What have your fan markings told us about the conditions at “Potsdam”?
Today we have a guest post by Tim Michaels. Tim is a research scientist at the SETI Institute who studies how the weather and climate of other worlds affects their surface features.
Have you ever wondered what the Planet Four science team has been able to discover from the many fan measurements that you all provided at the Potsdam fan site? Read on! This is a small part of our new paper in press (Portyankina et al.) at the Planetary Science Journal.
As shown on the topographic map above, the Potsdam site (81.68 S, 66.3 E; the red dot) is located on a broad equator-facing slope at the edge of the South Polar Layered Deposits (or SPLD). The SPLD are a huge layered pile of dirty water ice, dust, sand, and carbon dioxide ice (or “dry ice”) near the south pole of Mars — the pile is kilometers high! They are thought to be the result of many thousands (perhaps even millions) of Mars-years of shifting climate cycles. See the 10 December 2020 blog post below for more info. The black arrows represent the overall fan directions marked by you at Potsdam (for Mars Years 29 and 30).
Based on the topography and some knowledge about how the Mars atmosphere behaves, we can come up with a hypothesis: The fans are pointed in the same directions as katabatic flows would probably be — that is, cold winds rushing down from the higher elevation portions of the SPLD to the south (that is, nearer the pole), twisting toward the left (in this case, toward the west) because of the Coriolis effect. Note that katabatic flows (on Mars and Earth) are often strongest at night.
So, does the state-of-the-art computer climate model (MRAMS; Mars Regional Atmospheric Modeling System) that we are using agree with our hypothesis above? How well does the model output match with the fan measurements that all of you provided?
In the plot below there 3 early-spring seasonal windows shown — the first, Ls 180, is at the beginning of southern hemisphere spring. For each season panel, downwind direction (the direction in degrees that the wind is blowing toward; 90 = E, 270 = west) is on the vertical axis and wind speed is on the other axis (in units of meters per second). Fan directions from your fan markings are shown as horizontal red lines, while the vertical red dashed lines indicate conservative estimates of wind speeds derived from your fan markings. MRAMS (computer model) winds at 5 m above ground level (AGL) are shown as black dots, and winds at 91 m AGL (about the height of the CO2 gas jets that probably create the dark fans) are shown as cyan dots. A single Mars-day (or sol) of MRAMS winds is shown for each season.
The plot shows that MRAMS wind directions at Potsdam agree with (or are quite close to) P4 wind directions when the MRAMS wind speed is highest. The MRAMS output also tells us that these winds blow at night, and that the winds that blow more toward the east (90) and south (180) are daytime winds. This *does* strongly support our hypothesis that the fans at Potsdam are directed by katabatic winds! An added bonus is that most MRAMS wind speeds matching the P4 directions also are stronger than the conservative wind speed estimates derived from P4 fan markings, as we would expect.
Is that all that is possible to understand about Potsdam using your fan markings? No! We would like to know at what season (Ls) the fans stop being active, which would help us better confirm how they form. There are also clearly big differences in fan directions from year to year that may give us clues to how Mars’ atmosphere works. Please help us answer these questions (and others) at Potsdam and at the other south polar fan sites by continuing to mark fans on actual high-resolution spacecraft imagery using this platform!
You have a catalog of seasonal fans and blotches… Now what?
Today, I wanted to share a bit of the analysis we’re working on for Planet Four. Taking the Planet Four fan and blotch catalog from Season 1 and 2 of the HiRISE monitoring campaign, we’re now looking at what the average/dominant wind directions, derived for your classifications is telling us about the Martian south polar surface winds.
I wanted to show an example of what the science team is doing this. Tim Michaels has joined the science team and he’s an expert on climate modeling. We’re using the MRAMS (Mars Regional Atmospheric Modeling System) climate model/computer simulation to compare the fan directions to what direction is expected from the simulation. MRAMS is taking all the physics that we have about atmospheres and how we think these processes are working and computes what the atmosphere is doing and its conditions. We’re working on comparing the output of MRAMS to the wind directions we infer from the Planet Four fan directions.
Below is an example of one of the types of plots the team has been looking at. Here we show where the dominant fan direction is pointing in the full HiRISE frame from the Planet Four fan catalog. Think of this has telling you where the wind is headed. Each arrow represents a HiRISE observation image taken as part of the Spring/Summer monitoring season. The color of the arrows tell you which block of the Spring/Summer season the image was taken. For timekeeping on Mars, we use L_s, solar longitude, where Mars is located in in orbit around the Sun. L_s=180 is early Southern Spring. 220 is into early Southern Summer. We have 2 Mars Years as part of the current Planet Four catalog We plot the directions from each separately in the left and middle plot, and jointly all together in the right most plot. The left and middle plot show the topography that was used by the MRAMS model and the right most post shows the highest resolution topography measured by the Mars Global Surveyor’s Mars Orbiter Laser Altimeter.
Plots like this help the team look at the impact of topography and the structure of the local surface that might be contributing to how the wind blows. From this image we see that Giza is on the edge of an area where the elevation is dropping as we move more northward in latitude. Here we can see that the topography is likely playing a significant roll with the wind likely traveling from the highest elevations region (bottom of the plot) to the lower elevations. We’ll be able to compare with the detailed ouptut from the MRAMS simulation, but the topographic plots help us put the results from MRAMS in context. The simulation will tells us what direction it think the wind is blowing, but it won’t tell us necesarily why. These topographic plots help us add more explanation to the story.