Gullies on Mars and seasonal ices visualised using the Colour and Stereo Surface Imaging System (CaSSIS)

Young gullies on Mars were first reported by Malin and Edgett in 2000 and were hailed as evidence of recent liquid water flows on Mars. Since that time, monitoring of gullies has revealed they are active today at times of year when the martian surface is at its coldest and when carbon dioxide ice is condensed on to the surface. In order to further explore the relationship between surface frosts and gully-activity we focus on Sisyphi Cavi near the south pole of Mars, where gully-activity has already been studied and CaSSIS obtained a dense temporal coverage in 2018. We identified the following sequence of events:

1) In winter frost covers all surfaces and dark spots and flows can be seen across the slopes with gullies and preferentially around the gully channels. This is consistent with previous observations and has been interpreted to be the surface expression of gas-jets generated by the sublimation of CO₂ underneath a continuous slab of CO₂ ice on the surface. The jets occur when the pressure fractures the slab ice and the pressurized gas can escape with entrained particles.

2) As the surface temperature increases towards 200 K, the top of the slopes are the first to defrost followed by sun facing parts of the alcoves and channels.

3) As the surface temperature approaches and exceeds 250 K and the surrounding terrain is completely defrosted, the last parts of the gully to remain frost covered are the fans. We interpret this to be a result of the fans having slightly lower thermal inertia than the surrounding materials. This lower thermal inertia could be because the fans have a lower content of water ice (i.e. a thicker lag on top of the ice-table), because of recent depositional events. It is at this time of year when gullies are most active. Hence, we infer that gully activity increases when there is both frosted and defrosted surfaces available to drive vigorous sublimation of the CO₂ ice.

4) Finally, once defrosting has almost fully completed and surface temperatures have reached their seasonal maximum of ~270 K the only remaining surface frosts are in pole-facing niches at the base of gully-alcoves.

Our study has underlined that the colour capability of the CaSSIS instrument is particularly suited to studying and monitoring changes in surface ices. Our observations reveal that gully-alcoves defrost before the fans and gullies defrost later than surrounding terrain – suggesting activity is driven by the availability of “hot” sediment to trigger more efficient sublimation. Further work will examine whether surface frost patterns differ between gullies that have been shown to be active and inactive since spacecraft observations began.