Quantification of ice blockfall activity at a north polar scarp on Mars

Post contributed by Ernst Hauber and Lida Fanara, Institute of Planetary Research, German Aerospace Center (DLR), Berlin, Germany.

Mars is an active planet, and several processes are currently shaping its surface. Among those, gravity-driven mass wasting produces surface changes that can be quantified in image data acquired before and after discrete events. As such changes are typically small in their spatial dimensions, the prime dataset to recognize them are pairs of HiRISE images (High Resolution Imaging Science Experiment; McEwen et al., 2007), with scales of ~25-50 cm/px. The manual identification of surface changes in these huge images (a single HiRISE image can have a size of several Gigabytes) is challenging, however, and requires significant efforts. In order to circumvent this massive demand on human resources while yet taking advantage of all images, automated methods need to be developed. Here we show an example of such methods which was applied to ice block falls at a steep cliff in Mars` north polar region.

PGIM.Figure_01

Image 1: Block fall at a scarp on the north polar region of Mars near 83.796°N and 236.088°E. (a) «before» image acquired at 06 May 2014 (HiRISE image ESP_036453_2640). (b) «after» image acquired at 25 December 2019, showing a cluster of blocks that was displaced from the scarp to the right (east) (ESP_062866_2640). North is up in both images, scale bar is 100 m.

The kilometres-thick north polar ice dome (Planum Boreum) on Mars consists of dusty water ice and holds a record of the climatic changes on the planet (e.g., Byrne, 2009). Steep fractured scarps are prominent along the margins of Planum Boreum and can reach heights of 700 m and slope angles of up to 70°. They expose two geological units of Planum Boreum, the North Polar Layered Deposits (NPLD) and the sandier Basal Unit (BU). The CO2 ice that accumulates during winter at the north polar region sublimates first at the steep scarps due to their topography during spring. HiRISE images of these scarps revealed ongoing mass wasting activity, such as avalanches and ice block falls (Russell et al., 2008, 2012). In fact, some of these scarps are currently the most active places on Mars (Image 1). It has been suggested that the north polar steep scarps lose the dust lag from the previous summer due to avalanches in the spring which makes them vulnerable to the large daily temperature variations in the summer, leading to fracturing and block falls (Byrne et al., 2017)

Quantifying these block fall events over space and time is key to the study of the scarps’ dynamic evolution, however, the manual identification of newly emplaced, small, meter-sized blocks among the vast volume of available images, with each image covering tens of kilometres of scarp lengths, presents a challenging task. Recently, Fanara et al. (2020a) developed a change detection method for the identification of ice block falls that is based on a Support Vector Machine (SVM), trained using Histograms of Oriented Gradients (HOG), and blob detection (Image 2). The SVM detects potential new blocks between a set of images; the blob detection, then, confirms the identification of a block inside the area indicated by the SVM and derives the shape of the block. The results from the automatic analysis were compared with block statistics from visual inspection. The authors tested their method in six areas each consisting of 1000 × 1000 pixels, where several hundreds of blocks were identified. The results for the given test areas produced a true positive rate of ~75% for blocks with sizes larger than 0.5 m2 (i.e., approx. 3 times the available ground pixel size) and a false discovery rate of ~8.5%. Using blob detection, the study team was also able to recover the size of each block within 3 pixels of their actual size.

PGIM.Figure_02

Image 2: Detection of surface changes due to blockfall at polar scarps. (a–b) «before» (ESP_027750_2640) and «after» (ESP_036888_2640) images of a blockfall event in the Basal Unit with sources in black circles, c) block fall detection of the event outlined in white and source in black circles showing the formed (post-block fall) alcoves; (d–e) «before» and «after» images of an event in the NLD with sources in black circles showing how the cliff face retreated, f) block fall detection of the same event outlined in white and source in black circles (from Fanara et al., 2020b).

Based on these results, it was possible to estimate the erosion rate of a steep north polar scarp by automatically identifying newly apparent blocks throughout a time series of HiRISE images (Fanara et al., 2020b). The total depositional volume over three Mars years corresponds to a minimum erosion rate of ~0.3 m3 per Mars year per meter along the scarp, or a minimum average scarp retreat rate of ~0.2 m/kyr. This rate cannot balance published 0.01–1 m/yr viscous flow rates (Sori et al., 2016), implying that either lower rates of the latter occur or that other processes contribute more than block falls to the slopes’ steepness.

Further reading:

Byrne, S., 2009. The polar deposits of Mars. Annu. Rev. Earth Planet Sci. 37, 535–560.

Fanara, L., Gwinner, K., Hauber, E., Oberst, J., 2020a. Automated detection of block falls in the north polar region of Mars, Planetary and Space Science 180, 104733.

Fanara, L., Gwinner, K., Hauber, E., Oberst, J., 2020b. Present-day erosion rate of north polar scarps on Mars due to active mass wasting. Icarus 342, 113434.

McEwen, A.S., Eliason, E.M., Bergstrom, J.W., Bridges, N.T., Hansen, C.J., Delamere, W.A., Grant, J.A., Gulick, V.C., Herkenhoff, K.E., Keszthelyi, L., Kirk, R.L., Mellon, M.T., Squyres, S.W., Thomas, N., Weitz, C.M., 2007. Mars reconnaissance orbiter’s high resolution imaging science experiment (HiRISE). J. Geophys. Res. 112, E05S02.

Russell, P., Thomas, N., Byrne, S., Herkenhoff, K., Fishbaugh, K., Bridges, N., Okubo, C., Milazzo, M., Daubar, I., Hansen, C., McEwen, A., 2008. Seasonally active frost-dust avalanches on a north polar scarp of Mars captured by HiRISE. Geophys. Res. Lett. 35, L23204.

Russell, P.S., Byrne, S., Pathare, A., Herkenhoff, K.E., 2012. Active erosion and evolution of mars north polar scarps. Lunar and Planetary Science Conference 43, LPI Contribution No. 1659, abstract #2747.

Sori, M.M., Byrne, S., Hamilton, C.W., Landis, M.E., 2016. Viscous flow rates of icy topography on the north polar layered deposits of Mars. Geophys. Res. Lett. 43, 541–549.

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