Exploration of Ocean World Activity and Habitability with HWO

Many icy moons and volatile-rich dwarf planets could harbor saline reservoirs or global saline oceans beneath their icy exteriors. Liquid water in the interiors of these icy ocean worlds could reach their surfaces via fractures extending to liquid layers, as is likely the case on Jupiter’s moon Europa and the dwarf planet Ceres, depositing salts, organics, and other compounds that can help constrain internal environments. At Saturn’s moon Enceladus, ocean material is erupted into space forming a plume that sustains Saturn’s E ring and a large H2O torus. Investigation of the surfaces and exospheres of ocean worlds, and their surrounding space environments, can therefore provide invaluable insights into their internal geochemistries and habitability potential.

The ultraviolet (UV) wavelength region is particularly useful for measuring atomic H and O emission lines that are spectral tracers of H2O vapor, released by ongoing or transient ocean world activity. However, existing UV observations (< 350 nm) of ocean worlds are often limited to low signal-to-noise ratio (SNR), disk-integrated datasets and are inaccessible with ground-based telescopes and the James Webb Space Telescope (JWST). We will present on future observations of ocean worlds made with a UV/VIS (~90 – 700 nm) integral field spectrograph (IFS) on the Habitable Worlds Observatory (HWO). An IFS on HWO would allow for spatially-resolved (<0.02”/pixel), high spectral resolution (R > 3,000), and high SNR data, crucial for investigation of subsurface-surface exchange and exospheric activity, and for measuring sodium, carbon, and other components indicative of salts and organics, potentially derived from internal oceans.

As a serviceable facility that can receive instrument upgrades, propellant, and repairs, HWO will be a cutting-edge platform for multiple decades. HWO’s long lifespan will allow for monitoring variability in the intensity and composition of Enceladus’ eruptions and capturing sporadic H2O vapor release at Europa and Ceres, over long timescales. Thus, HWO will be able to collect high SNR and spatially-resolved data over UV wavelengths, which will complement measurements of H2O vapor made by JWST in the near-infrared, providing continuity in monitoring ocean world activity with space telescopes for multiple generations of scientists.