A significant portion of some of the oldest terrain on Venus, known as tesserae, has striations consistent with layering, according to new research led by North Carolina State University.
A simulated view from above Tellus Tessera, one of the regions on Venus where Byrne et al identified the presence of layering. Image credit: NASA.
Tesserae were first identified as a terrain type on Venus from radar images delivered by Venera 15 and 16 missions.
Originally referred to as ‘parquet’ terrain, they are areally extensive regions on the Venusian surface that are pervasively deformed by two or more intersecting sets of ridges and grooves.
Tesserae cover about 7.3% of the planet’s surface, and are always the oldest feature in their immediate surroundings, dating to about 750 million years old.
They are concentrated in the equatorial zone and at high northern latitudes; there is a distinct paucity of tessera terrain south of about 30° S, where the vast low-lying plains dominate the surface of Venus.
Although each patch of tesserae is distinct, most of the tesserae areas share several properties.
Tessera deformation occurs on scales of tens of miles to the limits of resolution of current imaging data.
Tessera terrain is rough, and usually elevated above surrounding areas, and its low free-air gravity anomaly indicates that this extra elevation is supported by a low-density crustal root.
Filling and embayment of low areas by radar-dark material within and on the margins of tesserae is common.
At present the origin of Venusian tesserae is unknown, but several hypotheses exist for their formation.
“There are generally two explanations for tesserae — either they are made of volcanic rocks, or they are counterparts of Earth’s continental crust,” said Dr. Paul Byrne, a planetary scientist in the Department of Marine, Earth, and Atmospheric Sciences at North Carolina State University.
“But the layering we find on some of the tessera isn’t consistent with the continental crust explanation.”
Dr. Byrne and colleagues analyzed images of Venus’ surface from NASA’s Magellan mission, which used radar to image 98% of the planet through its dense atmosphere.
While planetary researchers have studied the tesserae for decades, prior to this work the layering of the tesserae hasn’t been recognized as widespread.
“The layering would not be possible if the tesserae were portions of continental crust,” Dr. Byrne said.
“Continental crust is composed mainly of granite, an igneous rock formed when tectonic plates move and water is subducted from the surface.”
“But granite doesn’t form layers. If there’s continental crust on Venus, then it’s below the layered rocks we see.”
“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 m (roughly 3,000 feet) 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.”
The authors hope that their work will help to shed light on more of Venus’ complicated geological history.
“While the data we have now point to volcanic origins for the tesserae, if we were one day able to sample them and find that they are sedimentary rocks, then they would have had to have formed when the climate was very different — perhaps even Earth-like,” Dr. Byrne said.
“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.”
The study was published in the journal Geology.
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Paul K. Byrne et al. Venus tesserae feature layered, folded, and eroded rocks. Geology, published online September 4, 2020; doi: 10.1130/G47940.1
