Webb looks for Fomalhaut's asteroid belt and finds much more

Astronomers used NASA's James Webb Space Telescope to image the warm dust around a nearby young star, Fomalhaut, in order to study the first asteroid belt ever seen outside of our solar system in infrared light. But to their surprise, the dusty structures are much more complex than the asteroid and Kuiper dust belts of our solar system. Overall, there are three nested belts extending out to 14 billion miles (23 billion kilometers) from the star; that's 150 times the distance of Earth from the Sun. The scale of the outermost belt is roughly twice the scale of our solar system's Kuiper Belt of small bodies and cold dust beyond Neptune. The inner belts -- which had never been seen before -- were revealed by Webb for the first time.

The belts encircle the young hot star, which can be seen with the naked eye as the brightest star in the southern constellation Piscis Austrinus. The dusty belts are the debris from collisions of larger bodies, analogous to asteroids and comets, and are frequently described as 'debris disks.' "I would describe Fomalhaut as the archetype of debris disks found elsewhere in our galaxy, because it has components similar to those we have in our own planetary system," said András Gáspár of the University of Arizona in Tucson and lead author of a new paper describing these results. "By looking at the patterns in these rings, we can actually start to make a little sketch of what a planetary system ought to look like -- If we could actually take a deep enough picture to see the suspected planets."

The Hubble Space Telescope and Herschel Space Observatory, as well as the Atacama Large Millimeter/submillimeter Array (ALMA), have previously taken sharp images of the outermost belt. However, none of them found any structure interior to it. The inner belts have been resolved for the first time by Webb in infrared light. "Where Webb really excels is that we're able to physically resolve the thermal glow from dust in those inner regions. So you can see inner belts that we could never see before," said Schuyler Wolff, another member of the team at the University of Arizona.

Hubble, ALMA, and Webb are tag-teaming to assemble a holistic view of the debris disks around a number of stars. "With Hubble and ALMA, we were able to image a bunch of Kuiper Belt analogs, and we've learned loads about how outer disks form and evolve," said Wolff. "But we need Webb to allow us to image a dozen or so asteroid belts elsewhere. We can learn just as much about the inner warm regions of these disks as Hubble and ALMA taught us about the colder outer regions."

These belts most likely are carved by the gravitational forces produced by unseen planets. Similarly, inside our solar system Jupiter corrals the asteroid belt, the inner edge of the Kuiper Belt is sculpted by Neptune, and the outer edge could be shepherded by as-yet-unseen bodies beyond it. As Webb images more systems, we will learn about the configurations of their planets.

Fomalhaut's dust ring was discovered in 1983 in observations made by NASA's Infrared Astronomical Satellite (IRAS). The existence of the ring has also been inferred from previous and longer-wavelength observations using submillimeter telescopes on Mauna Kea, Hawaii, NASA's Spitzer Space Telescope, and Caltech's Submillimeter Observatory.

"The belts around Fomalhaut are kind of a mystery novel: Where are the planets?" said George Rieke, another team member and U.S. science lead for Webb's Mid-Infrared Instrument (MIRI), which made these observations. "I think it's not a very big leap to say there's probably a really interesting planetary system around the star."

"We definitely didn't expect the more complex structure with the second intermediate belt and then the broader asteroid belt," added Wolff. "That structure is very exciting because any time an astronomer sees a gap and rings in a disk, they say, 'There could be an embedded planet shaping the rings!'"

Webb also imaged what Gáspár dubs "the great dust cloud," which may be evidence for a collision occurring in the outer ring between two protoplanetary bodies. This is a different feature from a suspected planet first seen inside the outer ring by Hubble in 2008. Subsequent Hubble observations showed that by 2014 the object had vanished. A plausible interpretation is that this newly discovered feature, like the earlier one, is an expanding cloud of very fine dust particles from two icy bodies that smashed into each other.

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Modeling reveals how dwarf planet Ceres powers unexpected geologic activity

For a long time, our view of Ceres was fuzzy, said Scott King, a geoscientist in the Virginia Tech College of Science. A dwarf planet and the largest body found in the asteroid belt -- the region between Jupiter and Mars speckled with hundreds of thousands of asteroids -- Ceres had no distinguishable surface features in existing telescopic observations from Earth.

Then, in 2015, the hazy orb that was Ceres came into view. That view was stunning to scientists such as King. Data and images collected by NASA's Dawn mission gave a clearer picture of the surface, including its composition and structures, which revealed unexpected geologic activity.

Scientists had seen the general size of Ceres in earlier observations. It was so small it was assumed to be inactive. Instead, Dawn discovered a large plateau on one side of Ceres that covered a fraction of the dwarf planet, similar to what a continent might take up on Earth. Surrounding it were fractures in rocks clustered in one location. And there were visible traces of an ocean world: deposits all over the surface where minerals had condensed as water evaporated -- the mark of a freezing ocean.

A professor in the Department of Geosciences, King, who mostly studies larger bodies such as planets, wanted to know how a body as small as Ceres could generate the heat needed to power that kind of geological activity and account for the surface features picked up by Dawn.

Through modeling, he and a team of scientists from multiple universities as well as the United States Geological Survey and the Planetary Science Institute found that the decay of radioactive elements within Ceres's interior could keep it active. Their findings were recently published in American Geophysical Union Advances.

King's study of big planets such as Earth, Venus, and Mars had always shown him that planets start out hot. The collision between objects that form a planet creates that initial heat. Ceres, by contrast, never got big enough to become a planet and generate heat the same way, King said. To learn how it could still generate enough heat to power geologic activity, he used theories and computational tools previously applied to bigger planets to study Ceres's interior, and he looked for evidence that could support his models in data returned by the Dawn mission.

The team's model of the dwarf planet's interior showed a unique sequence: Ceres started out cold and heated up because of the decay of radioactive elements such as uranium and thorium -- which was alone enough to power its activity -- until the interior became unstable.

"What I would see in the model is, all of a sudden, one part of the interior would start heating up and would be moving upward and then the other part would be moving downward," King said.

That instability could explain some of the surface features that had formed on Ceres, as revealed by the Dawn mission. The large plateau had formed on only one side of Ceres with nothing on the other side, and the fractures were clustered in a single location around it. The concentration of features in one hemisphere signaled to King that instability had occurred and had left a visible impact.

"It turned out that you could show in the model that where one hemisphere had this instability that was rising up, it would cause extension at the surface, and it was consistent with these patterns of fractures," King said.

Based on the team's model, Ceres didn't follow a planet's typical pattern of hot first and cool second, with its own pattern of cool, hot, and cool again. "What we've shown in this paper is that radiogenic heating all on its own is enough to create interesting geology," King said.

He sees similarities to Ceres in the moons of Uranus, which a study commissioned by NASA and the National Science Foundation recently deemed high priority for a major robotic mission. With additional improvements to the model, he looks forward to exploring their interiors as well.

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Psyche, the iron giant of asteroids, may be less iron than researchers thought

The asteroid 16 Psyche, which NASA intends to visit with a spacecraft in 2026, may be less heavy metal and more hard rock than scientists have surmised, according to a new study by researchers from Brown and Purdue universities.

Psyche, which orbits the sun in the asteroid belt between Mars and Jupiter, is the largest of the M-type asteroids, which are composed chiefly of iron and nickel as opposed to the silicate rocks that make up most other asteroids. But when viewed from Earth, Psyche sends mixed signals about its composition.

The light it reflects tells scientists that the surface is indeed mostly metal. That has led to conjecture that Psyche may be the exposed iron core of a primordial planetary body -- one whose rocky crust and mantle were blasted away by an ancient collision. However, measurements of Psyche's mass and density tell a different story. The way its gravity tugs on neighboring bodies suggests that Psyche is far less dense than a giant hunk of iron should be. So if Psyche is indeed all metal, it would have to be highly porous -- a bit like a giant ball of steel wool with nearly equal parts void space and solid metal.

"What we wanted to do with this study was see whether it was possible for an iron body the size of Psyche to maintain that near-50% porosity," said Fiona Nichols-Fleming, a Ph.D. student at Brown and study's lead author. "We found that it's very unlikely."

For the study, published in Geophysical Research Letters, Nichols-Fleming worked with Alex Evans, an assistant professor at Brown, and Purdue professors Brandon Johnson and Michael Sori. The team created a computer model, based on known thermal properties of metallic iron, to estimate how the porosity of a large iron body would evolve over time.

The model shows that to remain highly porous, Psyche's internal temperature would have to cool below 800 Kelvin very shortly after its formation. At temperatures above that, iron would have been so malleable that Psyche's own gravity would have collapsed most of the pore space within its bulk. Based on what is known about conditions in the early solar system, the researchers say, it's extremely unlikely that a body of Psyche's size -- about 140 miles in diameter -- could have cooled so quickly.

In addition, any event that may have added porosity to Psyche after its formation -- a massive impact, for example -- would likely have also heated Psyche back up above 800 K. So any newly introduced porosity would have been unlikely to last.

Taken together, the results suggest that Psyche probably isn't a porous, all-iron body, the researchers conclude. More likely, it's harboring a hidden rocky component that drives its density down. But if Psyche does have a rocky component, why does its surface look so metallic when viewed from Earth? There are few possible explanations, the researchers say.

One of those possibilities is ferrovolcanism -- iron-spewing volcanoes. It's possible, the researchers say, that Psyche is actually a differentiated body with a rocky mantle and an iron core. But widespread ferrovolcanic activity may have brought large amounts of Psyche's core up to the surface, putting an iron coating atop its rocky mantle. Prior research by Johnson and Evans has shown that ferrovolcanism is possible on a body like Psyche.

Whatever the case, scientists will soon get a much clearer picture of this mysterious asteroid. Later this year, NASA plans to launch a spacecraft that will rendezvous with Psyche after a four-year journey to the asteroid belt.

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Earth and Mars were formed from inner Solar System material

Earth and Mars were formed from material that largely originated in the inner Solar System; only a few percent of the building blocks of these two planets originated beyond Jupiter's orbit. A group of researchers led by the University of Münster (Germany) report these findings today in the journal Science Advances. They present the most comprehensive comparison to date of the isotopic composition of Earth, Mars and pristine building material from the inner and outer Solar System. Some of this material is today still found largely unaltered in meteorites. The results of the study have far-reaching consequences for our understanding of the process that formed the planets Mercury, Venus, Earth, and Mars. The theory postulating that the four rocky planets grew to their present size by accumulating millimeter-sized dust pebbles from the outer Solar System is not tenable.

Approximately 4.6 billion years ago in the early days of our Solar System, a disk of dust and gases orbited the young Sun. Two theories describe how in the course of millions of years the inner rocky planets formed from this original building material. According to the older theory, the dust in the inner Solar System agglomerated to ever larger chunks gradually reaching approximately the size of our Moon. Collisions of these planetary embryos finally produced the inner planets Mercury, Venus, Earth, and Mars. A newer theory, however, prefers a different growth process: millimeter-sized dust "pebbles" migrated from the outer Solar System towards the Sun. On their way, they were accreted onto the planetary embryos of the inner Solar System, and step by step enlarged them to their present size.

Both theories are based on theoretical models and computer simulations aimed at reconstructing the conditions and dynamics in the early Solar System; both describe a possible path of planet formation. But which one is right? Which process actually took place? To answer these questions, in their current study researchers from the University of Münster (Germany), the Observatoire de la Cote d'Azur (France), the California Institute of Technology (USA), the Natural History Museum Berlin (Germany), and the Free University of Berlin (Germany) determined the exact composition of the rocky planets Earth and Mars. "We wanted to find out whether the building blocks of Earth and Mars originated in the outer or inner Solar System," says Dr. Christoph Burkhardt of the University of Münster, the study's first author. To this end, the isotopes of the rare metals titanium, zirconium and molybdenum found in minute traces in the outer, silicate-rich layers of both planets provide crucial clues. Isotopes are different varieties of the same element, which differ only in the weight of their atomic nucleus.

Meteorites as a reference

Scientists assume that in the early Solar System these and other metal isotopes were not evenly distributed. Rather, their abundance depended on the distance from the Sun. They therefore hold valuable information about where in the early Solar System a certain body's building blocks originated.

As a reference for the original isotopic inventory of the outer and inner Solar System, the researchers used two types of meteorites. These chunks of rock generally found their way to Earth from the asteroid belt, the region between the orbits of Mars and Jupiter. They are considered to be largely pristine material from the beginnings of the Solar System. While so-called carbonaceous chondrites, which can contain up to a few percent carbon, originated beyond Jupiter's orbit and only later relocated to the asteroid belt due to influence of the growing gas giants, their more carbon-depleted cousins, the non-carbonaceous chondrites, are true children of the inner Solar System.

The precise isotopic composition of Earth's accessible outer rock layers and that of both types of meteorites have been studied for some time; however, there have been no comparably comprehensive analyses of Martian rocks. In their current study, the researchers now examined samples from a total of 17 Martian meteorites, which can be assigned to six typical types of Martian rock. In addition, the scientists for the first time investigated the abundances of three different metal isotopes.

The samples of Martian meteorites were first powdered and subjected to complex chemical pretreatment. Using a multicollector plasma mass spectrometer at the Institute of Planetology at the University of Münster, the researchers were then able to detect tiny amounts of titanium, zirconium, and molybdenum isotopes. They then performed computer simulations to calculate the ratio in which building material found today in carbonaceous and non-carbonaceous chondrites must have been incorporated into Earth and Mars in order to reproduce their measured compositions. In doing so, they considered two different phases of accretion to account for the different history of the titanium and zirconium isotopes as well as of the molybdenum isotopes, respectively. Unlike titanium and zirconium, molybdenum accumulates mainly in the metallic planetary core. The tiny amounts still found today in the silicate-rich outer layers can therefore only have been added during the very last phase of the planet's growth.

The researchers' results show that the outer rock layers of Earth and Mars have little in common with the carbonaceous chondrites of the outer Solar System. They account for only about four percent of both planets' original building blocks. "If early Earth and Mars had mainly accreted dust grains from the outer Solar System, this value should be almost ten times higher," says Prof. Dr. Thorsten Kleine of the University of Münster, who is also director at the Max Planck Institute for Solar System Research in Göttingen. "We thus cannot confirm this theory of the formation of the inner planets," he adds.

Lost building material

But the composition of Earth and Mars does not exactly match the material of the non-carbonaceous chondrites either. The computer simulations suggest that another, different kind of building material must also have been in play. "The isotopic composition of this third type of building material as inferred by our computer simulations implies it must have originated in the innermost region of the Solar System," explains Christoph Burkhardt. Since bodies from such close proximity to the Sun were almost never scattered into the asteroid belt, this material was almost completely absorbed into the inner planets and thus does not occur in meteorites. "It is, so to speak, 'lost building material' to which we no longer have direct access today," says Thorsten Kleine.

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