Category: Uncategorized

In countless grade-school science textbooks, the Earth’s mantle is a yellow-to-orange gradient, a nebulously defined layer between the crust and the core.

To geologists, the mantle is so much more than that. It’s a region that lives somewhere between the cold of the crust and the bright heat of the core. It’s where the ocean floor is born and where tectonic plates die.

A new paper published today in Nature Geoscience paints an even more intricate picture of the mantle as a geochemically diverse mosaic, far different than the relatively uniform lavas that eventually reach the surface. Even more importantly, a copy of this mosaic is hidden deep in the crust. The study is led by Sarah Lambart, assistant professor of geology at the University of Utah, and is funded by European Union’s Horizon 2020 research and innovation program and the National Science Foundation.

“If you look at a painting from Jackson Pollock, you have a lot of different colors,” Lambart says. “Those colors represent different mantle components and the lines are magmas produced by these components and transported to the surface. You look at the yellow line, it’s not going to mix much with the red or black.”

Primitive minerals

Our best access to the mantle comes in the form of lava that erupts at mid-ocean ridges. These ridges are at the middle of the ocean floor and generate new ocean crust. Samples of this lava show that it’s chemically mostly the same anywhere on the planet.

But that’s at odds with what happens at the other end of the crust’s life cycle. Old ocean crust spreads away from mid-ocean ridges until it’s shoved beneath a continent and sinks back into the mantle. What happens after that is somewhat unclear, but if both the mantle and the old crust melt, there should be some variation in the chemical composition of the magmas.

So Lambart and her colleagues from Wales and the Netherlands, sought to discover what the mantle looks like before it rises as lava at a mid-ocean ridge. They examined cores, drilled through the ocean crust, to look at cumulate minerals: the first minerals to crystallize when the magmas enter the crust.

“We looked at the most primitive part of these minerals,” Lambart says, adding that once they located the primitive minerals they analyzed only the chemical composition from those very earliest minerals to form. “If you are not actually looking at the most primitive part you might lose the signal of this first melt that has been delivered to the crust. That is the originality of our work.”

They analyzed the samples centimeter by centimeter to look at variations in isotopes of neodymium and strontium, which can indicate different chemistries of mantle material that come from different types of rock. “If you have isotopic variability in your cumulates, that means that you have to have isotopic variability in the mantle too,” Lambart says.

When the blender turns on

That’s exactly what the team found. The amount of isotope variability in the cumulates was seven times greater than that in the mid-ocean ridge lavas. That means that the mantle is far from well-mixed and that this variability is preserved in the cumulates.

The likely reason, Lambart says, is that different rocks melt at different temperatures. The first rock to melt, for example the old crust, can create channels that can transport magma up to the crust. Melting of another type of rock can do the same. The end result is several networks of channels that converge towards the mid-ocean ridge but don’t mix — hearkening back to the streaks of paint on a Jackson Pollock painting.

To get at what this finding means for geology, picture a smoothie. No — go farther back than that and picture the blender carafe full of fruit, ice, milk and other ingredients. That’s like the mantle — discrete ingredients, as different from each other as a strawberry is from a blueberry. The fully blended smoothie is like the mid-ocean ridge lava. It’s fully mixed. At some point between the deep mantle and the mid-ocean ridge, Earth turns on the blender. Lambart says that her results show that at the very top of the mantle, the mixing hasn’t happened yet. The blender, it turns out, doesn’t turn on until somewhere in the crust.

Lambart’s work helps her and other geologists redefine their idea of how material moves up through the mantle to the surface.

“The problem is we need to find a way to model the geodynamic earth, including plate tectonics, to actually reproduce what is recorded in the rock today,” she says. “So far this link is missing.”

Now Lambart is setting up a new experimental petrology lab to study the conditions for the magmas to preserve their chemical compositions during their journey through the mantle and the crust.

Ashfall from ancient volcanic explosions is the likely source of a strange mineral deposit near the landing site for NASA’s next Mars rover, a new study finds. The research, published in the journal Geology, could help scientists assemble a timeline of volcanic activity and environmental conditions on early Mars.

“This is one of the most tangible pieces of evidence yet for the idea that explosive volcanism was more common on early Mars,” said Christopher Kremer, a graduate student at Brown University who led the work. “Understanding how important explosive volcanism was on early Mars is ultimately important for understand the water budget in Martian magma, groundwater abundance and the thickness of the atmosphere.”

Volcanic explosions happen when gases like water vapor are dissolved in underground magma. When the pressure of that dissolved gas is more than the rock above can hold, it explodes, sending a fiery cloud of ash and lava into the air. Scientists think that these kinds of eruptions should have happened very early in Martian history, when there was more water available to get mixed with magma. As the planet dried out, the volcanic explosions would have died down and given way to more effusive volcanism — a gentler oozing of lava onto the surface. There’s plenty of evidence of an effusive phase to be found on the Martian surface, but evidence of the early explosive phase hasn’t been easy to spot with orbital instruments, Kremer says.

This new study looked at a deposit located in a region called Nili Fossae that’s long been of interest to scientists. The deposit is rich in the mineral olivine, which is common in planetary interiors. That suggests that the deposit is derived from deep underground, but it hasn’t been clear how the material got to the surface. Some researchers have suggested that it’s yet another example of an effusive lava flow. Others have suggested that the material was dredged up by a large asteroid impact — the impact that formed the giant Isidis Basin in which the deposit sits.

For this study, Kremer and colleagues from Brown used high-resolution images from NASA’s Mars Reconnaissance Orbiter to look at the geology of the deposit in fine detail. Kremer’s co-authors on the work are fellow Brown graduate student Mike Bramble, and Jack Mustard, a professor in Brown’s Department of Earth, Environmental and Planetary Sciences and Kremer’s advisor.

“This work departed methodologically from what other folks have done by looking at the physical shape of the terrains that are composed of this bedrock,” Kremer said. “What’s the geometry, the thickness and orientation of the layers that make it up. We found that the explosive volcanism and ashfall explanation ticks all the right boxes, while all of the alternative ideas for what this deposit might be disagree in several important respects with what we observe from orbit.”

The work showed the deposit extends across the surface evenly in long continuous layers that drape evenly across hills, valleys, craters and other features. That even distribution, Kremer says, is much more consistent with ashfall than lava flow. A lava flow would be expected to pool in low-lying areas and leave thin or non-existent traces in highlands.

And the stratigraphic relationships in the area rule out an origin associated with the Isidis impact, the researchers say. They showed that the deposit sits on top of features that are known to have come after the Isidis event, suggesting that the deposit itself came after as well.

The ashfall explanation also helps to account for the deposit’s unusual mineral signatures, the researchers say. The olivine shows signs of widespread alteration through contact with water — far more alteration than other olivine deposits on Mars. That makes sense if this were ashfall, which is porous and therefore susceptible to alteration by small amounts of water, the researchers say.

All told, the researchers say, these orbital data strongly lean toward an ashfall origin. But the team won’t have to rely only on orbital data for long. NASA’s Mars2020 rover is scheduled to land in Jezero Crater, which sits within the olivine deposit. And there are exposures of the deposit within the crater. The olivine-rich unit will almost certainly be one of the rover’s exploration targets, and it might have the final say on what this deposit is.

“What’s exciting is that we’ll see very soon if I’m right or wrong,” Kremer said. “So that’s a little nerve wracking, but if it’s not an ashfall, it’s probably going to be something much stranger. That’s just as fun if not more so.”

If it does turn out to be ashfall, Kremer says, it validates the methodology used in this study as a means of looking at potential ashfall deposits elsewhere on Mars.

But whatever the rover finds will be important in understanding the evolution of the Red Planet.

“One of Mars 2020’s top 10 discoveries is going to be figuring out what this olivine-bearing unit is,” said Mustard, Kremer’s advisor. “That’s something people will be writing and talking about for a long time.”

Scientists from Ireland and France have announced a major new finding about how matter behaves in the extreme conditions of the Sun’s atmosphere.

The scientists used large radio telescopes and ultraviolet cameras on a NASA spacecraft to better understand the exotic but poorly understood “fourth state of matter.” Known as plasma, this matter could hold the key to developing safe, clean and efficient nuclear energy generators on Earth. The scientists published their findings in the leading international journal Nature Communications.

Most of the matter we encounter in our everyday lives comes in the form of solid, liquid or gas, but the majority of the Universe is composed of plasma — a highly unstable and electrically charged fluid. The Sun is also made up of this plasma.

Despite being the most common form of matter in the Universe plasma remains a mystery, mainly due to its scarcity in natural conditions on Earth, which makes it difficult to study. Special laboratories on Earth recreate the extreme conditions of space for this purpose, but the Sun represents an all-natural laboratory to study how plasma behaves in conditions that are often too extreme for the manually constructed Earth-based laboratories.

Postdoctoral Researcher at Trinity College Dublin and the Dublin Institute of Advanced Studies (DIAS), Dr Eoin Carley, led the international collaboration. He said: “The solar atmosphere is a hotbed of extreme activity, with plasma temperatures in excess of 1 million degrees Celsius and particles that travel close to light-speed. The light-speed particles shine bright at radio wavelengths, so we’re able to monitor exactly how plasmas behave with large radio telescopes.”

“We worked closely with scientists at the Paris Observatory and performed observations of the Sun with a large radio telescope located in Nançay in central France. We combined the radio observations with ultraviolet cameras on NASA’s space-based Solar Dynamics Observatory spacecraft to show that plasma on the sun can often emit radio light that pulses like a light-house. We have known about this activity for decades, but our use of space and ground-based equipment allowed us to image the radio pulses for the first time and see exactly how plasmas become unstable in the solar atmosphere.”

Studying the behaviour of plasmas on the Sun allows for a comparison of how they behave on Earth, where much effort is now under way to build magnetic confinement fusion reactors. These are nuclear energy generators that are much safer, cleaner and more efficient than their fission reactor cousins that we currently use for energy today.

Professor at DIAS and collaborator on the project, Peter Gallagher, said: “Nuclear fusion is a different type of nuclear energy generation that fuses plasma atoms together, as opposed to breaking them apart like fission does. Fusion is more stable and safer, and it doesn’t require highly radioactive fuel; in fact, much of the waste material from fusion is inert helium.”

“The only problem is that nuclear fusion plasmas are highly unstable. As soon as the plasma starts generating energy, some natural process switches off the reaction. While this switch-off behaviour is like an inherent safety switch — fusion reactors cannot form runaway reactions — it also means the plasma is difficult to maintain in a stable state for energy generation. By studying how plasmas become unstable on the Sun, we can learn about how to control them on Earth.”

The success of this research was made possible by the close ties between researchers at Trinity, DIAS, and their French collaborators.

Dr Nicole Vilmer, lead collaborator on the project in Paris, said: “The Paris Observatory has a long history of making radio observations of the Sun, dating back to the 1950s. By teaming up with other radio astronomy groups around Europe we are able to make groundbreaking discoveries such as this one and continue the success we have in solar radio astronomy in France. It also further strengthens scientific collaboration between France and Ireland, which I hope continues in the future.”

Dr Carley previously worked at the Paris Observatory, funded by a fellowship awarded by the Irish Research Council and the European Commission. He continues to work closely with his French colleagues today, and hopes to soon study the same phenomena using both French instruments and newly built, state-of-the-art equipment in Ireland.

Dr Carley added: “The collaboration with French scientists is ongoing and we’re already making progress with newly built radio telescopes in Ireland, such as the Irish Low Frequency Array (I-LOFAR). I-LOFAR can be used to uncover new plasma physics on the Sun in far greater detail than before, teaching us about how matter behaves in both plasmas on the Sun, here on Earth and throughout the Universe in general.”

The work was funded by the Irish Research Council.

Earth is bombarded every year by rocky debris, but the rate of incoming meteorites can change over time. Finding enough meteorites scattered on the planet’s surface can be challenging, especially if you are interested in reconstructing how frequently they land. Now, researchers have uncovered a wealth of well-preserved meteorites that allowed them to reconstruct the rate of falling meteorites over the past two million years.

“Our purpose in this work was to see how the meteorite flux to Earth changed over large timescales — millions of years, consistent with astronomical phenomena,” says Alexis Drouard, Aix-Marseille Université, lead author of the new paper in Geology.

To recover a meteorite record for millions of years, the researchers headed to the Atacama Desert. Drouard says they needed a study site that would preserve a wide range of terrestrial ages where the meteorites could persist over long time scales.

While Antarctica and hot deserts both host a large percentage of meteorites on Earth (about 64% and 30%, respectively), Drouard says, “Meteorites found in hot deserts or Antarctica are rarely older than half a million years.” He adds that meteorites naturally disappear because of weathering processes (e.g., erosion by wind), but because these locations themselves are young, the meteorites found on the surface are also young.

“The Atacama Desert in Chile, is very old ([over] 10 million years),” says Drouard. “It also hosts the densest collection of meteorites in the world.”

The team collected 388 meteorites and focused on 54 stony samples from the El Médano area in the Atacama Desert. Using cosmogenic age dating, they found that the mean age was 710,000 years old. In addition, 30% of the samples were older than one million years, and two samples were older than two million. All 54 meteorites were ordinary chondrites, or stony meteorites that contain grainy minerals, but spanned three different types.

“We were expecting more ‘young’ meteorites than ‘old’ ones (as the old ones are lost to weathering),” says Drouard. “But it turned out that the age distribution is perfectly explained by a constant accumulation of meteorites for millions years.” The authors note that this is the oldest meteorite collection on Earth’s surface.

Drouard says this terrestrial crop of meteorites in the Atacama can foster more research on studying meteorite fluxes over large time scales. “We found that the meteorite flux seems to have remained constant over this [two-million-year] period in numbers (222 meteorites larger than 10 g per squared kilometer per million year), but not in composition,” he says. Drouard adds that the team plans to expand their work, measuring more samples and narrowing in on how much time the meteorites spent in space. “This will tell us about the journey of these meteorites from their parent body to Earth’s surface.”

In 1969, a giant earthquake off the coast of Portugal kicked up a tsunami that killed over a dozen people. Some 200 years prior, an even larger earthquake hit the same area, killing around 100,000 people and destroying the city of Lisbon.

Two earthquakes in the same spot over a couple hundred years is not cause for alarm. But what puzzled seismologists about these tremors was that they began in relatively flat beds of the ocean — away from any faults or cracks in the Earth’s crust where tectonic plates slip past each other, releasing energy and causing earthquakes.

So what’s causing the rumbles under a seemingly quiet area? [In Photos: Ocean Hidden Beneath Earth’s Crust]

One idea is that a tectonic plate is peeling into two layers — the top peeling off the bottom layer — a phenomenon that has never been observed before, a group of scientists reported in April at the European Geosciences Union General Assembly held in Vienna. This peeling may be creating a new subduction zone, or an area in which one tectonic plate is rammed beneath another, according to their abstract.

The peeling is likely driven by a water-absorbing layer in the middle of the tectonic plate, according to National Geographic. This layer might have undergone a geological process called serpentinization, in which water that seeps in through cracks causes a layer to transform into soft green minerals. Now, this transformed layer might be causing enough weakness in the plate for the bottom layer to peel away from the top layer. That peeling could lead to deep fractures that trigger a tiny subduction zone, National Geographic reported.

This group isn’t the first to propose this idea, but it’s the first to provide some data on it. They tested their hypothesis with two-dimensional models, and their preliminary results showed that this type of activity is indeed possible — but is still yet to be proven.

This research has not yet been published in a peer-reviewed journal.