Solar system

Abstract:

Observational studies of exoplanets are suggestive of an ubiquitous presence of clouds. The current modelling techniques used in emission to account for the clouds tend to require prior knowledge of the cloud condensing species and often do not consider the scattering effects of the cloud. We explore the effects that thermal scattering has on the emission spectra by modelling a suite of hot Jupiter atmospheres with varying cloud single-scattering albedos (SSAs) and temperature profiles. We examine cases ranging from simple isothermal conditions to more complex structures and physically driven cloud modelling. We show that scattering from nightside clouds would lead to brightness temperatures that are cooler than the real atmospheric temperature, if scattering is unaccounted for. We show that scattering can produce spectral signatures in the emission spectrum even for isothermal atmospheres. We identify the retrieval degeneracies and biases that arise in the context of simulated JWST spectra when the scattering from the clouds dominates the spectral shape. Finally, we propose a novel method of fitting the SSA spectrum of the cloud in emission retrievals, using a technique that does not require any prior knowledge of the cloud chemical or physical properties.

Abstract:

Ground test data in HDF5 and Matlab object formats from the ground testing of the Lunar Thermal Mapper instrument, 2023.

Abstract:

An accurate description of how visible light scatters from the lunar surface enables 1) constraints to be placed on the physical and compositional properties of the surface, using a photometric model such as the Hapke BRDF model, which has nine free parameters related to compositional and physical properties, and 2) more realistic scattering function inputs to be set within thermal models. Until a recent study by Foote et al. in 2010, lunar visible light scattering functions had been theoretically derived using limited laboratory measurements. Within thermal models, unrealistic scattering functions may be partly responsible for modelled temperature discrepancies of up to ~15-50 K (dependent on location)—when compared to remote sensing data from Diviner, onboard the Lunar Reconnaissance Orbiter—in regions such as polar craters, where light scattering due to surface topography dominates heat transfer. In this project, a laboratory goniometer setup was developed, which was used to measure a suite of visible light scattering functions for Apollo 11 (10084) and Apollo 16 (68810) lunar regolith samples across a wider range of viewing angles than has previously been measured. These samples were characterized in terms of their surface roughness and porosity profiles, and this enabled two of the free parameters within the Hapke BRDF model to be constrained. By fitting the model to the dataset, Hapke parameters could be deduced for the two representative (mare and highlands) regolith samples, and further constraints could be placed on the ‘practical’ size-scale of the model’s slope angle parameter. Thus, the dataset enabled Diviner’s visible-wavelength off-nadir data to be interpreted in a novel way, due to the reduction of free terms within the model. This led to surface roughness and compositional deductions (via the Hapke parameters h_s, b and θ ̅) for seven Diviner targets. Finally, the dataset was used to set more realistic scattering functions within the Oxford 3D Thermal Model, and it was demonstrated that this 1) could affect modelled high-latitude lunar surface temperature profiles by up to ~30 K—as compared to using previously assumed scattering functions—and 2) could increase the minimum depth at which water ice is predicted to be stable in the lunar subsurface by up to ~0.8 m. Hence, this dataset may help to constrain the possible distribution of water ice on the lunar surface, and this may be crucial for future lunar exploration missions such as Luna-27 and Artemis.