A new analysis of Venus' surface shows evidence of tectonic motion in the
form of crustal blocks that have jostled against each other like broken
chunks of pack ice. The movement of these blocks could indicate that Venus
is still geologically active and give scientists insight into both exoplanet
tectonics and the earliest tectonic activity on Earth.
"We've identified a previously unrecognized pattern of tectonic deformation
on Venus, one that is driven by interior motion just like on Earth," says
Paul Byrne, associate professor of planetary science at North Carolina State
University and lead and co-corresponding author of the work. "Although
different from the tectonics we currently see on Earth, it is still evidence
of interior motion being expressed at the planet's surface."
The finding is important because 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.
Byrne and an international group of researchers 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 team 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," Byrne
says. "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,"
Byrne says. "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."
The researchers are optimistic that Venus' newly recognized "pack ice"
pattern could offer clues to understanding tectonic deformation on planets
outside of our solar system, as well as on a much younger Earth.
"The thickness of a planet's lithosphere depends mainly upon how hot it is,
both in the interior and on the surface," Byrne says. "Heat flow from the
young Earth's interior was up to three times greater than it is now, so its
lithosphere may have been similar to what we see on Venus today: not thick
enough to form plates that subduct, but thick enough to have fragmented into
blocks that pushed, pulled, and jostled."
NASA and the European Space Agency recently approved three new spacecraft
missions to Venus that will acquire observations of the planet's surface at
much higher resolution than Magellan. "It's great to see renewed interest in
the exploration of Venus, and I'm particularly excited that these missions
will be able to test our key finding that the planet's lowlands have
fragmented into jostling crustal blocks," Byrne says.
The work appears in Proceedings of the National Academy of Sciences.
Sean Solomon of Columbia University is co-corresponding author. Richard
Ghail of the University of London, Surrey; A. M. Celâl Sengör of Istanbul
Technical University; Peter James of Baylor University; and Christian
Klimczak of the University of Georgia also contributed to the work.
Reference:
Paul K. Byrne el al., "A globally fragmented and mobile lithosphere on
Venus," PNAS (2021).
www.pnas.org/cgi/doi/10.1073/pnas.2025919118
Tags:
Space & Astrophysics