There’s a very cool cloud on Mars right now. It’s downwind of an ancient martian volcano named Arsia Mons. What makes its such a head-turner is that it perfectly mimics a plume from a volcanic eruption. Naturally, it’s become the subject of much online speculation. The European Space Agency’s orbiting Mars Express has been taking photos of the feature since September. I checked today and it’s still there as of Oct. 24.
I wish it were a live eruption, a volcano come to life after millions of years of dormancy. How exciting that would be. But that’s not the case. Arsia Mons last erupted between 250 to 10 million years ago with peak activity occurring 150 million years ago. It’s been chillin’ ever since with no visible sign of volcanic activity.
Like the other giant volcanoes on Mars, Arsia Mons is a shield volcano built up layer upon layer by lava welling up from a “hot spot” in the martian crust. The Hawaiian Islands were and still are being created the same way. The only difference is that on Mars there are no moving crustal plates, so the lava keeps spilling out and piling up until a giant volcano is built: Arsia Mons is more than 12 miles high (20 km) and 270 miles (435 km) in diameter. That’s almost as big as Iowa!
OK, so what is it? We’re seeing an atmospheric feature called an orographic cloud that forms in the lee of mountains not only on Earth but also on Mars. It’s a common phenomenon in this region of the Red Planet as water vapor carried by the martian winds gets lifted up as it passes over the mountain. Once it reaches a high enough altitude the vapor condenses into a cloud of ice crystals. As the wind continues to blow and pass the mountain, more clouds can form downwind to create a long plume.
Mars just experienced its northern hemisphere winter solstice on Oct. 16. In the months leading up to the solstice, most cloud activity disappears over big volcanoes like Arsia Mons but returns during the solstice for the rest of the martian year. The cloud is no stranger. It recurs seasonally along the southwest flank of the cone-shaped volcano and was previously observed by Mars Express and other missions in 2009, 2012 and 2015.
The spacecraft has snapped hundreds of photos of the impressive, plume-like feature over the past few weeks, which extends over 930 miles (1,500 km). The cloud’s appearance varies throughout the martian day, growing in length during the morning hours downwind of the volcano and shrinking overnight. Days on Mars are just 37 minutes longer than they are on Earth. Water-ice clouds require dust or other materials to serve as sites for the water to condense to form a cloud. That makes them sensitive to the amount of dust present in the atmosphere. These photos were taken after the giant dust storm that blew across the entire planet this past summer and may provide valuable information on the effects of dust on martian cloud formation.
Although Mars only contains a trace of water in its atmosphere, that amount varies seasonally as sunlight vaporizes either polar ice cap during the martian spring and summers, pumping additional vapor into the dry, thin air to fuel cloud formation. By the way, the cloud is big enough to show in photos taken with amateur telescopes. Take a look!
To watch developments in the cloud, check out The Mars Webcam.