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!
A Planet Four Update
Today we have a post by Candy Hansen, principal investigator (PI) of Planet Four and Planet Four: Terrains. Candy also serves as the Deputy Principal Investigator for HiRISE (the camera providing the images of spiders, fans, and blotches seen on the original Planet Four project). Additionally she is a member of the science team for the Juno mission to Jupiter. She is responsible for the development and operation of JunoCam, an outreach camera that involves the public in planning images of Jupiter.
Dear Planet Four Volunteers,
Years ago we enlisted your help to measure spring fans on the surface of Mars’ seasonal CO2 polar cap. Our small science team had a vision of what we could learn from those fans about the weather on Mars, but we did not have the resources to make the needed measurements.
With your investment of your time we now have an extensive catalog of fan measurements. The catalog crosses space (the Mars south polar region) and time (8 years of Mars southern spring images). The contents of the catalog and potential science use was documented in a paper by Michael Aye, published in Icarus in 2019. We have a new paper by Anya Portyankina, in press at the Planetary Science Journal, that compares wind direction and speed (from your measurements of fans’ orientation and length) to one of the standard Mars weather models. From this we can explore where the models do and do not do well to predict the weather. This very important goal that we envisioned from the beginning has now been achieved thanks to your generous donation of your time to make these measurements.
Over the years we have realized that seasonal activity is affected by Mars’ dust storms. Using the catalog we can quantify the differences in the number and size of fans after a dust storm. That work is now underway so we have posted more images and we hope you will help us again by making more measurements. We know your time is valuable and we sincerely appreciate your willingness to help with this analysis of the weather on Mars.
One final note – we have explored doing this task via machine learning. People still win!