How the moon turned itself inside out
University of Arizona scientists combined computer simulations and spacecraft data to solve a long-standing mystery surrounding the moon's "lopsided" geology
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SCHEMATIC ILLUSTRATION WITH A GRAVITY GRADIENT MAP (BLUE HEXAGONAL PATTERN) OF THE LUNAR NEARSIDE AND A CROSS-SECTION SHOWING TWO ILMENITE-BEARING CUMULATE DOWNWELLINGS FROM LUNAR MANTLE OVERTURN.
view moreCREDIT: ADRIEN BROQUET/UNIVERSITY OF ARIZONA & AUDREY LASBORDES
About 4.5 billion years ago, a small planet smashed into the young Earth, flinging molten rock into space. Slowly, the debris coalesced, cooled and solidified, forming our moon. This scenario of how the Earth's moon came to be is the one largely agreed upon by most scientists. But the details of how exactly that happened are "more of a choose-your-own adventure novel," according to researchers in the University of Arizona Lunar and Planetary Laboratory who published a paper in Nature Geoscience. The findings offer important insights into the evolution of the lunar interior, and potentially for planets such as the Earth or Mars.
Most of what is known about the origin of the moon comes from analyses of rock samples, collected by Apollo astronauts more than 50 years ago, combined with theoretical models. The samples of basaltic lava rocks brought back from the moon showed surprisingly high concentrations of titanium. Later satellite observations found that these titanium-rich volcanic rocks are primarily located on the moon's nearside, but how and why they got there has remained a mystery – until now.
Because the moon formed fast and hot, it was likely covered by a global magma ocean. As the molten rock gradually cooled and solidified, it formed the moon's mantle and the bright crust we see when we look up at a full moon at night. But deeper below the surface, the young moon was wildly out of equilibrium. Models suggest that the last dregs of the magma ocean crystallized into dense minerals including ilmenite, a mineral containing titanium and iron.
"Because these heavy minerals are denser than the mantle underneath, it creates a gravitational instability, and you would expect this layer to sink deeper into the moon's interior," said Weigang Liang, who led the research as part of his doctoral work at LPL.
Somehow, in the millennia that followed, that dense material did sink into the interior, mixed with the mantle, melted and returned to the surface as titanium-rich lava flows that we see on the surface today.
"Our moon literally turned itself inside out," said co-author and LPL associate professor Jeff Andrews-Hanna. "But there has been little physical evidence to shed light on the exact sequence of events during this critical phase of lunar history, and there is a lot of disagreement in the details of what went down – literally."
Did this material sink as it formed a little at a time, or all at once after the moon had fully solidified? Did it sink into the interior globally and then rise up on the near side, or did it migrate to the near side and then sink? Did it sink in one big blob, or several smaller blobs?
"Without evidence, you can pick your favorite model. Each model holds profound implications for the geologic evolution of our moon," said co-lead author Adrien Broquet of the German Aerospace Center in Berlin, who did the work during his time as a postdoctoral research associate at LPL.
In a previous study, led by Nan Zhang at Peking University in Beijing, who is also a co-author on the latest paper, models predicted that the dense layer of titanium-rich material beneath the crust first migrated to the near side of the moon, possibly triggered by a giant impact on the far side, and then sunk into the interior in a network of sheetlike slabs, cascading into the lunar interior almost like waterfalls. But when that material sank, it left behind a small remnant in a geometric pattern of intersecting linear bodies of dense titanium-rich material beneath the crust.
"When we saw those model predictions, it was like a lightbulb went on," said Andrews-Hanna, "because we see the exact same pattern when we look at subtle variations in the moon’s gravity field, revealing a network of dense material lurking below the crust."
In the new study, the authors compared simulations of a sinking ilmenite-rich layer to a set of linear gravity anomalies detected by NASA's GRAIL mission, whose two spacecraft orbited the moon between 2011 and 2012, measuring tiny variations in its gravitational pull. These linear anomalies surround a vast dark region of the lunar near side covered by volcanic flows known as mare (Latin for "sea").
The authors found that the gravity signatures measured by the GRAIL mission are consistent with ilmenite layer simulations, and that the gravity field can be used to map out the distribution of the ilmenite remnants left after the sinking of the majority of the dense layer.
"Our analyses show that the models and data are telling one remarkably consistent story," Liang said. "Ilmenite materials migrated to the near side and sunk into the interior in sheetlike cascades, leaving behind a vestige that causes anomalies in the moon's gravity field, as seen by GRAIL."
The team's observations also constrain the timing of this event: The linear gravity anomalies are interrupted by the largest and oldest impact basins on the near side and therefore must have formed earlier. Based on these cross-cutting relationships, the authors suggest that the ilmenite-rich layer sank prior to 4.22 billion years ago, which is consistent with it contributing to later volcanism seen on the lunar surface.
"Analyzing these variations in the moon's gravity field allowed us to peek under the moon's surface and see what lies beneath," said Broquet, who worked with Liang to show that the anomalies in the moon’s gravitational field match what would be expected for the zones of dense titanium-rich material predicted by computer simulation models of lunar overturn.
Three views of the moon's nearside: the familiar sight from Earth (left), regions covered by titanium-rich volcanic flows (center) and polygonal pattern of gravity anomalies
The lunar near side with its dark regions, or “mare,” covered by titanium-rich volcanic flows (center) makes up the moon’s familiar sight from Earth (left). The mare region is surrounded by a polygonal pattern of linear gravity anomalies (blue in image on the right) interpreted to be the vestiges of dense material that sank into the interior. Their presence provides the first physical evidence for the nature of the global mantle overturn more than 4 billion years ago.
CREDIT
Adrien Broquet/University of Arizona
More than 50 years ago, Apollo astronauts brought basaltic lava rocks back from the moon with surprisingly high concentrations of titanium. Later, satellite observations found that these titanium-rich volcanic rocks are primarily located on the moon's nearside - but how and why they got there has remained a mystery – until now.
CREDIT
NASA
Lopsided moon
While the detection of lunar gravity anomalies provides evidence for the sinking of a dense layer in the moon’s interior and allows for a more precise estimate of how and when this event occurred, what we see on the surface of the moon adds even more intrigue to the story, according to the research team.
"The moon is fundamentally lopsided in every respect," Andrews-Hanna said, explaining that the near side facing the Earth, and particularly the dark region known as Oceanus Procellarum region, is lower in elevation, has a thinner crust, is largely covered in lava flows, and has high concentrations of typically rare elements like titanium and thorium. The far side differs in each of these respects. Somehow, the overturn of the lunar mantle is thought to be related to the unique structure and history of the near side Procellarum region. But the details of that overturn have been a matter of considerable debate among scientists.
"Our work connects the dots between the geophysical evidence for the interior structure of the moon and computer models of its evolution," Liang added.
"For the first time we have physical evidence showing us what was happening in the moon’s interior during this critical stage in its evolution, and that's really exciting," Andrews-Hanna said. "It turns out that the moon’s earliest history is written below the surface, and it just took the right combination of models and data to unveil that story."
"The vestiges of early lunar evolution are present below the crust today, which is mesmerizing," Broquet said. "Future missions, such as with a seismic network, would allow a better investigation of the geometry of these structures."
Liang added: "When the Artemis astronauts eventually land on the moon to begin a new era of human exploration, we will have a very different understanding of our neighbor than we did when the Apollo astronauts first set foot on it."
JOURNAL
Nature Geoscience
METHOD OF RESEARCH
Data/statistical analysis
SUBJECT OF RESEARCH
Not applicable
ARTICLE TITLE
Vestiges of a lunar ilmenite layer following mantle overturn revealed by gravity data
ARTICLE PUBLICATION DATE
8-Apr-2024
CSIRO telescope detects unprecedented behaviour from nearby magnetar
VIDEO:
ARTIST’S IMPRESSION OF A MAGNETAR WITH MAGNETIC FIELD AND POWERFUL JETS.
view moreCREDIT: CSIRO
Researchers using Murriyang, CSIRO’s Parkes radio telescope, have detected unusual radio pulses from a previously dormant star with a powerful magnetic field.
New results published today in Nature Astronomy describe radio signals from magnetar XTE J1810-197 behaving in complex ways.
Magnetars are a type of neutron star and the strongest magnets in the Universe. At roughly 8,000 light years away, this magnetar is also the closest known to Earth.
Most are known to emit polarised light, though the light this magnetar is emitting is circularly polarised, where the light appears to spiral as it moves through space.
Dr Marcus Lower, a postdoctoral fellow at Australia’s national science agency – CSIRO, led the latest research and said the results are unexpected and totally unprecedented.
"Unlike the radio signals we've seen from other magnetars, this one is emitting enormous amounts of rapidly changing circular polarisation. We had never seen anything like this before,” Dr Lower said.
Dr Manisha Caleb from the University of Sydney and co-author on the study said studying magnetars offers insights into the physics of intense magnetic fields and the environments these create.
"The signals emitted from this magnetar imply that interactions at the surface of the star are more complex than previous theoretical explanations.”
Detecting radio pulses from magnetars is already extremely rare: XTE J1810-197 is one of only a handful known to produce them.
While it’s not certain why this magnetar is behaving so differently, the team has an idea.
“Our results suggest there is a superheated plasma above the magnetar's magnetic pole, which is acting like a polarising filter,” Dr Lower said.
“How exactly the plasma is doing this is still to be determined.”
XTE J1810-197 was first observed to emit radio signals in 2003. Then it went silent for well over a decade. The signals were again detected by the University of Manchester's 76-m Lovell telescope at the Jodrell Bank Observatory in 2018 and quickly followed up by Murriyang, which has been crucial to observing the magnetar’s radio emissions ever since.
The 64-m diameter telescope on Wiradjuri Country is equipped with a cutting edge ultra-wide bandwidth receiver. The receiver was designed by CSIRO engineers who are world leaders in developing technologies for radio astronomy applications.
The receiver allows for more precise measurements of celestial objects, especially magnetars, as it is highly sensitive to changes in brightness and polarisation across a broad range of radio frequencies.
Studies of magnetars such as these provide insights into a range of extreme and unusual phenomena, such as plasma dynamics, bursts of X-rays and gamma-rays, and potentially fast radio bursts.
Lower, M. E., et al., Linear to circular conversion in the polarized radio emission of a magnetar, Nature Astronomy, vol. 8 (2024)
– ends –
CSIRO acknowledges the Wiradjuri People as the traditional custodians of the Parkes Observatory site where Murriyang, our Parkes radio telescope, is located.
Images and b-roll video are available here.
CAPTION
Murriyang, CSIRO’s Parkes radio telescope beneath the Milky Way.
CREDIT
Alex Cherney/CSIRO
Murriyang, CSIRO’s Parkes radio telescope in the field with wild kangaroos.
CREDIT
CSIRO
Artist’s impression of a magnetar.
CREDIT
Carl Knox, OzGrav/Swinburne University of Technology
Artist’s impression of a magnetar with magnetic field and powerful jets.
CREDIT
CSIRO
JOURNAL
Nature Astronomy
METHOD OF RESEARCH
Observational study
SUBJECT OF RESEARCH
Not applicable
ARTICLE TITLE
Linear to circular conversion in the polarized radio emission of a magnetar
ARTICLE PUBLICATION DATE
8-Apr-2024
Ellie Crabbe
Sun, 7 April 2024
Graphic by Lawrence Berkeley National Laboratory of the largest 3D map of our universe to date (Image: Lawrence Berkeley National Laboratory/PA)
Scientists have made the largest 3D map of the universe, measuring how fast it has expanded over billions of years by using the most precise measurements to date.
An international team, including researchers from Sussex university, used an instrument known as the Dark Energy Spectroscopic Instrument (Desi) to create the map.
Their aim was to measure the effects of dark energy, a mysterious force that is believed to be making the universe expand faster and faster.
The scientists said they were able to measure the expansion history of the universe spanning 11 billion years with a precision better than one per cent.
The map, comprising more than six million galaxies, is the largest 3D map of the cosmos constructed.
Dr Eva-Maria Mueller, Ernest Rutherford Fellow at the University of Sussex, who led part of the cosmological interpretation of the Desi data, said she could not initially believe the “fascinating” results.
READ MORE: Sussex university asking people to join its bee experiment
Dr Mueller said: “It was a moment I’d been eagerly anticipating since the start of my PhD.
“The findings were not just interesting – they were captivating, sparking fresh insights into the fundamental nature of our universe.
“It’s moments like these that remind me why I’m passionate about cosmology.”
The Desi instrument uses 5,000 tiny robots within a mountaintop telescope near Tucson, Arizona.
Scientists were able to map the cosmos as it was billions of years ago and traced its growth to what it is today, using light from distant objects in space which are only now reaching Desi.
Professor Carlos Frenk, of Durham University’s department of physics and a member of the Desi team, described the findings as “hugely exciting”.
He said: “Never before has mankind measured the basic properties of our universe with such precision.”
At present, Lambda CDM, a cosmological model that describes the structure and evolution of the universe, is seen by scientists as the leading framework determining how the universe is evolving.
It includes both a weakly interacting type of matter, known as cold dark matter (CDM), and dark energy – also referred to as Lambda.
According to the model, both matter and dark matter slow down the universe’s expansion, while dark energy speeds it up.
Stuart Clark
Sun, 7 April 2024
Interactive
Get ready for a “new” star to appear in the night sky. Not really new of course, but a star that is now below the naked-eye visibility limit is gearing up for an outburst that will bring it within sight of the unaided eye for the first time since the 1940s.
Such a star is called a nova, Latin for “new”. The star, T Coronae Borealis, is actually composed of two stars: a red giant and a white dwarf. The white dwarf is a dense stellar core about the size of the Earth and its gravity is pulling gas off the red giant. This gas accumulates on the white dwarf’s surface before detonating in a thermonuclear explosion, causing the star to temporarily brighten. Eventually, it returns to normal and the cycle repeats.
In the case of T Coronae Borealis, historical observations show that it explodes approximately every 80 years. Astronomers expect it to blow any time between now and September. The chart shows the view looking east from London at about 22.00 BST this week, and marks the location to keep an eye on. When it erupts, the star is expected to reach the same brightness as Alphecca, the brightest star in its home constellation.
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