Studying polar CO2 jet eruptions and the state of the lower martian atmosphere with help from the Planet Four citizen science project

The High Resolution Imaging Science Experiment (HiRISE) on the Mars Reconnaissance Orbiter (MRO) routinely observes the surface of martian polar regions during seasonal observational campaigns. The images reveal a variety of seasonal features related to CO₂ jet eruptions. The amount and quality of the data collected over 8 martian years provides abundant information on seasonal and inter-annual development of these transient surface features – fans and blotches – considered to be dust and sand deposits from the CO₂ jet eruptions. Directions and sizes of the fan deposits showcase information about eruption physics as well as about the state of the martian atmosphere at the time of eruption.

To analyze the HiRISE dataset numerically we have created a citizen science project Planet Four (P4). Participants of P4 mark seasonal deposits in sub-frames of HiRISE images with provided tools that have specific geometry. Locations, directions, and sizes of the markings are stored in a database. We have created a pipeline that works with the database, removes mistaken markings, and reduces multiple markings of the same object to the statistically best marking per object. The result of the pipeline is a catalog of fan and blotch markings that provide direct information about wind directions and speeds in the locations observed. The fan directions shift considerably during spring which is indicative of the volatile nature of weather in the southern polar regions in spring as the polar vortex develops. We will present derived wind speeds and directions in multiple regions of interest (ROIs) in the southern polar regions.

We have run the Mars Regional Atmospheric Modeling System (MRAMS) at the same locations and portions of spring to compare its results to values derived from P4. The MRAMS model is generally consistent with P4 wind directions and speeds derived from P4 fan sizes. MRAMS winds are most consistent with the observed directions in early spring, while after approximately L_s = 200° the model tends to be a bit less consistent with the P4 observations. This can be explained in part by the relative proximity of the sublimating edge of the seasonal polar cap at these times, which tends to increase the wind direction variability.

MRAMS winds are calculated every 10 Mars-minutes for multiple complete martian days, providing an opportunity to determine the timing of jet eruptions. In some of the ROIs we observe a correlation of P4-determined winds with modeled winds at specific times of day. This indicates that CO₂ jet eruptions happen during those specific times. The most probable time of jet eruptions determined from these correlations varies through spring. This is consistent with CO₂ jet modeling that suggested that timing of jet eruptions is connected to the amount of available solar energy. In the early spring those CO₂ jet models expect the eruptions to occur close to local noon or during the afternoon, while later in spring they are expected to shift to earlier hours of the day as the seasonal ice thins. Not all P4 ROIs exhibit this pattern when compared to the MRAMS simulations. Local wind patterns and the near-surface atmospheric environment (where eruptions happen) have to be examined in detail to understand and decouple the sporadic nature of CO₂ jet eruptions from the dynamic atmospheric behavior modeled by MRAMS.

Conclusions: HiRISE and consequently P4 data created several new ways to investigate seasonal processes and atmospheric dynamics on Mars with the potential for quantitative analysis.