Martian Timekeeping
A year on Earth is 365.25 days, but it takes Mars nearly twice as long to complete one revolution around the Sun. With 668.6 sols (a sol is a Martian day), how do scientists keep track of the Martian time and date?
A Martian sol happens to be 39 minutes and 35 seconds longer than Earth’s. Dating back to the days of the Viking landers, sols are kept on a 24 hour clock. With the extra 40 minutes, Martian hours, minutes and seconds are slightly longer than their Earthly counterparts. That can make it a bit difficult to have a Mars time wrist watch.
The Martian calendar is not broken up into months like we have on Earth. Instead planetary scientists use the position of Mars in its orbit to tell time and mark seasons. They use the solar longitude L_s (pronounced “L sub S”) which is the Mars-Sun angle to keep track of the year. L_s=0 degrees is when Mars is at the northern vernal (Spring) equinox. L_s=90 degrees when at the Northern Summer Solstice) 180 degrees at the Northern Autumnal Equinox and 270 degrees at the Northern Winter Solstice. Just think the opposite to get the specific season for Mars’s Southern Hemisphere.
This upcoming February 23rd will mark the Winter solstice in the Northern hemisphere (Summer solstice in the Southern hemisphere). So on the South Pole there should be some seasonal fans, like the ones you’re mapping on the main classification interface, currently visible to HiRES where there is still thawing carbon dioxide ice.
You can find out more about the Martian Calendar and the dates of future solstices with this post by the Planetary Society.
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.
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