The Distribution of Frosts on Mars: Links to Present‐Day Gully Activity

Abstract

Introduction 1.1. Gullies on Mars Gullies are predominantly found within latitudes of ∼30°-50° in either martian hemisphere (Malin & Edgett, 2001), with most gullies in this latitude range occurring on pole-facing slopes (e.g., Harrison et al., 2015). These gullies typically consist of a broad upslope alcove that feeds into a channel, leading to a depositional apron of debris downslope. Numerous mechanisms for gully formation have been suggested: release of liquid water/brine from shallow (

Gardening of the Martian Regolith by Diurnal CO2 Frost and the Formation of Slope Streaks

“…The spatial distribution of surface units at the CO 2 frost point where visible wavelength imagery is also available and without indications of calibration issues near dawn is shown in Figure 4. THEMIS surface temperatures acquired at sunrise are consistent with the presence of CO 2 frost at virtually all latitudes (black and white dots in Figure 4), confirming results presented by others with MCS (Piqueux et al, 2016) and THEMIS (Khuller, Christensen, Harrison, & Diniega, 2021) data. THEMIS images presenting surface temperature close to 𝐴𝐴 𝐴𝐴𝐶𝐶𝐶𝐶 2 cluster at the seasonal polar caps, at high latitudes in the North (up to 60°N) and middle latitudes in the South (down to 20°S), and the dusty terrains with low thermal inertia of Tharsis, Elysium, and Arabia Terra.…”

“…Despite more restrictive ice detection criteria than used in previous studies (Khuller, Christensen, Harrison, & Diniega, 2021;Piqueux et al, 2016), our approach shows a similar or higher sensitivity at visible wavelengths, given the lower latitudes of frost identifications. Yet, we still find 848 image pairs (out of 2,761, i.e., ∼30%) that do not show a signature of frost in visible wavelength imagery, while the ground is at CO 2 ice temperature.…”

“…This model of diurnal ice forming within the pores of the regolith is supported by the following two arguments: Low‐latitude CO 2 frost‐like surface temperatures have been identified at night in low thermal inertia terrains using Mars Climate Sounder (MCS; McCleese et al., 2007) data acquired in the thermal infrared at ∼32 μm (Piqueux et al., 2016) and Thermal Emission Imaging System (THEMIS; P. R. Christensen et al., 2004) data acquired at 12.57 μm (Khuller, Christensen, Harrison, & Diniega, 2021). At these wavelengths, the penetration depths D λ (m) (i.e., the distance traveled by photons in the regolith at a given wavelength λ (m)) are up to one order of magnitude longer than the typical regolith grain size where frost is observed (1–2 μm for atmospherically sedimented material found in these low thermal inertia regions; Lemmon et al., 2019; Presley & Christensen, 1997).…”

“…For this reason, these MCS and THEMIS observations show that temperatures conducive to diurnal CO 2 ice formation exist within a surface layer that is many regolith grains thick and not exclusively on top of it. Again, this reasoning only holds for very low thermal inertia regions associated with very small and loose regolith grains, and those happen to be the ones displaying overnight CO 2 ice‐like temperatures (Khuller, Christensen, Harrison, & Diniega, 2021; Piqueux et al., 2016); Wien's displacement law indicates that the peak infrared emission for ∼148 K surfaces, that is, the average CO 2 ice temperature on Mars (Leighton & Murray, 1966), is ∼18 μm.…”

Strong Seasonal and Regional Variations in the Evaporation Rate of Liquid Water on Mars

“…These conditions are the bare minimum for a liquid solution to be present on the surface of Mars. In the case of brine formation through melting water ice, surface frost of a near sub-surface water ice reservoir must be present (Byrne et al, 2009;Carrozzo et al, 2009;Khuller et al, 2021;Mellon et al, 2004;Schorghofer & Aharonson, 2005). Also as noted, the relative humidity is a determining factor for the case of deliquescence.…”