Near‐surface thermal gradients and their effects on mid‐infrared emission spectra of planetary surfaces

Abstract

We model the heat transfer by radiation and conduction in the top few millimeters of a planetary surface to determine the magnitude of near‐surface (≈100 μm) thermal gradients and their effects on mid‐infrared emission spectra for a number of planetary environments. The model is one‐dimensional and uses a finite difference scheme for ≈10‐μm layers. Calculations are performed for samples heated at the base and from above by sunlight. Our results indicate that near‐surface radiative cooling creates significant thermal gradients in the top few hundred microns of surfaces in which radiation is an important heat transfer mechanism. The effect is maximized in evacuated, underdense particulate media with sufficiently high temperatures. Near‐surface thermal gradients will be significant in fine‐grained particulate surfaces on the Moon (40–60 K/100 μm) and Mercury (≈80 K/100 μm), increasing spectral contrast and creating emission maxima in the transparent regions of the spectra. They will be of lesser importance on the surface of Mars, with a maximum value of around 5 K/100 μm in areas of low thermal inertia, and will be negligible on planets with more substantial atmospheres (<1 K/100 μm). We conclude that the effects that thermal gradients have on mid‐IR emission spectra are predictable and do not negate the utility of emission spectroscopy for remote determination of planetary surface composition.

Temperature-Dependent Near-Infrared Spectral Properties of Minerals, Meteorites, and Lunar Soil

“…Measurement of temperature effects on optical properties of powders is complicated by the fact that powders are highly insulating, which promotes the development of thermal gradients within the sample (Henderson and Jakosky 1994). Thermal gradients cause the temperature of the optical surface to differ from that of the sample interior or base where in situ temperature measurements might be made.…”

A Time-Dependent Model of Radiative and Conductive Thermal Energy Transport in Planetary Regoliths with Applications to the Moon and Mercury

“…Whereas continuously varying the physical characteristics of the medium is mathematically complex, creating layered models is not nearly so. Other modelers, such as Henderson and Jakosky (1994), have used layered models to approximate real planetary surfaces. They assumed that the top 2 mm of a regolith were not in radiative equilibrium and sat atop a slab which was in radiative equilibrium.…”

Particle Size Effects on Mid‐Infrared Spectra of Lunar Analog Minerals in a Simulated Lunar Environment

“…On Earth and Mars, heat transfer in the regolith is primarily accomplished through convection by air molecules in regolith pore spaces. On the Moon and other airless bodies in the solar system, there is no interstitial gas to facilitate heat transfer between grains of regolith, so heat is transferred through the less‐efficient processes of radiation and conduction between grain boundaries (Henderson & Jakosky, ; Logan & Hunt, ). As a result, while illuminated and heated by solar irradiation, the particles at the uppermost surface quickly lose that heat to space, but, moving deeper within the top several hundred microns of lunar regolith, the temperature of the particles increases.…”

“…Henderson and Jakosky () modeled the thermal gradient expected on the Moon and determined it to be up to 40 K/100 μm. Effects of this gradient on emission spectra have been modeled and measured in the lab in several additional studies (e.g., Donaldson Hanna, Wyatt, et al, ; Henderson & Jakosky, , ; Logan & Hunt, ; Millán et al, ) showing shifts in the CF and overall spectral contrast when compared to samples measured under terrestrial ambient conditions.…”