A composite image of Io in front of a Hubble Space Telescope photo of Jupiter. The observations for the first time show plumes of sulfur dioxide (yellow) rising up from Io’s volcanoes. Image © ALMA (ESO/NAOJ/NRAO), I. de Pater et al.; NRAO/AUI NSF, S. Dagnello; NASA/ESA
Fifth in line from the Sun, Jupiter is, by far, the largest planet in the solar system – more than twice as massive as all the other planets combined. It is primarily composed of hydrogen with a quarter of its mass being helium, though helium comprises only about a tenth of the number of molecules and have 79 known natural satellites where 63 are less than 10 kilometres in diameter and have only been discovered since 1975. Among those four largest moons- Io, Europa, Ganymede, and Callisto, are visible from Earth with binoculars on a clear night. These four moons are called the Galilean moons as they were first seen by Galileo Galilei in December 1609 or January 1610 and recognized by him as satellites of Jupiter in March 1610. Since then, Jupiter and its moon were a subject of interest for astronomers in many ways.
Do you know that Jupiter’s moon were the first objects found to orbit a planet other than the Earth?
Among all those mysterious objects out in the vast space and around Jupiter, astronomers have a special interest in Jupiter’s moon, Io, because of its more than 400 active volcanoes and its behaviour of the most geologically active object in the Solar System, however, studying Io is not an easy task.
Until now astronomers only know that several volcanoes produce plumes of sulfur (S2) and sulfur dioxide (SO2) that climb as high as 500 km above the surface of Io but they were unable to understand whether those volcanoes are the main contributors to the atmosphere or whether the main component is the accumulated cold SO2, much of which is frozen on the surface, but in sunlight evaporates or sublimates into the atmosphere but for the first time, they caught evidence to it.
A team of astronomers collaborating with Atacama Large Millimeter/submillimeter Array (ALMA) in Chile and led by astronomer Imke de Pater of the University of California, Berkeley, partially resolved that question. They took images of Io in radio and optical light as Io is eclipsed by Jupiter and comes out of the eclipse.
They got clear evidence of volcanic plumes on 20 March and 2 September and as the moon reemerged from Jupiter’s shadow (during observations on September 2 and 11 in 2018) the cold sulfur dioxide emissions returned within about 10 minutes.
de Pater said, “As soon as Io gets into the sunlight, the temperature increases, and you get all this SO2 ice subliming into gas, and you reform the atmosphere in about 10 minutes’ time, faster than what models had predicted.
The team observed the Io with the Atacama Large (sub)Millimeter Array (ALMA) on 20 March 2018 just before and after the satellite moved into eclipse. They typically scanned Io for about 6-7 minutes while towards the end of the observing sessions they usually lasted for only 1–2 minutes.
They noted that not all the cold SO2 froze out as the temperature dropped in Jupiter’s shadow. During the eclipse, in addition to abundant SO2 gas over some volcanoes, ALMA also detected low levels of SO2 globally in Io’s atmosphere, suggesting that many unseen volcanoes. These unseen volcanoes are so-called stealth volcanoes as they do not emit smoke or other particulates that can be easily seen.
de Pater said, “The SO2 that we see with ALMA when Io is in eclipse is at a very low level, and we can’t say if that is stealth volcanism or caused by SO2 not completely condensing out. But then, when we looked at the SO with Keck, we can only explain the SO emissions, which are widespread on the surface, through this stealth volcanism, because excitation of the SO requires a very high temperature.”
During the eclipse time, the tidal tug that Jupiter and two of its largest moons, Ganymede and Europa, exert on Io heats the moon’s interior, creating the volcanoes that bathe the surface in hot sulfur dioxide fumes. Io’s largest volcano, Loki Patera, spans more than 200 kilometres. Though the atmosphere is thin, later the volcanic SO2 eventually condenses on the surface to form a thick frozen layer of sulfur dioxide ice. This frozen SO2, often overlain by a layer of volcanic dust, is what gives Io its characteristic yellow, white, orange and red colours.
In addition, to distinguish the contributions of hot and cold SO2 on a higher level, de Pater and her colleagues, including Statia Luszcz-Cook from Columbia University in New York and Katherine de Kleer of the California Institute of Technology, observed the Io during its transition from sunlight into darkness during an eclipse and again when it reemerged into the light from eclipse. Though they had to make only two observations, it almost took six months because of the alignment of Io and Earth relative to Jupiter. Because of the alignment of Io and Earth relative to Jupiter, it’s impossible to observe both entry and exit of Jupiter’s moon from the same eclipse.
Luszcz-Cook said, “When Io passes into Jupiter’s shadow and is out of direct sunlight, it is too cold for sulfur dioxide gas, and it condenses onto Io’s surface. During that time, we can only see volcanically-sourced sulfur dioxide. We can, therefore, see exactly how much of the atmosphere is impacted by volcanic activity.”
The continuum maps for each of the 6 sessions, three in-sunlight and three in-eclipse, are very similar and do not show any structure other than that the maximum temperature is not centred on Io, but slightly displaced towards the afternoon.
They clearly observed that see the plumes of SO2 and SO rise up from the volcanoes, two of which - Karei Patera and Daedalus Patera - were erupting in March, while a third volcano was active in September. Based on the snapshots, they calculated that active volcanoes directly produce 30% to 50% of Io’s atmosphere, thanks to the ALMA’s exquisite resolution and sensitivity.
The ALMA images also showed a third gas coming out of volcanoes: potassium chloride (KCl). Both KCl and sodium chloride - NaCl, or common table salt - are common components of magma.
Luszcz-Cook explained; we see KCl in volcanic regions where we do not see SO2 or sulfur monoxide (SO). This is strong evidence that the magma reservoirs are different under different volcanoes.