Seasonal exospheres detected on Dione and Rhea.

Exospheres are gas envelopes around planets and moons too thin to be considered true atmospheres. Unlike atmospheres, the gasses in exospheres are not thought to interact with one another, rather individual molecules are thought to be lost from the body's surface, spend some time in the exosphere, then either be lost into space or resettle onto the surface. The presence of thin exosphere's of oxygen and carbon dioxide has been established on the Jovian moons Ganymede, Europa and Callisto, and such exospheres have also been detected on Saturn's moons Dione and Rhea. However the exospheres of Dione and Rhea have proven hard to understand, as repeated passes of the two moons by the Cassini spacecraft have detected exospheres with very different densities and proportions of the two gasses.

In a paper published in the journal Icarus on 2 March 2016, Ben Teolis and Hunter Waite of the Space Science Division at the Southwest Research Institute examine data from a number of passes of Dione and Rhea made by the Cassini spacecraft, and the development of a model that explains the behavior of the two moons' exospheres.

Cassini image of Dione (foreground) and Rhea (background), taken on 15 April 2015. NASA/JPL.

Teolis and Waite found that the density of Dione and Rhea's exospheres was related to the seasons; the gasses increased in density aound the moons' equinoxes, and lost density around the solstices, suggesting that the behaviour of the gasses is largelly determined by the amount of Sunlight different parts of the moons recieve.

They calculate that both gasses are lost from the surface of the moons as ice is warmed by direcr Sunlight on their daysides. Some of the carbon dioxide is precipitated back on the nightside of the moons, though this can be lost again as the nightside rotates back into sunlight. Around the equinoxes, when all of the moons recieve around the same amount of Sunlight each day, this is all that is happening, and the exosphere of the moons slowly becomes more dense. Hoever at the solstices, when of the poles is pointed away from the Sun and is in permanent darkness, he temperature falls low enough for both gasses to be precipitated, and as the polar regions remain in darkness for Earth years at a time (one Saturn year is euivalent to 29 Earth years) these gasses are lost from the exosphere for long enough to alter it loose density.

Schematic showing the mechanism responsible for the exospheric seasonal variability. The exosphere is cryopumped at the solstices onto the cold winter polar surface. As equinox approaches the Sun rises over the winter latitudes, warming the surface and desorbing the frost cap to produce a transient augmentation of the exosphere. Teolis & Waite (2016).

See also...

Fissure eruptions from the southern polar region of Saturn's moon Enceladus.  Observations of Saturn's moon Enceladus by the Cassini space probe have revealed a number of sulci (linear structures) in the...

Methane storms as a possible cause of Titan’s equatorial dune fields.                        Observations by the Cassini Space Probe have revealed vast dune fields, similar to those observed...

The origin of mini-jets in Saturn's F Ring.     The F Ring is the outermost and thinest of Saturn's rings. It was discovered by the Pioneer 11 Spacecraft in 1979, and has been studied by NASA's Cassini...

Follow Sciency Thoughts on Facebook.

NASA releases new Cassini images of Saturn's moons Enceladus, Janus and Dione.

Saturn has fifty-three named moons, of which thirteen are larger than 50 km in diameter, and seven large enough to have formed a roughly spherical shape due to their own mass. Another nine unnamed moons of Saturn have confirmed orbits; the exact number of moons orbiting the planet is hard to assess, since each of the billions of tiny particles that make up Saturn's rings could potentially be seen as a moon.

The 33 largest moons of Saturn, showing comparative size and the order in which they orbit. NASA/JPL.

The orbits of Saturn's moons. Lunaf.com.

The Cassini Probe has been orbiting Saturn since June 2004, sending back scientific data and images of the planet and its moons. This week NASA released new images of three of the moons, Enceladus, Janus and Dione sent back by Cassini in the last few days.

Enceladus is the sixth largest moon of Saturn, with a diameter of 500 km. It orbits at a distance of 238 000 km, within the planet's E ring. Enceladus is a geologically active body, on which volcanic activity was observed near its south pole by Cassini in 2005. The surface of Enceladus is made up of a number of distinct terrains, some thought to be less than 100 million years old.

Image of Enceladus from 111 809 km. Taken by the Cassini Probe on 27 March 2012, and received on Earth on 28 March.

Image of Enceladus from 232 197 km. Taken by the Cassini Probe on 27 March 2012, and received on Earth on 28 March.

Image of Enceladus from 48 759 km. Taken by the Cassini Probe on 27 March 2012, and received on Earth on 28 March.

Image of Enceladus from 31 881 km. Taken by the Cassini Probe on 27 March 2012, and received on Earth on 28 March.

Janus is a small irregularly shaped moon, roughly 203 × 185 × 152.6 km. It orbits Saturn at roughly 151 500 km, in an almost identical orbit to another moon, Epimetheus, which caused some confusion for astronomers trying to establish the orbital parameters of what they had assumed was a single moon. The surface of Janus is heavily cratered, with some craters exceeding 30 km in diameter. The moon has a low density, which combined with its irregular shape, leads some planetary scientists to conclude it is a rubble pile rather than a solid object.

Image of Janus taken by the Cassini Probe on 27 March 2012, and received on Earth on 28 March.

Image of Janus taken by the Cassini Probe on 27 March 2012, and received on Earth on 28 March.

Dione is the forth largest moon of Saturn, and the fifteenth largest moon in the Solar System (since it is bigger than all the known smaller moons combined, this is an important distinction, the fifteen largest moons really are a different class of object). Dione orbits Saturn at about 377 400 km. It is a spherical object with an icy, crater covered surface, though its high density suggests it has a large rocky core.

Image of Dione from 78 998 km. Taken by the Cassini Probe on 28 March 2012, and received on Earth on the same day.

Image of Dione from 80 171 km. Taken by the Cassini Probe on 28 March 2012, and received on Earth on the same day.

Image of Dione from 44 528 km. Taken by the Cassini Probe on 28 March 2012, and received on Earth on the same day.

Image of Dione from 44 028 km. Taken by the Cassini Probe on 28 March 2012, and received on Earth on the same day.

See also NASA releases new Cassini images of Saturn's moon Rhea, Saturn's moon Dione found to have an atmosphere and Saturn on Sciency Thoughts YouTube.

Saturn's moon Dione found to have an atmosphere.

Dione is was discovered in 1684 by the Genovan (coming from the Republic of Genova, part of modern Italy) astronomer Giovanni Domenico Cassini, the forth moon of Saturn to be discovered.

Dione orbits Saturn every 66 hours, at a distance of 377 000 km. It has a diameter of 1122 km, making it the fifteenth largest moon in the solar system. Its surface is made up of water ice, but planetary scientists calculate it is to massive to be icy throughout so it probably has an icy interior.

An image of Dione taken by the Cassini Space Probe.

In the 12 August edition of the journal Geophysical Research Letters contains a paper by a team lead by Sven Simon of the Institute of Geophysics and Meteorology at the University of Cologne, in which they announce the discovery of an atmosphere on Dione. This atmosphere was (unusually) not discovered by direct imaging but rather by its effect on the magnetic field of Saturn. As Dione moves through this field it creates a bow-wave ahead of itself, indicative of the presence of charged particles, particles which indicate the presence of an atmosphere, through in all probability a very thin one. This study gives no indication as to what the make-up of the atmosphere might be.

Dione is the third moon of Saturn where an atmosphere has been discovered.

Titan, Saturn's largest moon has long been known to have a thick, dense, atmosphere. This was first theorized by Josep Comas Solà, a Spanish astronomer at the Fabra Observatory in Barcelona in 1907, and confirmed in 1944 by Gerald Kuiper at the Yerkes Observatory. The atmosphere was studied directly by the Voyager Space Probes in the early 1980s and penetrated by the Huygens Probe, launched from the Cassini Spacecraft, in 2005. The atmosphere is 1.45 times as dense as that of Earth, and 1.19 times as massive. It is comprised mainly of nitrogen, with a small amount of methane; this methane content is about 1.6% of the total above a height of 32 km, increasing to 4.9% at 8 km, a concentration which remains constant to ground level.

Video taken during the Huygens Probe's decent into the atmosphere of Titan.

In 2010 NASA announced the discovery of a thin atmosphere on Rhea, the second largest moon of Saturn. This atmosphere is comprised of oxygen (60%) and carbon dioxide (40%). The oxygen content of the atmosphere is thought to be the result of the electrolysis of water ice on the surface of the moon, but the origin of the carbon dioxide is less clear; possibly the result of outgassing from somewhere inside the moon.

Video of the surface of Rhea from the Cassini Space Probe.

See also Running water on present day Mars? Pluto gains a fourth moon and Saturn on Sciency Thoughts YouTube.