Strange Cloud On Mars Turns Heads

Since Sept. 13, the European Space Agency’s Mars Express has been observing the evolution of a curious cloud formation that appears regularly in the vicinity of the 12.4-miles high (20 km) Arsia Mons volcano, close to the planet’s equator. This photo was taken on Oct. 10 when the cloud extended some 930 miles (1,500 km). Look closely and you’ll see that the cloud starts near but not inside the caldera of the volcano. ESA/GCP/UPV/EHU Bilbao, CC BY-SA 3.0 IGO

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.

This photo of Arsia Mons of a nearly identical cloud formation was taken 42 years ago during the Viking 2 mission to Mars. It also doesn’t originate from the ancient volcano itself but downwind. NASA

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.

This water ice cloud, which forms as air flows up and over the volcano slope, can be seen as the long white feature extending to the left of the Arsia Mons volcano. The cloud, which measures 570 miles (915 km) long in this view, also casts a shadow on the surface. This image was taken on Sept. 21 from an altitude of about 4,300 miles (6930 km). ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO

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.

This photo was taken by NASA’s Mars Global Surveyor some years back and shows orographic clouds extending from the tops of several other ancient martian volcanoes including Arsia Mons, which is named after the Roman forest Arsia Silvia. NASA

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.

Clouds scoot across the Martian sky in a movie clip taken by NASA’s Phoenix Mars Lander on Aug. 29, 2008. This clip accelerates the motion. Particles of water-ice make up these clouds, like ice-crystal cirrus clouds on Earth. NASA/JPL-Caltech

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.

5 Responses

  1. kevan hubbard

    It’s not high enough up to be the equivalent of one of Earth’s nacrous or nocculient clouds.although these are mainly made of meteor dust we think as very little watervapor exists where they are found.well they think it’s not coming out of the volcano which doesn’t mean it isn’t.mars must still have a hot core but I’m guessing that the volcanic vents to the surface have solidified by now?

    1. astrobob

      Hi Kevan,
      Mars probably has a molten core down deep but all its volcanoes have been extinct for millions of years. If you look at the photos, especially the second color closeup and the photo of similar clouds around ALL the volcanoes, it’s clear these are clouds and not pouring out of their ancient calderas.

      1. kevan hubbard

        I concur.as Mars has a weak residual localised magnetic field it suggests a ferous core. Venus has virtually no magnetic field yet evidence of volcanic activity perhaps ongoing?a few years back photos of very high clouds where taken over Mars I think they said they where 100 km up,and they reminded me of Earth’s nocculient,night shinning clouds,although lower nacrous,polar stratospheric clouds are similar but I’ve never seen nacrous clouds as you generally have to be way north about 70 degrees and in the winter too!or south for the Antarctic.

  2. it seems to me the problem with the idea it’s just a water vapour and dust cloud is it seems to be emanating in a line from a relatively small area on the side of the caldera. If it were simply a cloud formation surely one would expect to see similar formations over the entire caldera. It isn’t a very satisfactory explanation, because there’s no additional evidence which supports it. It seems to me therefore more likely to be some kind of volcanic activity, maybe some kind of geyser? But It’s unique location, isolation and scale doesn’t suggest simple cloud formations.

    1. astrobob

      Hi Amanda,

      I know it’s tempting to consider it might be volcanic, but the cloud has pops up seasonally and has been observed for the past 40-plus years. Similar clouds have been observed repeatedly over neighboring extinct volcanoes. If it were volcanic, spectra of the cloud would reveal volcano-related aerosols such as sulfur. Instead, we only see water vapor. Astronomers would shout it from the rooftops as one of the greatest discoveries of our time if it were a live volcano. As always when looking at photos, we have to be careful in assuming that because something looks like something else, it doesn’t necessarily mean it is that thing. Thank you very much for writing!

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