Exploring the seasonal variability at Louth crater ice mound with orbital colour imaging

Introduction :

Martian H₂O, CO₂ and dust cycles are key processes in modern climate dynamics. The polar caps [1], as well as their icy outliers (that can take the form of convex-shaped ice mounds inside deep craters) are the current, major, exposed volatiles reservoirs. Unanswered questions remain about the mass balance of the caps. Concerning outliers, such as the ice domes of Louth and Korolev craters[2,3], the prevailing hypothesis suggests a build up by atmospheric deposition [4]. If accurate, these icy mounds could record past climate  conditions while still responding to present-day Martian climate change because of their southerly location [5].  They therefore offer unique opportunities to study volatiles dynamics, with different boundary conditions than those of the north or south caps.

Research question :

Our study takes advantage of the Exomars Trace Gas Orbiter – Colour and Stereo Surface Imaging System  (CaSSIS) [6] colour imaging capabilities to evaluate pluri-annual ice changes, especially water frost and dust coverage at Louth and Korolev craters ice mounds. Those changes can serve as a proxy of local dynamics, which can reveal important information about ice and dust cycles.

General methods:

CaSSIS is the high-resolution stereoscopic camera of ESA’s ExoMars Trace Gas Orbiter (TGO). It possesses 4 broad band filters across the visible range : BLU (497nm), PAN (677nm), RED (835nm) and NIR (940nm). The orbit of TGO being non-Sun-synchronous, it allows us to acquire images of the same region of interest at different seasons and local times, at 4.6 m/pixel with high a  signal-to-noise ratio, and therefore allows to monitor diurnal phenomena until 74°N. The first step of the study consists of a morphological and spectral analysis of 25 Louth crater images, that cover MY34 to MY37.

Visible reflectance features of water ice include an average high reflectance, relatively constant over the VIS range, as well as asymmetric overtone absorption bands at 800nm, 840nm, and 1030nm. In CaSSIS colors, the strongest signature of water ice is the reduction of the strong visible red slope of the dust. It has been demonstrated experimentally that CaSSIS color criteria can help differentiate  between different types of ice and dust mixings [7].

 

Observational results  : 

In accordance with previous studies, [5,8], Louth ice mound appears generally homogeneous, with little to no significant spatial variations in its global albedo and colour. This is not the case in two particular observations, taken on 15.02.2022, and on 28.03.2022, respectively. A dark pattern appeared and displayed changes in the eastern direction (in Fig.1, the red circle shows the area of change). 

Fig.1 : Two CaSSIS images (MY36_018836_078 and MY36_018951_106) of Louth crater, 

taken in early martian autumn. The two images are NIR/PAN/BLU composites. 

We searched for reports of similar events in the literature, and only found one study [9] reporting an ‘unexplained dark patterning evolution’ in MRO - CTX images. We found other occurrences of dark patterns in MRO - HiRISE [10], in 2010, 2022, and 2023. Fig. 2 displays two examples of observations, both with HiRISE and CTX, in 2022 and 2023.

Fig.2: Two occurrences of dark patterns in 2022 (A) and 2023 (B)

The CTX observation shown in Fig.2 A was taken 14 days before the CaSSIS image MY36_018836_078 shown in Fig 1. A notable  eastward progression can be distinguished.Each occurrence is different in shape, shows  strong seasonality (appears at the end of summer, evolves through the autumn equinox), as well as rapid evolution (~tens of Martian days). The dark patterns disappear during each wintertime, showing a white  mound each following spring. This indicates that the mechanism that produces these patterns is related to seasonal effects.

Preliminary spectral measurements of the ice in CaSSIS images show that the signal is largely dominated by the dust signature. We hypothesize that the dark marks could be caused by either deposition from the adjacent dune field, or local sublimation events.

What’s next ?  

While continuing on spectral analysis with CaSSIS, we would like to explore the NOMAD [11] dataset to search if outliers in atmospheric water vapor content are correlated with the dark patterns occurrences. A ‘cloud’ can be seen in the MY36_018951_106 observation displayed in Fig.1. We do not know if this is linked to the formation of the North Polar Hood, or to the event itself. Thirdly, we will make use of the Mars Climate Database [12] to explore the temperature and relative humidity fields around Louth. Lastly, we will perform a similar study on Korolev crater. It is less clear than in Louth, but some structures appearing in early autumn resemble the ones that have been described here.

Acknowledgments:

This work has been developed in the framework of the National Center for Competence in Research PlanetS funded by the Swiss National Science Foundation (SNSF).CaSSIS is a project of the University of Bern and funded through the Swiss Space Office via ESA's PRODEX programme. The instrument hardware development was also supported by the Italian Space Agency (ASI) (ASI-INAF agreement no. 2020-17-HH.0), INAF/Astronomical Observatory of Padova, and the Space Research Center (CBK) in Warsaw.

References:

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