Recent Surface Meltwater and Glacial Assemblage in Eastern Hellas Basin, Mars.

      Introduction:

Stable liquid water existed on the surface of Mars prior to ~3.0 Ga [1]. After this period, surface pressure and temperature conditions decreased, leading to the end of stable liquid water conditions at the surface. However, the presence of features such as Fresh Shallow Valleys [2], meltwater system [3,4], as well as channels located near volcanic regions and/or craters [5,6], suggest that liquid water may have flowed during the Amazonian period of Mars history, even if at a generally smaller scale.

The sources required for such activity are hypothesized to be either volcanism, which may have temporarily created favorable atmospheric conditions [7], or the presence of subsurface or surface ice masses [3]. In the latter case, the process involves an ice mass exerting enough pressure so that, at its base, ice can melt and generate liquid water. This water can then flow and form channels under the ice. On Mars, subglacial channels have already been but often limited in extent. We aim to study channels that are hundreds of kilometers long, Amazonian in age, flowing from Est to West.

The study area is located east of the Hellas basin (Figure 1), near Reull Vallis, in Promethei Terra. This area is known to have been subject to different periods of ice accumulation and volcanism [9]. 

      Methods:

We used the Context Camera (CTX) MurrayLab mosaic (6 m/pixel) [10] to map the different structures and assess the regional context of the study area. We also used images and elevation data from the High-Resolution Imaging Science Experiment (HiRISE) (25 cm/pixel) [11], together with images from the Colour and Stereo Surface Imaging System (CaSSIS) (4–5 m/pixel) [12], to characterize points of interest. 

We used crater size-frequency distribution analysis, with the CraterStats software, to derive an absolute chronology. Craters were manually digitized on selected surfaces (mesas and basement) using ArcGIS software. Ages were estimated following the Hartmann & Daubar (2017) [13] chronology system, applying differential, cumulative, and Poisson fitting methods [13]. 

 

Fig. 1. Geomorphologic map of the study area. 

      Observations:

We mapped channels (Figure 1) in our region over several hundred kilometers flowing consistently toward the center of the Hellas basin (value ~0.1%). The valleys are sub-parallel and evolve from an upstream convergent drainage pattern, where a limited number of tributaries feed into a main channel, toward a more divergent downstream configuration, locally transitioning into braided channels. Other landforms have been identified (Figure 1), including the presence of crest/ridge oriented roughly north–south and extending over several hundred kilometers, although they are locally truncated.

Crater counting indicates ages of 3.6 Ga for the mesas and 3.4 Ga for the basal unit. Additionally, this dating highlights a possible resurfacing event at 900 Ma. Based on relative chronology, there are two distinct valley formation events. The absolute chronology suggests that the studied landforms likely formed during these two periods, corresponding to the Late Hesperian and the Late Amazonian.

The selected case study (Figure 2) highlights a channel that crosses two major ridges, each approximately 300 to 600 m wide and predominantly oriented north–south. After incising the second ridge, the channel terminates in a fan-shaped deposit extending over 4 km downstream and reaching a maximum width of 1.4 km. A longitudinal profile indicates that the fan deposits is present in a lower gradient area. No channel is observed cutting across this fan. The main valley is approximately 5 to 30 m deep, reaching its greatest depth where it intersects the second ridge, and is between 150 and 500 m wide. It is bordered by lateral channels that form a sub-parallel network on the valley margins. These lateral channels lack tributaries and, according to profile B–B’ in Figure 1, are perched 7 to 15 m above the floor of the main valley. They are generally less than 5 m deep, although they can locally reach up to 10 m where they intersect the ridges, and are around 200 m wide. They follow the same overall orientation as the main valley and join it at oblique angles. However, they do not appear to follow the local topographic gradient. Profile A–A’ further shows that some of these channels clearly cut across ridge crests, directly modifying the pre-existing relief.

 

Fig. 2. Fan-shaped deposit associated with potential moraines and lateral meltwater channels. 

    Discussion:

The observations present a large range of landforms that are commonly associated with glacial settings and may be indicative of an ice-margin environment [14]. Matter of fact, moraines and subglacial channels followed downstream by a fan deposit form a typical glacial marginal assemblage. In the glacial context, such fans are interpreted as ice marginal fans and correspond to fan-shaped sedimentary deposits formed by meltwater flows originating from under an ice mass, which transport and deposit sediments as flow velocity and pressure decrease upon reaching the ice margin [15]. Ridges are interpreted as possible moraines, based on their orientation, which is broadly parallel to one another and perpendicular to regional slope (hence inferred ice flow direction), as well as on their asymmetric cross-sectional profiles [16].

Finally, we interpreted the perched valleys as lateral meltwater channels. They form along ice margins and are thought to be primarily fed by supraglacial meltwater, mapping the location of former ice margins.

Overall, these observations support the hypothesis of a former subglacial, ice marginal and proglacial environment involving liquid water flow on the east of the Hellas Basin. The morphological and morphometric characteristics appear to match the definition of FSVs, making this one of the most extensive FSV systems identified on Mars. Based on cross-cutting relationships with edifices that have been estimated to be Amazonian in age [9,16,17], as well as crater counting, we suggest that liquid water may have been present on Mars around ~1 Gyr. To explain the occurrence of meltwater, we suggest that regional volcanic activity, particularly upstream of the study area, may have triggered basal melting beneath an ice mass. This is the first time that a large-scale, warm based Amazonian ice sheet marginal environment is described on Mars.