SPRITE: A Saturn probe new frontiers mission

If not for Saturn, Jupiter might have swept up the terrestrial planets, leaving our Sun's habitable zone barren. This startling hypothesis is based on modeling many exoplanet systems: orbital resonance with Saturn conceivably stopped Jupiter's inward migration, preserving humanity's future home. In situ measurements of the abundance pattern of Saturn's noble gases and other elements, and its isotope ratios, will reveal when and where it formed in relation to Jupiter. They are not remotely observable since noble gases have no measurable spectral signature and other elements are only well-mixed beneath the clouds. Neither Cassini nor Juno have answered these outstanding fundamental questions. Saturn's upper clouds also hide its chemistry, deep winds, and fundamental properties like temperature and pressure that govern vertical motion and circulation. In situ measurements by a probe will be a guidepost that tie the cloud-top view from remote sensing to the properties and processes below. We will measure the abundance of key disequilibrium (CO, PH 3 and C 2 H 6 ) and condensable (NH 3 ) species; cloud layer properties (particle density as a function of altitude) at 1%-scale-height resolution; and dynamic properties (P, T, vertical motion, and wind speed) to at least 10 bar (∼200 km deeper than the 1 bar level). These data, coupled with our near-simultaneous remote images and prior observations, are essential for validating the community's planetary cloud models and global climate simulations under different boundary conditions. SPRITE data will also provide context for our interpretation of time-varying observations of exoplanets and brown dwarfs. SPRITE is implemented with a three-element flight system comprised of a flyby Carrier Relay Spacecraft (CRSC) and an entry Probe consisting of an Aeroshell with engineering sensors and Descent Vehicle with science payload. There are four instruments: three on the Probe and one on the CRSC. The Probe's Quadrupole Mass Spectrometer and Tunable Laser Spectrometer directly measure noble gas and other elemental abundances, and isotopic ratios. This instrument suite has heritage from the SAM suite on MSL. The Probe's Atmospheric Structure Instrument measures vertical pressure, temperature, and acceleration profiles using the same approach as the Galileo Probe. A simple laser-backscatter nephelometer based on terrestrial instruments measures particle number density of tropospheric clouds. The Ultrastable Oscillators in the telecom subsystem enable retrieval of Saturn's deep winds. The CRSC hosts a Multi-Channel Imager to document the specific dynamic environment at the Probe site and to place the in situ measurements in context with prior remote imaging by Cassini and HST. All science analysis techniques needed were demonstrated on MSL, Cassini, HST, and other missions. Data quality of projected instrument performance is 2–5 times better than the measurement requirements; the instruments collect 1.5–2 times more samples than are required. The 120-minute descent also includes 33% unallocated time margin. SPRITE will provide definitive answers to all of these important scientific questions and represents the capstone of the Galileo, Cassini and Juno missions to the Giant Planets.