Centauri Dreams
Imagining and Planning Interstellar Exploration
Spiral Galaxies: A Common Path to Formation?
The galaxy UGC 10738 resonates with the galaxy described yesterday — BRI 1335-0417 — in that it raises questions about how spiral galaxies form. In fact, the team working on UGC 10738 thinks it goes a long way toward answering them. That’s because what we see here is a cross-sectional view of a galaxy much like the Milky Way, one that has both ‘thick’ and ‘thin’ disks like ours. The implication is that these structures are not the result of collisions with smaller galaxies but typical formation patterns for all spirals.
Nicholas Scott and Jesse van de Sande (ASTRO 3D/University of Sydney) led the study, which used data from the European Southern Observatory’s Very Large Telescope in Chile. As you can see from the image below, the galaxy, some 320 million light years away, presents itself to us edge on, offering a cross-section of its structure. Key to the work was the team’s assessment of stellar metallicity, as van de Sande explains:
“Using an instrument called the multi-unit spectroscopic explorer, or MUSE, we were able to assess the metal ratios of the stars in its thick and thin discs. They were pretty much the same as those in the Milky Way – ancient stars in the thick disc, younger stars in the thin one. We’re looking at some other galaxies to make sure, but that’s pretty strong evidence that the two galaxies evolved in the same way.”
Image: Galaxy UGC 10738, seen edge-on through the European Southern Observatory’s Very Large Telescope in Chile, revealing distinct thick and thin discs. Credit: Jesse van de Sande/European Southern Observatory.
Published in Astrophysical Journal Letters, the study argues that the thin and thick disks found in the Milky Way — and the similar structures found in UGC 10738 — indicate that rather than being the result of chance collisions with other galaxies, the different disks reveal a standard path of galaxy formation and evolution. They would be unlikely to be repeated elsewhere if the result of a rare and violent merger.
The ‘thick’ and ‘thin’ disks are interesting features, with the former primarily made up of ancient stars that show a low ratio of iron to hydrogen and helium. Tracing the metallicity of the thin disk stars reveals that they are younger and contain more metals. Our Sun is a thin disk star, and as the authors note, it is made up of about 1.5% elements heavier than helium (the definition of ‘metals’ in astronomical parlance). Stars of the thick disk show three to ten times less metal content, a striking difference.
Finding the same differences in the two disk structures in other spiral galaxies points to the likelihood that the Milky Way formed in a way common to such galaxies. Says Scott:
“It was thought that the Milky Way’s thin and thick discs formed after a rare violent merger, and so probably wouldn’t be found in other spiral galaxies. Our research shows that’s probably wrong, and it evolved ‘naturally’ without catastrophic interventions. This means Milky Way-type galaxies are probably very common. It also means we can use existing very detailed observations of the Milky Way as tools to better analyse much more distant galaxies which, for obvious reasons, we can’t see as well.”
So we have what the authors describe as a ‘tension’ between two formation scenarios, one stochastic, one common to spiral galaxies. But let’s get into the weeds a little. The paper goes on to widen the sample into other subtypes of galaxy:
This tension is further enhanced once early-type disk galaxies with [α/Fe]-enhanced thick disks are included in the population of present day galaxies with similar formation histories (Pinna et al. 2019b; Poci et al. 2019, 2021). That early-type disk galaxies are found to contain similar abundance patterns as found in the Milky Way (i.e. increased mean [α/Fe] off the plane of the disk) is unsurprising. Such galaxies represent one plausible end point for the Milky Way’s evolutionary history, suggesting a shared and generic evolutionary pathway for disk galaxies.
All of this harks back to Edwin Hubble’s classification of galaxies (the Hubble Sequence). The phrase “early-type” galaxies” refers to elliptical and lenticular galaxies, as opposed to spiral and irregular galaxies, without implying an evolutionary path from one type to another. So the authors are speculating when they talk about an ‘evolutionary end point’ for the Milky Way, but it’s an interesting speculation:
The Milky Way is often identified as a so-called ‘green valley’ galaxy (Mutch et al. 2011), a galaxy already undergoing a transition to the red sequence. When fully quenched (and assuming no dramatic structural changes in the mean time) the Milky Way will likely resemble a lenticular galaxy similar to FCC 170 (Pinna et al. 2019a).
The ‘green valley’ refers to galaxies where star formation has been slowed, either because they have exhausted their reservoirs of gas, or (in younger galaxies) have undergone mergers with other galaxies. Both the Milky Way and Andromeda are assumed to be green valley galaxies of the first kind.
FCC 170 (NGC 1381) is a lenticular galaxy found in the constellation Fornax, about 60 million light years from Earth, a member of the Fornax Cluster (hence the FCC designation, standing for Fornax Cluster Catalogue). As a lenticular, it is in a middle stage between a spiral and an elliptical galaxy, with a large scale disk but lacking spiral arms on the scale of a true spiral galaxy. Is this the Milky Way’s future?
And then, of course, there is the merger with Andromeda to look forward to. How much we have to learn about how galaxies change over time!
Image: This is NGC 1387, a lenticular galaxy likewise in the Fornax Cluster, and better angled to show the lack of spiral structure in such galaxies than FCC 170. Credit: Fabian RRRR. Based on observations made with the NASA/ESA Hubble Space Telescope, and obtained from the Hubble Legacy Archive, which is a collaboration between the Space Telescope Science Institute (STScI/NASA), the Space Telescope European Coordinating Facility (ST-ECF/ESA) and the Canadian Astronomy Data Centre (CADC/NRC/CSA). CC BY-SA 3.0.
The paper is Scott et al., “Identification of an [α/Fe]—Enhanced Thick Disk Component in an Edge-on Milky Way Analog,” Astrophysical Journal Letters Vol. 913, No. 1 (24 May 2021), L11 (abstract / preprint).
The Most Ancient Spiral Galaxy Yet Found
My fascination with the earliest era of star and galaxy formation leads me to a new paper on an intriguing find. The authors describe the distant object BRI 1335-0417 as “an intensely star-forming galaxy,” and its image as captured by the Atacama Large Millimeter/submillimeter Array (ALMA) is striking. This is a galaxy that formed a mere 1.4 billion years after the Big Bang, making it the most ancient galaxy with spiral structure ever observed.
Spirals make up perhaps 70 percent of the galaxies in our catalogs, but how they form is an open question. Indeed, the proportion of spiral galaxies declines the further back in the evolution of the universe we observe. The spiral structure observed here extends 15,000 light years from the center of the galaxy (about a third the size of the Milky Way), while the total mass of stars and interstellar matter roughly equals our own galaxy.
Image: ALMA image of a galaxy BRI1335-0417 in the Universe 12.4 billion years ago. ALMA detected emissions from carbon ions in the galaxy. There is a compact and bright area in the center of the galaxy, and spiral arms are visible on both sides of it. Credit: ALMA (ESO/NAOJ/NRAO), T. Tsukui & S. Iguchi.
The BRI1335-0417 observations were performed by Satoru Iguchi (SOKENDAI/National Astronomical Observatory of Japan), working with graduate student Takafumi Tsukui, who is lead author of the paper reporting the work. Huge amounts of dust exist here, obscuring the object in visible light, but ALMA’s prowess at detecting radio emissions from carbon ions makes the investigation possible.
Even so, Tsukui points out that getting the size of the galaxy right is a difficult matter:
“As BRI 1335-0417 is a very distant object, we might not be able to see the true edge of the galaxy in this observation. For a galaxy that existed in the early Universe, BRI 1335-0417 was a giant.”
The biggest issue is how such a large structure with obvious spiral structure emerged so soon after the Big Bang. This is a ‘starburst’ galaxy: There are areas of active star formation here and gas instabilities in the outer parts of the disk which point to possible interactions with smaller galaxies or other interstellar matter. The paper points out that dusty galaxies with high rates of star formation, like BRI 1335-0417, are the progenitors of elliptical galaxies, with the spiral structure possibly redistributing angular momentum so as to trigger gas inflows into the center of the galaxy.
The authors are quick to note, however, that gas inflows like this are beyond the detection threshold of these observations. What we do see is the possibility of tidal interactions. The authors describe the likelihood of such events:
The high star formation rates of z > 4 [the measure of redshift] galaxies like BRI 1335-0417 are commonly explained as the result of major mergers, which could produce distorted galactic kinematics. We find that BRI 1335-0417 has only slightly disturbed, rotation dominated kinematics, which can be well described by a rotating disk model. This suggests that the high star formation rate must have been maintained long enough for the disk to form after any major merger event. The Q parameter [a measure of stability in a rotating, gaseous accretion disc] shows the outer disk of BRI 1335-0417 is unstable, which could be caused by gas accretion along large-scale filaments of the cosmic web, and/or minor mergers with accreting satellites.
So we have spiral structure, a rotating galactic disk, and a centralized mass. Is BRI 1335-0417 the ancestor of a giant elliptical galaxy? Answering that question of galactic survival and change involves issues of galaxy evolution that remain wide open.
The paper is Tsukui & Iguchi “Spiral morphology in an intensely star-forming disk galaxy more than 12 billion years ago,” published online in Science 20 May 2021 (abstract).
Lights of the Nightside City
On the matter of city lights as technosignatures, which we looked at on Friday, I want to follow up with Thomas Beatty’s work on the issue in the context of an assortment of nearby stars. Beatty (University of Arizona, Tucson) assumes Earth-like planets examined via direct-imaging by LUVOIR, a future space telescope in planning, or HabEx, a different architecture for a likewise powerful instrument. What he’s done is to take data from the Soumi National Polar-orbiting Partnership satellite to find the flux from city lights and the spectra of currently available lighting. He goes on to model the spectral energy distribution from such emissions as applied to exoplanet settings at various distances.
Why look at city lights in the first place? Because they’re another form of technosignature that may be within the realm of detection, and we’d like to find out what’s possible and what any results would imply. In particular, Beatty reminds us, the National Academies’ Exoplanet Science Strategy and Astrobiology Strategy reports are on record as recommending space-based, direct imaging that is capable of directly detecting emissions from habitable zone planets. This would obviously support biosignature searches but would also open up a hunt for technosignatures.
Technosignature searches can take place within the context of ongoing biosignature investigations on the same planets. Both LUVOIR and HabEx should be capable of this, generating data sets that can be scanned for both biological and technological returns. One area of investigation has been satellites — could we detect satellite constellations in orbit around an Earth-like planet? Large-scale photovoltaic arrays would show a different signature than vegetation on the surface. Various forms of pollution in atmospheres are within LUVOIR’s range, so the field is wide.
A lack of a specific technosignature is itself interesting, as it helps us begin to constrain the field. Just as we started searching for planets around Proxima Centauri by first ruling out gas giants of a certain mass, then Jupiter-class objects, then ice giants of Neptune size, we first learn what is not there and then can specify what remains to be searched for. The lack of SETI detections at radio or optical frequencies, for example, makes it less likely that technological civilizations are broadcasting powerful beacons aimed at us from stars near the Sun, thus paring down earlier possibilities.
But back to city lights, which Jean Schneider and colleagues first studied in 2010 (citation below). We’ve learned through the work of Avi Loeb and Elisa Tabor that artificial illumination from the nightside of Proxima b could be detected, though with great difficulty, by the James Webb Space Telescope. LUVOIR will up the ante and widen the range. Beatty points out that city lights are compelling because they would presumably be long-lived artifacts of a technological culture and would offer a unique spectroscopic signature that is unlike anything produced by natural processes.
Image: This is Figure 1 from the paper. Caption: Figure 1. The nightside of Earth shows significant emission from city lights in the optical. This is a composite, cloud-free, image of Earth’s city lights compiled using Day/Night Band observations taken using the Visible Infrared Imaging Radiometer Suite instrument on the Soumi National Polar-orbiting Partnership satellite (Roman et al. 2018). Searching for the emission from city lights is a compelling technosignature because it requires very little extrapolation from current conditions on Earth, should be relatively long-lived presuming an urbanized civilization, and offers a very distinct spectroscopic signature that is difficult to cause via natural processes. This places the emission from nightside city lights high on the list of potential technosignatures to consider (Sheikh 2020).
Beatty considers the detectability of city lights first as a function of stellar distance and the amount of a planet’s surface covered by urbanization on Earth-like planets around G-, K- and M-dwarf stars. He then calculates their detectability on planets orbiting stars within 10 parsecs of the Sun, and finally estimates detectability on two dozen known, potentially habitable planets around stars close to the Sun. The tables within this paper are worth scanning, but here are some of the highlights:
We learn that LUVOIR should be able to detect city lights on Proxima b at an urbanization level of 0.5% (10 times Earth’s). Lalande 21185 b, Luyten’s Star b and Tau Ceti e and f would show detectable emission from city lights at urbanization levels of 3% to 10% in LUVOIR imaging.
Detection of city lights should be easiest on M-dwarf planets, and Beatty notes in particular planets around Proxima Centauri, Barnard’s Star, and Lalande 21185, but he points out how quickly the habitable zone around this kind of star moves within the inner working angle (IWA) of LUVOIR with distance, making it beyond the capacity of the instrument.
Earth-analog planets around Sun-like stars can be imaged at greater distances, but the distance drives the minimum detectable urbanization fraction higher. Here Beatty suggests Alpha Centauri A and B, Epsilon Eridani, Tau Ceti and Epsilon Indi A as potential targets.
And this brings up memories of Isaac Asimov’s global city Trantor: What Beatty calls an ‘ecumenopolis’ — a planet-wide city — would be detectable at much larger distances. The author surveys 80 nearby stars on which such a city would be at least marginally detectable.
Thus the work moves from the study of Earth’s urbanization fraction (0.05%) up to an ecumenopolis, showing how detectability scales with the amount of planetary surface covered. The paper assumes 100 hours of observing time for generic Earth-class planets around stars within 10 parsecs. Earth itself would not be detectable by LUVOIR in this range, but planets around M-dwarfs near the Sun would show detection for urbanization levels of 0.4% to 3%. City lights on planets orbiting nearby Sun-like stars would be detectable at urbanization levels in the range of 10 percent.
From the paper:
The possibility of directly detecting technosignatures on the surfaces of potentially habitable exoplanets is thus starting to be in the realm of practicality. Perhaps unsurprisingly, the 15m LUVOIR A architecture would be the most capable observatory for detecting city lights on the nightsides of nearby exoplanets, though LUVOIR B [smaller than LUVOIR A) or HabEx with a starshade would also have significantly sized detection spaces. Much of this proposed capability has been spurred by the goals of characterizing the atmospheres of and detecting biosignatures on potentially habitable exoplanets, but it also would afford us the opportunity to search for other, technological, signs of life.
In short, we’re going to be looking hard at many of these planets within a few decades as we search for biosignatures. The same data may show technosignatures, the strength of which we need to examine to see what’s possible. We are simply defining the limits of the search.
The paper is Beatty, “The Detectability of Nightside City Lights on Exoplanets,” in process at Monthly Notices of the Royal Astronomical Society (abstract). The Schneider et al. paper is “The Far Future of Exoplanet Direct Characterization,” Astrobiology Vol. 10, No. 1 (22 March 2010). Abstract. Thanks to my friend Antonio Tavani for the pointer to this paper.
Proxima Centauri b: Artificial Illumination as a Technosignature
Our recent look at the possibility of technosignatures at Alpha Centauri is now supplemented with a new study on the detectability of artificial lights on Proxima Centauri b. The planet is in the habitable zone, roughly similar in mass to the Earth, and of course, it orbits the nearest star, making it a world we can hope to learn a great deal more about as new instruments come online. The James Webb Space Telescope is certainly one of these, but the new work also points to LUVOIR (Large UV/Optical/IR Surveyor), a multi-wavelength space-based observatory with possible launch in 2035.
Authors Elisa Tabor (Stanford University) and Avi Loeb (Harvard) point out that a (presumably) tidally locked planet with a permanent nightside would need artificial lighting to support a technological culture. As we saw in Brian Lacki’s presentation at Breakthrough Discuss (see Alpha Centauri and the Search for Technosignatures), coincident epochs for civilizations developing around neighboring stars are highly unlikely, making this the longest of longshots. On the other hand, a civilization arising elsewhere could be detectable through its artifacts on worlds it has chosen to study.
We learn by asking questions and looking at data. In this case, asking how we would detect artificial light on Proxima b involves factoring in the planet’s radius, which is on the order of 1.3 Earth radii (1.3 R?) as well as that of Proxima Centauri itself, which is 0.14 that of the Sun (0.14 R?). We also know the planet is in an 11 day orbit at 0.05 AU. Other factors influencing its lightcurve would be its albedo and orbital inclination. Tabor and Loeb use recent work on Proxima Centauri c’s inclination (citation below) to ballpark an orbital inclination for the inner world.
Image: Northern Italy at night. City lights are an obvious technosignature, but can we detect them at interstellar distances? Credit: NASA/ESA.
The question then becomes whether soon to be flown technology like the James Webb Space Telescope could detect artificial lights if they were present at Proxima b. The authors detail in this paper their calculation of the lightcurves that would be involved, using two scenarios: Artificial lighting with the same spectrum found in LEDs on Earth, and a narrower spectrum leading to the “same proportion of light as the total artificial illumination on Earth.” The calculations draw on open source software source code called Exoplanet Analytic Reflected Lightcurve (EARL), and likewise deploy the JWST Exposure Time Calculator (ETC) to estimate the feasibility of detection.
What Loeb and Tabor find is that JWST could detect LED lighting “making up 5% of stellar power” with 85 percent confidence — in other words, 5% of the power the planet would receive at its orbital distance from Proxima Centauri. That would mean our space telescope could find a level of illumination from LEDs that is 500 times more powerful than found on Earth.
To detect the current level of artificial illumination (including but not limited to LEDs) on Earth, the spectral band would have to be 103 times narrower. “ In either case,” the authors add, “JWST will thus allow us to narrow down the type of artificial illumination being used.”
All of this demands maximum performance from JWST’s Near InfraRed Spectrograph (NIRSpec). Much depends upon what methods a civilization at Proxima b might use. From the paper:
Proxima b is tidally locked if its orbit has an eccentricity below 0.06, where for reference, the eccentricity of the Earth’s orbit is 0.017 (Ribas et al. 2016). If Proxima b has a permanent day and nightside, the civilization might illuminate the nightside using mirrors launched into orbit or placed at strategic points (Korpela et al. 2015). In that case, the lights shining onto the permanent nightside should be extremely powerful, and thus more likely to be detected with JWST.
That last comment calls to mind Karl Schroeder’s orbital mirrors lighting up brown dwarf planets in his novel Permanence (2002). A snip from the book, referring to a brown dwarf planet named Treya as seen by the protagonist, Rue:
A pinprick of light appeared on the limb of Treya and quickly grew into a brilliant white star. This seemed to move out and away from Treya, which was an illusion caused by Rue’s own motion. Treya’s artificial sun did not move, but stayed at the Lagrange point, bathing an area of the planet eighty kilometers in diameter with daylight. The sun was a sphere of tungsten a kilometer across. It glowed with incandescence from concentrated infrared light, harvested from Erythrion [the brown dwarf] by hundreds of orbiting mirrors. If it were turned into laser power, this energy could reshape Treya’s continents— or launch interstellar cargoes.
A flat line of light appeared on Treya’s horizon. It quickly grew into a disk almost too bright to look at. When Rue squinted at it she could make out white clouds, blue lakes, and the mottled ochre and green of grassland and forests. The light was bright enough to wash away the aurora and even make the stars vanish. Down there, she knew, the skies would be blue.
Back to Proxima b: The LUVOIR instrument should be able to confirm the presence or lack of artificial illumination with greater precision, serving as a follow-up to JWST observations with significantly higher performance. Loeb has previously worked with Manasvi Lingam to show the likelihood of detecting a spectral edge in the reflectance of photovoltaic cells on the planet’s dayside, so in terms of technosignatures, we’re learning what we will be able to identify based on a growing set of scenarios for any civilization there.
The paper is Tabor & Loeb, “Detectability of Artificial Lights from Proxima b,” (preprint). The paper on photovoltaic cells is Loeb & Lingam, “Natural and Artificial Spectral Edges in Exoplanets,” Monthly Notices of the Royal Astronomical Society Vol. 470, Issue 1 (September 2017), L82-L86 (abstract). The work on Proxima c’s orbital inclination is Kervella, Arenou & Schneider, “Orbital inclination and mass of the exoplanet candidate Proxima c,” Astronomy & Astrophysics Vol. 635, L14 (March 2020). Abstract / Full Text.
Exploring Ice Giant Oceans
Laboratory work on Earth is, as we saw yesterday, leading to hypotheses about how planets form and the effect of these processes on subsequent life. Whether in our own outer Solar System or orbiting other stars, planets in the ‘ice giant’ category, like Uranus and Neptune, remain mysterious, with Voyager 2’s flybys of the latter the only missions that have gone near them. We also know that sub-Neptune planets are common, many of these doubtless sharing the characteristics of their larger namesake.
Thus recent experiments probing ice giant interiors catch my eye this morning. Involving an international team of collaborators, the work looks at the interactions between water and rock that we would expect to find in the extreme conditions inside an ice giant. Planets like Uranus and Neptune are thought to house most of their mass in a deep water layer, a dense fluid overlaying a rocky core, a sharp departure from terrestrial worlds. What happens at that interface is ripe for examination.
The experiments were performed at Arizona State University’s DanShimLab, which is dedicated to the study of planetary materials at a wide range of pressures and temperatures using diamond-anvil and shockwave techniques. To probe this environment, the scientists immersed the rock-forming minerals olivine and ferropericlase in water and then compressed them to high pressures using a diamond-anvil. Heating the sample with a laser, the team could then track the water/mineral reaction under these conditions by way of X-ray measurements.
The result: High concentrations of magnesium, with implications for the composition of oceans much different from Earth’s, as study co-author Sang-Heon Dan Shim (Arizona State University) explains:
“We found that magnesium becomes much more soluble in water at high pressures. In fact, magnesium may become as soluble in the water layers of Uranus and Neptune as salt is in Earth’s ocean. If an early dynamic process enabled a rock-water reaction in these exoplanets, the topmost water layer may be rich in magnesium, possibly affecting the thermal history of the planet.”
Image: A diamond-anvil (top right) and laser were used in the lab on a sample of olivine to reach the pressure-temperature conditions expected at the top of the water layer beneath the hydrogen atmosphere of Uranus (left). In this experiment, the magnesium in olivine dissolved in the water. Credit: Shim/ASU.
Laser-heated diamond anvil cells can create pressures in the range of 1-5,000,000 bars by compressing materials between diamond ‘anvils’ that are transparent to X-rays, infrared and visible light. The lab’s laser heating systems can take samples to 1,000-5,000 K, all by way of exploring how materials behave under conditions thought to exist in planetary interiors. Thus the range of such laboratory work extends through rocky worlds and into the realm of not just the ice giants but gas giants like Jupiter.
Given the lab’s finding of high concentrations of magnesium under conditions of high pressure and temperature, Shim argues that the mineral may become as soluble in ice giant interiors as salt is in Earth’s oceans. Conceivably, this finding could explain why the atmosphere of Uranus is considerably colder than Neptune’s, for magnesium in larger amounts could block heat from escaping the interior. “This magnesium-rich water may act like a thermal blanket for the interior of the planet,” says Shim.
Image: An electron microscopy image of the olivine sample shows a large empty dome structure where magnesium under high-pressure water precipitated as magnesium oxide. Credit: Kim et al.
Oceans rich in magnesium may thus be common in ice giants, with a thick layer of water covering a rocky interior. Moreover, the idea that the interior of water worlds is sharply differentiated between rock and water has been challenged in recent work, with implications for the thermal evolution of these planets. To probe deeper into these matters, the study calls for an examination of other icy materials like CO2 and NH3, but note this cautionary remark in its conclusion:
Extrapolation of our results beyond the pressure range covered in our experiments should [be] treated with caution because of possible changes in the properties of H2O at very high pressures. Nevertheless, the models based on our experiments demonstrate that geochemical cycles and thermal history of water-rich planets could be sensitive to the size of the planet because of pressure-dependent chemical processes.
We have a useful methodology here to extend the study of the geochemical cycle on a range of planetary interiors. We have much to learn, for as the paper points out, the interactions of major rock-forming minerals at the interface between ocean and rock in ice giants have rarely been explored at the high pressures used in these experiments.
The paper is Kim et al., “Atomic-scale mixing between MgO and H2O in the deep interiors of water-rich planets,” Nature Astronomy 17 May 2021 (abstract).
Planet Formation Modes as a Key to Habitability
While a planet’s position in the habitable zone is thought critical for the development of life like ourselves, new work out of Rice University suggests an equally significant factor in planetary growth. Working at a high-pressure laboratory at the university, Damanveer Grewal and Rajdeep Dasgupta have explored how planets capture and retain key volatiles like nitrogen, carbon and water as they form The team used nitrogen as a proxy for volatile distribution in a range of simulated protoplanets.
Two processes are under study here, the first being the accretion of material in the circumstellar disk into a protoplanet, and the rate at which it proceeds. The second is differentiation, as the protoplanet separates into layers ranging from a metallic core to a silicate shell and, finally, an atmospheric envelope. The interplay between these processes is found to determine which volatiles the subsequent planet retains.
Most of the nitrogen is found to escape into the atmosphere during differentiation and is then lost to space as the protoplanet cools or, perhaps, collides with other protoplanets during the turbulent era of planet formation. The data, however, demonstrate the likelihood of nitrogen remaining in the metallic core. Says Grewal:
“We simulated high pressure-temperature conditions by subjecting a mixture of nitrogen-bearing metal and silicate powders to nearly 30,000 times the atmospheric pressure and heating them beyond their melting points. Small metallic blobs embedded in the silicate glasses of the recovered samples were the respective analogs of protoplanetary cores and mantles.”
Nitrogen, the researchers learned, is distributed in different ways between the core, the molten silicate shell and the atmosphere, with the extent of this fractionation being governed by the size of the body. The takeaway: If the rate of differentiation is faster than the rate of accretion for planetary embryos of Moon or Mars-size, then the planets that form from them will not have accreted enough volatiles to support later life.
Earth’s path would have been different. The scientists believe that the building blocks of Earth grew quickly into planetary embryos before they finished differentiating, forming within one to two million years at the beginning of the Solar System. The slower rate of differentiation allowed nitrogen, and other volatiles, to be accreted. Adds Dasgupta:
“Our calculations show that forming an Earth-size planet via planetary embryos that grew extremely quickly before undergoing metal-silicate differentiation sets a unique pathway to satisfy Earth’s nitrogen budget. This work shows there’s much greater affinity of nitrogen toward core-forming metallic liquid than previously thought.”
Image: Nitrogen-bearing, Earth-like planets can be formed if their feedstock material grows quickly to around moon- and Mars-sized planetary embryos before separating into core-mantle-crust-atmosphere, according to Rice University scientists. If metal-silicate differentiation is faster than the growth of planetary embryo-sized bodies, then solid reservoirs fail to retain much nitrogen and planets growing from such feedstock become extremely nitrogen-poor. Credit: Illustration by Amrita P. Vyas/Rice University.
This work takes the emphasis off the stellar nebula and places volatile depletion in the context of processes within the rocky body in formation, especially the affinity of nitrogen toward metallic cores. Here’s how the paper sums it up:
…we show that protoplanetary differentiation can explain the widespread depletion of N in the bulk silicate reservoirs of rocky bodies ranging from asteroids to planetary embryos. Parent body processes rather than nebular processes were responsible for N (and possibly C) depleted character of the bulk silicate reservoirs of rocky bodies in the inner Solar System. A competition between rates of accretion versus rates of differentiation defines the N inventory of bulk planetary embryos, and consequently, larger planets. N budget of larger planets with protracted growth history can be satisfied if they accreted planetary embryos that grew via instantaneous accretion.
And the nebular conclusion:
Because most of the N in those planetary embryos resides in their metallic portions, the cores were the predominant delivery reservoirs for N and other siderophile volatiles like C. Establishing the N budget of the BSE [bulk silicate Earth] chiefly via the cores of differentiated planetary embryos from inner and outer Solar System reservoirs obviates the need of late accretion of chondritic materials as the mode of N delivery to Earth.
Rajdeep Dasgupta, by the way, is principal investigator for the NASA-funded CLEVER Planets project (one of the teams in the Nexus of Exoplanetary Systems Science — NExSS — research network). CLEVER Planets, according to its website, is “working to unravel the conditions of planetary habitability in the Solar System and other exoplanetary systems. The overarching theme of our research is to investigate the origin and cycles of life-essential elements (carbon, oxygen, hydrogen, nitrogen, sulfur, and phosphorus – COHNSP) in young rocky planets.”
All of which reminds us that the essential elements for life must be present no matter where a given planet exists in its star’s habitable zone.
The paper is Grewal et al., “Rates of protoplanetary accretion and differentiation set nitrogen budget of rocky planets,” Nature Geoscience 10 May 2021 (abstract / preprint).
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