Investigating the Solar System's Ocean Worlds with Next-Generation Space Telescopes

Spurred by discoveries of this past decade and intense scientific and political interest, the search for life on the solar system’s ocean worlds has begun. This search must include next-generation space telescopes, which are uniquely capable of observing numerous distant targets at spatial resolutions equivalent to a not-too-distant flyby, for nearly arbitrary amounts of time, unimpeded by the Earth’s atmosphere. This white paper describes the specific questions – drawn from the NASA Roadmap to Ocean Worlds – that can be best addressed by future space telescopes. It also identifies key measurements to help plan the design and operation of these assets in order to ensure the best use of their capabilities in the ocean worlds exploration endeavor. Ocean worlds in the solar system, defined for the purposes of this white paper as bodies with a current local or global liquid ocean (Lunine 2017; Hendrix, Hurford et al. 2019), have become the subject of intense scientific interest in the past decade as possible abodes for life beyond Earth. Ten years ago, there was only strong evidence for subsurface oceans on Jupiter’s moons Europa, Ganymede, and Callisto (Khurana et al. 1998; Kivelson et al. 2000, 2002), the H​2​O plume on Enceladus (e.g. Porco et al. 2006), and Triton’s N​2​ plume (Smith et al. 1989). It is now known that Enceladus’ plume is sourced from a global ocean (Thomas et al. 2016) at conditions compatible with life (Sekine et al. 2015), providing energy and bioessential element sources (Waite et al. 2017; Postberg et al. 2018). Europa too may be intermittently erupting water to space (Roth et al. 2014; Sparks et al. 2016, 2017; Jia et al., 2018). Titan is now recognized as a world of two oceans: surface light hydrocarbons and subsurface water (Iess et al. 2012). Indirect evidence indicates that Pluto and Saturn’s moon Dione too may have a deep ocean (Nimmo et al. 2016; Beuthe et al. 2016), and that the last pockets of briny water may be freezing on Ceres (Neveu and Desch 2015). There may be many more possible ocean worlds in the solar system, yet to be investigated. The interest has widened from purely scientific to also political, with the U.S. Congress mandating that NASA establish “an Ocean World Exploration Program whose primary goal is to discover extant life on another world” (H. Rept., 2015). In response, the community has developed a NASA Roadmap to Ocean Worlds (Hendrix, Hurford et al. 2019). The Roadmap outlines key science goals as milestones in assessing the potential for ocean worlds to host life (Fig. 1). Ideally, these goals would be addressed by robotic exploration at each ocean world. However, the sheer number of targets (Fig. 2) and necessity for monitoring at practical timescales of days to decades makes complementary telescopic observations essential. Figure 1. Science goals for ocean world exploration. White lines across roadmap milestones depict the current state of knowledge for each world or class of worlds from past exploration by the Cassini, Galileo, Voyager, Dawn, and New Horizons spacecrafts (from top to bottom). Upcoming missions such as Europa Clipper (NASA) to Europa and JUICE (ESA) to Ganymede will make further progress along this roadmap, but there is a need for a higher cadence of observations of a large number of worlds that requires complementary telescopic observations. Modified from Hendrix, Hurford et al. (2019).