Outer Solar System: Many Worlds to Explore Outer Planets Assessment Group (OPAG) View of Decadal Survey Progress May 2017 Alfred McEwen, OPAG Chair LPL, University of Arizona This presentation will address (relevant to OPAG): • 1. Most important new discoveries 2011-2017 – (V&V writing was completed in late 2010) • 2. Progress made in implementing Decadal advice – Flight investigations • Flagship Missions • New Frontiers – R&A and infrastructure – Technology • 3. Other issues relevant to the committee’s statement of task – Smallsats for Outer Planet Exploration – How to make the Discovery Program useful for Outer Planets – Europa Lander – Coordination with ESA JUICE mission – Adding Ocean Worlds to New Frontiers 4 – Future mission studies to prepare for next Decadal • 4. Summary grade recommendations on Decadal progress • 5. OPAG top recommendations to mid-term review 1. Most Important New Discoveries 2011-present • Jupiter system: – Europa plate tectonics (Prockter et al., 2014) – Europa cryovolcanism (Quick et al., 2017; Prockter et al., 2017) – Europa plumes (Roth et al., 2014; Sparks et al., 2017) – Confirmation of subsurface ocean in Ganymede (Saur et al., 2015) – Evidence for extensive melt in Io’s mantle (Khurana et al., 2011; Tyler et al., 2015) – Fabulous results from Juno (papers submitted) • Saturn System: – MUCH from Cassini—see upcoming slides • Uranus and Neptune systems: – Standard interior models do not fit observations; Uranus and Neptune may be quite different (Nettelmann et al. 2013) – Intense auroras seen at Uranus (Lamy et al., 2012, 2017) – Weather on Uranus and Neptune confined to a “thin” layer (<1,000 km) (Kaspi et al. 2013) – Ice Giant growing around nearby TW Hydra (Rapson et al., 2015) – Triton’s tidal heating and possible subsurface ocean (Gaeman et al., 2012; Nimmo and Spencer, 2015) • Pluto system—lots of results from New Horizons – The Pluto system is complex in the variety of its landscapes, activity, and range of surface ages. (Stern et al., 2015; Weaver et al., 2016; Moore et al., 2016; Grundy et al., 2016; Gladstone et al., 2016; McKinnon et al., 2016) • Planet 9 (or X) predicted to exist (Trujillo and Sheppard, 2014; Batygin and Brown, 2016) 1. Most Important New Discoveries 2011-present (Continued) • Giant Planets Comparative Atmospheric Sciences – Intriguing similarities and differences in the polar atmospheric dynamics (Sromovsky et al. 2012, review by Sayanagi et al. 2016/in press) • Theory: Atmospheric Dynamics – Explanation of Saturn’s Hexagon (Morales-Juberias et al. 2011, 2015) – Explanation of Giant Planet Polar Vortices (O’Neil et al. 2015, 2016) – Moist convection’s effect on energy balance (Li and Ingersoll, 2015) – These theories explain observed phenomena • Theory: Interior Structure – Relationship between Interior Structure and Rotation Rate (Helled and Guillot 2013) – Atmospheric Jet Stability may predict interior rotation rate (Read et al. 2009) – These theories have implications for the moment of intertia of giant planets measurable through gravity. • Theory: Outer Solar System Evolution – Nice Model (Levison et al. 2011, Bottke et al. 2012…) – Grand Tack Model (Walsh et al. 2011, 2012…) – These theories have implications for the bulk composition and isotopic ratios of giant planets Cassini Accomplishments • 1. Post-2011 Enceladus: – CDA finds salty water: H2O, Na and K salts discovered in plume particles (Postberg et al., 2011) – Internal heat source: warm south polar tectonic fractures ("Tiger Stripes” ) (Spencer et al., 2006; Howett et al., 2011; Goguen et al., 2013; Spencer et al., 2013) – Plume activity correlated with tidal stresses (Hedman et al., 2013) – RSS gravity data show that Enceladus interior is differentiated (Iess et al., 2014) – RSS gravity and ISS libration show a global liquid water ocean (Iess et al., 2014; McKinnon et al., 2015, Thomas et al., 2015) – ISS images show >100 jets and/or curtains erupt from tiger stripes (Porco et al., 2014; Spitale et al., 2015) – CDA finds SiO2 nanograins condense from core-heated H2O (Hsu et al., 2015) – CDA and INMS find CH4 in plume from warm-water chemistry at rock-core interface (Bouquet et al., 2015) – INMS finds H2 in plume: food for life (Waite et al, 2017) • 2. Titan: Cassini and Huygens showed us one of the most Earth-like worlds we’ve ever encountered, with weather, climate and geology that provide new ways to understand our home planet; see later slide • 3. Rings: Cassini showed us the complexity of Saturn’s rings and the dramatic processes operating within them. • 4. Icy Moons: Cassini revealed Saturn’s moons to be unique worlds with their own stories to tell. • 5. Saturn and Titan: The length of Cassini’s mission has enabled us to observe weather and seasonal changes, improving our understanding of similar processes at Earth, and potentially those at planets around other stars. See https://saturn.jpl.nasa.gov/mission/top-tens/images/ for top-10 Cassini results for each year. Seafloor Dust Captured by Cassini Silica nanoparticles captured by Cassini provides first evidence for ongoing seafloor hydrothermal activity. Hydrothermal activity occurs when seawater infiltrates and reacts with a rocky core, emerging as a heated, mineral- Earth laden liquid. Titan Discoveries Year paper Summary of Discovery Accepted (V&V priorities were finalized Sept 2010) 2010 Changing shoreline of northern seas 2011 Titan south polar vortex 2011 Methane rain storm 2012 Global subsurface ocean 2012 Seasonal change in atmosphere circulation 2012 Tropical lakes 2013 Definitive detection of a plastic ingredient 2013 Confirmation of complex hydrocarbons in Titan's upper atmosphere 2013 Large Abundances of Polycyclic Aromatic Hydrocarbons in Titan's Upper Atmosphere 2013 Titan’s ionospheric density linked to solar activity 2014 First determination of depth of a Titanian sea 2014 Titan's "Magic Islands": initial discovery 2014 Titan's ocean as salty as Dead Sea 2014 Methane Ice Cloud in Titan’s Stratosphere 2014 Titan Observed Outside of Saturnian Magnetosphere 2015 Titan dissolves to form small lake basins 2016 "Magic Islands" ongoing observations Seasonal Rains Transform Titan's Surface (Turtle et al., 2011) Cassini Mission 56 Flybys 12 Flybys Cassini Makes Historic Passage On April 26, 2017, Cassini made the first historic passage between Saturn and its rings marking the beginning of Cassini’s Grand Finale. Unique science goals during the Grand Finale include studying Saturn’s interior structure, measuring the mass and composition of the rings, and analyzing composition of uppermost atmosphere. Read more about it here: https://go.nasa.gov/2poRfE1 2. Progress made in implementing Decadal advice • A. Flight investigations – Flagships – New Frontiers – Discovery (none) discussed later • B. R&A and infrastructure (Earth-based observing, DSN, Pu-238) • C. Technology A. Outer Planet Mission recommendations in V&V • The recommended program: – • Discovery program funded at the current level adjusted for inflation, • How are we doing so – • Mars Trace Gas Orbiter conducted jointly with ESA, far? – • New Frontiers Missions 4 (includes Saturn • Descoped JEO Europa probes) and 5 (adds Io Observer) Clipper: NASA deserves – • MAX-C (descoped to $2.5 billion), an “A” • Uranus Orbiter and probe: – • Jupiter Europa Orbiter (descoped), and unlikely to get a new start – • Uranus Orbiter and Probe. this decade, “D” • The cost-constrained program: • New Frontiers 4 underway – • Discovery program funded at the current level but 5 may not make this adjusted for inflation, decade: I (incomplete) – • Mars Trace Gas Orbiter conducted jointly with • Bonus points for adding ESA, Enceladus and Titan – • New Frontiers Mission 4 and 5, • Improved budget picture – • MAX-C (descoped to $2.5 billion), and should lead to NF-6 this – • Uranus Orbiter and Probe. decade • Discovery: I (incomplete) • Improved budget picture: – 1. An increase in funding for the Discovery program, – 2. Another New Frontiers mission, and – 3. Either the Enceladus Orbiter mission or the Venus Climate Mission. OPAG Extended Missions: Cassini • From V&V: ”Continue missions currently in flight, subject to approval obtained through the appropriate senior review process. These missions include the Cassini mission to the Saturn system, ... Ensure a level of funding that is adequate for successful operation, analysis of data, and publication of the results of these missions, and for extended missions that afford rich new science return. • NASA has followed this advice OPAG Flagship missions 1. Europa Clipper • This mission satisfies V&V recommendation for descoped JEO • Entering Phase B; 9 science instruments accommodated • OPAG strongly supports this mission. Pop Quiz What major (known) planets in our Solar System have never had a dedicated spacecraft mission? Uranus and Neptune The Ice Giants: An unexplored class of planet HST image of Neptune and Triton Uranus: It isn’t always bland OPAG Flagship Missions: 2. Uranus Orbiter and Probes • V&V quotes: – The third-highest-priority flagship mission is the Uranus Orbiter and Probe mission. – …many exoplanets are ice-giant size. – Exploration of the ice giants Uranus and Neptune is therefore the obvious and important next step in the exploration of the giant planets. – ... a Uranus mission is favored for the decade 2013-2022 for practical reasons (no Jupiter gravity assist; cruise time to Neptune exceeded ASRG lifetime) – Many unknowns remain as to how the outer planets formed out of the solar nebula and if and when they migrated into different