Potential Ocean Worlds

Abstract

The Roadmap for Ocean Worlds (ROW) provides an overarching goal for ocean worlds exploration: Identify ocean worlds, characterize their oceans, evaluate their habitability, search for life, and ultimately understand any life we find. The first part of the goal-to determine which bodies have oceans and understand how to determine whether other bodies host current oceans-drives the search for and exploration of potential ocean worlds. Based on the ROW goals document (Hendrix et al., 2018), we highlight the state of knowledge for potential ocean worlds and support their exploration in the next decade to confirm if they possess oceans. Triton:Triton is believed to be a captured Kuiper Belt Object (KBO; Agnor and Hamilton, 2006). This origin scenario leads to two unique opportunities to describe and constrain the early histories of ocean worlds. First, the capture event likely resulted in substantial tidal heating early in Triton's history (Jankowski et al., 1989;Ross and Schubert, 1990), which presents the potentially unique opportunity to study a subsurface ocean on a body for which tidal heating was relatively significant during its early thermal history. This early heating would afford an opportunity to assess the relative importance of various heat sources to create and maintain a subsurface ocean (Gaeman et al., 2012). Second, as a KBO, Triton is likely similar in bulk composition and origin to Pluto and other KBOs, which allows comparisons among these bodies and extrapolation of information learned from Triton to other bodies that are less accessible.A variety of data hint at the possibility of an internal ocean, or at least near-surface liquid water at Triton today. The surface of Triton is young and active, which is evident from several observed surficial features. These include the observations of the young crater-retention age of Triton which gives a surface age of <100 Myr (possibly <10 Myr) (Schenk and Zahnle, 2007), possible cryovolcanic flows (Croft et al., 1995) and tectonic features (Croft et al., 1995;Prockter et al., 2005). Triton's unique surface morphology (e.g., the "cantaloupe" terrain) and young surface may indicate ongoing diapiric activity today (Schenk and Jackson, 1993). The ice shell of Triton may be capable of internally-driven resurfacing, making it potentially unique among ocean worlds (Howell and Pappalardo, 2019).Possibly the most intriguing evidence of activity on Triton are the active geysers that have been observed (Soderblom et al., 1990); while these have been attributed to solar-powered sublimation of N2 ice (Kirk et al., 1990), the existence of water plumes from the tiny moon, Enceladus (Dougherty et al., 2006; see also Appendix D2 of this report) provides reason to question this conclusion (Hansen and Kirk, 2015). Endogenic plumes that sample a subsurface ocean would be very exciting astrobiological targets if indeed the geysers on Triton are endogenic rather than solar-driven. Their composition can be sampled not just directly near the geysers sources, but throughout the atmo...