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What is Winter Like on Mars?
It’s not only winter in the northern hemisphere here on Earth—it’s also winter in the northern hemisphere of our Red Planet neighbour.
At the time of writing this, the solar longitude on Mars is 315°. Solar longitude, written shorthand as Ls (said aloud as “L sub S”), is a way we keep time on Mars. It’s a measure of where Mars is in its orbit around the sun in terms of a 360° ellipse, marking the current season on the Red Planet. Seasons on Mars are quite dynamic with its axial tilt toward the sun of 25°, close to Earth’s current tilt of 23.5°. Changes in season result in changes in weather, surface temperature, and even the distribution of ice on the martian surface. Ls 315° puts us in late winter in the northern hemisphere and late summer in the southern hemisphere.
Mars’ polar caps essentially have two components: “Residual” and “seasonal” caps. The residual polar caps are the bulk of the polar cap that persist year after year. The residual northern polar cap remains pretty consistent over time, while the residual southern polar cap is changing quite a bit…but we’ll save the complexities of the southern cap for their own article.
The residual northern polar cap is made up predominantly of water ice with some interspersed layers of dust and sand. In the central portion, it is about 2 kilometres thick—on par with the average thickness of the Antarctic ice sheet. The residual cap covers an area nearly twice the size of France, with a volume about half that of the Greenland ice sheet.
In winter however, the areal extent of the cap changes dramatically with the formation of the seasonal polar cap. The seasonal cap forms when carbon dioxide and water vapour in the atmosphere condense onto the martian surface as frost. Depending on latitude, this frost layer can be up to a metre thick—problematic for surface missions at high latitudes. The northern seasonal cap extends as far south as ~53°N latitude.
