Radiant Energy Budgets and Internal Heat of Planets and Moons

Abstract

Knowledge of the radiant energy budgets and internal heat of planets and moons is of wide interest in the planetary science community. Some progress has been achieved with recent studies, but there are still significant limitations in current observations and studies. We recommend future exploration to better understand the radiant energy budgets and internal heat of planets and moons in our solar system. 1. Big picture and significance As a fundamental parameter of planets and moons, the radiant energy budget is determined by the absorbed solar energy and the emitted thermal energy (1, 2). Such an energy budget plays an important role in determining the thermal structures of planets and moons (3-6). It can help us understand the geology (e.g., polar ices of Mars) (7), internal heat related to the formation and evolution of giant planets [8-10], and sub-surface internal heat driving the jet plumes on some moons (11-15). For bodies with atmospheres, the radiant energy budgets at the top of atmospheres also set critical boundary conditions for the atmospheric systems (3). The transfer and distribution of radiant energies within the atmospheric systems modify the thermal structure to generate available potential energy. The available potential energy can be converted into kinetic energy to drive atmospheric circulation and the related weather and climate (3-6, 16, 17). Unfortunately, the global radiant energy budget has not been well determined for most of planets and moons (4-6) in our solar system, mainly because the observations of the radiant energies are limited (1, 2, 18, 19). With the advance of space exploration, we expect to get a much better picture of the global radiant energy budget and internal heat for the planets and moons in our solar system. 2. Methodology and observations To determine the radiant energy budgets and hence the internal heat of planets and moons, we have to measure two radiant energies – the absorbed solar energy and the emitted thermal energy. The emitted thermal energy of planets and moons in our solar system is concentrated in the infrared wavelengths, which can be measured by an instrument in the wavelength range 5-400 microns. On the other hand, the solar energy from the Sun is mainly concentrated in the ultraviolet, visible and near-infrared wavelengths (0-5 microns). Generally, we measure the reflected solar energy and then compute the absorbed solar energy. The precise measurements of the emitted thermal energy and the reflected solar energy require accurate observations with complete coverage of wavelength and viewing angles (e.g., emission angle and phase angle). The basic methodology of computing the radiant energies is to integrate the radiance over wavelength and viewing angle, which is described in detail in our previous studies (18, 19). The difference between the emitted thermal energy and the absorbed solar energy is generally used to estimate the internal heat of planets and moons