SCIENCE BLOG | May 22, 2016
Seasonal Changes in the Polar Caps of Mars Imaged by Hubble Telescope
Elon Musk's Spacex to Mars in 2018

The beautiful Mars image on the right was taken a few days ago (on May 12, 2016) by the Hubble Space Telescope. This image reveals a lot of Martian detail both because Mars was relatively close to the Earth on May 12 but also because Hubble took the picture. The image shows a wealth of seasonal and daily effects. I decided to go back into the NASA image archives [1] to find other images of Mars taken by Hubble to bring out one of these seasonal effects and write a science blog to interest you.

The middle image was taken by Hubble on February 25, 1995, when again the Earth and Mars were relatively close to each other. I choose to show you this image because February 25, 1995 was 11.28 Martian years before May 12, 2016. The 11 years are not so important, rather the additional 0.28 years is important for it shifts the middle image back a full Martian season relative to the rightmost image. The right image was taken when the northern hemisphere of Mars was in its late Summer, while the middle one was taken when that hemisphere was in late Spring. The smaller image on the left was taken by Hubble on September 18, 1996, with a much greater distance between the Earth and Mars (so this image is not so detailed). September 18, 1996 was 10.44 Martian years ago, so the left image was taken with the northern hemisphere of Mars in early Spring (close to an equinox of Mars).

The most visible changes these three seasonal images illustrate is a very large shrinkage in the area covered by the white cap around the North pole between early Spring and late Summer. What is actually happening to cause this shrinkage is not obvious and is beyond the normal experience of Earthlings.

In the depths of the North Mars winter the North polar region gets so cold that huge amounts of carbon dioxide just freeze out of the atmospheric gas [2]. This frozen carbon dioxide, often called "dry ice," forms the top, visible layer of the white polar caps seen in the left and middle images. Similar freezing occurs half a Martian year later (and earlier) around the opposite South polar region of Mars in its Winter season. Note that 96% of the atmosphere of Mars is carbon dioxide, so this carbon dioxide freezing around the poles freezes the majority of the atmosphere and substantially drops the atmospheric pressure around a winter-time pole (try imaging the atmosphere around you just freezing). This gas-freezing-driven polar pressure drop causes the atmosphere from other latitudes to rush toward the winter-time pole at speeds that can approach 400 km/h (250 mph). These speedy winds can actually be sustained for quite long periods because stores of frozen carbon dioxide can be sublimated into gaseous carbon dioxide at the planet's opposite pole (which is warming up in its own Spring and Summer seasons). The shrinkage in the white (dry ice covered) North polar cap seen from the left to the right images is mostly caused by such sublimation of frozen carbon dioxide through the relatively warm (northern) Spring and Summer seasons. About an eighth of the entire Martian atmosphere gets frozen and sublimated in one of these cycles, and the mass of carbon dioxide moved in a cycle, between the planets' poles, is large enough that it causes detectable changes in the gravitational field of Mars.

The coldest air temperature ever recorded (outside of laboratories) on the surface of the Earth was -89.2°C (-128.6°F) on July 21, 1983, close to the Earth's South Pole [3].

Just how cold does it need to be to freeze gaseous carbon dioxide at the Martian poles? You have to factor in the gas pressure at the poles, if you assume the average atmospheric pressure of Mars (i.e. 600 Pa) then gas-to-solid sublimation freezing of carbon dioxide occurs at a mind-numbing -125°C (-193°F), while at a lower 300 Pa pressure this phase transition temperature drops to about -130°C (-202°F)[4]. Since the large majority of the Martian atmosphere is carbon dioxide, the local temperature in polar region locations where carbon dioxide freezing occurs will tend to force the local pressure to be the value just where sublimation-freezing of carbon dioxide occurs at that local temperature. In polar locations where carbon dioxide freezing is occurring the local pressure is likely to be well under the average, 600 Pa, pressure and the local temperature will be under -125°C (-193°F). Note, the standard atmospheric pressure on Earth is a little over 100,000 Pa, and at this pressure the freezing-sublimation temperature of carbon dioxide is -78.5°C; so, the small fraction of carbon dioxide in the Earth's atmosphere would have frozen solid close to the Earth's South Pole on July 21, 1983.

That's enough for one science blog. However, there is enough illustrated in these three images of Mars for several more such blogs. You might be seeing these images again.


References:
1. http://nssdc.gsfc.nasa.gov/photo_gallery/photogallery-mars.html
2. https://en.wikipedia.org/wiki/Martian_polar_ice_caps
3. https://en.wikipedia.org/wiki/Lowest_temperature_recorded_on_Earth
4. http://webbook.nist.gov/cgi/cbook.cgi?ID=C124389&Mask=4&Type=ANTOINE&Plot=on#ANTOINE

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