The Global Radiant Energy Budgets of Titan and Mars

The radiant energy budget, which is determined by the emitted thermal energy and absorbed solar energy, is fundamental to understanding a planet or moon, as it has impacts to thermal structure, atmospheric circulation, and weather and climate patterns. Here, we present our work measuring the energy budgets of Titan and Mars. Titan is the only satellite in our solar system with a thick atmosphere, made of mostly nitrogen, as well as an active methane cycle that produces large permanent liquid bodies on the surface. Mars has many unique features that affect energy transport mechanisms, such as polar ice caps, large-scale dust storms, large orbital eccentricity (0.0935), and large obliquity (25.19º). Both of these terrestrial bodies have complex characteristics that create a very interesting energy budget picture.

To study seasonal variations of Titan’s global energy budget, we use long-term multi-instrument Cassini observations covering three Titan seasons (2004-2017). The Cassini observations provides an unprecedented opportunity to measure temporal variations of Titan’s radiant energy budget for the first time. We acquire data from Cassini from the following three instruments: Composite Infrared Spectrometer (CIRS), Visual and Infrared Mapping Spectrometer (VIMS), and Imaging Science Subsystem (ISS). The wavelength coverage is 7-1000 μm, 0.35-5.1 μm, and 0.26-1.0 μm for CIRS, VIMS, and ISS respectively. Averaged for the Cassini epoch, we found that global-average emitted energy is not balanced by the global-average absorbed solar energy. On the scale of Earth years, the imbalance is even larger. Such an energy imbalance has significant impacts to the weather and climate on Titan.

For Mars, we use observations from the Thermal Emission Spectrometer (TES) onboard the Mars Global Surveyor (MGS) from 1997 to 2005 to examine the emitted power. The bolometric thermal radiance spectrometer on TES has wavelength coverage from 5.3-100 μm. These observations span from Mars Year (MY) 24 to MY 28. This is the first step in our observational study of Mars’ global radiation budget. While studies have explored the radiant energy budget in the polar region, and utilized models to estimate Mars’ global energy budget, there have been no systematical, long-term, observational studies done to measure the spatio-temporal variations of the radiation budget. We found that the emitted power shows strong diurnal and seasonal variations. Additionally, we examine the impacts of dust storms on the emitted energy. Given the dynamic nature of the seasonal variations of Mars’ emitted energy, it is unlikely the seasonal variations in absorbed solar power will match the seasonal variations of emitted energy. Therefore, we can expect a radiant energy imbalance on the time scale of seasons, which probably plays a role in developing dust storms on Mars.