December 6, 2018 : A New Study Reveals That Saturn's Rings, Titan And The Other Moons Of The Gas Giant May Surprisingly Contain The Same Type Of Water As The Earth Except For Phoebe Which Contains A Heavier Water

A new study entitled  Isotopic Ratios of Saturn's Rings and Satellites: Implications for the Origin of Water and Phoebe , recently published in the journal Icarus and proposed by a group of researchers involving Planetary Science Institute Senior Scientist Roger N. Clark as well as the collaborators Robert H. Brown (University of Arizona), Dale P. Cruikshank (NASA) and Gregg A. Swayze from the USGS, shows that Saturn's rings, Titan and the other moons of the second largest planet in the Solar System, except for Phoebe, surprisingly contain the same type of water as the Earth. The outcome is unexpected and surprising because Phoebe is composed of a heavier water than the other objects evolving around Saturn and because the other moons of the Gas Giant, from Mimas to Rhea or Titan, contain the same type of water molecule as our own planet. The conclusion was made possible thanks to the development of a new method for detemining isotopic ratios of water and carbon dioxide from the spacecraft or the orbiter.

Isotopes correspond to different forms of the same element. Each element has a precise number of protons but it can contain a different number of neutrons. Therefore, the same element can be a different type of the same element that is to say a different isotope. If the element incorporates more neutrons, it will be heavier and it will be likely to have an impact on the formation mechanisms of any planet, comet or moon. The planetologists have focused their attention on the isotopes of water, a well-known molecule with the chemical formula H2O and composed of two hydrogen atoms and one oxygen atom. If a neutron is incorporated into the hydrogen atom of the water molecule, the hydrogen atom becomes Deuterium (D) and the water molecule will be heavier with a mass increased by approximately 5 percent. The water molecule with a heavier hydrogen atom represents a heavy water known as HDO.

This relatively small change in the weight of the water molecule has a relatively significant impact on the way the planets, the moons or the comets form and develop. One can identify the isotopic differences between various worlds from outer space. The Cassini orbiter which arrived at Saturn in 2004 and which ended its mission in 2017 with a crash against the Gas Giant gathered a huge amount of data regarding the composition and the dynamics of the multitude of worlds evolving in the Saturn System. Water is widespread in the System of Saturn which represents, to a certain extent, a mini Solar System. The level of water evaporation after the formation process of a given world will be influenced by the concentration of heavy water, HDO or water incorporating heavy hydrogen. Researchers can gather clues regarding the conditions of the formation process of any world on the basis of the deuterium to hydrogen ratio (D/H).

Planetologists are in a position to determine or deduce the dynamics of the formation process of any planet, moon or comet. They can determine the environmental temperature of the world and its thermal evolution. The surface of the body will tend to be richer in deuterium or heavy water if the level of water evaporation is higher. What is the level of water evaporation over long periods of time ? That's a major question on a parameter which can have a strong influence on the deuterium to hydrogen ratio for any world. Prevailing models or theories regarding the formation or the development of the Solar System suggest that the bodies of the Outer Solar System, where the level of energy received from our star is lower than the solar energy received by the bodies of the Inner Solar System where the Earth formed and developed, should have a much higher deuterium to hydrogen ratio or should contain a higher concentration of deuterium and heavy water than the bodies of the Inner Solar System.

Deuterium or heavy hydrogen appears to be more abundant in cold molecular clouds where stars can take shape. Some models or simulations have determined that the deuterium to hydrogen ratio (D/H) should be 10 times higher for the System of the Ringed Planet than for the  Blue Planet . However, the new measurements upon Saturn's rings and moons performed by the team of Roger N. Clark demonstrate that reality is apparently not in line with prevailing models regarding the deuterium to hydrogen ratio. Curiously, the new outcome reveals that the small moon Phoebe has a remarkably different deuterium to hydrogen ratio from Saturn's rings and the other moons of the Gas Giant which contain the same type of water as the Earth. The identification of an unusual deuterium to hydrogen isotopic ratio (D/H) for the relatively dark moon Phoebe implies that it may have formed much farther from the Sun than where it evolves today and that it may have migrated to the Saturn System in a way that has to be determined.

The group of planetologists also determined the carbon-13 to carbon-12 (13C/12C) ratio on the two-toned moon Iapetus and on the irregular moon Phoebe. The researchers show that Iapetus, which evolves much farther from Saturn than Titan or Rhea, has a deuterium to hydrogen ratio (D/H) similar to that of our own planet and has a carbon-13 to carbon-12 ratio (13C/12C) close to the level of the Blue Planet. However, once again, the team of scientists demonstrates that Phoebe has another unusual or unexpected isotopic ratio, that is to say a carbon-13 to carbon-12 ratio which appears almost five times higher than expected, compared to the other moons of the Gas Giant. The level of carbon dioxide identified on that small world brings information on the potential level of evaporation of the original object after formation and implies that the small irregular moon may have formed in a particularly cold environment in the outer limits of our Solar System. Let's point out that many worlds have already been found in the Solar System far beyond Uranus, Neptune or Pluto.

The new outcome implies that Phoebe may have formed much farther from the Sun than the System of Saturn which evolves at a mean distance of about 1.4 billion kilometers from the Sun. In fact, Phoebe may have formed in a particularly harsh environment where the level of energy received from our star is much smaller than the level of energy absorbed by the celestial objects evolving in the Saturn System. The new outcome also suggests that the orbit of the proto-Phoebe may have been disrupted by a cosmic event triggering a migration of the proto-Phoebe toward its present orbit or environment. The gravitational influence of Saturn may have allowed the Gas Giant to capture the proto-Phoebe in its environment. The planetologists don't know exactly where the small irregular moon originated but they are convinced that the object probably formed well beyond Saturn. Did Phoebe form in the Kuiper Belt ? Did Phoebe form in the Scattered Disk ? Did Phoebe form in the Oort Cloud ? The debate is far from being over.

Researchers hope to perform measurements regarding the deuterium to hydrogen ratio and the carbon-13 to carbon-12 ratio (13C/12C) for Dwarf Planets, asteroids, comets or small icy bodies well beyond Uranus or Neptune in order to try to deduce the potential origin of Phoebe. No measurements of key isotopes (D/H or 13C/12C) for the icy surface of the Dwarf Planet Pluto or Kuiper Belt Objects, beyond Neptune, are available today. The new technique or methodology is likely to allow the team of Roger N. Clark to determine key isotopic ratios of the icy surface of Pluto, Charon or even worlds beyond like Eris or Makemake. The measurements regarding the deuterium to hydrogen ratio and the carbon-13 to carbon-12 ratio were performed with the Visual and Infrared Mapping Spectrometer (VIMS) of the Cassini orbiter which was proposed by NASA, ESA and ASI. The VIMS which was provided by the Italian space agency (Agenzia Spaziale Italiana) has been very useful during the long mission of the Cassini spacecraft.

The new technique for measuring isotopic ratios on solids like water ice or carbon dioxide ice using reflectance spectroscopy from outer space or from the orbiter will allow planetologists to determine the level of isotopic ratios for comets, dwarf planets, planetary bodies or other worlds in the Solar System. Therefore, planetologists will have new clues, new parameters or new constraints at their disposal to develop new models of Solar System formation. The determination of a similar deuterium to hydrogen ratio (D/H) between the Earth and the rings and the moons of Saturn, except for Phoebe, implies that the water of all those bodies located in the Inner Solar System or in the Outer Solar System is from the same cosmic source. Researchers or planetologists will have to produce new models of the Solar System formation and development which account for those new parameters or observations. We had thought that worlds like Tethys, Titan or Rhea would be richer in deuterium in terms of concentration but that's not the case, apparently. The Saturn System is rich in water ice and a world like Titan is thought to harbor a subsurface ocean of water. Is that water similar to the water we encounter on Earth ?

 

The image above corresponds to a mosaic of views of Rhea, Titan and Phoebe obtained from the Cassini orbiter during its mission in the Saturn System. Those three moons of the Gas Giant Saturn are represented at scale, here. The original images of the mosaic of Rhea were taken with the Narrow-Angle Camera of the Cassini spacecraft on November 26, 2005. The original view of Saturn's largest moon Titan was generated on the basis of data collected on August 25, 2009 with the Wide-Angle Camera of the Cassini orbiter. The original images of the mosaic of Phoebe were acquired on June 11, 2004 from the Cassini probe. Rhea, Titan and Phoebe may be rich in water. The water of Rhea and Titan where a subsurface ocean of liquid water may exist appears to be the same type of water as on Earth. That's not the case for Phoebe where the type of water appears heavier than on the other moons of Saturn.

Credit for the original view of Rhea: NASA/JPL/Space Science Institute.
Credit for the original view of Titan: NASA/JPL/Space Science Institute.
Credit for the original view of Phoebe: NASA/JPL.
Montage credit: Marc Lafferre, 2018.

- To get further information on that news, go to: https://www.asi.it/en/news/phoebes-heavy-water.

 

 

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