Planetary Stratigraphy and Sedimentology
Located north of Mars’ hemispheric dichotomy, Aeolis Dorsa is home to some of the most extensive preserved fluvial deposits on the surface of Mars. They come in the form of topographically-inverted ridges, and are thought to be over 3.5 billion years old.
We examined a particular set of stacked branching fluvial networks in the southeast Aeolis Dorsa region. We analyzed superposition relationships and stratigraphic architecture to determine that these branching networks were the preserved remnants of a long-lived river delta system. These results, along with some others nearby, strongly indicate the presence of a large and long-lived standing body of water in the northern lowlands. This is exciting because some hypothesize that the north lowlands on Mars could have been the home to an ocean!
Hughes, C. M., Cardenas, B. T., Goudge, T. A., & Mohrig, D. (2019). Deltaic deposits indicative of a paleo-coastline at Aeolis Dorsa, Mars. Icarus, 317, 442-453.
During my PhD research, I have taken another look at these rocks. Using a relationship established for terrestrial deltas by Fernandes et al. (2016), where normalized channel belt widths decrease with proximity to the coast (see below) and reaches a characteristic width range from ~2-5 at one backwater length. Using this relationship we have been able to estimate the distance upstream that these rivers started to ‘feel’ the receiving basin at their terminus.
Part of the reason it is so exciting to have an estimate for backwater length, is that on Mars, estimating grainsize for these river systems is very challenging. Unfortunately, if we want to make any calculations that would allow us to estimate the longevity of these systems, or really anything about their paleo-hydrology, we need a grainsize estimate. Fortunately, from an estimated backwater length, we can get to an estimate for grainsize! So, these new results provide an entirely new method for backing our way into grainsize, which will avail new and refined estimates for how much water was flowing through this system billions of years ago on Mars.
Planetary Spectroscopy and Remote Sensing
Eberswalde crater is home to one of the most pristinely preserved fluvial deposits on the surface of Mars. The deposit, widely thought to be another ancient deltaic system, has been a perennial contender for rover mission locations during the planning stages.
We took a close look at the minerals in Eberswalde crater, and in the area around it. We found that there was an abundance of clay minerals (Saponite and Nontronite) that form when water interacts with basaltic minerals. These ‘aqueous alteration minerals’ were found in the large plateau to the west, in the drainage basin that leads to the delta, and in the delta itself. Where they outcrop in the plateau, they look like they may have formed from processes similar to how soil forms on Earth. These clay minerals are also potential hosts for biomarkers of past life! Therefore, with sediment containing these minerals being concentrated in the delta, it is possible that there may be evidence of past life in a small, rover-accessible area, in Eberswalde crater!
Terrestrial analogues for planetary science
One of the most difficult challenges planetary scientists face is that most of our data come from orbital spacecraft and that the rocks we analyse are very far away. However, planet Earth provides us with an abundance of analogous processes and outcrops by which to compare, and if we can develop a deep understanding of the great detail on Earth then we can expect to gain a more informed understanding of what we observe from orbit on other planets’ surfaces.
Cardenas, B. T., Mohrig, D. C., Goudge, T. A., Hughes, C. M., Levy, J., Swanson, T., & Mason, J. (2018, December). Anatomy of exhumed river channel-belts. In AGU Fall Meeting Abstracts.