Maat Mons, on Venus. Image: NASA

Heavy metal snow on Venus is lead, bismuth sulfides
by Carolyn Jones Otten / January 1, 2002

“Lead sulfide — also known by its mineral name, galena — is a naturally occurring mineral found in Missouri, other parts of the world, and now … other parts of the solar system. That’s because recent thermodynamic calculations by University researchers provide plausible evidence that “heavy metal snow,” which blankets the surface of upper altitude Venusian rocks, is composed of both lead and bismuth sulfides. The findings — by Laura Schaefer, research assistant in the Planetary Chemistry Laboratory, and M. Bruce Fegley Jr., Ph.D., professor of earth and planetary sciences in Arts & Sciences — discount previous hypotheses that the snow was made of elemental tellurium. They are important also because lead sulfide “snow” could allow the dating of Venus by lead isotopes, provided a soil sample can be obtained in a future mission. Schaefer and Fegley’s work was published in a recent issue of Icarus, the official journal of the Division of Planetary Sciences of the American Astronomical Society.

“We calculated the equilibrium compositions for 20 trace metals in Venus’ lower atmosphere, looking for something that condenses at this altitude of 2.6 kilometers,” Schaefer said. Previous analyses, she added, simply “didn’t consider any chemistry. When we looked at the chemistry, we found that the best candidates were actually lead and bismuth sulfides.” Discovery of the metallic snow dates back to 1995, when Raymond E. Arvidson, Ph.D., the McDonnell Distinguished University Professor and chair of earth and planetary sciences, and other researchers were analyzing the vast archives of data taken from NASA’s Magellan mission to Venus in 1989.

“The top of this mountain is covered with a thin layer of substance that reflects radar”

Magellan’s primary objective was to map the surface of Venus using a technique known as synthetic aperture radar (SAR). SAR images taken of Aphrodite Terra and other mountainous regions in Venus’ highlands revealed a mysterious brightening effect. Using computers to factor in physical parameters such as elemental abundances — what elements are present and in what amounts, altitudes, temperatures and pressures — researchers surmised that the brightening effect was due to a metal-containing “snow” only a few millimeters in thickness frosting the mountains’ rugged surfaces.

But even as the hypothesis of metallic snow was circulating throughout the planetary community, its chemical composition remained largely an educated guess — one among many on the short-list of 98 possible metal-containing compounds that commonly exist around volcanic vents on Earth. “An old idea we had was that you have compounds of these trace metals being erupted and condensing around volcanoes on Earth,” Fegley said. “Now on Venus, which is much hotter than Earth, you’d have a similar process: You’d be erupting these trace metals, which would then stay in the gas phase until they reached a high enough atmospheric level where they’d condense. “Because you have a decrease in temperature with altitude, places like the Maxwell Montes on Venus — similar to Mauna Loa in Hawaii — get cold enough that some of these things would start to condense out.”

The researchers took the list of possibilities and used their expertise in chemical thermodynamics to help them narrow the pool of suspects. In this case, whether a particular compound remained a plausible candidate was governed by two factors: thermodynamics — the rules that predict chemical stability based on environment — and the chemical profile of Venus, which was obtained from earlier American and Russian data-gathering missions. “One of my old professors from MIT (Gordon Pettengill, the principal investigator for the Magellan SAR project) did an experiment that proved our model for the (existence of) metallic snow, but he suggested tellurium,” Fegley said.

“I decided to re-examine the issue in early 2003.” Schaefer and Fegley carefully considered what could happen to tellurium after it was introduced into the Venusian atmosphere by a volcanic event. But they went a step further by allowing it to undergo reactions with other volatile species present in the atmosphere. As it turns out, sulfur dioxide is the third-most-abundant gas on Venus and is a major contributor to the thick layer of sulfuric acid clouds that envelope the planet. According to thermodynamic equations, any significant concentration of volatile tellurium would react with these sulfur-containing compounds to make tellurium sulfide, a relatively stable gas.

“So it can’t just condense out because it’s undergoing chemical reactions instead,” Fegley said. Lead sulfide and bismuth sulfide were identified as front-runners thanks to a specific physical property called a dielectric constant — an intrinsic value describing a material’s electrical conductivity — that Magellan’s SAR measured in 1991. “Typical volcanic rocks have a dielectric constant of a few, maybe 4, but the stuff that Magellan saw in the highlands of Venus was much higher, about 100,” Fegley said. “In order to have a dielectric constant that high, you have to have something that’s either a semiconductor or a conductor, and actually, these minerals that we’ve proposed condensing, the galena (lead sulfide) and the bismuth sulfide, have dielectric constants that are basically — BANG! — right on.” If Schaefer and Fegley are right, having “snow” made of lead sulfide could have implications beyond confirming their own work; it could be used as a means of dating the beginning of Venus’ existence.

So how exactly would that work? By the same process that scientists have used to date the age of the Earth — lead dating — using the ratios of different lead isotopes (which differ only in number of neutrons). All of these lofty dreams rest on there being an actual sample of dirt to analyze; a dream that could become reality with one of NASA’s New Frontiers Missions, a competitive $650 million endeavor to be selected for funding in the next year or two. Venus aficionados like Fegley are pushing for a more comprehensive probe of the Earth’s nearest neighbor. Their mission would include a detachable landing module that could perform geochemical analyses in the highlands using techniques like X-ray fluorescence and X-ray diffraction. “All these ideas — these calculations — can be tested by one of these New Frontiers spacecrafts, if the Venus mission is picked,” Fegley said. “What makes this type of work exciting is the fact that these ideas could be tested by spacecrafts that are on the drawing boards today.”

Scientists find evidence of extremely heavy elements in ancient stars
by Prachi Patel  /  December 12, 2023

“Spectral analysis of 42 stars shows a correlation in the proportions of certain elements, suggesting they were formed by the fission of elements much heavier than uranium
Scientists believe that many of the elements found in the Universe that are heavier than iron are created when stars merge or explosively die, but they are still unsure about the cosmic origin of several elements and the full array of processes that form heavy elements. The source of naturally-formed elements heavier than uranium is especially unclear, and no one has directly detected such elements in space. Now, by analyzing the chemical composition of 42 ancient stars in the Milky Way, researchers show that the stars produced elements with atomic masses larger than 260, heavier than any element found naturally on Earth (Science 2023, DOI: 10.1126/science.adf1341) .

Fission of these radioactive nuclei is a major cosmic source of elements heavier than iron, they suggest. Scientists look at light spectra from stars to identify the proportions of various elements inside. Studies so far have focused on the detailed elemental composition of one star or compared relative amounts of a few elements in several stars, says Ian Roederer, a physicist and astronomer at North Carolina State University. To reveal a more complete picture, he and his colleagues analyzed the element patterns of “every star we could find in published literature with high levels of heavy elements,” he says.

Specifically, they looked at elements from selenium to platinum, atomic numbers 34 to 78. They found a direct correlation between the amounts of elements with atomic numbers 44 to 47 (ruthenium to silver), and of slightly heavier ones with atomic numbers 63 to 78 (europium to platinum). There was no similar behavior among elements adjacent to the two groups. The only reasonable explanation is that the correlated elements came from a common source, he says. Adding the atomic mass numbers of the two element sets indicates that the stars had produced elements with atomic mass over 260, and these radioactive transuranic nuclei split into two fragments, a smaller one with lighter elements from the silver to ruthenium group and a larger one with the slightly heavier element group.”

“Most of the elements that make up the human body were formed in stars” 

Too much heavy metal stops stars producing more
by ARC Centre (ASTRO 3D)  /  January 11, 2022

“Stars are giant factories that produce most of the elements in the universe—including the elements in us, and in Earth’s metal deposits. But how do stars produce changes over time? Two new papers published in Monthly Notices of the Royal Astronomical Society (MNRAS) shed light on how the youngest generation of stars will eventually stop contributing metals back to the universe.

The authors are all members of ASTRO 3D, the ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions. They are based at Monash University, the Australian National University (ANU), and the Space Telescope Science Institute. “We know the first two elements of the periodic table—hydrogen and helium—were created in the Big Bang,” says Amanda Karakas, first author of a paper studying metal-rich stars. “Over time, the stars that came after the Big Bang produce heavier elements.” These “metal-rich” stars, like our sun, spew out their products into space, enriching the composition of the galaxy over time.

These objects affect us directly as around half of the carbon and all elements heavier than iron are synthesized by stars like our sun. About 90 percent of all the lead on Earth, for example, was made in  that also produce elements such as strontium and barium. But this ability to produce more metals changes depending on the composition of a star at its birth. “Introducing just a tiny bit more metal into the stars’ gas has really large implications on their evolution,” says Giulia Cinquegrana.

Her paper uses modeling from the earlier paper to study the chemical output of metal-rich stars. “We discovered that at a certain threshold of initial metal content in the gas, stars will stop sending more metals into the universe over their lifetime,” Cinquegrana says. The sun, born about 4.5 billion years ago, is a typical “middle-aged” star. It is “metal-rich” compared to the first stellar generations and has a heavy element content similar to many other stars in the center of the Milky Way.

“Our papers predict the evolution of younger stars (most-recent generations) which are up to seven times more metal-rich than the sun,” says Karakas. “My simulations show that this really high level of chemical enrichment causes these stars to act quite weirdly, compared to what we believe is happening in the sun,” says Cinquegrana. “Our models of super metal-rich stars show that they still expand to become red giants and go on to end their lives as white dwarfs, but by that time they are not expelling any heavy elements. The metals get locked up in the white dwarf remnant,” she says. “But the process of stars constantly adding elements to the universe means that the make-up of the universe is always changing. In the far distant future, the distribution of elements will look very different to what we see now in our solar system,” says Karakas.”

More information: Amanda I Karakas et al, The most metal-rich asymptotic giant branch stars, Monthly Notices of the Royal Astronomical Society (2021). DOI: 10.1093/mnras/stab3205

Giulia C Cinquegrana et al, The most metal-rich stars in the universe: chemical contributions of low- and intermediate-mass asymptotic giant branch stars with metallicities within 0.04 ≤ Z ≤ 0.10, Monthly Notices of the Royal Astronomical Society (2021). DOI: 10.1093/mnras/stab3379

Astronomers turn up the heavy metal to shed light on star formation
by Jess Reid, ICRAR  /  October 6, 2020

“Astronomers from The University of Western Australia’s node of the International Center for Radio Astronomy Research (ICRAR) have developed a new way to study star formation in galaxies from the dawn of time to today. “Stars can be thought of as enormous nuclear-powered processing plants,” said lead researcher Dr. Sabine Bellstedt, from ICRAR. “They take lighter elements like hydrogen and helium, and, over billions of years, produce the heavier elements of the periodic table that we find scattered throughout the universe today.

“A selection of the 7,000 galaxies used by the researchers in this work”

The carbon, calcium and iron in your body, the oxygen in the air you breathe, and the silicon in your computer all exist because a star created these heavier elements and left them behind,” Bellstedt said. “Stars are the ultimate element factories in the universe.” Understanding how galaxies formed stars billions of years ago requires the very difficult task of using powerful telescopes to observe galaxies many billions of light-years away in the distant universe. However, nearby galaxies are much easier to observe. Using the light from these local galaxies, astronomers can forensically piece together the history of their lives (called their star-formation history). This allows researchers to determine how and when they formed stars in their infancy, billions of years ago, without struggling to observe galaxies in the distant universe.

Traditionally, astronomers studying star formation histories assumed the overall metallicity—or amount of heavy elements—in a galaxy doesn’t change over time. But when they used these models to pinpoint when stars in the universe should have formed, the results didn’t match up with what they were seeing through their telescopes. “The results not matching up with our observations is a big problem,” Bellstedt said. “It tells us we’re missing something. That missing ingredient, it turns out, is the gradual build-up of heavy metals within galaxies over time.” Using a new algorithm to model the energy and wavelengths of light coming from almost 7000 nearby galaxies, the researchers succeeded in reconstructing when most of the stars in the universe formed—in agreement with telescope observations for the first time.

“Artist’s impression. of the ProSpect code analysing a galaxy. Credit: ICRAR”

The designer of the new code—known as ProSpect—is Associate Professor Aaron Robotham from ICRAR’s University of Western Australia node. “This is the first time we’ve been able to constrain how the heavier elements in galaxies change over time based on our analysis of these 7000 nearby galaxies,” Robotham said. “Using this galactic laboratory on our own doorstep gives us lots of observations to test this new approach, and we’re very excited that it works. With this tool, we can now dissect nearby galaxies to determine the state of the universe and the rate at which stars form and mass grows at any stage over the past 13 billion years. It’s absolutely mind-blowing stuff.”

This work also confirms an important theory about when most of the stars in the universe formed. “Most of the stars in the universe were born in extremely massive galaxies early on in cosmic history—around three to four billion years after the Big Bang,” Bellstedt said. “Today, the universe is almost 14 billion years old, and most new stars are being formed in much smaller galaxies.” Based on this research, the next challenge for the team will be to expand the sample of galaxies being studied using this technique, in an effort to understand when, where and why galaxies die and stop forming new stars.

Bellstedt and Robotham, along with colleagues from Australia, the UK and the United States, are reporting their results in the scientific journal the Monthly Notices of the Royal Astronomical Society. The Galaxy And Mass Assembly (GAMA) is a decade-long project to probe the evolution of mass, energy and structure on scales ranging from 1kpc to 1Mpc—measuring properties of the internal structures of galaxies, interacting pairs and mergers, the group environment and large-scale structure.”

More information: Sabine Bellstedt et al. Galaxy And Mass Assembly (GAMA): a forensic SED reconstruction of the cosmic star formation history and metallicity evolution by galaxy type, Monthly Notices of the Royal Astronomical Society (2020). DOI: 10.1093/mnras/staa2620

Earth’s heavy metals result of supernova explosion
by University of Guelph  /  June 13, 2019

“That gold on your ring finger is stellar—and not just in a complimentary way. In a finding that may overthrow our understanding of where Earth’s heavy elements such as gold and platinum come from, new research by a University of Guelph physicist suggests that most of them were spewed from a largely overlooked kind of star explosion far away in space and time from our planet. Some 80 per cent of the heavy elements in the universe likely formed in collapsars, a rare but heavy element-rich form of supernova explosion from the gravitational collapse of old, massive stars typically 30 times as weighty as our sun, said physics professor Daniel Siegel. That finding overturns the widely held belief that these elements mostly come from collisions between neutron stars or between a neutron star and a black hole, said Siegel. His paper co-authored with Columbia University colleagues appears today in the journal Nature.

Using supercomputers, the trio simulated the dynamics of collapsars, or old stars whose gravity causes them to implode and form black holes. Under their model, massive, rapidly spinning collapsars eject heavy elements whose amounts and distribution are “astonishingly similar to what we observe in our solar system,” said Siegel. He joined U of G this month and is also appointed to the Perimeter Institute for Theoretical Physics, in Waterloo, Ont. Most of the elements found in nature were created in nuclear reactions in stars and ultimately expelled in huge stellar explosions. Heavy elements found on Earth and elsewhere in the universe from long-ago explosions range from gold and platinum, to uranium and plutonium used in nuclear reactors, to more exotic chemical elements such as neodymium found in consumer items such as electronics.

Until now, scientists thought that these elements were cooked up mostly in stellar smashups involving neutron stars or black holes, as in a collision of two neutron stars observed by Earth-bound detectors that made headlines in 2017. Ironically, said Siegel, his team began working to understand the physics of that merger before their simulations pointed toward collapsars as a heavy element birth chamber. “Our research on neutron star mergers has led us to believe that the birth of black holes in a very different type of stellar explosion might produce even more gold than neutron star mergers.”

What collapsars lack in frequency, they make up for in generation of heavy elements, said Siegel. Collapsars also produce intense flashes of gamma rays. “Eighty per cent of these heavy elements we see should come from collapsars. Collapsars are fairly rare in occurrences of supernovae, even more rare than neutron star mergers—but the amount of material that they eject into space is much higher than that from neutron star mergers.” The team now hopes to see its theoretical model validated by observations.


Siegel said infrared instruments such as those on the James Webb Space Telescope, set for launch in 2021, should be able to detect telltale radiation pointing to heavy elements from a collapsar in a far-distant galaxy. “That would be a clear signature,” he said, adding that astronomers might also detect evidence of collapsars by looking at amounts and distribution of heavy element s in other stars across our Milky Way galaxy. Siegel said this research may yield clues about how our galaxy began. “Trying to nail down where heavy elements come from may help us understand how the galaxy was chemically assembled and how the galaxy formed.

This may actually help solve some big questions in cosmology as heavy elements are a nice tracer.” This year marks the 150th anniversary of Dmitri Mendeleev‘s creation of the periodic table of the chemical elements. Since then, scientists have added many more elements to the periodic table, a staple of science textbooks and classrooms worldwide. Referring to the Russian chemist, Siegel said, “We know many more elements that he didn’t. What’s fascinating and surprising is that, after 150 years of studying the fundamental building blocks of nature, we still don’t quite understand how the universe creates a big fraction of the elements in the periodic table.”

More information: Daniel M. Siegel et al, Collapsars as a major source of r-process elements, Nature (2019). DOI: 10.1038/s41586-019-1136-0

ScienceShot: Stars of Heavy Metal
by Sid Perkins  /  August 1, 2013

“The most lead-rich stars known to science may represent a brief stage in stellar evolution that scientists have theorized but previously haven’t seen. The small suns, known as HE 2359-2844 (artist’s representation shown) in the constellation Sculptor and HE 1256-2738 in Hydra, were among nine identified as being helium-rich in a previous survey of stars. But new analyses show that these two are doubly unusual because they also sport atmospheres with lead concentrations about 10,000 times those seen in the atmosphere of our sun, the researchers report online today in Monthly Notices of the Royal Astronomical Society.

“Many stars in the center of the Milky Way have high heavy metal content”

The surface temperatures of these two bluish stars are estimated to be about 38,000°C (far hotter than our sun’s surface temperature of about 5500°C), so hot that lead atoms in their atmospheres have been stripped of three electrons. The stars may be passing through a stage of stellar evolution that lasts no more than a few tens of thousands of years, the scientists say—a phase between red giants (about 30 or 40 times the size of our sun) and blue subdwarfs (stars about one-fifth the size of our sun but seven times hotter and 70 times brighter). The lead surrounding the stars—which was part of the original cloud of gas and dust from which these stars formed, not generated by reactions in the evolving stars themselves—may be dispersed within an atmospheric layer as much as 100 kilometers thick (depicted patchily in pink) that altogether weighs up to 100 billion metric tons.”

“A single pre-solar grain viewed through an electron microscope.Invisible to the human eye, a single speck of this very pure, original stardust (known as a pre-solar grains, because they are older than our Sun) is only a few microns in size – 100 times smaller than the width of a human hair.”

Heavy metal stars produce Earth-Like planets
by Nola Taylor Redd  /  September 30, 2011

“New research reveals that, like their giant cousins, rocky planets are more likely to be found orbiting high metallicity stars. Furthermore, these planets are more plentiful around low mass stars. This could have important implications for the search for life outside of Earth. Kevin Schlaufman and Gregory Laughlin, both of the University of California at Santa Cruz, studied the 997 stars with candidate planets thought to be in orbit around them, as reported by Kepler’s science team last February. Schlaufman and Laughlin confirmed that both large and small planets were more likely to be found around stars with higher metallicities. For astronomers, elements other than hydrogen and helium are considered “metals.”

“NASA’s Kepler has discovered 1,235 candidate planets orbiting 997 stars”

Stars with high metallicities contain a significant amount of other elements. These metals were first formed when early stars, composed of the two basic gases hydrogen and helium, died in a violent supernova, spewing their contents into space. Sally Dodson-Robinson, of the University of Texas at Austin, noted that it wasn’t surprising to find that terrestrial planets tend to form around more metallic stars. “Planets formed from the same raw materials as their star does,” she explained.

Stars form from the gravitational compression of gas and dust, and the spinning disk of leftover material that orbits the new star is where planets are made. Before Kepler, enough gas giant planets had been located for astronomers to say with certainty that these  behemoths were linked to metal-rich stars. But it was not known if this applied to rocky planets as well, since so few had been found in the galaxy. That changed in February, when NASA announced the discovery by Kepler of 68 Earth-sized candidates and 288 super-Earths. This planetary goldmine provided a wealth of systems to study, and enough stars to make firmer correlations about the types of stars that less massive planets orbit.

Because all types of planets are more likely to exist around high-metallicity stars, Schlaufman said this gives a rough time frame for when planets first began to appear in the galaxy. After all, they would have to wait for the first generation of stars to speed through their life cycle and explode, providing the metals required for planetary formation. Each cycle of stars would have created more metals, making it easier for planets to coalesce. The process would have taken a few billion years. This provides constraints on finding advanced civilizations, since planets – and thus life – would not have formed in the early years of the universe. Schlaufman added that a stronger case could be made as more extrasolar planets are found in the future, helping astronomers better understand the links between planets and their stars.

“Kepler 10b was the first rocky planet found by the spacecraft. With a temperature of over 2,500 degrees Fahrenheit, the planet is unlikely to have life as we know it”

But in their study, Schlaufman and Laughlin examined more than the metallicity of stars. They also determined that terrestrial planets were more likely to be found around low mass stars. The reason is simple: gas giants require a lot of mass to form. “The total mass in the disk is proportional to how massive the star is,” Schlaufman said. Larger disks are more likely to yield massive planets, while smaller stars and their disks seem to result in less massive, rockier satellites.

Schlaufman was quick to note the possibilities for life. Stars more massive than the Sun last only a few billion years, while their lower-mass siblings have much longer lifetimes. This gives a planet more time to develop life – and for that life to evolve into an advanced civilization – before the death of its sun. The odds of finding life may increase with the more planets that are discovered, especially rocky planets like the Earth. Kepler’s confirmation that such planets are more likely to form around high-metal stars should help in this search.

Schlaufman points out that Kepler has seven million stars in its field of view, but can only examine about a 160,000 at a time. Although this introduces a bias in the search for new planets, he praises the results Kepler is producing. Dodson-Robinson agrees. “If your goal is to find planets, it means you want to look at the most metal-rich stars.”



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