Brilliant Venus and fainter Mars seem inseparable. They’ve been sky buds for at least a month, hanging out within a few fingers of each other at dusk. While Venus is certainly one of the hottest places in the solar system with a surface temperature around 860° F (460° C), Mars is downright frigid with an average temperature of –80° F (–62° C). For perspective, Earth averages 61° F (16° C). But though 80 below sounds pretty darn cold, that’s nothing compared to the Martian polar regions where it dips to –195° F (–126° C) during winter.
In synchrony for the moment with Earth’s seasons, it’s also winter right now in Mars northern hemisphere. To celebrate this chilliest of Martian seasons, the European Space Agency (ESA) just released a new mosaic photo of Mars’ north polar cap. The agency combined photographs taken during 32 passes by the orbiting Mars Express spacecraft between 2004 and 2010 into a single, detailed image.
Both north and south polar ice caps are permanent features of the planet. During northern winter, temperatures are cold enough for around 30% of the carbon dioxide in Mars’ atmosphere to precipitate onto the cap as frost and snow, adding a seasonal layer up to about 3 feet (a meter) thick. During the warmer summer months most of the carbon dioxide ice turns directly into gas and escapes into the atmosphere, leaving behind the water-ice layers. When Mars’ polar caps first pop out of thick cloud cover in late winter and early spring, they appear big enough to spot in a smaller telescope, but once much of the carbon dioxide has vaporized in the spring and summer heat, the caps become tiny dots that require larger telescopes to distinguish.
Strong winds are thought to have played an important role in shaping the ice cap over time, blowing from the elevated center of the cap towards its lower edges and twisted by the same Coriolis Effect that causes hurricanes to spiral on Earth. Essentially, the spinning Earth — or in this case Mars — deflects the winds coming off the north pole in as they travel southward, turning them to form the spiral pattern.
The Martian polar region features a prominent 310-mile-long (500 km), 1.2 mile-deep (2 km) canyon that almost cuts the cap in two. Known as Chasma Boreale, it’s thought to have predated the ice-dust spirals. Amateur astronomers with medium to large telescopes can spot it during certain seasons as a narrow, dark gash in the otherwise white cap.
Radar instruments onboard Mars Express and NASA’s Mars Reconnaissance Orbiter have penetrated the surface and revealed that the ice cap is made up of many individual layers of ice and dust reaching a depth of about 1.2 miles (2 km). We can picture these layers as the pages of what I like to call the Red Planet’s Great Book of Climate Change. They record how the planet’s climate has changed as its tilt and orbit have varied over hundreds of thousands of years due to the gravitational effects of the giant planets Jupiter and Saturn.
Earth’s orbit is likewise altered by those planets, and the tilt of its axis affected by the gravity of the sun and moon. Since Mars lacks a large moon to help stabilize its axis, variations in the tilt are much more extreme, leading to more dramatic changes in climate over the long haul. The tilt of Earth’s axis varies modestly from 22.1 to 24.5°, while Mars can tip anywhere from less than 10° up to more than 60° in a very chaotic manner. Tilt affects season lengths and how much carbon dioxide condenses out of the Martian atmosphere over the polar regions and beyond.
Glance up at Mars perched near Venus the next clear night and imagine all those weather extremes picturesquely recorded in whirls of dusty snow.