Using data collected by the synthetic aperture radar on NASA’s Magellan spacecraft, a team of planetary researchers has identified a pattern of tectonic deformation on Venus that suggests that many of the planet’s lowlands have fragmented into discrete crustal blocks, and that these blocks have moved relative to each other in the geologically recent past.
The largest crustal block Byrne et al. identified in the Venus lowlands: (A) Magellan radar image mosaic of the block; named landforms are shown; the black and white arrows mark impact craters and prominent lava flows, respectively; the approximate outline of the campus is marked by a dashed yellow line; (B) radar image (left) and structural sketch (right) of extensional faults curving into the main groove belt that delineates the southwestern margin of this campus, here with a right-lateral sense of slip; (C) sigmoidal, positive-relief landforms we interpret as right-lateral transpressive structures; (D) another example of sigmoidal, positive-relief ridges we regard as denoting right-lateral transpression, with a different strike from those structures in (C); in these and subsequent structural sketches, prominent extensional structures are in purple (with ball-and-bar symbols shown on example down-thrown blocks), and prominent shortening structures are in teal (with sawtooth symbols on example upper blocks); minor and/or poorly expressed fractures of various types are shown as thin, black lines; wrinkle ridges are generally not recorded; exemplar extensional and shortening landforms are marked with gold and red arrows, respectively; the radar look direction is from the left in (A), and from the right in (B-D). All images are in azimuthal equidistant projection, centered at 24.5°S, 146.5°E (A); 20.0°S, 138.3°E (B); 27.2°S, 141.1°E (C); and 18.0°S, 145.0°E (D). Image credit: Byrne et al., doi: 10.1073/pnas.2025919118.
Despite close similarities in mass and composition, Earth and Venus have followed different evolutionary paths, at least over the recent history of Solar System.
Given the detection of ever more Earth-mass exoplanets at distances from their host stars in the so-called ‘Venus zone,’ it is increasingly important to understand the geological character and history of our nearest planetary neighbor.
Venus has long been assumed to have an immobile solid outer shell, or lithosphere, just like Mars or Earth’s Moon.
In contrast, Earth’s lithosphere is broken into tectonic plates, which slide against, apart from, and underneath each other on top of a hot, weaker mantle layer.
The Venusian surface has been extensively deformed, and convection of the underlying mantle, possibly acting in concert with a low-strength lower crust, has been suggested as a source of some surface horizontal strains.
The extent of surface mobility on the planet driven by mantle convection, however, and the style and scale of its tectonic expression have been unclear.
“We’ve identified a previously unrecognized pattern of tectonic deformation on Venus, one that is driven by interior motion just like on Earth,” said Dr. Paul Byrne, a planetary researcher in the Department of Marine, Earth, and Atmospheric Sciences at North Carolina State University.
“Although different from the tectonics we currently see on Earth, it is still evidence of interior motion being expressed at the planet’s surface.”
In the current study, Dr. Byrne and colleagues used radar images from NASA’s Magellan mission to map the surface of Venus.
In examining the extensive Venusian lowlands that make up most of the planet surface, they saw areas where large blocks of the lithosphere seem to have moved: pulling apart, pushing together, rotating and sliding past each other like broken pack ice over a frozen lake.
The scientists created a computer model of this deformation, and found that sluggish motion of the planet’s interior can account for the style of tectonics seen at the surface.
“These observations tell us that interior motion is driving surface deformation on Venus, in a similar way to what happens on Earth,” Dr. Byrne said.
“Plate tectonics on Earth are driven by convection in the mantle. The mantle is hot or cold in different places, it moves, and some of that motion transfers to Earth’s surface in the form of plate movement.”
“A variation on that theme seems to be playing out on Venus as well. It’s not plate tectonics like on Earth — there aren’t huge mountain ranges being created here, or giant subduction systems — but it is evidence of deformation due to interior mantle flow, which hasn’t been demonstrated on a global scale before.”
The deformation associated with these crustal blocks could also indicate that Venus is still geologically active.
“We know that much of Venus has been volcanically resurfaced over time, so some parts of the planet might be really young, geologically speaking,” Dr. Byrne said.
“But several of the jostling blocks have formed in and deformed these young lava plains, which means that the lithosphere fragmented after those plains were laid down.”
“This gives us reason to think that some of these blocks may have moved geologically very recently — perhaps even up to today.”
A paper on the findings was published in the Proceedings of the National Academy of Sciences.
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Paul K. Byrne et al. 2021. A globally fragmented and mobile lithosphere on Venus. PNAS 118 (26): e2025919118; doi: 10.1073/pnas.2025919118
