Slope streaks are intriguing features on the surface of Mars, but we are still not sure how they form. A recent paper in Reviews of Geophysics explores the proposed mechanisms for their formation and suggests a terrestrial analogue site in Bolivia which may improve our understanding of the processes on Mars. Here, the authors of the paper give an overview of what is known about slope streaks and their formation, and suggest where further research is needed.
What do slope streaks look like, and where and when do they occur on Mars?
Slope streaks are prevalent on the surface of Mars, but they come in a multitude of shapes and sizes.
What we know from observations is the following: Slope streaks range from about a few meters to several kilometers long. They usually have a starting point upslope with gradual widening towards the downslope termini, thus indicating the possible involvement of some flow or mass movement. They are capable of following very gentle slopes and are reportedly able to climb even a few meters of obstacles in their flow paths. Slope streaks can appear anytime of the year in the equatorial and subequatorial regions of Mars. They appear to be singular events formed within a short temporal span, and their recurrence, or lengthening, is extremely rarely observed. They gradually fade over decadal timescales.
How has our understanding of these landforms advanced over recent years?
The earliest sightings of slope streaks were made on some of the highest resolution Viking orbiter images during the 1970s. Since then, observational capabilities have immensely improved with the advent of higher resolution sensors, such as the Mars Orbiter Camera (MOC), High Resolution Stereo Camera (HRSC), Context (CTX) Camera, and High Resolution Imaging Science Experiment (HiRISE). The per-pixel resolutions of images have increased, from several hundreds of meters in the 1970s to several centimeters now, and we are able to study the morphology and topography of slope streaks in unprecedented detail.
In addition, recent developments in our understanding of the seasons, climate, terrain, and geochemistry of Mars, through various global datasets, have enabled us to correlate the presence of slope streaks in a particular region with the other prevalent landforms, atmospheric conditions, and geochemical conditions.
What are the different explanations for how they form?
Although we can now examine slope streaks in great detail, the exact mechanisms behind their formation and development are still unknown. Several hypotheses have been proposed based on their physicochemical and geomorphological properties, but the scientific community is divided between those proposing “wet” or “dry” mechanisms.
The proposed wet mechanisms suggest the possible involvement of liquid water in slope streak formation, perhaps relating to groundwater springs, melting surface frost or ice, brine formation and flow, or low‐albedo iron oxide precipitates of brines.
The dry mechanism hypotheses explain their formation by processes including: dust avalanching due to air fall deposits, subsurface melting, and localized disturbances caused by rockfalls, impact blasts, and quakes that eventually led to dry mass movements.
What are some of the unresolved questions where additional observations, data, or modeling are needed?
Both the proposed wet and dry mechanisms have limitations in explaining some observations of slope streaks.
The wet mechanism explanations are challenged by observations showing a lack of global consistency between slope streak occurrences and bedrock geology, the absence of significant seasonality in slope streak formations, the ability of slope streaks to climb over obstacles, and the requirement to overcome a slope threshold for their initiation.
Meanwhile, the main issue with dry mechanism theories is that observations show undisturbed topography alongside the majority of slope streaks and the absence of debris accumulation at the extremities of formations, which would surely be evident if some dry mass transfer took place in their formation. Also, dry mechanisms cannot fully explain the ability of the perceived dry mass movements to travel kilometers on gentle slopes under Martian gravity. There is also an absence of appropriate dry terrestrial analogues.
While available remote sensing data has vastly improved, as well as our knowledge of Martian mineralogy, climate, and atmosphere, we still need further investigations to advance our understanding. In this regard, targeting slope streak regions during future robotic or manned Mars missions would be advantageous.
How might a more holistic approach provide a better explanation for slope streaks?
Since neither wet nor dry mechanisms in isolation can account for all the observations, our review suggests developing models which consider both mechanisms having an influence on the start and formation phases of slope streaks.
While gathering more data from remote sensing images and Martian rover explorations would provide additional insights, identifying more wet and dry terrestrial analogues for Martian slope streaks, and studying them at high spatiotemporal resolutions, can greatly improve our understanding. For example, we have investigated a wet analogue site in Salar de Uynui, Bolivia, where seasonal brines form and flow under similar atmospheric and surface conditions as in equatorial regions of Mars. The results of this drone-based investigation are currently pending publication.
There’s still so much to discover and learn about slope streaks on Mars, and with the upcoming Mars missions equipped with more atmospheric sensors and remote sensing capabilities, it will be interesting to further explore these perplexing land features in the coming years.
—Anshuman Bhardwaj (email: [email protected]), Lydia Sam, F. Javier Martín-Torres, and María-Paz Zorzano, Luleå University of Technology, Sweden