Post by Dr. Norbert Schorghofer. Senior Scientist, Planetary Science Institute
Recurring slope lineae (RSL) are dark narrow streaks on Mars that have puzzled scientists since their discovery in 2011. Image 1 shows a 3-dimensional perspective of a landscape with some of these flow features. RSL form and grow annually and mostly in the warm season, so the mechanism by which they form and grow is tantalizing. To what extent are RSL related to temperature or water? In rugged terrain there are stark temperature contrasts between pole‐facing and equator‐facing slopes that infrared cameras on Mars-orbiting spacecraft cannot spatially resolve. New modeling capabilities make it possible to overcome this limitation and provide surface temperatures at high spatial resolution.
Image 1: An image combining orbital imagery with 3-D modeling shows RSL on a slope inside Newton Crater. Image credit: NASA/JPL-Caltech/University of Arizona.
The new thermal model can calculate temperatures for topographic domains consisting of millions of pixels, and it includes the effect of horizons (also known as “terrain shadowing”). The model computes temperatures, the extent of shadows, and frost accumulation as the sun moves through the Martian sky day after day and year after year. Image 2 shows two variables from the model output: the mean surface temperature and the duration of continuous water frost accumulation. The longer the duration of continuous frost accumulation, the more frost is expected. Equator-facing slopes accumulate no frost, while pole-facing slopes continuously accumulate frost over hundreds of sols. (On Mars, one solar day or “sol” is 24 hours and 40 minutes long, and a Mars year has 669 sols). It turns out RSL are unrelated to CO2 frost, subsurface water ice, or seasonal water frost. Hence, RSL are likely dry features.
RSL are morphologically similar to so-called “slope streaks”, but they occur in different regions of Mars and are smaller and shorter-lived than slope streaks. Nevertheless, their morphological similarities suggest the two features may be somehow related.
Image 2: A section at the southeast side of Palikir Crater (42°S), where many RSL are found. North is up and toward the equator. (a) A portion of High Resolution Imaging Science Experiment Image ESP_022834_1380. (b) Model result for the mean annual surface temperature at a resolution of 1 meter per pixel. (c) Model result for the longest period of uninterrupted H2O accumulation. The dots are the start (top) locations of recurring slope lineae. Contours are elevation spaced by a vertical distance of 5 m. Some of the densely spaced contours are due to artefacts in the digital elevation map.
Further Reading
McEwen, A. S., Ojha, L., Dundas, et al. (2011). Seasonal flows on warm Martian slopes. Science 333, 740-743. doi:10.1126/science.1204816
McEwen, A. S. (2018). Are recurring slope lineae habitable? In: “From Habitability to Life on Mars” (pp. 249-274). Elsevier. doi:10.1016/B978-0-12-809935-3.00008-6
Schorghofer, N., J.S. Levy, T.A. Goudge (2019). High-resolution thermal environment of recurring slope lineae in Palikir Crater, Mars, and its implications for volatiles. Journal of Geophysical Research: Planets 124, in press. https://doi.org/10.1029/2019JE006083
Schorghofer, N. (2014). Slope Streak (Mars). In: “Encyclopedia of Planetary Landforms”, eds. Hargitai, H., Kereszturi, Á., 8pp. Springer. doi:10.1007/978-1-4614-9213-9_344-1
Stillman, D. E., & Grimm, R. E. (2018). Two pulses of seasonal activity in martian southern mid-latitude recurring slope lineae (RSL). Icarus 302, 126-133. doi:10.1016/j.icarus.2017.10.026
Tebolt, M., Levy, J., Goudge, T., Schorghofer, N. (2019). Slope, elevation, and thermal inertia trends of martian recurring slope lineae initiation and termination points: Multiple possible processes occurring on coarse, sandy slopes. Icarus 338, in press. doi:10.1016/j.icarus.2019.113536
Planetary Geomorphology Image of the Month 