Scientists are watching a persistent plume that has formed over the 20 km (12 mi) high Arsia Mons volcano near the equator of Mars. First seen by ESA's Mars Express orbiter on September 13, the elongated cloud formation isn't a sign of volcanic activity – it's made of water ice produced by winds blowing over Arsia in the dust-laden atmosphere of the Red Planet.
If you live or have traveled near mountains, you may have, from time to time, seen long, white plumes that seem to stream away from the peaks, making them look like active volcanoes ready to blow their tops. In fact, these are orographic or lee clouds. They're caused by winds blowing over the mountains that, like air flowing over a wing, experience a sudden change in pressure as they reach the summit. The result is the condensation of water vapor, which turns into ice crystals at high altitudes.
According to scientists, a similar phenomenon is occurring on Mars. Extending some 1,500 km (932 mi) westward from the 250 km (155 mi) wide Arsia Mons, the plume has been monitored by Mars Express' Visual Monitoring Camera for last month. It's a regular event that Mars went through as regional clouds started to return in the run up to the northern hemisphere winter solstice on October 16. Similar plumes have been seen in 2009, 2012, and 2015.
The plume is anything but static. If changes in length through the Martian day and sometimes is large enough to be seen by telescopes on Earth. Like their terrestrial counterparts, Martian plumes are a matter of condensation, but on Mars they are controlled by the amount of dust in the air.
Since Mars has just come out of a global dust storm that socked in the planet for weeks, conditions are ideal for some very dramatic plumes and they could tell scientists a lot about cloud development. To aid in this, Mars Express has also turned its OMEGA visible and near-infrared mapping spectrometer, and the High Resolution Stereo Camera to the task of learning more.
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