Evidence of active volcanoes – finally


Venus: Evidence of Active Volcanoes - Finally

A perspective view of Maat Mons on Venus, based on Magellan radar data. Credits: NASA/JPL

Venus is almost the same size, mass, and density as Earth. So it should generate heat in its interior (from the decay of radioactive elements) at much the same rate as Earth. On Earth, one of the main ways this heat leaks out is through volcanic eruptions. In an average year, at least 50 volcanoes erupt.

But despite decades of searching, we haven’t seen clear signs of volcanic eruptions on Venus so far. A new study by geophysicist Robert Herrick of the University of Alaska, Fairbanks, which he reported this week at the Lunar & Planetary Science Conference in Houston and published in the journal Sciencehas finally caught one of the planet’s volcanoes in the act.

It is not easy to study the surface of Venus because it has a dense atmosphere with a continuous cloud layer at an altitude of 45-65 km that is opaque to most wavelengths of radiation, including visible light. The only way to get a detailed view of the ground from above the clouds is through a radar pointed down from an orbiting spacecraft.

A technique known as aperture synthesis is used to build an image of the surface. This combines the varying strength of the radar echoes reflected back from the ground, including the time delay between transmission and reception, plus small frequency shifts corresponding to whether the spacecraft is getting closer to or farther from the origin of a given echo. The resulting image is very similar to a black and white photograph, except that the bright areas usually correspond to rougher surfaces and the dark areas to smoother surfaces.

Venus: Evidence of Active Volcanoes - Finally

Venus seen in ultraviolet light by Japan’s Akatsuki spacecraft in December 2016. The surface is not visible. Credit: ISAS/JAXA

NASA’s Magellan probe orbited Venus from August 1990 to October 1994 and used this kind of radar technique to map the planet’s surface with a spatial resolution of about a hundred meters at most. It showed that more than 80% of the surface is covered by lava flows, but how recent the latest eruptions and whether there are any eruptions today remained a mystery for the next three decades.

There have been several hints of activity from spacecraft peering into, and sometimes through, the clouds – suggesting that the rocks there are so young that their minerals have not yet been changed by reaction with the acidic atmosphere and that includes newly erupted lava . Thermal anomalies that may correspond to active lava flows have also been detected, as have temporary local hiccups in atmospheric sulfur dioxide concentration – another potential sign of volcanic eruptions. But none of these were completely convincing.

Volcanic vent spotted

Venus: Evidence of Active Volcanoes - Finally

90-mile wide Magellan radar image of Venus showing lava flows (bright because they are rough) beginning to enter an older impact crater. Credits: NASA/JPL

The new study now seems to have settled the matter by revealing surface changes that really must be due to volcanic activity. The authors spent hundreds of hours comparing Magellan radar images of parts of Venus imaged more than once to look for new or altered surface features.

They focused on the most promising volcanic regions and eventually discovered an example where details in an image taken in October 1991 differ from those in an image taken in February of the same year. The changes they saw are best explained by a volcanic eruption within that time frame.

It is difficult to use radar images to verify surface changes because the appearance of even an immovable surface can differ depending on the slope of the surface and the direction of view. However, the researchers ran simulations to verify that the observed changes cannot be due to these things.

Venus: Evidence of Active Volcanoes - Finally

Close-ups of the active volcanic vent north of the summit of Maat Mons in February and October 1991. Between those dates, the vent enlarged and changed shape, and new lava flows appear to be forming. Credits: NASA/JPL

The combined images show an initially nearly circular volcanic crater about 1.5 km across that doubled in size between February and October by expanding eastward. It also became shallower, and the authors suggest that the crater is a volcanic vent that partially collapsed and was largely filled with fresh lava in October.

There are also likely new lava flows extending several kilometers downhill north of the crater, either overflowing the crater rim or leaking from an associated fissure. The active crater is located high on Maat Mons, one of the largest volcanoes on Venus, whose summit rises 5 km above the surrounding plains.

Future missions

Most planetary scientists already expected Venus to be volcanically active. Attention will now surely turn to how often and in how many locations eruptions occur on Venus. The biggest surprise of all of this is that it took so long for anyone to find evidence for surface changes that had been lurking in the Magellan data for 30 years.

Venus: Evidence of Active Volcanoes - Finally

Mate Bergen. The arrow points to the location of the volcanic eruption that erupted in 1991, which is too small to appear at this scale. Credits: NASA/JPL

The likelihood of finding and studying persistent volcanism is one of the main drivers for NASA’s Veritas mission and Esa’s EnVision mission (both approved in 2021). Elk has better imaging radar than Magellan. EnVision is scheduled to enter orbit around Venus in 2034. Originally, Veritas was supposed to be there several years earlier, but the schedule has been delayed.

With NASA’s DaVinci mission likely to arrive a year or two early and provide optical images from beneath the clouds as it descends, we’re in for an exciting time in about a decade or so.

Presented by The Conversation

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