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).
I’m not clear in what form the nitrogen is. Is it in gaseous N2 form during planetary formation, or bound as nitrates and other solids? If the former, then the concept of a frost line makes no sense to me, so I assume it must be the latter. If the latter, how is gaseous N2 formed which eventually becomes the major atmospheric constituent, whether directly as N2 or produced via NH4.
I would suspect that N2 in the atmosphere is the result of volcanic outgassing and geological processes.
While this paper adds another factor to consider in planetary formation, I have a problem with one of tits assumptions:
“Our calculations show that forming an Earth-size planet via planetary embryos that grew extremely quickly before undergoing metal-silicate differentiation…”
Why would the planetary embryos around the early sun be growing any faster that those for any similar system? Maybe a denser nebula would produce faster planetesimal accretion, but the surely that would also produce larger planets.
And if we are looking at planets forming closer in around smaller stars, surely the planetesimals would accrete faster as the orbital distances are smaller leading to planets with a higher volatile percentage.
Then there is this paper:
Bifurcation of planetary building blocks during Solar System formation
Science 22 Jan 2021:
Vol. 371, Issue 6527, pp. 365-370
DOI: 10.1126/science.abb3091
which has the highly tangental conclusion that our inner system formed planetesimals quickly compared to the outer systems—as you expect from orbital mechanics—but because it formed out of material from a nearby supernova rich in AL26, the planetesimals were hot and differentiated quickly thereby leading to a volatile depleted inner system.
So while the mechanism on the paper we are commenting on may be a factor in planetary formation, other evidence would argue that Earth came out in the volatile depleted end of the spectrum rather than the volatile rich.
MAY 17, 2021
Shrinking planets could explain mystery of universe’s missing worlds
by Thomas Sumner, Simons Foundation
https://phys.org/news/2021-05-planets-mystery-universe-worlds.html
Searching beyond the solar system for life on exoplanets
18th May 2021
LIFE – a future space mission to characterise the atmospheres of terrestrial exoplanets and search for life outside the Solar System.
https://www.innovationnewsnetwork.com/searching-beyond-the-solar-system-for-life-on-exoplanets/11631/
Reading the paper, it indicates that it is N2 gas. So my mental model of volatile behavior in the solar disk is wrong. Rather than being depleted by temperature due to proximity to the sun, it is bound through solubility in chondritic material 1. Mental model adjustment in progress. ;)
The generally accepted view in astrophysics or planetary geology is that the first differentiation described in this paper has no relationship to the amount of gases in the early atmosphere of the fully formed rocky, inner planets. Most of the atmosphere including the atmophiles comes from the heating of the elements, chemicals and rock trapped inside the crust, core and mantle of the rocky, inner planets after the accretion process of first differentiation is complete.
The Nitrogen cycle is not included in this paper, and it is thought that Earth’s early atmosphere had much less Nitrogen and mostly carbon dioxide. Life through the nitrogen cycle gave us the abundance of nitrogen we have today. There was some nitrogen in the early atmosphere, but most of it might have come from the nitrogen cycle, the denitrifying bacteria which change NO2, nitrite, NO3, Nitrate and NH3 ammonium into N2 nitrogen gas.
Plate tectonics and subduction also play a role in the nitrogen cycle. https://arstechnica.com/science/2014/10/earths-nitrogen-rich-atmosphere-linked-to-plate-tectonics/#:~:text=Nitrogen%20bubbles%20up,chemicals%20into%20the%20rock%20above.
As the day approaches when spectral imaging of terrestrial planets comes in reach, one wonders what we are most likely to see. If our Earth is exceptional in the success of life to transform it, then there is likelihood we would get many returns of earth like planets where life never get started – but yet had some kind of atmosphere. I would assume that there would be much diatomic nitrogen present. And yet that perhaps is an arguable facet too.
By no means expert on this matter, but curious, I do note that Titan has a preponderance of nitrogen in its atmosphere; something like 95% to 5% methane and other hydrocarbon compounds, whereas Mars reverses this ratio with CO2 and N2. Were we discussing this matter decades ago, I think geologists and biologists would have argued for an early atmosphere enriched by volcanism with ammonia… Outer planet and satellite exploration suggests presence of much free diatomic nitrogen in those depths of the solar system, but it is difficult to translate what that means in the inner regions with the Venus – Earth – Mars sampling we have. Granted that there is an increase of luminosity by the sun over gigayears, that might make local stability for N2 easier than we perceive it now. If Earth did not have the impact history it has had, then maybe there would be more N2 surviving in the atmosphere, but that does not seem to argue well for an early state similar to Titan.
Perhaps just something moved further toward it on a dial?
Having just entered an argument for presence of N2 in lifeless exoplanets analogous to Earth, I neglected to address the issue of detecting N2.
That could be a problem…
May 18, 2021
09:24 pm EDT
Astronomers Rule Out Super-Earth Around Barnard’s Star
Bruce Dorminey
A super-earth thought to orbit Barnard’s Star, the second closest star to our own Sun, actually doesn’t exist. Or so say the authors of a new paper accepted for publication in The Astronomical Journal.
The University of California, Irvine-led team used archival data and new measurements of Barnard Star’s radial velocity, or how it moves towards or away from us along our line of sight to make the determination. They credit the Habitable-Zone Planet Finder (HPF) instrument attached to the 10-meter Hobby-Eberly Telescope at McDonald Observatory in Texas with providing enough data to determine that signal first interpreted as a super-earth actually is an alias of the star’s 145-day rotation period.
The astronomers suggest that signals initially appearing to be from a “super-earth” measuring 3.3 times the size of Earth are more likely the result of aliasing from an incomplete sampling of stellar activity bearing similarity to the spots on our own sun, says UCI.
As I wrote here earlier, the team who claimed the 2018 detection in a paper in the journal Nature, combined twenty years of data from seven different telescopic instruments to detect the planet using Doppler spectroscopy, which measures how the star’s light is either redshifting away from us, or blueshifting towards us along our line of sight.
Our observations with the new HPF instrument do not show a signal at the proposed planet’s period of 233 days, lead author Jack Lubin, a UCI doctoral student in physics, told me. We see that the 233-day signal is strongest only in three consecutive observing seasons, from 2011 to 2013, which represents only about 25 percent of the data, he says.
Full article here:
https://www.forbes.com/sites/brucedorminey/2021/05/18/astronomers-nix-idea-of-super-earth-around-barnards-star/?sh=3cd35ed8842a
This theory does not go into the details what they mean about larger planets and instantaneous differentiation. Does it assume that Earth instantaneously differentiated, but the other rocky planets did not because Earth is a larger planet? Why would larger planets differentiate faster? The first differentiation might be faster with a larger planet due to the greater mass and gravity but the second differentiation might slower because it is inside the planet and the heavier material has to sink. The amount of material, that is gas, dust, and pebbles might be more in the life belt than outside it which are some other things worth considering.
Earth might have gained some more nitrogen from it’s giant impact with Theia.
Thinking about this more carefully, a larger, denser planet might differentiate faster due to gravity, but that does not necessarily mean that is the reason why Earth retained it’s nitrogen and the other planets retained less. The mass and size of the body and it temperature or distance from the Sun are variables controlled by Jeans escape.
Excuse me for the lack of preciseness. The mass an size of a planet and temperature are variables which affect how long a planet can retain at atmosphere and the temperature or distance from the Sun controls what gases heavy or light it can retain which is called Jeans escape.
Astronomers Confirm Third-Nearest Star With a Planet – And It’s Rocky Like Earth
By LOUISE LERNER, UNIVERSITY OF CHICAGO
MAY 17, 2021
MAROON-X instrument built by University of Chicago team measures its first planet.
In the past two decades, scientists have discovered more and more planets orbiting distant stars—but in some sense, they’re still just dots on a map.
“It’s kind of like looking at a map of Europe and seeing the dot that’s labeled ‘Paris,’” said University of Chicago astrophysicist Jacob Bean. “You know where it is, but there’s a whole lot that you’re missing about the city.”
Scientists are developing new telescopes and instruments to fill in more and more of that picture. Bean led the creation of one such instrument called MAROON-X, which was installed at the Gemini Telescope in Hawaii last year. It allowed scientists to not only confirm the existence of the third-nearest star with a transiting exoplanet, but to take extraordinarily precise measurements of that planet and discover that it is rocky like Earth.
The new planet, called Gliese 486 b, is located just over two dozen light-years from Earth in the direction of the constellation Virgo, and is also made out of rock—though it is hotter and three times larger than our home.
“This is the third-nearest system with a transiting exoplanet, and it should be just the first in a long line of them for MAROON-X,” said Bean, an associate professor in the Department of Astronomy and Astrophysics. “We’re really happy. We’re going to learn a lot about terrestrial exoplanets over the coming years.”
https://scitechdaily.com/astronomers-confirm-third-nearest-star-with-a-planet-and-its-rocky-like-earth/
Only asteroids that hit a certain mineral trigger a mass extinction
25 May 2021
By Kerry Hebden
https://www.newscientist.com/article/2278447-only-asteroids-that-hit-a-certain-mineral-trigger-a-mass-extinction/