A THOUSAND AND ONE ICE WONDERS

• BY ANNIE LABRECQUE, WITH THE CONTRIBUTION OF MARC JOBIN, ASTRONOMER AND ASTRONOMICAL INFORMATION OFFICER AT PLANÉTARIUM THE RIO TINTO ALCAN

There is ice almost everywhere on our planet. It can cool your glass of water, form on the surface of lakes during the winter or dominate the landscape of the Arctic and Antarctic.

On Earth, we refer to ice as water in a solid state. Elsewhere in the Solar System, pressure and temperature are different—they can be extreme! —and they affect the state of the molecules (solid, liquid, gas). On some moons or planets, gases such as ammonia or carbon dioxide are present in the form of ice because of the ambient conditions.

Ice is thus a general term encompassing a vast array of substances, which can come in surprising forms. Let's take a look at these icescapes!

PROBING THE ICE IN THE SOLAR SYSTEM

Planetologists have long studied the ice caps of Mars. These ice caps are composed mainly of water ice, but also of carbon dioxide in solid form, better known as "dry ice". It is mostly found at the poles, where temperatures below -78°C enable carbon dioxide to change directly into a solid state.

Another celestial object that arouses a lot of curiosity is Enceladus, a small icy moon barely 500 km in diameter orbiting Saturn. Enceladus has a unique landscape with its four "tiger stripes", geological structures located near the South Pole. These stripes, first spotted by the Cassini spacecraft in 2005, are associated with parallel cracks 130 km long. Water and organic molecules escape from these fissures, indicating the presence of a liquid ocean under the icy crust of the moon.

A rare phenomenon also occurs on Enceladus and other icy satellites: eruptions of cryovolcanoes, or "ice volcanoes," that emit plumes of pressurized water into the tenuous atmosphere of these moons, where temperatures can reach -130°C.

Exploration of the dwarf planet Pluto by the New Horizons probe revealed a large, heart-shaped impact basin. Part of this depression, called Sputnik Planitia, is filled with nitrogen ice. In the images taken by the probe, you can see that the surface of this region has multiple polygonal shapes next to each other. The formation of this icy quilt intrigued scientists. Using numerical simulations, they believe that this landscape appeared under the action of the sublimation of nitrogen, which transforms directly from solid to gas without passing by the liquid state.

Pluto is much farther from the Sun than the Earth and it is extremely cold, with an average temperature of -223 °C. However, in some areas, the environment of the dwarf planet is strangely reminiscent of Earth, with high snow-capped peaks. But these mountains appear to be made of water ice, as hard as stone. And the "snow" they are covered with would be more a methane frost.

Comets do not go unnoticed and offer one of the most beautiful celestial spectacles. When they approach the Sun, their nucleus of water ice, ammonia and carbon dioxide, mixed in with small rocky particles, heats up and releases the gases and dust. This is what produces their tails that sometimes stretch for millions of kilometers. ↑ A liquid ocean is probably hiding under the icy crust of Enceladus, a moon of Saturn.

A liquid ocean is probably hiding under the icy crust of Enceladus, a moon of Saturn.

ELSEWHERE IN THE MILKY WAY

Located 33 light-years away from our Solar System, the exoplanet Gliese 436 b is also worth mentioning. Comparable in size to Neptune, the temperature on Gliese 436 b is estimated at 439 °C! Given its size and density, scientists speculate that it is largely composed of "X ice," a rare form of water ice. Despite the high temperature, it remains solid because of the incredible pressure generated by the planet's intense gravity.

As we delve into the cosmos, we keep discovering worlds with extreme and varied conditions, where ice takes on unusual shapes—to the amazement of scientists!