Evidence of active volcanoes – finally

Venus has almost the same size, mass and density as Earth. So it should generate heat in its interior (by decaying radioactive elements) at about the same rate as Earth does. On Earth, one of the most important ways this heat escapes is via volcanic eruptions. During an average year, at least 50 volcanoes erupt.

But despite decades of searching, we haven’t seen clear signs of volcanic eruptions on Venus – until now. 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 including an unbroken 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 picture of the ground from above the clouds is with radar pointed down from an orbiting spacecraft.

A technique known as aperture synthesis is used to build up an image of the surface. This combines the varying strength of the radar echoes returned from the ground – including the time delay between transmission and reception, plus small shifts in frequency that correspond to whether the spacecraft is getting closer or further from the origin of a particular echo. The resulting image looks pretty much like a black-and-white photograph, except that the lighter areas usually correspond to rougher surfaces and the darker areas to smoother surfaces.

NASA’s Magellan probe orbited Venus from August 1990 to October 1994 and used this type of radar technique to map the planet’s surface with a spatial resolution of at best around a hundred meters. It showed that over 80% of the surface is covered by lava flows, but how recently the youngest of them erupted, and whether any eruptions continue today, remained a mystery for the next three decades.

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

Volcano vent discovered

The new study now appears to have settled the matter, revealing surface changes that must indeed be the result of volcanic activity. The authors spent hundreds of hours comparing Magellan radar images of parts of Venus that had been imaged more than once to look for new or altered surface features.

They concentrated on the most promising volcanic areas, eventually discovering an example where the details of an image taken in October 1991 are different from those of an image from February of the same year. The changes they saw are best explained by a volcanic eruption within that time window.

Using radar images to verify surface changes is difficult because the appearance of even an unchanging surface can vary according to surface slopes and viewing direction. However, the researchers ran simulations to confirm that the observed changes could not be due to these things.

The paired images show an originally nearly circular volcanic crater about 1.5 km across that between February and October doubled in size by stretching eastwards. It also became shallower, and the authors suggest that the crater is a volcanic vent that partially collapsed and was mostly filled by fresh lava during October.

There are probably also new lava flows extending several kilometers down the slope, north of the crater, which either flooded over the crater rim or leaked out of an associated fissure. The active crater is located high on Maat Mons, one of Venus’ largest volcanoes whose summit is 5 km above the surrounding plains.

Future missions

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

The likelihood of finding and studying ongoing volcanism is one of the main drivers for NASA’s Veritas mission and Esa’s EnVision mission (both approved in 2021). Each has a better imaging radar than Magellan. EnVision is supposed to reach its orbit around Venus in 2034. Originally, Veritas was supposed to be there several years in advance, but there have been delays in the schedule.

With NASA’s DaVinci mission likely to arrive a year or two before them, providing optical images from below the clouds during descent, we’re in for an exciting time ten years from now.

This article is republished from The Conversation under a Creative Commons license. Read the original article.