Unveiling Exoplanet Surfaces: Lessons from Jupiter and Enceladus’ Opposition Effect

When planets and moons in our Solar System reflect sunlight, their brightness increases dramatically when viewed directly opposite the Sun—a phenomenon called the "opposition effect." This effect is caused by two main mechanisms: shadow hiding (SH) and coherent backscattering (CB). SH occurs when surface roughness casts shadows that disappear at opposition, while CB is a result of light waves constructively interfering as they bounce off particles.

In this study, Jones et al. investigate how this effect appears on Jupiter and Enceladus, using images taken by the Cassini spacecraft. They aim to determine whether the opposition effect could be used to detect solid surfaces on exoplanets—planets outside our Solar System.

Methods

The authors analyze reflected light from Jupiter and Enceladus using phase curves, which show how brightness changes as the viewing angle shifts. They use previously published Cassini data from multiple wavelengths and apply a mathematical model to fit the phase curves.

This model, based on the work of Heng et al. (2021), calculates reflected light using a Henyey-Greenstein scattering function, which describes how light is scattered by particles. By adjusting parameters in the model, the authors determine whether SH or CB best explains the opposition effect for each body.

Results from Jupiter and Enceladus

Opposition Effect is Present in Both

The study confirms that both Jupiter and Enceladus experience a strong opposition effect. Comparing different models, they find that for Jupiter, CB is preferred in 7 out of 8 wavelengths, suggesting that multiple scattering events dominate. Since Jupiter is a gas giant with no solid surface, shadows (which drive SH) cannot form as they do on rocky bodies. For Enceladus, a mix of SH and CB is observed, indicating that both mechanisms contribute. This aligns with what we know about Enceladus—it has an icy, reflective surface that could support both types of scattering.

Surface Clues from Opposition Peaks

A key difference emerges between the two objects: Jupiter’s opposition peak is much broader than Enceladus’s. The authors measure the full-width at half-maximum (FWHM) of the opposition peak, which is about 10 times larger for Jupiter than Enceladus. This suggests that solid surfaces produce sharper peaks, while gaseous planets have wider peaks. If the same pattern holds for exoplanets, this could provide a way to distinguish between rocky and gas planets using their phase curves.

Can This Method Work for Exoplanets?

The authors test whether this opposition effect could be detected on distant exoplanets using current and future telescopes. They simulate phase curves for K2-141b, a rocky exoplanet, and attempt to recover an opposition peak similar to Enceladus’s. Their results show that detecting the opposition peak would require an extremely high number of observations—far beyond current capabilities. For Jupiter-like exoplanets, the detection is easier because their peaks are broader, making them more visible. However, even for these planets, thousands of observations would be needed.

Discussion and Conclusion

The study highlights the opposition effect as a potential tool for identifying solid exoplanet surfaces, but current telescopes like JWST are not sensitive enough to measure it. Future space telescopes with improved precision may make this method feasible.

Additionally, the study reinforces our understanding of how different surfaces scatter light—gas giants like Jupiter show strong CB, while icy moons like Enceladus show both CB and SH. These findings help us interpret reflected light curves of exoplanets, advancing our ability to study distant worlds.

Source: Jones