MAPPING EXPOSED DUSTY WATER ICE IN NORTHERN MARTIAN GULLIES

The detection of near-surface, mid-latitude water ice on Mars is key to the exploration of the planet’s surface and the potential presence of liquid water. Gullies are found in the mid-latitudes, and numerous processes have been proposed for their formation, ranging from CO₂ frost-triggered avalanches to melting of dusty water ice. These gullies are often associated with smooth mantles thought to have originated as dusty snow that was subsequently buried [1]. Within this mantle, using HiRISE images, [2] documented the presence of dusty (< ~1%) water ice being exposed by slumping in the form of light-toned patches within gullies. These exposures confirmed that the mantle is indeed composed of buried, dusty ice. Although only 15 such locations were documented in the southern hemisphere, to date, no global survey of water ice exposures within gullies has been conducted.

In this work, we map exposed dusty water ice in the form of light-toned material within gullies using HiRISE images in the northern mid-latitudes (29 - 65°N) and a global gully map [3]. We distinguish between exposed subsurface water ice and potential surface frosts by only looking at images between L_S 70 - 200, when surface temperatures are too warm for frosts to form.

While it is easier to identify water ice in HiRISE color images, most locations only have single-band data. Thus, water ice was identified as isolated, light-toned patches exposed within the mantle, unlike frosts that drape the surface. When available, images from different Mars Years were also examined to look for potential changes or new/past exposures.

We found water ice being exposed in 15 out of 500 (3%) northern gully sites. The lowest latitude of such a detection was 46.4°N. We also found 7 other potential ice locations that may appear lighter toned because of local lighting conditions rather than the presence of water ice. Overall, these sites fall between 35 - 61°N latitude.

A similar southern hemisphere gully survey is currently ongoing. We are also in the process of analyzing the three-point HiRISE spectra of locations where color data are available, to qualitatively assess the ice’s dust content [2].

References:

[1] Christensen, P. (2003). Nature, 422(6927), 45–48.

[2] Khuller, A., & Christensen, P. (2021). JGR: Planets, 126(2).

[3] Harrison, T. N. et al.. (2015). Icarus, 252, 236-254.