Frost Covered Dunes and Phase Changes

This week’s image takes us back to Mars, and because it comes from the HiRISE instrument we can have a look at a full colour version in wonderful detail. The addition of colour makes this scene even more spectacular than the black and white version. What we are looking at here is part of the circumpolar dune field, which surrounds the northern icecap of Mars. 

The North Pole itself is covered by a large icecap, made up primarily of water ice. This remains frozen year round, but during the winter a thin layer of Carbon Dioxide ice forms on top of it. This seasonal layer is only around a meter thick. Carbon Dioxide is also responsible for the bright streaks in our image. The glistening patches are covered by CO₂ frost, which accumulated during the northern winter. As the HiRISE team explain in the caption to this image, the satellite captured this scene during the early spring. This means that the seasonal frost layer was disappearing at this time. The feathery patterns which we can see in this image indicate places where the frost is sheltered from the sun, and so has yet to evaporate. 

Martian frost doesn’t melt, as water ice does on Earth, but rather goes straight from the solid state to a gaseous one, in a process called sublimation. Sublimation is very important on Mars, because the temperature and pressure conditions there mean that several of the substances which shape the martian landscape aren’t stable in their liquid phase. This is particularly true of water, which can only exist as a liquid for short periods of time on Mars, or under unusual conditions. It is fairly intuitive that the cold temperatures on Mars will make the liquid water freeze again, but what is equally important is the low pressure. 

Pressure has a large effect on the position of the freezing and boiling points of a substance. These are the temperatures at which it will change phase from a solid to a liquid and from a liquid to a gas, respectively. We are used to seeing these phase changes take place at set temperatures on Earth. With our fairly high atmospheric pressure water almost always freezes at 0^oC, and boils at 100 ^oC. The fact that these are nice round numbers is no coincidence. The Celsius, or centigrade, scale was deliberately calibrated based on these commonly observed properties. 

Although the modern centigrade scale is named after him, it doesn’t work quite the same way as the system which Swedish physicist and astronomer Anders Celsius first proposed in the early 1740’s. Celsius made measurements in the opposite direction, so that zero degrees represented the boiling point of water while 100 was calibrated to fall at the freezing point. The system we use today is actually based on a scale that was first proposed by the French physicist Jean-Pierre Christin. He appears to have come up with the idea independently of Celsius, at around the same time. The ascending scale took off when Celsius’ scale was reversed by another Swedish scientist; Carolus Linaeaus for use in his botanical research. The 0-100 degree system quickly became the most popular and is now a scientific standard. 

If he had lived on Mars, Celsius would likely have had a much harder time calibrating his temperature scale. The low atmospheric pressure means that there is no easily defined interval in which liquid water is stable. It will begin to boil the moment it is exposed to the thin atmosphere. This naturally produces water vapour, but ironically it can also turn the bulk of the boiling water back into ice. Evaporation requires energy, and this is drawn from the boiling liquid. When very hot water boils there is plenty of energy to go around, and the liquid will keep on evaporating until none is left. When cold water boils this energy is in short supply, and so the liquid gets colder and colder until it freezes again. This process is called evaporative cooling, and could result in ice covered rivers, where the exposed water boils away, until a cap of more stable ice forms above it. When we look at outflow channels on Mars, or the debris flows left by martian gullies, we need to take this process into account, as it will have a substantial effect on the appearance of the landscape such flows of water leave behind. 

Image Credit: NASA/HiRISE/University of Arizona https://www.uahirise.org/ESP_050703_2560