The Radiant Energy Budgets of Titan and Mars
Abstract
The radiant energy budget of planets and moons is of wide interest in the fields of geoscience and planetary science, as it is essential to understanding surface and atmospheric processes. In this work, we report the seasonal variations of the energy budget for both Titan and Mars and examine the surface emissivity of high latitude regions on Mars.
Based on the Cassini multi-instrument observations, we find the global‐average emitted power decreased by 6.8 ± 0.4%, while the absorbed solar power decreased 18.7% ± 0.5% during the Cassini period (2004–2017). We find Titan's radiant energy budget is not balanced from 2004 to 2017, with the absorbed solar energy being (1.208 ± 0.008) x 10^23 J and the emitted thermal energy being (1.174 ± 0.005) x 10^23 J. There is an energy imbalance of 2.9 ± 0.8% of the emitted thermal energy. Titan's global radiant energy budget is not balanced at the timescales of Earth's years and Titan's seasons, and the imbalance can be beyond 10% of the emitted thermal energy at the timescale of an Earth year.
Based on the observations from Mars Global Surveyor, Curiosity, and InSight, we find Mars’ global-average emitted power is 111.7 ± 2.4 Wm-2. There are strong seasonal variations in the emitted energy, and energy imbalances at the time scale of Mars’ seasons (e.g., ∼15.3% of the emitted power in the Northern autumn for the Southern Hemisphere), which could play a role in generating dust storms on Mars. We find the 2001 global dust storm decreased the global-average emitted power during daytime but increased the global-average emitted power at nighttime. This suggests that global dust storms play a significant role in modifying Mars’ radiant energy budget.
Using data from the Mars Climate Sounder, we compute a solid-angle integrated emissivity and relative emissivity for the Southern and Northern high latitude regions, respectively, with 5 degree resolution in latitude and 10 degree resolution in longitude. We find the solid-angle integrated emissivity is typically a few percent less than the emissivity at nadir, with no strong correlation to known surface features (e.g., cryptic vs. anti-cryptic terrain).