Ancient Terrain on Venus Looks Like it Was Formed Through Volcanism

Ever since NASA’s Magellan orbiter was able to peak beneath Venus’ dense cloud layer and map out the surface, scientists have puzzled over the planet’s geological history. One of the greatest mysteries is the role volcanic activity has played in shaping Venus’ surface. In particular, there are what is known as “tesserae,” tectonically deformed regions on the surface that often stand above the surrounding landscape.

These features comprise about 7% of the planet’s surface and are consistently the oldest features in their immediate surroundings (dating to about 750 million years ago). In a new study, an international team of geologists and Earth scientists showed how a significant portion of these tesserae appear to be made up of layered rock, which is similar to features on Earth that are the result of volcanic activity.

The study, titled “Venus Tesserae Feature Layered, Folded, and Eroded Rocks,” recently appeared in Geology, a publication maintained by the Geological Society of America (GSA). The study was led by Paul K. Byrne, an associate professor of planetary science at North Carolina State University, who was joined by researchers from the UK, US, Canada, Turkey, Russia, and the NASA Jet Propulsion Laboratory (JPL).

For the sake of their study, the team analyzed images of Venus’ surface taken by NASA’s Magellan mission. Between 1990 and 1994, this orbiter used synthetic aperture radar to penetrate Venus’ atmosphere and map 98% of the planet. In the course of its six mapping cycles, Magellan detected numerous interesting features that were indicative of past volcanic activity (i.e. lava plains, domes, and shield volcanoes).

While researchers have studied the tesserae for decades, they did not believe that the layering of it was widespread – which is a good indication that the tesserae are not portions of the continental crust. As Prof. Byrne explained in a recent NC State University News release:

“There are generally two explanations for tesserae – either they are made of volcanic rocks, or they are counterparts of Earth’s continental crust. But the layering we find on some of the tessera isn’t consistent with the continental crust explanation.”

For the most part, continental crust is composed of granite, an igneous rock that is formed when tectonic plates move, and water is subducted from the surface granite. However, as the name implies, granite is coarse-grained in structure and doesn’t form in layers. What this implies is that if Venus has a continental crust, then it’s likely below the layered rocks that are seen on the surface.

Such formations can be explained through one other geological process (sediment deposited by flowing water) but Prof. Byrne and his colleagues were able to rule that out with a fair degree of certainty. As he indicated:

“Aside from volcanic activity, the other way to make layered rock is through sedimentary deposits, like sandstone or limestone. There isn’t a single place today on Venus where these kinds of rocks could form. The surface of Venus is as hot as a self-cleaning oven and the pressure is equivalent to being 900 meters (about 985 yards) underwater. So the evidence right now points to some portions of the tesserae being made up of layered volcanic rock, similar to that found on Earth.”

Byrne and his colleagues hope that this research will help to shed light on Venus’ complicated geological history. For example, a future mission to the surface could obtain samples from the tesserae, which would be able to confirm or deny their volcanic origin. If they are in fact sedimentary in nature, it would indicate that they formed when the surface of Venus was much different than it is today.

To date, studies have been conducted that indicate that Venus may have once been habitable, with oceans covering much of its surface. These oceans (and even lifeforms) could have existed for billions of years until roughly 700 million years ago. At this point, a massive resurfacing event is believed to have caused a runaway greenhouse effect that caused its atmosphere to become extremely hot and dense as it is today.

However, other research based on the examination of lava flows (like Ovda Fluctus) has cast doubt that the highlands on Venus are likely to be composed of basaltic lava rock instead of granite. This contradicts previous theories that the Ovda Regio highlands plateau formed in the presence of water, which is the most compelling argument for the planet once having oceans on its surface.

In this respect, these research findings could be a step in the direction of resolving the debate of what caused Venus’ surface features: Oceans? Volcanoes? A little from column A, a little from column B? Said Bryne:

“Venus today is hellish, but we don’t know if it was always like this. Was it once like Earth but suffered catastrophic volcanic eruptions that ruined the planet? Right now we cannot say for certain, but the fact of the layering in the tesserae narrows down the potential origins of this rock.”

Further Reading: NCSU, Geology