladus's water vapour plumes. The researchers analysed Webb's data, and found that about 30 per cent of the water stays within the torus, while the remaining 70 per cent escapes to supply the rest of the Saturnian system of water.
The authors concluded that as scientists prepare to send new spacecraft into the outer solar system, these observations demonstrate the unique ability of Webb in providing critical support to the exploration of distant icy bodies and cryovolcanic plumes.