Exploring Giant Planet Atmospheres with Habitable Worlds Observatory

TL;DR

The paper argues that to advance giant planet science, a future large UV–visible observatory must provide spatially resolved spectroscopy and time-domain imaging of Jupiter, Saturn, Uranus, and Neptune across $80$–$900$ nm, with capabilities to track moving targets, avoid saturation, and operate over wide dynamic ranges. It outlines imaging and spectroscopy objectives, translating them into concrete observables (cloud dynamics, aerosol properties, tropospheric and stratospheric chemistry, and auroral processes), and specifies measurement requirements (field of view, spectral resolution, and cadence) to enable discovery-level insights. By detailing campaigns, instrument modes (IFS with multiple FoVs), and rapid-response strategies, the paper makes a compelling case for HWO as a transformative platform for comparative planetology, energy balance, and magnetosphere–atmosphere coupling across the Solar System and beyond. The proposed capabilities would particularly empower Uranus and Neptune studies, offering constraints and context long before dedicated orbiters or probes, and aligning with multiple decadal science priorities across planetary science, heliophysics, and astrophysics.

Abstract

Visible and ultraviolet imaging and spectroscopy of Solar System giant planets can set the paradigm for the atmospheric, ionospheric, and magnetospheric processes shaping the diversity of giant exoplanets, brown dwarfs, and their interactions with stellar hosts. Spectra of their molecular absorptions, aerosol scattering, airglow, and auroral emissions can reveal these dynamic atmospheres in three dimensions. Given that giant planets are extended, bright, moving, and rotating objects, with extreme dynamic range and highly variable appearances, they impose specific mission and instrumentation requirements on future large space-based optical/UV observatories like the proposed Habitable Worlds Observatory (HWO). We advocate that HWO must have the capability to track non-sidereal targets like the giant planets and their satellites; should be able to view auroras and atmospheres without saturation (e.g., through the use of filters or fast read-out modes); and with a high dynamic range to explore faint objects near bright discs. HWO should enable spatially-resolved spectroscopy from $\sim80$ nm to $\sim900$ nm, capturing H$_2$ Lyman and Werner band series and H Lyman-$α$ in the far-UV; molecular absorptions and scattering in the mid-UV/visible; and deep hydrogen/methane absorptions in the 800-900 nm for cloud characterisation and CH$_4$ mapping. Imaging should enable time-resolved observations, from seconds to create auroral movies, to hours for cloud tracking and winds, to months and years for atmosphere/ionosphere variability. We advocate that an imager should have sufficient field of view to capture Jupiter ($>50$\arcsec), and that UV/visible integral field spectrographs be considered with both narrow ($3$\arcsec) and wide ($>10$\arcsec) field capabilities to provide efficient mapping of atmospheres and auroras. [Abbr]

Figures (5)

• Figure 2: The four giant planets as seen by Hubble WFC3/UVIS at different near-UV wavelengths, and processed by Judy Schmidt (note - these images are not shown to scale). The Jupiter and Saturn images are constructed from F343N (red), F275W (green), and F225 (blue), whereas a lack of observations for Uranus and Neptune required us to shift to redder wavelengths: F547M (red), F467M (green), and F336W (blue) for the Ice Giants. Jupiter and Ganymede were observed 2017-02-02 at 12:16UT, Saturn was observed at 2018-06-07 at 00:06UT, Uranus on 2015-08-30 at 10:08UT, and Neptune on 2015-09-02 at 07:58UT. Strong UV absorption is visible at the poles of Jupiter and Saturn due to the nature of high-altitude aerosols and the potential link to auroral chemistry. Red vortices, such as Jupiter's Great Red Spot and Oval BA (lower left), appear dark due to the blue-absorbing chromophore responsible for their red colours. Uranus' bright north polar cap of aerosols, and its narrow rings, are just visible in this image. Neptune's banding, and some bright high clouds (appearing in pink) can be seen in both northern and southern hemispheres. In addition, Saturn's rings appear reddish due to UV absorption. Credit: NASA/ESA/CSA/Hubble/Judy Schmidt.
• Figure 3: Montage of Jupiter spectra from the UV 20melin_uv99edgington, visible 94karkoschka, and near-infrared 09rayner03encrenaz, adapted from 23fletcher. Key atmospheric absorptions and emissions have been labelled. We define the spectral ranges as follows: extreme UV (50-120 nm); far-UV (121-200 nm), mid-UV (200-300 nm), near-UV (300-400 nm), visible ($>400$ nm), near-IR ($>900$ nm), and the start of the mid-IR at $\sim5$$\mu$m.
• Figure 4: Two-way transmission functions, indicating the approximate penetration depth to an optical depth of unity in an aerosol-free atmosphere, reproduced with permission for Jupiter 18sromovsky_jup, Saturn 20sromovsky and Uranus 19sromovsky. The full 0.4-5.2 $\mu$m range is shown for Jupiter and Saturn, but we restrict to 0.4-1.7 $\mu$m for Uranus (which also indicates the location of filters used by Hubble and Keck to measure the distribution of CH$_4$). Multiple gases are shown for Jupiter and Saturn, only CH$_4$ and the H$_2$, He continuum are shown for Uranus. Rayleigh scattering dominates at shorter wavelengths.
• Figure 5: Dark ovals on Uranus and Neptune, which are best explored with HWO's exceptional spatial resolution at blue wavelengths. Uranus and its dark spot 09hammel was observed on 2006-08-23 in three filters - red 775 nm, green 658 nm, blue 550 nm [Credit NASA, ESA, L. Sromovsky and P. Fry (University of Wisconsin), H. Hammel (Space Science Institute), and K. Rages (SETI Institute)]. The shrinkage of Neptune's southern dark spot (SDS-2015, not to be confused with the Great Dark Spot observed by Voyager) is shown in three-colour images top right (red 763 nm, green 547 nm, blue 467 nm), and in just the blue filter bottom right to reveal the dark oval 18wong [Credit: NASA, ESA, and M.H. Wong and A.I. Hsu (UC Berkeley)].
• Figure 6: Absorption cross-sections for a number of important chemical species in giant planet atmospheres, compared to the solar luminosity (red, dashed line), reproduced from 20melin_uv. Laboratory measurements at different temperatures are shown for acetylene (C$_2$H$_2$) and phosphine (PH$_3$), with single measurements for ethane (C$_2$H$_6$), ammonia (NH$_3$), and propane (C$_3$H$_8$). Methane (CH$_4$), carbon monoxide (CO), diacetylene (C$_4$H$_2$), and water (H$_2$O) also have UV absorptions in this region.