Although we weren’t here to observe the birth of our own solar system, astronomers have developed a relatively informed picture of how it likely happened, based on observations of our present-day home and the infant planets forming around other stars. But every so often, something throws a wrench in our theories, and that may have just happened — researchers have discovered interesting new properties of Saturn and its moons that contradict our current models for how the solar system formed.
In a paper accepted November 28 to the journal Icarus, researchers from the Planetary Science Institute, the University of Arizona, NASA Ames Research Center, and the U.S. Geological survey report their measurements of isotope ratios in the Saturn system. Isotopes are atoms of a single element with the same number of protons and electrons, but differing numbers of neutrons. Studying the abundance of certain isotopes helps astronomers piece together an object’s history. Some isotopes were more or less common, particularly in certain areas of the solar nebula as it formed planets; others are more likely to stick around or appear after processes such as heating or evaporation. These researchers found that, based on spectroscopic observations of the Saturn system from Cassini, the water in Saturn’s rings and moons is surprisingly like the water on Earth — an unexpected result, given their disparate locations. Even stranger, the water on Saturn’s moon Phoebe (and only Phoebe) is unlike the rest of the water in the Saturn system, suggesting it formed even farther out in the solar nebula, rather than in place around the ringed planet.
To make these discoveries, the team developed a new way to measure isotope abundances in the readings taken by Cassini’s Visual and Infrared Mapping Spectrometer (VIMS). In particular, they studied the ratio of deuterium to hydrogen (D/H) in the water in the Saturn system. Deuterium is an isotope of hydrogen with one proton and one neutron in its nucleus; D2O, which is water that contains deuterium instead of hydrogen, is commonly known as heavy water because of the added mass from the extra neutrons.
Each spectrum taken by VIMS breaks the light reflected from the surface of Saturn, its rings, or its moons into its constituent parts to identify the fingerprints of the elements contained within them. Based on a comparison of laboratory spectra with the VIMS results, the team discovered the bulk of the Saturn system’s water, including the water in the planet’s rings and on its moons (except Phoebe), has a D/H ratio similar to terrestrial water. However, our current models for the formation of the solar system state that the D/H ratio farther out in the solar system should be higher than closer in; if Earth formed close to the Sun and Saturn far from it, why is their water so similar?
According to the team, the similarities indicate that the same type of water may have been found in the inner and outer solar system during its formation, which would require us to change our current models. “The terrestrial-like D/H of Saturn’s rings and satellites may indicate a similar water source for the inner and outer solar system, or at least a change in models where the D/H varies less from inner to outer solar system, less than a factor of two from Earth to Saturn,” the paper states.