Using quantitative topographic analysis to understand the role of water on transport and deposition processes on crater walls
The amount of water runoff need to evolve landscapes is rarely assessed. Empirical studies correlate erosion rate to runoff or mean annual precipitation, but rarely is the full history of a landscape known such that it is possible to assess how much water was required to produce it. While this may not seem to be of primary importance on Earth where water is commonly plentiful, on Mars the amount of water to drive landscape evolution is a key question. Here we tackle this question through a series of five chapters, one devoted to field work at Meteor Crater, another to laboratory experiments about controlling processes, and then two chapters on analysis of landforms and implications of water runoff on Mars (associated with the Mars Science Laboratory mission to Gale Crater), and then we complete this effort with a consideration of how we can reliably assign relative timing between events resulting in small depositional features. What follows below is a summary of what is found in each chapter.
Meteor Crater, a 4.5 km2 impact crater that formed ~50,000 years ago in northern Arizona, has prominent gully features on its steep walls that appear similar to some gullies found on Mars. At the crater bottom, there are over 30 meters of lake sediments from a lake that disappeared ~10,000 to 11,000 years ago, indicating the transition from the Pleistocene to the current, drier climate. A combination of fieldwork, cosmogenic dating, and topographic analysis of LiDAR data show that debris flows, not seepage erosion and fluvial processes as previously suggested in the literature, drove gully incision during their formation period of ~40,000 years before the onset of the Holocene. Runoff from bare bedrock source areas high on the crater wall cut into lower debris mantled slopes, where the runoff bulked up and transformed into debris flows that carried boulders down to ~5 to 8 degree slopes, leaving distinct boulder lined levees and lobate tongues of terminal debris deposits that crisscrossed on the lower slopes. We hypothesize that the fine material, likely generated in the impact, and deposited with the coarse debris on the lower portion of the crater wall, is key to this bulking up process as flows cut across the deposits. Fluvial processes following the debris flow gullies extended alluvial deposits to the crater floor and contributed to lake infilling. Cosmogenic dating confirms that most of the modification of the crater walls occurred before the early Holocene. To account for the 75 distinct deposits currently lying on the crater floor, debris flow frequency would be about 1 event every 17 years, assuming debris flow activity terminated ~10,000 years ago. Assuming a water-to-rock ratio of 0.2 at the time of transport, it would have taken ~100,000 m3 of water to transport the ~500,000 m3 of debris flow deposits on the crater floor. Given the 4.5 km2 size of the crater, this extensive erosion would require less than 0.02 m of total runoff, or the equivalent of just 0.001 mm/year over a 40,000 year period. This insignificant amount of water was likely packaged into short-lived storm or snow-melt events when debris flows were generated. Much more runoff did occur, as evidenced by the lake and fluvial deposits, as well as the likely cool, wet conditions of the late Pleistocene. This suggests only a small fraction of the total runoff is needed to do considerable geomorphic evolution, producing strongly gully-scared crater walls. Currently, only minor fluvial modification of the gully networks occurs.
To test the hypothesis that fine-grained material is a necessary component behind sustained mobilization in granular debris flows, we performed a series of experiments in a 4 m diameter vertically rotating drum. Specifically, we assessed the role of fines and coarse grain size distribution on the rate of pore fluid pressure generation and its dissipation upon cessation of movement of the granular mass. We varied the amount of fines, from no fines to amounts found in debris flow deposits, and varied the coarse grain size distribution, from a single grain size (10 mm) to a range found in a natural flow deposit. The mixtures with fines contents close to those found in actual debris flows had elevated pore pressures, indicating they were almost fully liquefied, though the highest excess pore pressures were generated with the combination of fine material and a wide coarse grain size distribution. A boulder was also placed upon the fines-rich mixtures just after cessation of motion and it was observed that the pore fluid pressure rose instantly, bearing most of the grain's weight, and then the pressure slowly declined. When the same boulder was placed on the water-gravel flows, there was no change in fluid pressure, as the mass was supported entirely by grain-grain contacts. Our observations, combined with observations of others in the literature, inform the conceptual model presented herein where we hypothesize that the dilational separation of particles during debris flow shear leads to coarse particles (in this case particles >12 mm) slowly settling through the highly viscous non-Newtonian fluid. As the fines-rich mixture has a yield strength of ~5 Pa, we calculate that particles ~12 mm and smaller remain suspended. This combined effect requires the fluid to sustain the weight of the particles, leading to pore pressures equal to the entire weight of the solid and fluid mass.
As part of the MSL science team, we focused on understanding the geomorphology and its implications for the hydrologic history of the landing site for the Curiosity rover, which was located at the distal end of the Peace Vallis fan in Gale Crater. The Peace Vallis fan covers 80 km2 and is fed by a 730 km2 catchment. Valley incision into accumulated debris delivered sediment through a relatively low-density valley network to a mainstem channel to the fan. An estimated total fan volume of 0.9 km3 matches the calculated volume of removal due to valley incision (0.8 km3). The fan profile is weakly concave up with a mean slope of 1.5% for the lower portion. Runoff (discharge/watershed area) to produce the fan is estimated to be more than 600 m, perhaps as much as 6000 m, indicating a hydrologic cycle that likely lasted at least 1000's of years. Atmospheric precipitation, possibly snow, not groundwater seepage produced the runoff. Based on topographic data, Peace Vallis fan likely onlapped the rise onto which Curiosity landed, Bradbury Rise, and spilled into a topographic low to the east of the rise. This argues that the light-toned fractured terrain within this topographic low corresponds to the distal deposits of Peace Vallis fan and in such a setting lacustrine deposits were predicted (and later confirmed by observations from the rover).
Previous studies had suggested that there might have been large lakes in Gale. This history is important to understanding Mars' climate history and the volumes of water that may have passed through or covered the sediments encountered by Curiosity. Here we use improved imagery, topography, and ground-based observations from the Curiosity rover to map deltas, fans and gullies, which leads to a proposed history of lakes in Gale. We report evidence for at least three distinct large lakes within Gale, all which occurred after the crater's central mound, Aeolis Mons, reached close to its current topographic form. From corresponding deltaic deposits off the southern rim of Gale crater and the southern flank of Aeolis Mons, we identified the highest lake level at -3280 meters, which would have had a mean depth of 700 meters. The larger context provided by craters near Gale suggests that this lake was derived from regionally sourced water from the south. The next lake level, which was established after a period of drying with subsequent lake level decline before rewetting and lake level rise, is defined by four deltaic features as well as the termination of gullies around the northern rim of Gale. This second lake corresponds to an elevation of -3980 m, has a mean depth of 300 meters, and was likely sourced from water more local to Gale. Lake levels then rose to the third lake level, at -3780 m, with a mean depth of 400 meters. Two deltaic deposits derived from sediment from the rim of Gale define this lake. This last lake declined sufficiently quickly to prevent incision into any of the deltaic forms around the crater, though hydrologic activity likely continued as evidenced by a time of fan building, including the Peace Vallis fan, around the crater. Quantification of the fan and delta volumes and their gully sources on the crater wall and rim, and on Aeolis Mons, suggest that these discrete deposits contain essentially all of the sediment mobilized from the upslope source areas. Importantly for the MSL mission, this suggests that most of the sediments the rover has and will encounter experienced at least a few cycles of drying and wetting.
The depositional features (i.e. alluvial fans and deltas) within Gale Crater are small but they likely record a time history of lakes and other depositional events within the crater. An obvious question to ask is: can crater counting, which has been used to date much of Mars' surface, be used on such small surfaces to provide accurate age dating? Here we introduce a simple simulation model to quantify the effects of sample area size and crater obliteration effects on age estimates derived from crater size-frequency distributions. Our results show, quantitatively, that crater counting to estimate age of small surfaces (<1000 km2) has unavoidable large uncertainties. This arises because the small craters, which are most numerous, are eliminated due to erosion and infilling and there is a low probability of counting large craters on small sample areas. These effects on small and large craters leads to a narrow range of crater sizes in which the correct age may be reflected in the crater density functions that are used to date surfaces. This range decreases with decreasing surface area and increasing erosion and infilling. Steps in the isochron data, in which crater density data are shifted downward for some crater sizes to a different density size relationship, are often cited as indicators of resurfacing events. We find, however, that such steps occurred randomly in ~5% of the crater size-frequency distributions we generated. This can lead to large errors when determining which tangential isochron is used to assign a surface age. Our modeling, which accounts for obliteration of craters and a reduced chance of encountering large craters on small and young surfaces, suggests that in general the least reliable ages occur between 1 and 3 Ga years. Younger surfaces preserve the smaller craters and older surfaces ones collect larger ones, both improving the probability of obtaining the correct age. For areas less than 1000 km2 and true ages of 1 to 3 Ga, there is only a 20 to 40% probability of the calculated age lying within 0.25 Ga of the true value, where as for surfaces smaller than 500 km2 (most mapped Mars fans are smaller) the corresponding probabilities drop to about 10 to 20%. To highlight the constraints on dating small deposits, we applied this model to cratering data from the Peace Vallis fan (~80 km2) and one of the larger deltaic deposits within Gale crater (117 km2), which deposited at different elevations and likely record different hydrologic events. Even though portions of the observed crater count data in each case follow the proposed crater density isochrons, the uncertainties for such small areas are so large as to make the age assignment unreliable.