Science from a Saturn Entry Probe Mission

Data from atmospheric entry probe missions to the giant planets can uniquely discriminate between competing theories of solar system formation and the origin and evolution of the giant planets and their atmospheres. This provides for important comparative studies of the gas and ice giants, and provides a laboratory for studying the atmospheric chemistries, dynamics, and interiors of all the planets including Earth. Additionally, the giant planets also represent a valuable link to extrasolar planetary systems. For these reasons, a Saturn Probe mission with a shallow probe is ranked as a high priority for a New Frontiers class mission in the recent Planetary Decadal Survey. Atmospheric constituents needed to constrain theories of solar system formation and the origin and evolution of the giant planets can be accessed and sampled by shallow entry probes. Many of the most important constituents are either spectrally inactive or are beneath an atmospheric overburden that is optically thick at useful wavelengths and are therefore not remotely accessible by flyby or orbiting spacecraft such as Cassini. A small, scientifically focused shallow entry probe mission can make critical abundance measurements of key constituents, and can measure profiles of atmospheric structure and dynamics at a vertical resolution that is significantly higher than can be achieved by remote sensing techniques. The Galileo mission began the detailed study of the solar system's two major gas giants by dropping an entry probe into the atmosphere of Jupiter and deploying an orbiter around Jupiter. In 2016-2017 the Juno mission will make detailed measurements of Jupiter's deep oxygen abundance, and gravitational and magnetic fields. In the same epoch, the Cassini Saturn Orbiter will begin a set of Juno-like orbits to make comparable gravitational and magnetic field measurements of Saturn. A Saturn atmospheric entry probe would complete the quartet of missions needed for a comparative study of the two planets leading to a better understanding of the formation of the solar system. A highly focused Saturn entry / descent probe mission carrying a minimal science payload can address unique and critical science while fitting within existing program budget caps. Fundamental measurements include abundances of the noble gases He, Ne, Ar, Kr, and Xe, abundances of key isotopic ratios 4He/3He, D/H, 15N/14N, 18O/16O, and 13C/12C. Detection of disequilibrium species such as CO, PH3, AsH3, and GeH4 are diagnostic of deeper internal processes and dynamics of the atmosphere along the probe descent path. Abundances of all these key constituents, as well as carbon which does not condense at Saturn, sulfur which is expected to be well-mixed below the 4 to 5-bar ammonium hydrosulfide (NH4SH) cloud, and gradients of nitrogen below the NH4SH cloud and oxygen in the upper layers of the water and water-ammonia solution cloud, can be measured by a shallow entry probe descending through 5 - 10 bars. In concert with the results from Galileo, Cassini, and Juno, a shallow Saturn probe capable of measuring abundances of key constituents not remotely accessible by an orbiter or flyby mission will provide measurements critical to enabling a comparison of composition and dynamical processes on the giant planets in our solar system, thereby providing an improved context for understanding exoplanets as well.