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!