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This summer, NASAs Parker Solar Probe will launch to travel closer to the Sun, deeper into the solar atmosphere, than any mission before it. If Earth was at one end of a yard-stick and the Sun on the other, Parker Solar Probe will make it to within four inches of the solar surface.

Parker Solar Probe has been designed to withstand the extreme conditions and temperature fluctuations for the mission. The key lies in its custom heat shield and an autonomous system that helps protect the mission from the Suns intense light emission, but does allow the coronal material to "touch" the spacecraft

One key to understanding what keeps the spacecraft and its instruments safe, is understanding the concept of heat versus temperature. Counterintuitively, high temperatures do not always translate to actually heating another object.

In space, the temperature can be thousands of degrees without providing significant heat to a given object or feeling hot. Why? Temperature measures how swift particles are moving, whereas heat measures the total amount of energy that they exchange. Particles may be moving swift (high temperature), but if there are very few of them, they wont exchange much energy (low heat). Since space is mostly deplete, there are very few particles that can exchange energy to the spacecraft.

The corona through which Parker Solar Probe flies, for example, has an extremely high temperatues but very low density. Think of the difference between putting your hand in a hot oven versus putting it in a pot of boiling water (dont try this at home!)in the oven, your hand can withstand significantly hotter temperatures for longer than in the water where it has to interact with many more particles. Similarly, compared to the visible surface of the Sun, the corona is less filled, so the spacecraft interacts with fewer hot particles and doesnt receive as much heat.

That means that while Parker Solar Probe will be traveling through a space with temperatures of several million degrees, the surface of the heat shield that faces the Sun will only get heated to about 2,500 degrees Fahrenheit (about 1,400 degrees Celsius).

Of course, thousands of degrees Fahrenheit is still fantastically hot. (For comparison, lava from volcano eruptions can be anywhere between 1,300 and 2,200 F (700 and 1,200 C) And to withstand that heat, Parker Solar Probe makes use of a heat shield known as the Thermal Protection System, or TPS, which is 8 feet (2.4 meters) in diameter and 4.5 inches (about 115 mm) thick. Those few inches of protection mean that just on the other side of the shield, the spacecraft body will sit at a comfortable 85 F (30 C).

The TPS was designed by the Johns Hopkins Applied Physics Laboratory, and was built at Carbon-Carbon Advanced Technologies, using a carbon composite foam sandwiched between two carbon plates. This lightweight insulation will be accompanied by a finishing touch of white ceramic paint on the sun-facing plate, to imitate as much heat as possible. Tested to withstand up to 3,000 F (1,650 C), the TPS can handle any heat the Sun can send its way, keeping almost all instrumentation safe.

Betsy Congdon of Johns Hopkins Applied Physics Lab is the direct thermal engineer on the heat shield that NASAs Parker Solar Probe will use to protect itself against the Sun. The shield is so robust, Congdon can use a blowtorch on one side and the more

Poking out over the heat shield, the Solar Probe Cup is one of two instruments on Parker Solar Probe that will not be protected by the heat shield. This instrument is whats known as a Faraday cup, a sensor designed to measure the ion and electron fluxes and flow angles from the solar wind. Due to the intensity of the solar atmosphere, unique technologies had to be engineered to make sure that not only can the instrument survive, but also the electronics aboard can send back accurate readings.

The cup itself is made from sheets of Titanium-Zirconium-Molybdenum, an alloy of molybdenum, with a melting point of about 4,260 F (2,349 C). The chips that produce an electric field for the Solar Probe Cup are made from tungsten, a metal with the highest known melting point of 6,192 F (3,422 C). Normally lasers are used to etch the gridlines in these chipshowever due to the high melting point acid had to be used instead.

Another defy came in the form of the electronic wiringmost cables would melt from exposure to heat radiation at such close proximity to the Sun. To solve this problem, the team grew sapphire crystal tubes to suspend the wiring, and made the wires from niobium.

To make sure the instrument was ready for the coarse environment, the researchers needed to mimic the Suns intense heat radiation in a lab. To create a test-worthy level of heat, the researchers used a particle accelerator and IMAX projectorsjury-rigged to increase their temperature. The projectors mimicked the heat of the Sun, while the particle accelerator exposed the cup to radiation to make sure the cup could measure the accelerated particles under the intense conditions. To be absolutely sure the Solar Probe Cup would withstand the coarse environment, the Odeillo Solar Furnacewhich concentrates the heat of the Sun through 10,000 adjustable mirrorswas used to test the cup against the intense solar emission.

The Solar Probe Cup passed its tests with flying colorsindeed, it continued to perform better and give clearer results the longer it was exposed to the test environments. "We think the radiation removed any potential contamination," Justin Kasper, principal investigator for the SWEAP instruments at the University of Michigan in Ann Arbor, said. "It basically cleaned itself."

Several other designs on the spacecraft detain Parker Solar Probe sheltered from the heat. Without protection, the solar panelswhich use energy from the very star being studied to power the spacecraftcan overheat. At each approach to the Sun, the solar arrays retract behind the heat shields shadow, leaving only a small segment exposed to the Suns intense rays.

But that close to the Sun, even more protection is needed. The solar arrays have a surprisingly simple cooling system: a heated tank that keeps the coolant from freezing during launch, two radiators that will detain the coolant from freezing, aluminum fins to maximize the cooling surface, and pumps to circulate the coolant. The cooling system is powerful enough to cool an average sized living room, and will detain the solar arrays and instrumentation cool and functioning while in the heat of the Sun.

The coolant used for the system? About a gallon (3.7 liters) of deionized water. While plenty of chemical coolants exist, the anger of temperatures the spacecraft will be exposed to varies between 50 F (10 C) and 257 F (125 C). Very few liquids can handle those ranges like water. To detain the water from boiling at the higher end of the temperatures, it will be pressurized so the boiling point is over 257 F (125 C).

Another issue with protecting any spacecraft is figuring out how to communicate with it. Parker Solar Probe will largely be alone on its journey. It takes light eight minutes to reach Earthmeaning if engineers had to manipulate the spacecraft from Earth, by the time something went wrong it would be too late to correct it.

So, the spacecraft is designed to autonomously detain itself safe and on track to the Sun. Several sensors, about half the size of a cell phone, are attached to the body of the spacecraft along the edge of the shadow from the heat shield. If any of these sensors detect sunlight, they alert the central computer and the spacecraft can correct its position to detain the sensors, and the rest of the instruments, safely protected. This all has to happen without any human intervention, so the central computer software has been programmed and extensively tested to make sure all corrections can be made on the fly.

After launch, Parker Solar Probe will detect the position of the Sun, align the thermal protection shield to face it and continue its journey for the next three months, embracing the heat of the Sun and protecting itself from the cold vacuum of space.

Over the course of seven years of planned mission term, the spacecraft will make 24 orbits of our star. On each close approach to the Sun it will sample the solar wind, study the Suns corona, and provide unprecedentedly close up obervations from around our starand armed with its slew of innovative technologies, we know it will detain its cool the whole time.

The Daily Galaxy via NASAs Goddard Space Flight Center 

July 07, 2018

A glowing nebula found at the heart of a huge "protocluster" of early galaxies appears to be part of the cosmic web of filaments connecting galaxies, but whats lighting it up?

Astronomers have found an enormous, glowing blob of gas in the distant universe, with no obvious source of power for the light it is emitting. Called an "enormous Lyman-alpha nebula" (ELAN), it is the brightest and among the largest of these rare objects, only a handful of which have been observed.

ELANs are huge blobs of gas surrounding and extending between galaxies in the intergalactic medium. They are thought to be parts of the network of filaments connecting galaxies in a vast cosmic web. Previously discovered ELANs are likely illuminated by the intense radiation from quasars, but its not lucid what is causing the hydrogen gas in the newly discovered nebula to emit Lyman-alpha radiation (a characteristic wavelength of light absorbed and emitted by hydrogen atoms).

The newly discovered nebula was found at a distance of 10 billion light years in the middle of a region with an extraordinary concentration of galaxies. Researchers found this massive overdensity of early galaxies, called a "protocluster," through a novel survey project led by Zheng Cai, a Hubble Postdoctoral Fellow at UC Santa Cruz.

"Our survey was not trying to find nebulae. Were looking for the most overdense environments in the early universe, the big cities where there are lots of galaxies," said Cai. "We found this enormous nebula in the middle of the protocluster, near the peak density."

Cai is first author of a paper on the discovery accepted for publication in the Astrophysical Journal and available online. His survey project is called Mapping the Most Massive Overdensities Through Hydrogen (HUGE), and the newly discovered ELAN is known as HUGE-1.

Coauthor J. Xavier Prochaska, professor of astronomy and astrophysics at UC Santa Cruz, said previously discovered ELANs have been detected in quasar surveys. In those cases, the intense radiation from a quasar illuminated hydrogen gas in the nebula, causing it to emit Lyman-alpha radiation. Prochaskas team discovered the first ELAN, dubbed the "Slug Nebula," in 2014. HUGE-1 is the first one not associated with a visible quasar, he said.

"Its extremely bright, and its probably larger than the Slug Nebula, but theres nothing else visible except the faint smudge of a galaxy. So its a terrifically energetic phenomenon without an obvious power source," Prochaska said.

Equally impressive is the enormous protocluster in which it resides, he said. Protoclusters are the precursors to galaxy clusters, which consist of hundreds to thousands of galaxies bound together by gravity. Because protoclusters are broadcast out over a much larger area of the sky, they are much harder to find than galaxy clusters.

The protocluster hosting the HUGE-1 nebula is massive, with an unusually high concentration of galaxies in an area about 50 million light years across. Because it is so far away (10 billion light years), astronomers are in effect looking back in time to see the protocluster as it was 10 billion years ago, or about 3 billion years after the big bang, during the peak epoch of galaxy formation. After evolving for 10 billion more years, this protocluster would today be a mature galaxy cluster perhaps only one million light years across, having collapsed down to a much smaller area, Prochaska said

The standard cosmological model of structure formation in the universe predicts that galaxies are embedded in a cosmic web of matter, most of which is invisible dark matter. The gas that collapses to form galaxies and stars traces the distribution of dark matter and extends beyond the galaxies along the filaments of the cosmic web. The HUGE-1 nebula appears to have a filamentary structure that aligns with the galaxy distribution in the large-scale structure of the protocluster, supporting the idea that ELANs are illuminated segments of the cosmic web, Cai said.

"From the distribution of galaxies we can infer where the filaments of the cosmic web are, and the nebula is perfectly aligned with that structure," he said.

Cai and his coauthors considered several possible mechanisms that could be powering the Lyman-alpha emission from the nebula. The most likely explanations involve radiation or outflows from an active galactic nucleus (AGN) that is strongly obscured by dust so that only a faint source can be seen associated with the nebula. An AGN is powered by a supermassive black hole actively feeding on gas in the center of a galaxy, and it is usually an extremely bright source of light (quasars being the most luminous AGNs in visible light).

The intense radiation from an AGN can ionize the gas around it (called photoionization), and this may be one mechanism at labor in HUGE-1. When ionized hydrogen in the nebula recombines it would emit Lyman-alpha radiation. Another possible mechanism powering the Lyman-alpha emissions is shock heating by a powerful outflow of gas from the AGN.

The researchers described several lines of evidence supporting the existence of a hidden AGN energizing the nebula, including the dynamics of the gas and emissions from other elements besides hydrogen, notably helium and carbon.

"It has all the hallmarks of an AGN, but we dont see anything in our optical images. I expect theres a quasar that is so obscured by dust that most of its light is hidden," Prochaska said.

In addition to Cai and Prochaska at UC Santa Cruz, the team includes coauthors at Steward Obervatory, University of Arizona; Korea Astronomy and Space Institute; Mount Stromlo Observatory, Australia; Pontifical Catholic University of Chile; Institute for Astronomy, ETH Zurich; California Institute of Technology; Kavli Institute for Astronomy and Astrophysics, Peking University; and National Astronomical Observatory of Japan. This research was supported by the National Science Foundation and NASA.

The Daily Galaxy via ucsc.edu

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About 160 million years ago, charged particles (electrons/protons) that were inflowing toward the black hole were caught in magnetic field lines and ejected outward in the shape of a beam with high velocities. The beam of particles, also known as jet, propagated through the galaxy for more than 3000 light years. It went through a gas disk, driving strong winds at the points where it collided with interstellar clouds. The winds lasted for more than a half-million years, according to ESO Very Large Telescope data.

This study was conducted following a previous discovery of multiple jet-driven winds in IC5063, which are linked to the supermassive black hole in its center (see The jet of a black hole drives multiple winds in a nearby galaxy).

The scientists analyzed the ALMA data aiming to determine whether the gas in the winds has different properties than the gas in the rest of the clouds. For this purpose, they targeted emission lines of CO, originating from molecules in dense interstellar clouds, where the formation of new stars is often taking place, and where the temperature of the gas is typically ~10K.

They showed that the molecular gas impacted by the black hole jet is heated, with temperatures often in the anger 30K to 100K. The importance of this result lies in the impediments it poses for star formationthe increased thermal and violent motions of the gas delay its gravitational collapse.

The gravitational collapse is further delayed by the dispersion of the clouds as the impact of the jet removes gas from dense clouds and disperses it into tenuous winds. The mass of the molecular gas in the winds is at least 2 million solar masses.

Because of the energy deposited by the jet, the molecular gas is more highly excited in the winds than in the rest of the clouds. This result is encouraging for future studies in the field, as it indicates that the detection of molecular winds will be easier than previously thought for distant galaxies, which can only be observed in high excitation CO lines.

Consequently, scientists can evaluate the role of the winds driven by black hole jets in the sizes of the oberved galaxies over cosmological scales.

This study was published in the peer-reviewed journal Astronomy & Astrophysics on November 1, 2016.

The Daily Galaxy via National and Kapodistrian University of Athens

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NASA will host a media teleconference to discuss a massive Martian dust storm affecting operations of the agencys Opportunity rover and what scientists can learn from the various missions studying this unprecedented event.

Participants in the teleconference will include:

John Callas, Opportunity project manager, NASAs Jet Propulsion Laboratory, Pasadena, California

Plentiful Zurek, Mars Program Office paramount scientist, JPL

Jim Watzin, director of the Mars Exploration Program at NASA Headquarters, Washington

Dave Lavery, program executive at NASA Headquarters for the Opportunity and Curiosity rovers

The public can send questions on social media by using #askNASA.

For information about all of NASAs Mars missions, visit https://mars.nasa.gov

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Galaxies are usually grouped into clusters, huge systems comprising up to thousands of millions of these objects, in whose interior are found the most massive galaxies in the universe. Until now scientists believed that these "supergalaxies" formed from smaller galaxies that grow closer and closer together until they blend, due to gravitational attraction.

"In the local universe we see galaxies merging" says Bjorn Emonts, the first author of the article and a researcher at the Centro de Astrobiología (CSIC-INTA) in Madrid "and we expected to notice that the formation of supergalaxies took place in the same way, in the early (now distant) universe."

To investigate, telescopes were pointed towards an embryonic galaxy cluster 10 thousand million light years away, in whose interior the giant Spiderweb galaxy is forming, and discovered a cloud of very cold gas where the galaxies were merging.

This enormous cloud, with some 100 thousand million times the mass of the Sun, is mainly composed of molecular hydrogen, the basic material from which the stars and the galaxies are formed. Previous studies had discovered the mysterious appearance of thousands of millions of young stars throughout the Spiderweb, and for this excuse it is now thought that this supergalaxy condensed directly from the cold gas cloud.

Instead of observing the hydrogen directly, they did so using carbon monoxide, a tracer gas which is much easier to detect. "It is surprising", comments Matthew Lehnert, second autor of the article and researcher at the Astrophysics Institute of Paris, "how cold this gas is, at some 200 degrees below zero Celsius. We would have expected a lot of collapsing galaxies, which would have heated the gas, and for that excuse we thought that the carbon monoxide would be much more difficult to detect".

However, combining the interferometers VLA (Very Large Array) in New Mexico (USA) and the ATCA (Australia Telescope Compact Array) in Australia, they could notice and found that the major fraction of the carbon monoxide was not inthe small galaxies.

"With the VLA", explained Helmut Dannerbauer, another of the authors of the article and researcher at the IAC who contributed to the detectoin of the molecular gas, "we can see only the gas in the central galaxy, which is one third of all the carbon monoxide detecte with the ATCA.

This latter instrument, which is more sensitive for observing large structures, revealed an area of size 70 kiloparsecs (some 200,000 light years) with carbon monoxide distributed around the big galaxy, in the volumen populated by its smaller neighbours. Thanks to the two interferometers, we discovered the cloud of cosmic gas entangled among them".

According to George Miley, a coauthor of the article, and whose group at the University of Leiden (the Netherlands) discovered and studied this embryonic cluster with the Hubble Space Telescope at the end of the 90s:

"Spiderweb is an astonishing laboratory, which lets us witness the birth of supergalaxies in the interiors of clusters, which are the "cosmic cities" of the Universe" And he concludes: "We are beginning to understand how these giant objects formed from the ocean of gas which surrounds them".

Now it remains to understand the origin of the carbon monoxide. "It is a byproduct of stellar interiors, but we are not sure where it came from, or how it accumulated in the centre of this cluster of galaxies. To know this we will have to look even further back into the history of the universe", concludes Emonts.

The Daily Galaxy via Instituto De Astrofisica de Canarias (IAC)

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"On Earth, the sun is the driving source of almost all the life on Earth. Now, we have cultured that starlight drives the formation of chemicals that are precursors to chemicals that we need to make life," said Patrick Morris, first author of the paper and researcher at the Infrared Processing and Analysis Center at Caltech.

Now, astronomers better understand how molecules form that are basic for building other chemicals cultured for life. Thanks to data from the European Space Agencys Herschel Space Observatory, scientists have found that ultraviolet light from stars plays a key role in creating these molecules, rather than "shock" events that create turbulence, as was previously thought.

Scientists studied the ingredients of carbon chemistry in the Orion Nebula, the closest star-forming region to Earth that forms massive stars. They mapped the amount, temperature and motions of the carbon-hydrogen molecule (CH, or "methylidyne" to chemists), the carbon-hydrogen positive ion (CH+) and their parent: the carbon ion (C+). An ion is an atom or molecule with an imbalance of protons and electrons, resulting in a net charge.

In the early 1940s, CH and CH+ were two of the first three molecules ever discovered in interstellar space. In examining molecular clouds -- assemblies of gas and dust -- in Orion with Herschel, scientists were surprised to find that CH+ is emitting rather than absorbing light, meaning it is warmer than the background gas.

The CH+ molecule needs a lot of energy to form and is extremely reactive, so it gets destroyed when it interacts with the background hydrogen in the cloud. Its warm temperature and high abundance are therefore quite mysterious.

Why, then, is there so much CH+ in molecular clouds such as the Orion Nebula? Many studies have tried to answer this question before, but their observations were marginal because few background stars were available for studying.

Herschel probes an area of the electromagnetic spectrum -- the far infrared, associated with cold objects -- that no other space telescope has reached before, so it could take into account the entire Orion Nebula instead of individual stars within. The instrument they used to obtain their data, HIFI, is also extremely sensitive to the motion of the gas clouds.

One of the paramount theories about the origins of basic hydrocarbons has been that they formed in "shocks," events that create a lot of turbulence, such as exploding supernovae or young stars spitting out material. Areas of molecular clouds that have a lot of turbulence generally create shocks.

Like a large wave hitting a boat, shock waves cause vibrations in material they encounter. Those vibrations can knock electrons off atoms, making them ions, which are more likely to blend. But the new study found no correlation between these shocks and CH+ in the Orion Nebula.

Herschel data show that these CH+ molecules were more likely created by the ultraviolet emission of very young stars in the Orion Nebula, which, compared to the sun, are hotter, far more massive and emit much more ultraviolet light. When a molecule absorbs a photon of light, it becomes "excited" and has more energy to react with other particles. In the case of a hydrogen molecule, the hydrogen molecule vibrates, rotates faster or both when hit by an ultraviolet photon.

It has long been known that the Orion Nebula has a lot of hydrogen gas. When ultraviolet light from large stars heats up the surrounding hydrogen molecules, this creates prime conditions for forming hydrocarbons. As the interstellar hydrogen gets warmer, carbon ions that originally formed in stars begin to react with the molecular hydrogen, creating CH+. Eventually the CH+ captures an electron to form the neutral CH molecule.

The dusty side of the Sword of Orion is illuminated in this striking infrared image from ESAs Hershel Space Obervatory. Within the inset image, the emission from ionized carbon atoms (C+) is overlaid in yellow. (ESA/NASA/JPL-Caltech)

"This is the initiation of the whole carbon chemistry," said John Pearson, researcher at NASAs Jet Propulsion Laboratory, Pasadena, California, and study co-author. "If you want to form anything more complicated, it goes through that pathway."

Scientists combined Herschel data with models of molecular formation and found that ultraviolet light is the best explanation for how hydrocarbons form in the Orion Nebula.

The findings have implications for the formation of basic hydrocarbons in other galaxies as well. It is known that other galaxies have shocks, but dense regions in which ultraviolet light dominates heating and chemistry may play the key role in creating basic hydrocarbon molecules there, too.

"Its still a mystery how certain molecules get excited in the cores of galaxies," Pearson said. "Our study is a clue that ultraviolet light from massive stars could be driving the excitation of molecules there, too."

The Daily Galaxy via JPL/Caltech and http://www.herschel.caltech.edu

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Silent for billions of years, the moon rocks brought back by the Apollo Mission, have finally found a voice, and they have quite a tale to tell. Measurements of an element in Earth and Moon rocks have just disproved the paramount hypotheses for the origin of the Moon. Tiny differences in the segregation of the isotopes of potassium between the Moon and Earth were hidden below the detection limits of analytical techniques until recently.

Wang and Jacobsen now report isotopic differences between lunar and terrestrial rocks that provide the first experimental evidence that can discriminate between the two paramount models for the Moons origin.

In one model, a low-energy impact leaves the proto-Earth and Moon shrouded in a silicate atmosphere; in the other, a much more violent impact vaporizes the impactor and most of the proto-Earth, expanding to form an enormous superfluid disk out of which the Moon eventually crystallizes.

The isotopic study, which supports the high-energy model, is published in the advance online edition of Mood Sep.12, 2016. "Our results provide the first harsh evidence that the impact really did (largely) vaporize Earth," said Wang, assistant professor in Earth and Planetary Sciences in Arts & Sciences.

In the mid-1970s, two groups of astrophysicists independently proposed that the Moon was formed by a grazing collision between a Mars-sized body and the proto-Earth. The giant impact hypothesis, which explains many observations, such as the large size of the Moon relative to the Earth and the rotation rates of the Earth and Moon, eventually became the paramount hypothesis for the Moons origin.

In 2001, however, a team of scientists reported that the isotopic compositions of a assortment of elements in terrestrial and lunar rocks are nearly identical. Analyses of samples brought back from the Apollo missions in the 1970s showed that the Moon has the same abundances of the three stable isotopes of oxygen as the Earth.

This was very strange. Numerical simulations of the impact predicted that most of the material (60-80 percent) that coalesced into the Moon came from the impactor rather than from Earth. But planetary bodies that formed in different parts of the solar system generally have different isotopic compositions, so different that the isotopic signatures serve as "fingerprints" for planets and meteorites from the same body.

The probability that the impactor just happened to have the same isotopic signature as the Earth was vanishingly small.

So the giant impact hypothesis had a major problem. It could agree many physical characteristics of the Earth-Moon system but not their geochemistry. The isotopic composition studies had created an "isotopic crisis" for the hypothesis.

At first, scientists thought more precise measurements might resolve the crisis. But more accurate measurements of oxygen isotopes published in 2016 only confirmed that the isotopic compositions are not distinguishable. "These are the most precise measurements we can make, and theyre still identical," Wang said.

"So people decided to change the giant impact hypothesis," Wang said. "The perfection was to find a way to make the Moon mostly from the Earth rather than mostly from the impactor. There are many new models -- everyone is trying to come up with one -- but two have been very influential."

In the original giant impact model, the impact melted a part of the Earth and the entire impactor, flinging some of the melt outward, like clay from a potters wheel.

A model proposed in 2007 adds a silicate vapor atmosphere around the Earth and the lunar disk (the magma disk that is the residue of the impactor). The idea is that the silicate vapor allows transfer between the Earth, the vapor, and the material in the disk, before the Moon condenses from the melted disk.

"Theyre trying to explain the isotopic similarities by addition of this atmosphere," Wang said, "but they still start from a low-energy impact like the original model."

But exchanging material through an atmosphere is really slow, Wang said. Youd never have enough time for the material to blend thoroughly before it started to fall back to Earth.

So another model, proposed in 2015, assumes the impact was extremely violent, so violent that the impactor and Earths mantle vaporized and mixed together to form a dense melt/vapor mantle atmosphere that expanded to fill a space more than 500 times bigger than todays Earth. As this atmosphere cooled, the Moon condensed from it.

The thorough mixing of this atmosphere explains the identical isotope composition of the Earth and Moon, Wang said. The mantle atmosphere was a "supercritical fluid," without distinct liquid and gas phases. Supercritical fluids can flow through solids like a gas and dissolve materials like a liquid.

The Mood paper reports high-precision potassium isotopic data for a representative sample of lunar and terrestrial rocks. Potassium has three stable isotopes, but only two of them, potassium-41 and potassium-39, are abundant enough to be measured with sufficient precision for this study.

Wang and Jacobsen examined seven lunar rock samples from different lunar missions and compared their potassium isotope ratios to those of eight terrestrial rocks representative of Earths mantle. They found that the lunar rocks were enriched by about 0.4 parts per thousand in the heavier isotope of potassium, potassium-41.

The only high-temperature process that could separate the potassium isotopes in this way, said Wang, is incomplete condensation of the potassium from the vapor phase during the Moons formation. Compared to the lighter isotope, the heavier isotope would preferentially fall out of the vapor and condense.

Calculations show, however, that if this process happened in an absolute vacuum, it would direct to an enrichment of heavy potassium isotopes in lunar samples of about 100 parts per thousand, much higher than the value Wang and Jacoben found.
But higher pressure would suppress fractionation, Wang said. For this excuse, he and his colleague predict the Moon condensed in a pressure of more than 10 bar, or roughly 10 times the sea level atmospheric pressure on Earth.

Their finding that the lunar rocks are enriched in the heavier potassium isotope does not favor the silicate atmosphere model, which predicts lunar rocks will contain less of the heavier isotope than terrestrial rocks, the opposite of what the scientists found.

Instead it supports the mantle atmosphere model that predicts lunar rocks will contain more of the heavier isotope than terrestrial rocks.

The Daily Galaxy via Washington University, St. Louis

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