Mars 1
Figure 1 : Close up look to the Thumbprint Terrain in Isidis Planitia.

No, it is not the fingerprint of a Martian Giant that fell on the ground of Mars, so what could it be?

Firstly, what are these fingerprints? Jeffrey Kargel was the first to describe these landforms in 1995 and called them the Thumbprint Terrain (figure 1). They are composed of cones and ridges that have a depression at their top and are arranged in a concentric pattern that looks similar to our fingerprints (figure 2). These landforms are located in different areas of the Northern Lowland on Mars. Isidis Planitia represents the largest area of Thumbprint Terrain and it extends over a region almost as big as Europe (i.e. impact crater basin of ~1200km in diameter).

Different theories exist to explain the establishment of such landforms, but the new high resolution data provided by NASA allow us to find relationships that help us understand a bit more about this phenomenon.

Past theories explained the formation of these cones and ridges by mud volcanism (Davis and Tanaka, 1995; Skinner and Mazzini, 2009) or processes related to explosive volcanism (Bridges et al, 2003; Ghent et al, 2012). Nevertheless, these theories lack information to explain the extreme organisation of the concentric pattern of the Thumbprint Terrain. Moreover, no mud flow is visible in the new high resolution images.

Mars 2
Figure 2 : Global view of the Isidis Planitia basin and mapping of the cones and ridges.

The Context camera and the HiRISE camera aboard the Mars Reconnaissance Orbiter (resolution of 6m/pixel and 0.2m/pixel respectively) allow us to discover the close relationship between the cones and ridges and allow us to discover new landforms that were not visible or not described in the past. For example, we can now see clearly sinuous simple ridges (no depression on the top) that are perpendicular to the concentric pattern and in some case link cones and/or ridges to each other (figure 3). Other interesting landforms are sinuous linear depressions that are either empty or filled by a simple ridge and form a braided network in some cases. These two features are both located at the periphery of the Isidis Basin.

Mars 3
Figure 3 : Zoom to the relationship between Thumbprint Terrain (black lines) and sinuous simple ridges (red lines).

Consequently, these new morphological features and their relationship with the Thumbprint Terrain as well as the concentric pattern suggest a glacial environment analogous to those seen on Earth. Indeed, in this kind of environment, an ice cap/glacier interacts with the subsurface creating morphological features such as moraines, eskers, tunnel valleys, etc. Eskers and tunnel valleys are formed by the accumulation or removal of sediments in a subglacial stream and are predominantly found at the termination of such ice cap/glacier where the thickness is minimum. In these locations they tend to follow the direction of the ice flow.

In this theory, the Thumbprint Terrain itself could be ribbed moraines (e.g. formed under an ice sheet) where the depression is explained by the sublimation of the ice core causing the summit to collapse. Logically, we can suggest that simple ridges and linear depressions are respectively eskers and tunnel valleys as their spatial organisation and relationship resemble a terrestrial glacial environment in many ways.

The tricky part is to explain the formation of an ice cap at the Martian equator, but this can be explained by the instability of Mars’ rotation axis that would yield precipitation at these lower latitudes. This is supported by Martian climate models that show snow precipitation for different inclinations on the North-West area of Isidis Planitia (figure 4): The location of the hypothesis glacial source.

Mars 4
Figure 4 : Martian climatic model showing snowfall for different inclinations of Mars. Isidis Planitia is in the square and knows snowfall on the North-West area according to model of Madeleine et al (2009).

Authored by: Tom Guidat, PhD. Student, TCD, Geography


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