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Origins of meteorites?

Origins of meteorites?


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I am aware that the vast majority of meteorites come from the asteroid belt (including 4 Vesta), with some from the Moon and Mars. Are there any other locations from which meteorites have originated (the Kuiper belt? Ceres?). Here it mentions that carbonaceous chondrites formed farthest from the Sun. Is that still within the asteroid belt?


Kuiper belt (and beyond) objects do sometimes come into the inner solar system: we call them "comets". However comets have a large proportion of volatiles (water etc) and the non-volatile matter tends to be in the form of small dust particles, neither of which form useful meteorites Ceres is also covered mostly in ice, so even if some was ejected into space and reach the Earth, it wouldn't survive the descent.

An older paper wonders if very old dead comets might be a source of meteorites, but notes that none have been found. A much more recent paper describes a small (100 µm) particle of cometary dust found embedded in another meteorite. This dust particle would have come from the outer solar system.


Top 10 Largest Meteorite Strikes in History

This article will take a look at 10 of the world&aposs largest meteorite strikes.

When a child sees a shooting star in the night sky, he makes a wish. Then, in astronomy lessons, he is told that these rapidly moving points of fire are just space debris.

Large and small cosmic bodies continuously bombard the Earth. Most of them burn up without residue in the upper atmosphere, but especially large ones reach the surface. Such "travelers" are capable of causing quite a stir.

The Chelyabinsk meteorite, which landed in 2013, stripped the windows of an entire block. The crater from the Chelyabinsk meteorite has not been found to this day. However, traces left by its larger brethren survived on the surface of the Earth. The depressions created by the impacts of space wanderers make it possible to judge the size of the meteorites.

We present to your attention the 10 largest meteorite impacts known to humankind.


Another Water-carrying Candidate

This example of an enstatite chondrite meteorite is on display in the Vale Inco Limited Gallery of Minerals at the Royal Ontario Museum.
Wikimedia Commons / CC BY 3.0

Enstatite chondrites formed in the first million years of the solar system, close to the Sun in an environment rich in gaseous carbon and oxygen compounds. Previous studies have found that isotopic ratios of a number of elements — including oxygen, calcium, titanium, neodymium, chromium, nickel, molybdenum, and ruthenium — are almost indistinguishable between these particular asteroids and Earth and the Moon.

"We consider [enstatite chondrites] the best analog we have for the Earth's building blocks," says Piani.

Piani’s team collected and analyzed 13 samples of rare enstatite meteorites, finding traces of water that could have been accreted into the young Earth. Water accounted for 0.08% to 0.5% of the analyzed meteorites, but Piani says the highest values are the most relevant ones, because they came from the meteorites showing the fewest signs of alteration since their formation. That 0.5% concentration is far short of carbonaceous chondrite meteorites, whichh hold up to 10% of their mass in water. But Piani points out that surface oceans account for only 0.02% of the Earth's mass, and total water only about 0.2%.

The enstatite meteorites found on Earth are the only samples we have of the primordial enstatite chondrites that formed near the sun. If those objects formed most of the building blocks of Earth, they easily could have provided most terrestrial water, says Piani. Her group proposes enstatite chondrites supplied all the water in the mantle and 95% of that in the oceans, with the rest coming from wet asteroids.

Finding the new water source means "we do not need to invoke complicated, unlikely dynamical models of bringing water-rich asteroids from the outer solar system to supply Earth with water," says Anne Peslier (NASA Johnson Space Flight Center), who wrote an accompanying perspective piece in Science. "Water was simply acquired like most other materials when the planet formed from accretion of dust in the inner solar nebula."

The new evidence suggests Earth grew from building blocks that were wet or at least damp, with some later contributions from other sources. One question that remains is how the original water came to be in rocks that formed close to the Sun at low pressure. In theory, it should have evaporated under those conditions. To find out, Piani's group plans a new series of experiments to study more of the hydrogen-bearing material in the meteorites and investigate its behavior as well as that of other volatiles that may have been present in the early solar system.


Study reveals secret origins of asteroids and meteorites

Illustration of a large asteroid splintering. Credit: Don Davis

Most asteroids and meteorites originate from the splintering of a handful of minor planets formed during the infancy of our solar system, a new study shows.

A study appearing online today in Nature Astronomy found at least 85 percent of 200,000 asteroids in the inner asteroid belt—the main source of Earth's meteorites—originate from five or six ancient minor planets. The other 15 percent may also trace their origins to the same group of primordial bodies, said Stanley Dermott, lead author and a theoretical astronomer at the University of Florida.

The discovery is important for understanding the materials that shaped our own rocky planet, Dermott said.

The finding provides a more robust understanding of the evolutionary history of asteroids and the materials that form them—information Dermott says could prove essential to protecting the Earth and ourselves from meteorites the size of the Statue of Liberty and asteroids more powerful than atomic bombs.

"These large bodies whiz by the Earth, so of course we're very concerned about how many of these there are and what types of material are in them," said Dermott, professor emeritus in UF's College of Liberal Arts and Sciences. "If ever one of these comes towards the earth, and we want to deflect it, we need to know what its nature is."

UF astronomer Stanley Dermott discusses his new paper on the origins of asteroids. Credit: University of Florida

Dermott's team demonstrated that the type of orbit an asteroid has depends on the size of the asteroid. This finding suggests that differences in meteorites found on Earth appear because of the evolutionary changes that occurred inside a few large, precursor bodies that existed more than four billion years ago, Dermott said.

"I wouldn't be surprised if we eventually trace the origins of all asteroids in the main asteroid belt, not just those in the inner belt, to a small number of known parent bodies," Dermott said.

Building knowledge of the evolutionary history of bodies that formed our early solar system helps theoretical astronomers answer questions related to where planets like our own might exist in the universe, Dermott said. But, first, he said we have to understand the processes that produced the planet we live on.


History

The American Meteor Society was founded in 1911 by the late Dr. Charles P. Olivier, as an offshoot from the American Astronomical Society (AAS). Dr. Olivier’s goal was to organize a collaborative effort between amateur and professional astronomers, for the purpose of conducting visual observations of both meteor showers and meteors appearing at random (sporadic meteors). Over his 65-year career, Dr. Olivier attracted a wide variety of observers who helped him explore a number of areas within “classical” meteoric astronomy.

Dr. Olivier’s work revolved around the single visual meteor observer, usually an amateur astronomer, who joined the AMS with the understanding that he (or she) would, as occasion permitted, submit observations on meteors and related phenomena. During the visual program’s heyday, the AMS consisted of hundreds of such amateur observers, directed by volunteer regional coordinators, and often organized into state wide affiliate groups. Over the years, the observations submitted by these affiliate members served as the basis for hundreds of short scientific papers, along with a score of long, important works, printed in various astronomical journals, AMS bulletins, and parts of several observatory publications. Dr. Olivier’s 1925 professional text, “Meteors,” has become one of the classic reference sources for this field.

During the 1950’s, the advent of more sophisticated techniques and a shift in emphasis in the professional community to photographic and radar techniques led Dr. Olivier to shift the research emphasis of the AMS to a long term statistical study of the sporadic meteor flux. Containing many years of previous observations, the AMS Visual Database formed the basis for four catalogs giving the average visual meteor rates seen for each hour of the night during the year (Olivier, 1960, 1965, 1974a, and 1974b). Three of these catalogs were for the northern hemisphere and one for the southern hemisphere. The northern hemisphere catalogs were average rates over the years 1901-1958, 1959-1963, and 1964-1972. Dr. Olivier continued this work throughout his retirement, and was completing the manuscript for a fifth catalogue at the time of his death in 1976.

Only 10 days before his death, Dr. Olivier passed the leadership of the AMS over to Dr. David D. Meisel. Dr. Meisel had literally grown up within the AMS, conducting amateur observations as a teenager and moving on to pursue a professional career in astronomy. Following the receipt of his Ph.D from Ohio State University, Dr. Meisel had continued to collaborate with Dr. Olivier in the area of comet and meteor studies, and the two had developed a close working relationship and friendship.

Upon receiving the mantle, Dr. Meisel’s first challenge was to secure a suitable home and headquarters for the AMS. Dr. Meisel’s institution, the State University of New York, College at Geneseo graciously accepted the organization, and since 1976, the AMS headquarters have been located in the Department of Physics and Astronomy there. For some 17 years, the public service and scientific activities of the AMS were supported in part by SUNY – Geneseo as a part of the astronomy program at the College.

These first years of Dr. Meisel’s leadership were difficult ones for the AMS. The professional community was turning away from meteor work of any sort, and given the meager funds then available, the organization was only able to continue a few of Dr. Olivier’s original goals. Nonetheless, the AMS continued to have a dedicated staff of amateur coordinators to help with its various modest research programs. The expenses for these programs often were paid by the coordinators themselves with no help from the society, and it was this volunteer effort which kept the AMS going during this rough transition period.

One such effort, the “AMS Radio Scatter Program,” was founded in 1977. The Kansas Meteor Group of the AMS led by Walter Scott Houston had conducted pioneering radiometeor experiments in the late 1950’s. Much work was done in the professional community on radio- and radar- meteor observation, and by the late 1970’s, advances in technology made sensitive receiving equipment practical for amateur use. The formal AMS program began to explore a new avenue in which amateur scientists could contribute to meteor astronomy beyond traditional visual observations. Many promising experiments were conducted in this area throughout the 1980’s and early 1990’s, with a working automated station developed by 1993.

Also in 1993, the organization was able to become the American Meteor Society, Ltd., thanks to a generous permanent endowment from the estate of Dr. Clinton B. Ford. In the United States, Dr. Ford was a well-known supporter of amateur astronomy with particular interest in variable stars. Throughout his life-time, he was a member of the AMS, having started observing meteors as a boy. His generosity has made possible the reorganization and modernization of the AMS along the lines described in this bulletin. Our new structure will enable the society to further the cause of meteor science for many years to come, while continuing the tradition of amateur-professional collaboration.


Meteorite’s origins point to possible undiscovered asteroid

The Bunburra Rockhole meteorite was discovered in Australia. Credit: Imperial College London

A new analysis of a meteorite called Bunburra Rockhole has revealed that the rock originated from a previously unknown parent asteroid, allowing scientists to understand the geology of the parent body.

The parent body was differentiated, meaning that it was large enough to separate into a core, mantle and crust, and was roughly spherical in shape, though not as large as a planet. Identifying a new differentiated asteroid is vital for understanding the formation of asteroids and planets in the Solar System. Most of the large asteroids in the Asteroid Belt are already known, so this means that either the meteorite originated on an asteroid that has been eroded, or there is another large asteroid out there.

Bunburra Rockhole was the first meteorite to be recovered using the Desert Fireball Network, a network of cameras across Australia that observe where meteoroids enter the atmosphere. These cameras make it possible to determine the orbit of a meteorite prior to its descent to Earth. Models of the orbit of Bunburra Rockhole placed its origin within the innermost, main asteroid belt, interior to Vesta, the second-largest body in the Asteroid Belt between Mars and Jupiter.

The oxygen isotopes of a meteorite can act as a fingerprint to identify the parent body it originated from. The group of meteorites known as HED (howardite, eucrite, and diogenite) are thought to emanate from Vesta, as their oxygen isotope signatures are the same. Bunburra Rockhole was originally classed as a eucrite, however its oxygen composition is very different from that of the other HEDs.

When an asteroid or a protoplanet accretes enough material, it will start to become roughly spherical in shape. The heaviest material will sink into the core, and the body will be become split up into the core, mantle, and crust. This process is known as differentiation. Credit: Smithsonian Museum of Natural History

In a new study, astro-geologist Gretchen Benedix of Curtin University in Australia and colleagues, performed a more detailed analysis of the meteorite. The paper, “Bunburra Rockhole: Exploring the geology of a new differentiated asteroid,” was recently published in the journal Geochimica et Cosmochimica Acta. The research was funded by the NASA Emerging Worlds and Cosmochemistry programs. Some of the international consortium were also funded by the Australian Research Council (ARC) and some European grants.

“The initial data were collected on one piece, which gave intriguing results, thus, we examined several different pieces to make sure that the original piece wasn’t an anomaly,” said Benedix.

Their results revealed that all of the different pieces also have anomalous oxygen compositions, showing that the initial analysis on a single piece was correct. The composition measured does not equate to that seen in meteorites from Vesta.

Even though the oxygen composition is different to that of Vesta, the bulk composition of Bunburra Rockhole is remarkably similar, raising even more questions about the meteorite’s origin.

Three scenarios were proposed by the scientists in order to attempt to explain the anomalies of this meteorite. The first was that the rock had been contaminated by other material, the second that it originated from a previously un-sampled part of Vesta, and the third that its parent body is an undiscovered differentiated asteroid.

If contamination had occurred, an estimated 10 percent of the material in the meteorite would have to be contaminants in order to explain the anomalous oxygen, and this is something that would have been obvious in computer tomography scans (CT scans) due to the density differences between materials. In addition, fragments of the contaminant should also have been present, and yet none were seen. This evidence was used to rule out the contamination theory.

If the meteorite came from an un-sampled part of Vesta, it would imply that Vesta is heterogeneous, meaning the composition varies across the asteroid. However, there is no evidence, based on the HED meteorites, to suggest that Vesta is heterogeneous as all have the same oxygen isotope composition. This means that the oxygen composition was homogenous across Vesta prior to the formation of the basalt that the eucrites come from. Therefore, Vesta cannot be the parent body of Bunburra Rockhole.

This leaves standing the theory that a previously undiscovered, differentiated asteroid is the most likely origin of Bunburra Rockhole.

“The bulk chemical composition of a meteorite tells a lot about how much thermal or aqueous alteration it has experienced,” explained Benedix. “This is because heat and water tend to move different elements around at different rates. So if a body has been differentiated, like Earth, it will separate into a metal rich core, a dense mineral rich mantle and a light mineral rich crust because of the elements that make up those minerals.”

Bunburra Rockhole is a basalt meteorite, which indicates that melting occurred in the parent body as the layers became separated and the asteroid differentiated. If the parent body hadn’t differentiated, then more metals would have been present.

As the bulk composition of Bunburra Rockhole and Vesta are similar, it is likely that the Bunburra Rockhole’s parent body and Vesta formed within a similar part of the Solar System. However, it is currently impossible to pinpoint which asteroid Bunburra Rockhole originated from.

“All the larger asteroids in the belt and in near Earth space are classified,” explained Benedix. “So either there is another big asteroid that we haven’t found yet or the asteroid that Bunburra Rockhole originated from has evolved over time through space weathering and impact processing.”

The parent asteroid would have been a similar size to Vesta, although slightly smaller. Rare earth elements and bulk major elements in the meteorite show similar levels of partial melting, as Vesta does, but the variations in the oxygen isotopes in the meteorite are consistent with quicker cooling than Vesta, indicating a body around 100 kilometers smaller.

Interestingly, another strange meteorite, called Asuka 881394, has similar oxygen and chromium isotope abundances to Bunburra Rockhole (though there are enough subtle differences to indicate that it is not the same parent asteroid), which suggests that there could be yet another differentiated body out there that would have formed around the same time and in the same region as the Bunburra Rockhole parent. Analyzing Asuka will be a future project for the team of scientists.


Space rock found on Earth traced back to origins on Vesta

Shown here is the team that found the first meteorite from the breakup of asteroid 2018 LA in Botswana. They point to the space rock found there, later named Motopi Pan. Image via Meteoritics and Planetary Science/ Peter Jenniskens.

In 2018, an international team of scientists tracked a small asteroid as it streaked toward Earth and crashed into southern Africa. The team was able to find and collect pieces of the asteroid that were scattered across a Botswana game reserve. Now, the scientists believe they know the source of that asteroid. They say the space rock came from Vesta, the brightest and second-most massive object in the asteroid belt, a region of our solar system between Jupiter and Mars occupied by a great many solid, irregularly-shaped bodies of many sizes.

During the early days of the solar system, 1 to 2 billion years ago, enormous collisions nearly shattered Vesta, creating a slew of fragments that occasionally get close enough to Jupiter to hurl them in toward Earth. The researchers say it’s one of these pieces of space debris that landed in Africa in 2018.

The study of the findings was published April 23, 2021, in the peer-reviewed journal Meteoritics & Planetary Science.

Rocks from space that reach Earth’s surface are called meteorites. Most of the meteorites found on Earth were once part of asteroids from the asteroid belt that chipped off the parent body in a collision. Some rarer sources of meteorites are the moon and Mars.

When the asteroid – now known as 2018 LA – was discovered, it was only the second asteroid in space detected before hitting land. An international team guided by SETI Institute meteor astronomer Peter Jenniskens eventually found 23 fragments of the asteroid, which they estimate to originally have been 5 feet (about 1.5 meters) in diameter. The first meteorite found was named Motopi Pan after a nearby watering hole.

Pieces of Vesta on Earth. Asteroid 2018 LA in space (top left image by the Catalina Sky Survey) and the 1st 23 meteorites recovered on the ground as photographed in situ. Meteorites are shown in the order they were found with Motopi Pan at top left. Image via Meteoritics and Planetary Science/ Peter Jenniskens.

Jenniskens explained how they tied 2018 LA to Vesta:

Combining the observations of the small asteroid in space with information gleaned from the meteorites shows it likely came from Vesta, second largest asteroid in our solar system and target of NASA’s Dawn mission. Billions of years ago, two giant impacts on Vesta created a family of larger, more dangerous asteroids. The newly recovered meteorites gave us a clue on when those impacts might have happened.

NASA’s Dawn mission, which visited the asteroid belt and studied Vesta in 2011, found that the asteroid’s surface is covered in coarse-grained basaltic and silicate-rich rocks. The rocks are types known as howardite, eucrite and diogenite (HED). Other meteorites that scientists have recovered and studied on Earth with this same makeup are known as HED meteorites. Analysis of Motopi Pan conducted at University of Helsinki, Finland, showed it to also be an HED meteorite. Tomas Kohout of the University of Helsinki said:

We managed to measure metal content as well as secure a reflectance spectrum and X-ray elemental analysis from a thinly crusted part of the exposed meteorite interior. All the measurements added well together and pointed to values typical for HED type meteorites.

The scientists eventually studied more of the 23 space rocks that they found, although they did find some variability between the meteorites. Roger Gibson of Witts University in Johannesburg, South Africa, said:

We studied the petrography and mineral chemistry of five of these meteorites and confirmed that they belong to the HED group. Overall, we classified the material that asteroid 2018 LA contained as being howardite, but some individual fragments had more affinity to diogenites and eucrites.

When Dawn explored Vesta, it imaged the Antonia impact crater that was created in a collision 22 million years ago. One-third of all HED meteorites found on Earth were ejected 22 million years ago. The scientists wanted to know if Motopi Pan was also ejected from Vesta in the collision that created the Antonia crater at that time. Kees Welten of University of California, Berkeley, said:

Noble gas isotopes measurements at ETH in Zürich, Switzerland, and radioactive isotopes measured at Purdue University showed that this meteorite too had been in space as a small object for about 23 million years, but give or take 4 million years, so it could be from the same source crater on Vesta.

The researchers examined lead isotopes in zircon minerals to find that Motopi Pan experienced melting events 4.563 billion years ago and again 4.234 billion years ago. These two big impact events on Vesta billions of years ago helped to create the material of asteroid 2018 LA that was subsequently launched from the surface of Vesta in another impact some 22 million years ago. The billion-year-old impacts coincide with information already known about Vesta: Vesta experienced two significant impact events that created the Rheasilvia impact basin and the underlying and older Veneneia impact basin. Jenniskens elaborated:

We now suspect that Motopi Pan was heated by the Veneneia impact, while the subsequent Rheasilvia impact scattered this material around. If so, that would date the Veneneia impact to about 4234 million years ago.

Another possible origin for Motopi Pan on Vesta is a different crater from Antonia. This crater is named Rubria and is associated with the Rheasilvia impact. Jenniskens said:

On top of Rheasilvia impact ejecta is the 6.5-mile (10.3-km) diameter Rubria impact crater, slightly smaller than the 10.3-mile (16.7-km) Antonia crater, and slightly younger at 19 +/- 3 million years, but a good candidate for the origin crater of Motopi Pan.

View larger. | This high-resolution geological map of Vesta comes from Dawn spacecraft data. Brown colors represent the oldest, most heavily cratered surface. Purple colors in the north and light blue represent terrains modified by the Veneneia and Rheasilvia impacts, respectively. Light purples and dark blue colors below the equator represent the interior of the Rheasilvia and Veneneia basins. Image via NASA.

While the exact crater for the source of 2018 LA is yet to be determined, it appears that Earth has received space rocks that once were a part of Vesta.

Bottom line: In 2018, an asteroid smashed into Earth over Botswana. Pieces were discovered in the Central Kalahari Game Reserve, and now researchers have revealed that the analysis shows them to have come from Vesta.


An asteroid traveled for 23 million years before crashing into Earth &mdash and now scientists know where it came from

A small asteroid crashed into Earth three years ago &mdash and now scientists know where it came from. Researchers have traced the origins of the resulting rare meteorite fragments, which began the journey to Earth some 23 million years ago.

The asteroid, called 2018 LA, shot across the sky like a fireball before landing in Botswana on June 2, 2018. Researchers subsequently recovered 23 meteorites from the Central Kalahari Game Reserve, a huge area known for its diverse wildlife.

"The meteorite is named 'Motopi Pan' after a local watering hole," Mohutsiwa Gabadirwe, the senior curator of the Botswana Geoscience Institute, said in a statement, referring to the first sample they found. "This meteorite is a national treasure of Botswana."

Scientists first spotted the asteroid using the University of Arizona's Catalina Sky Survey , which tracks asteroids as part of NASA's Planetary Defense program . It marked just the second time scientists have been able to study an asteroid in space before it reaches Earth &mdash typically, they don't know about them until after it's happened.

Fragment of asteroid 2018 LA recovered in Central Kalahari Game Reserve in central Botswana. SETI Institute

At the time, the asteroid was estimated to be about 6 feet across &mdash small enough to safely break apart in Earth's atmosphere. It arrived at the fast speed of 38,000 miles per hour, according to NASA.

"This is only the second time we have spotted an asteroid in space before it hit Earth over land," said Jenniskens. "The first was asteroid 2008 TC3 in Sudan ten years earlier."

Space & Astronomy

Through precisely mapping the boulder-sized asteroid's orbit and path to Earth, as well as analyzing the samples at the University of Helsinki, researchers determined that they belong to the group of Howardite-Eucrite-Diogenite (HED) meteorites, named for their composition. They published their findings in the journal Meteoritics and Planetary Science.

This group of meteorites is likely to have come from Vesta , the second-largest asteroid in our solar system, located in the asteroid belt between Mars and Jupiter.

"Combining the observations of the small asteroid in space with information gleaned from the meteorites shows it likely came from Vesta, second-largest asteroid in our Solar System and target of NASA's DAWN mission ," said lead author Peter Jenniskens. "Billions of years ago, two giant impacts on Vesta created a family of larger, more dangerous asteroids. The newly recovered meteorites gave us a clue on when those impacts might have happened."

Researchers now believe the Veneneia impact basin formed about 4.2 billion years ago.

These are the discovery observations of asteroid 2018 LA from the Catalina Sky Survey, taken June 2, 2018. About eight hours after these images were taken, the asteroid entered Earth's atmosphere and disintegrated in the upper atmosphere near Botswana, Africa. NASA / JPL

Researchers observed more diversity in the appearance of the meteorites than expected. They classified the asteroid as a breccia, a mixture of rock pieces from various parts on Vesta.

"We studied the petrography and mineral chemistry of five of these meteorites and confirmed that they belong to the HED group," said co-author Roger Gibson. "Overall, we classified the material that asteroid 2018 LA contained as being Howardite, but some individual fragments had more affinity to Diogenites and Eucrites."

One-third of all HED meteorites that arrive on Earth were ejected from the asteroid approximately 22 million years ago.

Further research "showed that this meteorite too had been in space as a small object for about 23 million years," said Kees Welten of UC Berkeley, "but give or take 4 million years."

Researchers say they are excited to uncover more secrets surrounding the mysterious Vesta asteroid. A more recent expedition, in November 2020, led to researchers locating another Motopi Pan meteorite &mdash at 2.3 ounces, it's the largest found to date.

First published on April 29, 2021 / 11:42 AM

© 2021 CBS Interactive Inc. All Rights Reserved.

Sophie Lewis is a social media producer and trending writer for CBS News, focusing on space and climate change.


MIT Geologists Trace Mercury’s Origins to Rare Meteorite

An image, taken by MESSENGER during its Mercury flyby on January 14, 2008, of Mercury’s full crescent.

A newly published study shows that Mercury underwent a period of rapid cooling and that the planet likely has the composition of an enstatite chondrite — a type of meteorite that is extremely rare here on Earth.

Around 4.6 billion years ago, the universe was a chaos of collapsing gas and spinning debris. Small particles of gas and dust clumped together into larger and more massive meteoroids that in turn smashed together to form planets. Scientists believe that shortly after their formation, these planets — and particularly Mercury — were fiery spheres of molten material, which cooled over millions of years.

Now, geologists at MIT have traced part of Mercury’s cooling history and found that between 4.2 and 3.7 billion years ago, soon after the planet formed, its interior temperatures plummeted by 240 degrees Celsius, or 464 degrees Fahrenheit.

They also determined, based on this rapid cooling rate and the composition of lava deposits on Mercury’s surface, that the planet likely has the composition of an enstatite chondrite — a type of meteorite that is extremely rare here on Earth.

Timothy Grove, the Cecil and Ida Green Professor of Geology in MIT’s Department of Earth, Atmospheric, and Planetary Sciences, says new information on Mercury’s past is of interest for tracing Earth’s early formation.

“Here we are today, with 4.5 billion years of planetary evolution, and because the Earth has such a dynamic interior, because of the water we’ve preserved on the planet, [volcanism] just wipes out its past,” Grove says. “On planets like Mercury, early volcanism is much more dramatic, and [once] they cooled down there were no later volcanic processes to wipe out the early history. This is the first place where we actually have an estimate of how fast the interior cooled during an early part of a planet’s history.”

Grove and his colleagues, including researchers from the University of Hanover, in Germany the University of Liége, in Belgium and the University of Bayreuth, in Germany, have published their results in Earth and Planetary Science Letters.

Compositions in craters

For their analysis, the team utilized data collected by NASA’s MESSENGER spacecraft. The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) probe orbited Mercury between 2011 and 2015, collecting measurements of the planet’s chemical composition with each flyby. During its mission, MESSENGER produced images that revealed kilometer-thick lava deposits covering the entire planet’s surface.

An X-ray spectrometer onboard the spacecraft measured the X-ray radiation from the planet’s surface, produced by solar flares on the sun, to determine the chemical composition of more than 5,800 lava deposits on Mercury’s surface.

Grove’s co-author, Olivier Namur of the University of Hanover, recalculated the surface compositions of all 5,800 locations, and correlated each composition with the type of terrain in which it was found, from heavily cratered regions to those that were less impacted. The density of a region’s craters can tell something about that region’s age: The more craters there are, the older the surface is, and vice versa. The researchers were able to correlate Mercury’s lava composition with age and found that older deposits, around 4.2 billion years old, contained elements that were very different from younger deposits that were estimated to be 3.7 billion years old.

“It’s true of all planets that different age terrains have different chemical compositions because things are changing inside the planet,” Grove says. “Why are they so different? That’s what we’re trying to figure out.”

A rare rock, 10 standard deviations away

To answer that question, Grove attempted to retrace a lava deposit’s path, from the time it melted inside the planet to the time it ultimately erupted onto Mercury’s surface.

To do this, he started by recreating Mercury’s lava deposits in the lab. From MESSENGER’s 5,800 compositional data points, Grove selected two extremes: one representing the older lava deposits and one from the younger deposits. He and his team converted the lava deposits’ element ratios into the chemical building blocks that make up rock, then followed this recipe to create synthetic rocks representing each lava deposit.

The team melted the synthetic rocks in a furnace to simulate the point in time when the deposits were lava, and not yet solidified as rock. Then, the researchers dialed the temperature and pressure of the furnace up and down to effectively turn back the clock, simulating the lava’s eruption from deep within the planet to the surface, in reverse.

Throughout these experiments, the team looked for tiny crystals forming in each molten sample, representing the point at which the sample turns from lava to rock. This represents the stage at which the planet’s solid rocky core begins to melt, creating a molten material that sloshes around in Mercury’s mantle before erupting onto the surface.

The team found a surprising disparity in the two samples: The older rock melted deeper in the planet, at 360 kilometers, and at higher temperatures of 1,650 C, while the younger rock melted at shallower depths, at 160 kilometers, and 1,410 C. The experiments indicate that the planet’s interior cooled dramatically, over 240 degrees Celsius between 4.2 and 3.7 billion years ago — a geologically short span of 500 million years.

“Mercury has had a huge variation in temperature over a fairly short period of time, that records a really amazing melting process,” Grove says.

The researchers determined the chemical compositions of the tiny crystals that formed in each sample, in order to identify the original material that may have made up Mercury’s interior before it melted and erupted onto the surface. They found the closest match to be an enstatite chondrite, an extremely rare form of meteorite that is thought to make up only about 2 percent of the meteorites that fall to Earth.

“We now know something like an enstatite chondrite was the starting material for Mercury, which is surprising, because they are about 10 standard deviations away from all other chondrites,” Grove says.

“This is a very significant paper that synthesizes geologic, chronologic, and geochemical observations from the recently completed MESSENGER mission about the nature of volcanic surface units on Mercury, and connects these with modelling and experimental petrology results,” says James Head, professor of geological sciences at Brown University. “It reaches very important conclusions about the depth and significance of melting in the mantle of Mercury, and how it changed rapidly with time. These results give us fundamental new insights into the style of mantle melting and provide a logical explanation for the nature and timing of volcanic units on Mercury.”

Grove cautions that the group’s results are not set in stone and that Mercury may have been an accumulation of other types of starting materials. To know this would require an actual sample from the planet’s surface.

“The next thing that would really help us move our understanding of Mercury way forward is to actually have a meteorite from Mercury that we could study,” Grove says. “That would be lovely.”


An origin story for a family of oddball meteorites

Most meteorites that have landed on Earth are fragments of planetesimals, the very earliest protoplanetary bodies in the solar system. Scientists have thought that these primordial bodies either completely melted early in their history or remained as piles of unmelted rubble.

But a family of meteorites has befuddled researchers since its discovery in the 1960s. The diverse fragments, found all over the world, seem to have broken off from the same primordial body, and yet the makeup of these meteorites indicates that their parent must have been a puzzling chimera that was both melted and unmelted.

Now researchers at MIT and elsewhere have determined that the parent body of these rare meteorites was indeed a multilayered, differentiated object that likely had a liquid metallic core. This core was substantial enough to generate a magnetic field that may have been as strong as Earth's magnetic field is today.

Their results, published in the journal Science Advances, suggest that the diversity of the earliest objects in the solar system may have been more complex than scientists had assumed.

"This is one example of a planetesimal that must have had melted and unmelted layers. It encourages searches for more evidence of composite planetary structures," says lead author Clara Maurel, a graduate student in MIT's Department of Earth, Atmospheric, and Planetary Sciences (EAPS). "Understanding the full spectrum of structures, from nonmelted to fully melted, is key to deciphering how planetesimals formed in the early solar system."

Maurel's co-authors include EAPS professor Benjamin Weiss, along with collaborators at Oxford University, Cambridge University, the University of Chicago, Lawrence Berkeley National Laboratory, and the Southwest Research Institute.

Oddball irons

The solar system formed around 4.5 billion years ago as a swirl of super-hot gas and dust. As this disk gradually cooled, bits of matter collided and merged to form progressively larger bodies, such as planetesimals.

The majority of meteorites that have fallen to Earth have compositions that suggest they came from such early planetesimals that were either of two types: melted, and unmelted. Both types of objects, scientists believe, would have formed relatively quickly, in less than a few million years, early in the solar system's evolution.

If a planetesimal formed in the first 1.5 million years of the solar system, short-lived radiogenic elements could have melted the body entirely due to the heat released by their decay. Unmelted planetesimals could have formed later, when their material had lower quantities of radiogenic elements, insufficient for melting.

There has been little evidence in the meteorite record of intermediate objects with both melted and unmelted compositions, except for a rare family of meteorites called IIE irons.

"These IIE irons are oddball meteorites," Weiss says. "They show both evidence of being from primordial objects that never melted, and also evidence for coming from a body that's completely or at least substantially melted. We haven't known where to put them, and that's what made us zero in on them."

Magnetic pockets

Scientists have previously found that both melted and unmelted IIE meteorites originated from the same ancient planetesimal, which likely had a solid crust overlying a liquid mantle, like Earth. Maurel and her colleagues wondered whether the planetesimal also may have harbored a metallic, melted core.

"Did this object melt enough that material sank to the center and formed a metallic core like that of the Earth?" Maurel says. "That was the missing piece to the story of these meteorites."

The team reasoned that if the planetesimal did host a metallic core, it could very well have generated a magnetic field, similar to the way Earth's churning liquid core produces a magnetic field. Such an ancient field could have caused minerals in the planetesimal to point in the direction of the field, like a needle in a compass. Certain minerals could have kept this alignment over billions of years.

Maurel and her colleagues wondered whether they might find such minerals in samples of IIE meteorites that had crashed to Earth. They obtained two meteorites, which they analyzed for a type of iron-nickel mineral known for its exceptional magnetism-recording properties.

The team analyzed the samples using the Lawrence Berkeley National Laboratory's Advanced Light Source, which produces X-rays that interact with mineral grains at the nanometer scale, in a way that can reveal the minerals' magnetic direction.

Sure enough, the electrons within a number of grains were aligned in a similar direction -- evidence that the parent body generated a magnetic field, possibly up to several tens of microtesla, which is about the strength of Earth's magnetic field. After ruling out less plausible sources, the team concluded that the magnetic field was most likely produced by a liquid metallic core. To generate such a field, they estimate the core must have been at least several tens of kilometers wide.

Such complex planetesimals with mixed composition (both melted, in the form of a liquid core and mantle, and unmelted in the form of a solid crust), Maurel says, would likely have taken over several million years to form -- a formation period that is longer than what scientists had assumed until recently.

But where within the parent body did the meteorites come from? If the magnetic field was generated by the parent body's core, this would mean that the fragments that ultimately fell to Earth could not have come from the core itself. That's because a liquid core only generates a magnetic field while still churning and hot. Any minerals that would have recorded the ancient field must have done so outside the core, before the core itself completely cooled.

Working with collaborators at the University of Chicago, the team ran high-velocity simulations of various formation scenarios for these meteorites. They showed that it was possible for a body with a liquid core to collide with another object, and for that impact to dislodge material from the core. That material would then migrate to pockets close to the surface where the meteorites originated.

"As the body cools, the meteorites in these pockets will imprint this magnetic field in their minerals. At some point, the magnetic field will decay, but the imprint will remain," Maurel says. "Later on, this body is going to undergo a lot of other collisions until the ultimate collisions that will place these meteorites on Earth's trajectory."

Was such a complex planetesimal an outlier in the early solar system, or one of many such differentiated objects? The answer, Weiss says, may lie in the asteroid belt, a region populated with primordial remnants.

"Most bodies in the asteroid belt appear unmelted on their surface," Weiss says. "If we're eventually able to see inside asteroids, we might test this idea. Maybe some asteroids are melted inside, and bodies like this planetesimal are actually common."


Watch the video: Ο Τάκης Θεοδοσίου, ειδικός στους μετεωρίτες, στο (May 2022).