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Starts With A Bang

Why Does Mars Appear To Have Smoke Plumes In Its Atmosphere?

It’s not volcanic activity, and it’s definitely not from a fire.

Mars, our red planetary neighbor, is a vastly different world from Earth.

Mars and Earth, to scale, shows how much larger and more friendly to life our planet is than our red neighbor. Mars, the red planet, has no magnetic field to protect it from the solar wind, meaning that it can lose its atmosphere in a way that Earth doesn’t. (NASA)

Mars’s atmosphere is too sparse (0.007 bar) to allow fresh water on its surface.

Mars, along with its thin atmosphere, as photographed from the Viking orbiter in the 1970s. The methane found in the atmosphere has a recent origin, and could be either geological or biological. There is only a tiny amount of oxygen: about 1 part in 700 for Mars’ atmosphere, and the atmosphere in total is just 0.7% the pressure of Earth’s. (NASA/VIKING 1)

There’s also practically no free molecular oxygen (O_2) there, as it’s only produced by ultraviolet radiation striking carbon dioxide.

NASA’s Mars orbiter, MAVEN, was able to measure the atoms and ions escaping from Mars’ atmosphere due to the solar wind and radiation. Oxygen was definitively detected by MAVEN, with the determination that it was being created by the photodissociation of atmospheric carbon dioxide. (NASA/UNIV. OF COLORADO/MAVEN MISSION)

Without those ingredients, how can we explain these apparent plumes of smoke on Mars?

With this high-resolution view from ESA’s Mars Express, it’s very clear that it’s not a volcanic plume arising from the top of the extinct volcano Arsia Mons, but rather some other feature. In this image, you can reconstruct that the atmospheric feature is 915 km across, and casts a shadow that’s visible on the Martian surface.(ESA/DLR/FU BERLIN,CC BY-SA 3.0 IGO)

On Earth, such plumes typically indicate one of two things: fires or volcanic eruptions.

Fires can be captured from a number of Earth-orbiting observatories in incredible detail, such as this photograph of a natural wildfire in Yellowstone in 2009 taken from the International Space Station. Large plumes that are blown by the prevailing winds are frequent sights in dry regions during summer months on Earth. (NASA / ISS)

Without carbon-based material or copious amounts of available oxygen, we can rule out fire.

NASA’s Terra satellite captured this image of the erupting Raikoke volcano on the morning of June 22, 2019, after the Sun had risen. At the time, the most concentrated ash was on the western edge of the plume, above Raikoke. As spectacular as this terrestrial sight is, the phenomenon of volcanism cannot account for the plumes we see on Mars. (NASA / TERRA SATELLITE / MODIS INSTRUMENT)

Mars possesses the Solar System’s largest volcano in Olympus Mons, but it appears to be extinct.

The Tharsis region of Mars. Tharsis is a volcanic plateau located on the western hemisphere of the red planet. It is home to several volcanoes, including Olympus Mons (bottom right), the largest volcanic feature in the Solar System. On the left limb of the planet can be seen the three smaller but still substantial extinct volcanoes Arsia Mons (bottom), Pavonis Mons (middle) and Ascraeus Mons (top). These three volcanoes range in size from 350 to 450 kilometres (km) across and they each have a height of approximately 15–20 km. (GETTY)

Although there is some circumstantial evidence that Mars may be volcanically active, we’ve never witnessed an eruption.

This orbital view of the Olympus Mons volcano on Mars, the largest known volcano in the Solar System, is shown in an assigned color to remove the redness of the atmosphere. It measures 375 miles across at its base. Despite being extremely large, there is no evidence of a recent eruption from this or any other Martian volcano. Whether Mars is still volcanically active or whether it’s entirely extinct is still an open question. (CORBIS/Corbis via Getty Images)

Instead, these plumes are a simple atmospheric phenomenon: clouds.

Water-ice clouds are often found in the atmosphere of Mars, and tend to preferentially form over the summit of tall mountains or volcanoes, such as Arsia Mons, annotated by Tanya Harrison in the image shown here. (NASA / MARS RECONNAISSANCE ORBITER / TANYA HARRISON)

Mars has water vapor just like Earth, which circulates through the Martian atmosphere.

As the Martian winds encircle the globe, they carry atmospheric constituents over the mountains, through the valleys, and into and out of the deep basins of Mars. This topographic map shows the extreme changes in elevation that occur in particular at equatorial latitudes. (NASA / CHRISTINE M. RODRIGUE / CSU-LONG BEACH)

As the air climbs into cooler, low-pressure regions to rise above intervening mountain, it cools.

Martian clouds come in a variety of formations and can be triggered by a variety of phenomena, from temperature changes to air rising over a mountain to meteor-induced clouds, such as the ones captured earlier this year from the Martian surface. (JPL-NASA/CORNELL UNIVERSITY)

If that air cools sufficiently, it drops below the dew point, forming mountaintop clouds.

Arsia Mons is the largest shield volcano on the southern hemisphere of Mars, and frequently forms this particular type of cloud which could be mistaken for a smoke plume by an uninformed observer. This 2015 image clearly shows a cloud formation with a plume-like appearance. (ISRO / ISSDC / JUSTIN COWART)

With rapid winds in its low-density atmosphere, Martian clouds can persist for hundreds of kilometers or more.

About 4 weeks after they first appeared, ESA’s Mars Express captured this image of an enormous cloud formation forming over the shield volcano Arsia Mons. Rising to a height of approximately 20 kilometers, the white clouds are made of water vapor and extend for approximately 1500 km in this photo. (ESA/GCP/UPV/EHU BILBAO)

The atmospheric phenomenon responsible, lee waves, occur on Earth, too.

On Earth, when winds carry water vapor over a high enough mountain, the water vapor can transition into a liquid phase as the temperature falls below the dew point. So long as the temperatures remain that way, the clouds will persist, but when the conditions are no longer right, the clouds will disappear as the water droplets transition back into the gaseous phase. (PXHERE PHOTO #727519)

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.

Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.


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