The Hubble OPAL Program: 10 years of time-variable phenomena on Jupiter and the other giant planets (invited)

Introduction: The Outer Planet Atmospheres Legacy (OPAL) program began in 2014 as part of the Hubble 2020 legacy initiative (Simon et al. 2015; DOI: 10.1088/0004-637X/812/1/55). These observations were meant to cement long-term legacy of the Hubble Space Telescope (HST) by ensuring a regular cadence of giant planet observations to fill temporal gaps between individual programs. The giant planets have highly dynamic atmospheres, so long-term trends tied to seasonal or other evolutionary cycles require regular data collected using the same instruments and filters.

In addition to building up a long data base of consistent observations on an annual cadence, serendipitous discoveries have been made along the way. Filters extend from the near-UV (F225W at 225 nm) to the near-IR (FQ889N at 889 nm), and each planet is imaged to cover all longitudes over a period of two planetary rotations. All raw data are immediately available to the public, and the team also hosts high level science products in the form of global maps at the MAST Archive (Simon 2015; DOI: 10.17909/T9G593).

OPAL at Jupiter: Hubble’s exquisite spatial resolution and OPAL’s global and temporal coverage allow detailed study of Jupiter’s long-lived vortices, high speed narrow wind jets, and alternating, variable, bands of colored clouds. OPAL results have included studies of vortices including the Great Red Spot (GRS), zonal wind speeds, small atmospheric waves, long-term color trends, and UV-dark ovals in the polar hoods.

Space missions: OPAL data have extended the science return of several space missions, with Jupiter observations commencing one year before Juno arrived at Jupiter. OPAL wind and cloud structure measurements have been used in diverse analyses of phenomena from the gravitational anomaly of the GRS, to deep zonal atmospheric structure revealed by microwave emission, to convective cycles in cyclonic vortices. Wave, jet, and vortex features previously observed by Voyager and Cassini have also been studied in greater detail with the long-term OPAL program.

Earth-based observatories: High-resolution visible-wavelength observations from OPAL target the planets near solar opposition to maximize spatial resolution, as do many Earth-based programs. Multi-observatory studies include correlations between cloud color from OPAL and microwave brightness from the VLA, comparisons between Doppler velocimetry from the ground and time-series imaging from OPAL, calibration, validation, and context for spectroscopic measurements, and deep context for stratospheric aerosol anomalies.

Conclusion: The results cited here are a small subset of the Jupiter results achieved with the OPAL monitoring of the outer planets, with additional discoveries at Saturn, Uranus, and Neptune. As of January 2025, 62 papers have cited OPAL data. With more than 10 years of data in hand, and continuing for the life of Hubble, we expect the scientific return to increase exponentially. OPAL serves as a model for future long-term programs at other observatories.

Acknowledgments: This research is based on HST observations (with NASA support; see Simon et al. 2015). GSO was additionally supported by NASA through contract 80NM0018D0004 to JPL.