A moon-forming cataclysm could have also triggered Earth’s plate tectonics
THE WOODLANDS, TEXAS — Vestiges of a moon-forming cataclysm could have kick-started plate tectonics on Earth.
The leading explanation for the origin of the moon proposes that a Mars-sized planet, dubbed Theia, struck the nascent Earth, ejecting a cloud of debris into space that later coalesced into a satellite (SN: 3/2/18). New computer simulations suggest that purported remains of Theia deep inside the planet could have also triggered the onset of subduction, a hallmark of modern plate tectonics, geodynamicist Qian Yuan of Caltech reported March 13 at the Lunar and Planetary Science Conference.
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The story offers a cohesive explanation for how Earth gained both its moon and its moving tectonic plates, and it could aid in the search for other Earthlike worlds. But others caution that it’s much too early to say that this is, in fact, what happened.
Of all the worlds yet discovered, ours is the only one known to have plate tectonics (SN: 1/13/21). For billions of years, Earth’s creeping plates have spread, collided and plunged beneath one another, birthing and splitting continents, uplifting mountain ranges and widening oceans (SN: 4/22/20, SN: 1/11/17). But all this reshaping has also erased most of the clues to the planet’s early history, including how and when plate tectonics first began.
Many hypotheses have been proposed to explain the initiation of subduction, a tectonic process in which one plate slides under another (SN: 5/2/22; SN: 6/5/19; SN: 1/2/18). Yuan and his colleagues chose to focus on two continent-sized blobs of material in Earth’s lower mantle known as large low-shear velocity provinces (SN: 5/12/16). These are regions through which seismic waves are known to move anomalously slow. Researchers had previously proposed these regions could have formed from old, subducted plates. But in 2021, Yuan and colleagues alternatively proposed that the mysterious masses could be the dense, sunken remnants of Theia.
Building off that previous work, the researchers used computers to simulate how Theia’s impact, and its lingering remains, would impact the flow of rock inside the Earth.
They found that once these hot alien blobs had sunk to the bottom of the mantle, they could have compelled large plumes of warm rock to upwell and wedge into Earth’s rigid outer layer. As upwelling continued to feed into the risen plumes, they would have ballooned and pushed slabs of Earth’s surface beneath them, triggering subduction about 200 million years after the moon formed.
While the simulations suggest the large low-shear velocity provinces could have had a hand in starting subduction, it’s not yet clear whether these masses came from Theia. “The features … are a fairly recent discovery,” says geodynamicist Laurent Montési of the University of Maryland in College Park. “They’re very fascinating structures, with a very unknown origin.” As such, he says, it’s too early to say that Theia triggered plate tectonics.
“It’s provoking. This material down there is something special,” Montési says of the large low-shear velocity provinces. “But whether it has to be originally extraterrestrial, I don’t think the case is made.”
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However, if confirmed, the explanation could have implications that reach beyond our solar system. “If you have a large moon, you likely have a large impactor,” Yuan said. Scientists have yet to confirm the discovery of such an exomoon (SN: 4/30/19). But keeping an eye out, Yuan said, could help us uncover another world as tectonically active as our own.
Saturn’s Rings are Heating Gas Giant’s Upper Atmosphere, Astronomers Say | Sci.News
“Though the slow disintegration of the rings is well known, its influence on the atomic hydrogen of the planet is a surprise,” said Dr. Lotfi Ben-Jaffel, a researcher at the Institute of Astrophysics in Paris and the Lunar & Planetary Laboratory at the University of Arizona.
“From the Cassini probe, we already knew about the rings’ influence. However, we knew nothing about the atomic hydrogen content.”
“Everything is driven by ring particles cascading into the atmosphere at specific latitudes. They modify the upper atmosphere, changing the composition.”
“And then you also have collisional processes with atmospheric gasses that are probably heating the atmosphere at a specific altitude.”
The team’s conclusion required pulling together archival ultraviolet-light (UV) observations from four space missions that studied Saturn.
This includes observations from NASA’s Voyager probes that flew by Saturn in the 1980s and measured the UV excess.
At the time, astronomers dismissed the measurements as noise in the detectors.
NASA’s Cassini mission, which arrived at Saturn in 2004, also collected UV data on the atmosphere (over several years).
Additional data came from Hubble and the International Ultraviolet Explorer, which launched in 1978.
But the lingering question was whether all the data could be illusory, or instead reflected a true phenomenon on Saturn.
The key to assembling the jigsaw puzzle came in the team’s decision to use measurements from Hubble’s Space Telescope Imaging Spectrograph (STIS).
Its precision observations of Saturn were used to calibrate the archival UV data from all four other space missions that have observed Saturn.
The astronomers compared the STIS UV observations of Saturn to the distribution of light from multiple space missions and instruments.
“When everything was calibrated, we saw clearly that the spectra are consistent across all the missions,” Dr. Ben-Jaffel said.
“This was possible because we have the same reference point, from Hubble, on the rate of transfer of energy from the atmosphere as measured over decades.”
“It was really a surprise for me. I just plotted the different light distribution data together, and then I realized, wow — it’s the same.”
Four decades of UV data cover multiple solar cycles and help astronomers study the Sun’s seasonal effects on Saturn.
By bringing all the diverse data together and calibrating it, Dr. Ben-Jaffel and colleagues found that there is no difference to the level of UV radiation.
“At any time, at any position on the planet, we can follow the UV level of radiation. This points to the steady ‘ice rain’ from Saturn’s rings as the best explanation,” Dr. Ben-Jaffel said.
“We are just at the beginning of this ring characterization effect on the upper atmosphere of a planet.”
“We eventually want to have a global approach that would yield a real signature about the atmospheres on distant worlds.”
“One of the goals of this study is to see how we can apply it to planets orbiting other stars. Call it the search for exo-rings.”
A paper on the findings was published in the Planetary Science Journal.
Lotfi Ben-Jaffel et al. 2023. The Enigmatic Abundance of Atomic Hydrogen in Saturn’s Upper Atmosphere. Planet. Sci. J 4, 54; doi: 10.3847/PSJ/acaf78
Venus is Home to At Least 85,000 Volcanoes, Planetary Researchers Say | Sci.News
“Our paper provides the most comprehensive map of all volcanic edifices on Venus ever compiled,” said Dr. Paul Byrne, a planetary scientist at Washington University in St. Louis.
“It provides researchers with an enormously valuable database for understanding volcanism on that planet — a key planetary process, but for Venus is something about which we know very little, even though it’s a world about the same size as our own.”
In their research, Dr. Byrne and his colleague, Washington University in St. Louis graduate student Rebecca Hahn, used radar imagery from NASA’s Magellan mission to catalog volcanoes across Venus at a global scale.
Their database contains 85,000 volcanoes, about 99% of which are less than 5 km in diameter.
“Since NASA’s Magellan mission in the 1990s, we’ve had numerous major questions about Venus’ geology, including its volcanic characteristics,” Dr. Byrne said.
“But with the recent discovery of active volcanism on Venus, understanding just where volcanoes are concentrated on the planet, how many there are, how big they are, etc., becomes all the more important — especially since we’ll have new data for Venus in the coming years.”
The team’s study includes detailed analyses of where volcanoes are, where and how they’re clustered, and how their spatial distributions compare with geophysical properties of the planet such as crustal thickness.
Taken together, the work provides the most comprehensive understanding of Venus’ volcanic properties — and perhaps of any world’s volcanism so far.
That’s because, although we know a great deal about the volcanoes on Earth that are on land, there are still likely a great many yet to be discovered under the oceans.
Lacking oceans of its own, Venus’ entire surface can be viewed with Magellan radar imagery.
Although there are volcanoes across almost the entire surface of Venus, the scientists found relatively fewer volcanoes in the 20-100 km (12-62 miles) diameter range, which may be a function of magma availability and eruption rate.
They also wanted to take a closer look at smaller volcanoes on Venus, those less than 5 km across that have been overlooked by previous volcano hunters.
“They’re the most common volcanic feature on the planet: they represent about 99% of our dataset,” Hahn said.
“We looked at their distribution using different spatial statistics to figure out whether the volcanoes are clustered around other structures on Venus, or if they’re grouped in certain areas.”
The team’s paper was published in the Journal of Geophysical Research: Planets.
Rebecca M. Hahn & Paul K. Byrne. A Morphological and Spatial Analysis of Volcanoes on Venus. Journal of Geophysical Research: Planets, published online March 24, 2023; doi: 10.1029/2023JE007753
Impact Glass Beads from the Moon Contain Solar Wind-Derived Water, Study Shows | Sci.News
It has long been argued that there could be water and other volatile species at the surface of the Moon.
Renewed lunar exploration and advances in remote-sensing measurements in the 1990s allowed the neutron spectrometer on board NASA’s Lunar Prospector mission to confirm the existence of water ice at the lunar poles.
Following this, the Moon mineralogy mapper instrument on board India’s Chandrayaan-1 spacecraft detected the absorption bands of hydroxyl and/or water on the lunar surface.
Furthermore, NASA’s Lunar Crater Observation and Sensing Satellite impact experiment carried out in 2009 provided direct evidence for high water-ice abundances in permanently shadowed regions within Cabeus crater.
Elevated water-ice abundance in lunar polar regions was further supported by the neutron flux measurements performed by the Lunar Exploration Neutron Detector on board NASA’s Lunar Reconnaissance Orbiter spacecraft.
Recently, the neutral mass spectrometer on NASA’s Lunar Atmosphere and Dust Environment Explorer detected exospheric water liberated by meteoroid impacts, and ground-based telescope observations detected molecular water on the lunar surface.
Today, there is little doubt that most of the Moon’s surface harbors water in one form or another.
However, the origins of this water and its distribution and evolution during regolith gardening remain largely unknown, despite key implications for future lunar surface exploration and for better understanding the surface water reservoir and processing on solar system airless bodies.
“We proposed that impact glass beads, a ubiquitous component in lunar soils with an amorphous nature, are a potential candidate for investigation of the unidentified hydrated layer or reservoir in lunar soils,” said first author Huicun He, a doctoral student at the Key Laboratory of Earth and Planetary Physics and the University of Chinese Academy of Sciences.
In their research, He and her colleagues analyzed the water content within glass beads produced by impact events, extracted from Chang’e-5 lunar soil samples.
They identified water stored within these impact glass that is consistent with a solar wind origin.
Furthermore, the distribution of water within individual beads indicates that water can rapidly accumulate in glass beads by diffusion, over timescales of only a few years, and be rapidly released.
“These impact glass beads have homogeneous chemical compositions and smooth exposed surfaces,” they said.
“They are characterized by water abundance up to about 2,000 μg/g, with extreme deuterium-depleted characteristics.”
“The negative correlation between water abundance and hydrogen isotope composition reflects the fact that water in the impact glass beads comes from solar wind.”
“The impact glass beads acted as a sponge for buffering the lunar surface water cycle,” they added.
“We estimate that the amount of water hosted by impact glass beads in lunar soils may reach up to 2.7*1014 kg.”
“Our direct measurements of this surface reservoir of lunar water show that impact glass beads can store substantial quantities of solar wind-derived water on the Moon and suggest that impact glass may be water reservoirs on other airless bodies.”
The findings appear in the journal Nature Geoscience.
H. He et al. A solar wind-derived water reservoir on the Moon hosted by impact glass beads. Nat. Geosci, published online March 27, 2023; doi: 10.1038/s41561-023-01159-6
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