Except for Saturn’s Moon Phoebe, Which is Out of This World
Planetary Science Institute | 2018 Nov 03
By developing a new method for measuring isotopic ratios of water and carbon dioxide remotely, scientists have found that the water in Saturn’s rings and satellites is unexpectedly like water on the Earth, except on Saturn’s moon Phoebe, where the water is more unusual than on any other object so far studied in the Solar System. ...
- Lower left: Cassini VIMS infrared view of Saturn. Blue is infrared light where water ice reflects relatively brightly. Red is longer wavelength thermal emission showing heat from deep inside the planet. Green is infrared wavelengths where aurora emit light. Upper right: Phoebe in visible light. Phoebe is very dark, like charcoal whereas the rings are very bright in visible light like slightly dirty snow. Phoebe is not to scale relative to Saturn. (Credits: NASA, JPL, VIMS Team, ISS Team, U. Arizona, D. Machacek, U. Leicester)
Isotopes are different forms of elements but with differing numbers of neutrons. Adding a neutron adds mass to the element, and that can change processes of how a planet, comet, or moon is formed. Water is composed of two hydrogen (H) atoms and one oxygen atom, H2O. Adding a neutron to one hydrogen atom, then called deuterium (D), increases the mass of a water molecule (HDO) by about 5 percent, and that small change results in isotopic differences in the formation of a planet, moon, or comet, and changes the evaporation of water after formation. The deuterium to hydrogen ratio (D/H) is a fingerprint of the formation conditions, including temperature and evolution over time. Evaporating water enriches deuterium in the remaining surface.
Models for the formation of the Solar System indicate that the D/H should be much higher in the colder outer Solar System than in the hotter inner system where the Earth formed. Deuterium is more abundant in cold molecular clouds. Some models predict the D/H should be 10 times higher for the Saturn system than on Earth. But the new measurements show this is not the case for Saturn’s rings and satellites except Saturn’s moon Phoebe. ...
Isotopic Ratios of Saturn's Rings and Satellites: Implications for the Origin of Water and Phoebe ~ Roger N. Clark et al
- Icarus (online 29 Nov 2018) DOI: 10.1016/j.icarus.2018.11.029