Two full Mars years have passed since the Curiosity rover was delivered via sky crane to the surface of Gale Crater in August 2012. Wait a sec. That’s four years, right? Yes, four Earth years but only two if you’re a Martian. Since Mars orbits beyond Earth, it revolves more slowly around the sun, taking 687 days or nearly two years to complete an orbit. During that time, the rover has drilled into the crust, laser-zapped rocks and taken selfies (among other images!). But it’s also served as our friendly meteorologist on the Red Planet. So what’s it like to “take in the air” on Mars anyway?
After two full Martian years, we know have a good idea about what the weather’s like at Gale Crater, an impact crater 96 miles across once home to rivers and lakes where a visitor might stand and hear the slap of waves and rush of water. Today, save for the wind, it’s a silent, cold desert where an equatorial sun casts enough ultraviolet light to give you a sunburn in a hurry.
Curiosity’s Rover Environmental Monitoring Station (REMS) has measured air temperatures from 60.5° F (15.9° C) on a summer afternoon, to –148° F (–100° C) on a winter night.These temperatures are the extremes; a typical “warm” day at Gale Crater would see the mercury rise to around the freezing point and dip to –130° F overnight. REMS has made measurements nearly every hour of every day or more than 34 million data points.
Were it not for the planet’s extremely low atmospheric pressure (less the 1/100 that of Earth), you could walk around on a summer day dressed in a fall coat, light gloves and an oxygen mask. While the similar tilts of Earth (23.5°) and Mars (25°) give both planets a yearly rhythm of seasons, the difference between day and night temperatures is great due to the planet’s extremely thin atmosphere and lack of heat-holding oceans. It may warm up during the day, but there’s little to hold that heat in at night.
You can imagine how these temperature extremes would act to fracture and erode Martian rocks over time. It’s also the reason NASA makes sure to keep the rover’s innards warm. To function, Curiosity’s “vital organs” must not exceed temperatures of –40° F to +104° F (–40°C to 40° C). Essentials, such as batteries, electronics and the computer, stay safe inside a Warm Electronics Box or WEB usually referred to simply as the “rover body.” Heat is provided by the radioactive decay of plutonium, the same source that the fictional Mark Watney used to stay warm and power the rover while trekking across Mars in the movie The Martian.
Mars is also much drier than our planet, and particularly in Gale Crater, located near the equator, where the water vapor content is 1,000 to 10,000 times less than Earth. In much of the U.S. nights are often dewy or frosty with water condensing on grass and telescope optics alike. Such would rarely be a concern in Curiosity’s backyard. Though relative humidities of up to 70% have been recorded on chilly winter nights, researchers have yet to see frost forming on the ground.
Curiosity’s air-pressure measurements confirm a strong seasonal trend previously seen by other missions due to the capture and release of carbon dioxide by the planet’s seasonal polar caps. Most of the Martian atmosphere is composed of carbon dioxide (95.3%). During each pole’s winter, millions of tons of this gas migrates poleward and freezes solid, lowering atmospheric pressure. Come spring and warmer temperatures, it vaporizes and recharges the atmosphere.
These very un-Earthlike seasonal variations cause the atmosphere pressure to vary about 25% over a year. Compare that to Earth, where the pressure variation at sea level is less than 10%.
Curiosity also discovered another seasonal pattern: the local atmosphere is clear in winter, dustier in spring and summer, and windy in autumn. Visibility in Gale Crater is as low as 20 miles (30 kilometers) in summer, and as high as 80 miles (130 kilometers) in winter.
Then there’s the quirky methane finding. For most of the two Martian years, the rover has measured methane concentrations between 0.3 and 0.8 parts per billion, but for several weeks during the first autumn, the level spiked, reaching 7 parts per billion. The mission checked carefully for a repeat of that spike during the second autumn, but concentrations stayed at lower background levels, telling us that the methane jump must not be seasonal but some kind of one-off. Whether Mars methane has a biological origin or not is still unknown.
It’s fun to keep Martian weather in mind as the planet heads for opposition next Sunday May 22, when it will be at its closest to Earth since 2005. We’ll have more about this special event in an upcoming blog.