Centauri Dreams
Imagining and Planning Interstellar Exploration
Earth in Formation: The Accretion of Terrestrial Worlds
It would be useful to have a better handle on how and when water appeared on the early Earth. We know that comets and asteroids can bring water from beyond the ‘snowline,’ that zone demarcated by temperatures beyond which volatiles like water, ammonia or carbon dioxide are cold enough to condense into ice grains. For our Solar System, that distance in our era is 5 AU, roughly the orbital distance of Jupiter, although the snowline would have been somewhat closer to the Sun during the period of planet formation. So we have a mechanism to bring ices into the inner Solar System but don’t know just how large a role incoming ices played in Earth’s development.
Knowing more about the emergence of volatiles on Earth would help us frame what we see in other stellar systems, as we evaluate whether or not a given planet may be habitable. Usefully, there are ways to study our planet’s formation that can drill down to its accretion from the materials in the original circumstellar disk. A new study from Caltech goes to work on the magmas that emerge from the planetary interior, finding that water could only have arrived later in the history of Earth’s formation.
Published in Science Advances, the paper involves an international team working in laboratories at Caltech as well as the University of the Chinese Academy of Sciences, with Caltech grad student Weiyi Liu as first author. When I think about studying magma, zircon comes first to mind. It appears in crystalline form as magma cools and solidifies. I’m no geologist, but I’m told that the chemistry of melt inclusions can identify factors such as volatile content and broader chemical composition of the original magma itself. Feldspar crystals are likewise useful, and the isotopic analysis of a variety of rocks and minerals can tell us much about their origin.
So it’s no surprise to learn that the Caltech paper uses isotopes, in this case the changing ratio of isotopes of xenon (Xe) as found in mid-ocean ridge basalt vs. ocean island basalt. Specifically, 129Xe* comes from the radioactive decay of the extinct volatile 129I, whose half-life is 15.7 million years, while 136Xe*Pu comes from the extinction of 244Pu, with a halflife of 80 million years. So the 129Xe*/136Xe*Pu ratio is a useful tool. As the paper notes, this ratio:
…evolves as a function of both time and reservoirs compositions (i.e., I/Pu ratio) early in Earth’s history. Hence, the study of the 129Xe*/136Xe*Pu in silicate reservoirs of Earth has the potential to place strong constraints on Earth’s accretion and evolution.
The ocean island basalt samples, originating as far down as the core/mantle boundary, reveal this ratio to be low by a factor of 2.8 as compared to mid-ocean ridge basalts, which have their origin in the upper mantle. Using computationally intensive simulations drawing on what is known as first-principles molecular dynamics (FPMD), the authors find that the low I/Pu levels were established in the first 80 to 100 million years of the Solar System (thus before 129I extinction), and have been preserved for the past 4.45 billion years. Their calculations assess the I/Pu findings under different accretion scenarios, drawing on simulated magmas from the lower mantle, which runs from 680 kilometers below the surface, to the core-mantle boundary (2,900 kilometers), and also from the upper mantle beginning at 15 kilometers and extending downward to 680 kilometers.
The result: The lower mantle reveals an early Earth composed primarily of dry, rocky materials, with a distinct lack of volatiles, with the later-forming upper mantle numbers showing three times the amount of volatiles found below. The volatiles essential for life seem to have emerged only within the last 15 percent, and perhaps less, of Earth’s formation. In the caption below, the italics are mine.
Image: This is Figure 4 from the paper. Caption: Schematic representation of the heterogeneous accretion history of Earth that is consistent with the more siderophile behavior of I and Pu at high P-T [pressure-temperature] conditions (this work). As core formation alone does not result in I/Pu fractionations sufficient to explain the ~3 times lower 129Xe*/136Xe*Pu ratio observed in OIBs [ocean island basalt] compared to MORBs [mid-ocean ridge basalt], a scenario of heterogeneous accretion has to be invoked in which volatile-depleted differentiated planetesimals constitute the main building blocks of Earth for most of its accretion history (phase 1), before addition of, comparatively, volatile-rich undifferentiated materials (chondrite and possibly comet) during the last stages of accretion (phase 2).Isolation and preservation, at the CMB [core mantle boundary], of a small portion of the proto-Earth’s mantle before addition of volatile-rich material would explain the lower I/Pu ratio of plume mantle, while the mantle involved in the last stages of the accretion would have higher, MORB-like, I/Pu ratios. Because the low I/Pu mantle would also have an inherently lower Mg/Si, its higher viscosity could help to be preserved at the CMB until today. Credit: Liu et al.
We’re a long way from knowing in just what proportions Earth’s water has derived from incoming materials from beyond the snowline. But we’re making progress:
…our model sheds light on the origin of Earth’s water, as it requires that chondrites represent the main material delivered to Earth in the last 1 to 15% of its accretion. Independent constraints from Mo [molybdenum] nucleosynthetic anomalies require these late accreted materials to come from the carbonaceous supergroup. Together, these results indicate that carbonaceous chondrites [the most primitive class of meoteorites, containing a high proportion of carbon along with water and minerals] must have represented a non-negligible fraction of the volatile-enriched materials in phase 2 and, thus, play a substantial role in the water delivery to Earth.
All this from the observation that mid-ocean ridge basalts had roughly three times higher iodine/plutonium ratios (inferred from xenon isotopes) as compared to ocean island basalts. The key to this paper, though, is the demonstration that the ratio difference is likely from a history of accretion that began with dry planetesimals followed by a secondary accretion phase driven by infalling materials rich in volatiles.
Thus Earth presents us with a model of planet formation from dry, rocky materials, one that presumably would apply to other terrestrial worlds, though we’d like to know more. To push the inquiry forward, Caltech’s Francois Tissot, a co-author on the paper, advocates looking at rocky worlds within our own Solar System:
“Space exploration to the outer planets is really important because a water world is probably the best place to look for extraterrestrial life. But the inner solar system shouldn’t be forgotten. There hasn’t been a mission that’s touched Venus’ surface for nearly 40 years, and there has never been a mission to the surface of Mercury. We need to be able to study those worlds to better understand how terrestrial planets such as Earth formed.”
And indeed, to better measure the impact of ices brought from far beyond the snowline to the infant worlds of the inner system. Tissot’s work demonstrates how deeply we are now delving into the transition between planetary nebulae and fully formed planets. working across the entire spectrum of what he calls ‘geochemical problematics,’ which includes studying the isotopic makeup of meteorites and their inclusions, the reconstruction of the earliest redox conditions in the Earth’s ocean and atmosphere, and the analysis of isotopes to investigate ancient magmas. At Caltech, he has created the Isotoparium, a state-of-the-art facility for high-precision isotope studies.
That we are now probing our planet’s very accretion is likely not news to many of my readers, but it stuns me as another example of extraordinary methodologies driving theory forward through simulation and laboratory work. And as we don’t often consider work on the geological front in these pages, it seems a good time to point this out.
The paper is Weiyi Liu et al., “I/Pu reveals Earth mainly accreted from volatile-poor differentiated planetesimals,” Science Advances Vol. 9, No. 27 (5 July 2023) (full text).
On Retrieving Dyson
One of the pleasures of writing and editing Centauri Dreams is connecting with people I’ve been writing about. A case in point is my recent article on Freeman Dyson’s “Gravitational Machines” paper, which has only lately again come to light thanks to the indefatigable efforts of David Derbes (University of Chicago Laboratory Schools, now retired). See Freeman Dyson’s Gravitational Machines for more, as well as the follow-up, Building the Gravitational Machine. I was delighted to begin an email exchange with Dr. Derbes following the Centauri Dreams articles, out of which emerges today’s post, which presents elements of that exchange.
I run this particularly because of my continued fascination with the work and personality of Freeman Dyson, who is one of those rare individuals who seems to grow in stature every time I read or hear about his contributions to physics. It was fascinating to receive from Dr. Derbes not only the background on how this manuscript hunter goes about his craft, thereby illuminating some of the more hidden corners of physics history, but also to learn of his recollections of the interactions between Dyson and Peter Higgs, whose ‘Higgs mechanism’ has revolutionized our understanding of mass and contributed a key factor to the Standard Model of particle physics. I’m also pleased to make the acquaintance of a kindred spirit, who shares my fascination with how today’s physics came to be, and the great figures who shaped its growth.
by David Derbes
I have a lifelong interest in the history of physics, particularly the history of physicists. Somehow I got through graduate school (in the UK; but I’m American) with only a very shaky acquaintance with Feynman diagrams and calculations in QED [quantum electrodynamics, the relativistic quantum theory of electrically charged particles, mutually interacting by exchange of photons]. This led me to a program of self-study (resulting in “Feynman’s derivation of the Schrödiinger equation”, Amer. Jour. Phys. 64 (1996) 881-884, two editions of Dyson’s AQM [Advanced Quantum Mechanics], and, with Richard Sohn, David J. Griffiths, and a cast of thousands, Sidney Coleman’s Lectures on Quantum Field Theory).
Along the way I stumbled onto David Kaiser’s Drawing Theories Apart, a sociological study of Feynman’s diagrams. Kaiser, who is now a friend, is a very remarkable fellow; he has two PhD’s, one in physics ostensibly under Coleman but actually under Alan Guth, and another in the history and philosophy of science). Kaiser mentioned the Cornell AQM notes of Dyson, never published, and I thought, hmmm… I found scans of them online at MIT, and (deleting a few side trips here) contacted Dyson about LaTeX’ing them for the arXiv (where they may be found today).
Image: Physicist, writer and teacher David Derbes, recently retired from University of Chicago Laboratory Schools. Credit: Maria Shaughnessy.
Dyson was quite enthusiastic. It probably helped that I had been a grad student of Higgs’ under Nick Kemmer at Edinburgh; Kemmer had steered Dyson towards physics and away from mathematics at Cambridge after the war. Ultimately (in my opinion) it is Dyson who was (very quietly) responsible for the recognition of Higgs’s work, and its incorporation by Weinberg into the Standard Model. Dyson had seen Higgs’s short pieces from 1964, learned (maybe from Kemmer) that he was at UNC Chapel Hill for 1965-66, wrote Higgs to give a talk at the IAS, which led to his giving a talk to Harvard (with Coleman, Glashow, and maybe Weinberg, then at MIT, in the audience).
Typing up Dyson’s Cornell lectures killed two birds: I learned more about QED, and I learned LaTeX from scratch. In retirement, “manuscript salvage” is my main hobby. (There are at least a couple of other oddballs who are doing much the same thing: David Delphenich, and there’s a guy in Australia, Ian Bruce, who has done a bunch of stuff from the 17th and 18th century, among other things a new translation of the Principia.)
Flash forward to shortly after LIGO’s results were announced. A letter in Physics Today drew attention to Dyson’s “Gravitational Machines”, so I went looking for it in the Cameron collection. I have a copy of Dyson’s Selected Works, and as you report the paper is not there. Couldn’t find it anywhere else, either. Cameron’s collection was mostly published in ephemeral paperback (I think there were a small number of hardbacks for libraries, but the U of Chicago’s copy is in paper covers).
So I wrote Dyson, with whom I had developed a very friendly relationship (there is a second edition of AQM, and it was more work than the first, due to the ~200 Feynman diagrams in the supplement), and asked if he would consent to my retyping (and redrawing the illustration for) his article for the arXiv. He was pleased by this. I very much regret that I couldn’t get it done before he died. The reason for that was copyright problems.
I’m going to give you only bullet points for that. Cameron died in 2005. His Interstellar Communication was published by W. A. Benjamin, then purchased by Cummings, Cummings was purchased by Addison-Wesley, and most of A-W’s assets purchased by Pearson; some by Taylor & Francis (UK). Took about four years to unravel. Neither Pearson (totally unhelpful) nor T&F (much better) had any record of the Cameron collection. As this may be helpful to you down the road, here was the resolution:
A work which was in copyright prior to January 1, 1964 had to have its copyright renewed in the 28th year after original copyright or lose its US copyright protection forever. Cameron’s collection was copyrighted in 1963. It took hours, but by scouring the online catalog at the US Copyright Office (you can do it in person near the Library of Congress) I was able to convince myself that the copyright had never been renewed. As far as US copyright goes, “Gravitational Engines” is in the public domain, and so I was clear of corporate entanglements (more to the point, so is the arXiv).
However, as I learned from Dyson’s Selected Papers, the article had originally been entered into an annual contest by the Gravity Research Foundation. The contributors to this contest read historically like a Who’s Who of astrophysics, general relativists and astronomers. So I got in touch with that organization’s director, George Rideout Jr. Rideout’s father had been appointed director by Roger W. Babson. who made a pile of money and set the foundation up. The story behind this is very sad: His beloved older sister drowned, and he blamed gravity. So he thought, well, if people could only invent anti-gravity, that might prevent future disasters. So he set up the foundation. (I think they also provided some funding for GR1 [Conference on the Role of Gravitation in Physics], the first international general relativity conference, Chapel Hill, 1957.)
I quickly obtained permission from George Rideout, satisfied the arXiv officials that they were free and clear to post “Gravitational Engines,” and here we are. (As I mentioned in the arXiv posting, the abstract comes from the original Gravity Research Foundation submission; it is absent in the Cameron collection.)
Incidentally, in chasing down other things, I found something I’d been seeking for a long time, the report from the Chapel Hill conference:
https://edition-open-sources.org/publications/#sources
https://edition-open-sources.org/sources/5/index.html
(So as you can see, there are several of us oddball manuscript hunters out there.)
Theoretical physics was not that large a community in 1965, and the British community even smaller. The physicists of Dyson’s generation typically went to Cambridge (which remains the main training ground for math and physics in the UK), with smaller spillover at Oxford, Imperial College London, and Edinburgh.
Kemmer hired Higgs at Edinburgh (Peter had been in the same department as Maurice Wilkins and Rosalind Franklin at King’s College, London. He was an expert at the time on crystal structure via group theory. He did not have any direct involvement with the DNA work, though subsequently he wrote an article that had a lot to do post facto with explaining the helical structure. The big boss at the lab (not Wilkins) was apparently quite annoyed with Higgs that he didn’t want to work on DNA.) Higgs wrote a Kemmer obit for the University of Edinburgh bulletin. He had been at Edinburgh for a couple of years in the 1950s in a junior position before he returned for good in 1960 (I think).
If I recall correctly, as Peter tells the story, Sheldon Glashow (who Higgs had known since a Scottish Summer School (conference) in Physics, 1960, I think) told Higgs that if he were ever planning to be in the Northeast, Glashow would arrange for Peter to give a talk at Harvard on whatever he liked. Independently of Glashow, Dyson wrote Peter to give a talk on what is now famously the Higgs mechanism at IAS, and Peter called Glashow to say something like, “Well, I’m driving from Chapel Hill to Princeton, and I see that Cambridge is only another few hours, so…” and that led to Higgs giving pretty much the same talk at Harvard, a really important event. But if Dyson hadn’t asked Peter to come to Princeton, he would not have gone to Harvard.
[Thus the contingencies of history, always telling a fascinating tale, in this case of a concept that rocked the world of physics, and wouldn’t you know Freeman Dyson would be in the middle of it.- PG]
Sunshade: A New Trek through ‘Daedalus Country’
Letting the imagination roam has philosophical as well as practical benefits. From the interstellar perspective, consider the Daedalus starship, designed with loving detail by members of the British Interplanetary Society in the 1970s. The mammoth (54,000 ton) vehicle was never conceived as remotely feasible at our stage of technology. But ‘our stage of technology’ is exactly the point the project illustrated. Daedalus demonstrated that there was nothing in physical law to prevent the construction of a starship. The question was, when would we reach the level of building it? For as Robert Forward frequently pointed out, interstellar flight could no longer be considered impossible.
We can’t know the answer to the question, but recall that before Daedalus, there was a lot of ‘informed’ opinion that interstellar flight was a chimera, and that all species were necessarily restricted to their home systems. Daedalus made the point debatable. If a civilization had a thousand year jump on us in terms of tech, could they build this thing? Probably, but they’d also surely come up with far better methods than we in the 1970s could imagine. Daedalus was, then, a possibility maker, a driver for further imaginings.
Fortunately, the Daedalus impulse – and the broader concept of thought experiments that so captivated Einstein – remains with us. I think, for example, of Cliff Singer’s pellet-driven starship, one that would demand a particle accelerator fully 100,000 miles long. Crazy? Sure, but a few decades later we were talking about slinging nanochip satellites in swarms using Jupiter’s magnificent magnetic fields, finding a way to do with nature what was evidently impossible for us to build with our own hands.
Robert Forward used to conceive of enormous laser sails for interstellar exploration, sails whose outbound laser flux would be amplified by an even larger 560,000-ton Fresnel lens built between the orbits of Saturn and Uranus. But I discovered in a new paper from Greg Matloff (New York City College of Technology, CUNY) that it was James Early who introduced another extraordinary idea, that of using a gigantic sail-like structure not for propulsion but as a sunshade. Early’s 1989 paper in the Journal of the British Interplanetary Society specifically addressed the ‘greenhouse effect,’ which even then concerned scientists in terms of its effect on global climate. Could technology tame it?
Once again we’re in Daedalus country, or Forward country, if you will. Imagine a true megastructure, a 2000 kilometer sunshade located at the L1 Lagrange region between the Earth and the Sun, approximately 1.5 million kilometers from Earth. The five Lagrange points allow a spacecraft to remain in a relatively fixed orbital position in relation to two larger masses, in the case of L1 the Earth and the Sun. But L1 is not stable, which means that a structure like the sunshade would require thrusting capability for course correction to maintain its optimum position in relation to the Earth. Bear in mind as well the effect of solar radiation pressure on the shade.
Image: Physicist and prolific writer Greg Matloff, author of The Starflight Handbook (Wiley, 1989) and many other books and papers including the indispensable Deep Space Probes (Springer, 2005).
Would a 2000-kilometer shade be sufficient, assuming the intention of reducing the Earth’s effective temperature (255 K) by one K? We learn that solar flux would need to be reduced by 1.5 percent to reduce Earth’s EFF to 254 K. 2000 kilometers does in fact somewhat overshoot the need, reducing solar influx by about 2 percent. That’s a figure that changes over astronomical time, of course, for like any active star, the Sun experiences increased luminosity as it ages, but 2000 km certainly serves for now.
But how to build such a thing? Matloff looks at two versions of the technology, the first being a fully opaque, thick sunshade which would be constructed of lunar or perhaps asteroidal materials. Think in terms of a square sunshade with a thickness of 10-4 meters, and a density of 2,000 kg/m3, producing a mass of 8 X 1011 kg. Building such a thing on Earth is a non-starter, so we can think in terms of assembly in lunar orbit, with the shade materials taken from an asteroid of 460 meters in radius. Corrective thrusting via solar-electric methods with an exhaust velocity of 100 km/s adds up to an eye-opening fuel consumption of 400 kg/s.
But we have other options. Matloff goes on to consider a transparent diffractive film sail (Andreas Hein has recently explored this possibility). Here the sail is imprinted with a diffraction pattern that diverts incoming sunlight from striking the Earth. This is a sail that experiences low solar radiation pressure, its mass reaching 6.4 X 108 kg. But thinner transparent surfaces are feasible as the technology matures, reducing the mass on orbit to 107 kg. Such a futuristic sunshade could be built on Earth and delivered to LEO through 100 flights of today’s super-heavy launch vehicles. Presumably other options will emerge by the time we have the assembly capabilities.
Either of these designs would divert 5.6 X 1015 watts of sunlight from the Earth, energy that if directed to other optical devices would offer numerous possibilities. Matloff considers powering up laser arrays for asteroid mitigation, an in-space defensive system that would work with energy levels much higher than those available through currently envisioned systems like the proposed Breakthrough Starshot Earth-based laser array. A space-based system would also have the advantage of not being confined to a single hemisphere on the surface.
Other possibilities emerge. A laser near the sunshade could tap some of the solar flux and direct it to power stations in geosynchronous Earth orbit, where it would be converted into a microwave frequency to which the Earth’s atmosphere is transparent. You can see the political problem here, which Matloff acknowledges. Any such instrumentation clearly has implications as a weapon, demanding international governance, although through what mechanisms remains to be determined.
But let’s push this concept as hard as we can. How about accelerating a starship? Matloff works the math on a crewed generation ship accelerated to interstellar velocities, with travel time to the nearest star totaling about four centuries. The point is, this is an energy source that makes abundant solar power available while producing the desired reduction in temperatures on Earth, a benefit that could drive development of these technologies not only by us but conceivably by other civilizations as well. If such is a case, we have a new kind of technosignature:
If sufficiently large telescopes are constructed on Earth or in space, astronomers might occasionally survey the vicinity of nearby habitable planets for momentary visual glints. If these sporadic events correspond to the planet-star L1 point, they might constitute an observable technosignature of an existing advanced extraterrestrial civilization.
When considering technosignatures from ET sunshades, it is worth noting that a single monolithic sunshade might be replaced by two or more smaller devices. Also, an advanced extraterrestrial civilization may choose to place its sunshade in a location other than planet-star L1.
There is a Bob Forward quality to this paper that reminds me of Forward’s pleasure in delving into the feasibility of projects from the standpoint of physics while leaving open the issue of how engineers could create structures that at present seem fantastic. That quality might be described as ‘visionary,’ calling up, say, Konstantin Tsiolkovsky in its sheer sweep. Matloff, who knew Forward well, preserves Forward’s exuberance, the pleasure of painting what will be possible for our descendants, who as they one day leave our system will surely continue the exploration of their own ‘Daedalus country.’
The paper is Matloff, “The Lagrange Sunshade: Its Effectiveness in Combating Global Warming and Its Application to Earth Defense from Asteroid Impacts, Beaming Solar Energy for Terrestrial Use, Propelling Interstellar Migration by Laser-Photon Sails and Its Technosignature,” JBIS Vol. 76, No. 4 (April 2023). The Early paper is “Space-based solar shield to offset greenhouse effect,” JBIS Vol. 42, Dec. 1989, p. 567-569 (abstract).
What We’re Learning about TRAPPIST-1
It’s no surprise that the James Webb Space Telescope’s General Observers program should target TRAPPIST-1 with eight different efforts slated for Webb’s first year of scientific observations. Where else do we find a planetary system that is not only laden with seven planets, but also with orbits so aligned with the system’s ecliptic? Indeed, TRAPPIST-1’s worlds comprise the flattest planetary arrangement we know about, with orbital inclinations throughout less than 0.1 degrees. This is a system made for transits. Four of these worlds may allow temperatures that could support liquid water, should it exist in so exotic a locale.
Image: This diagram compares the orbits of the planets around the faint red star TRAPPIST-1 with the Galilean moons of Jupiter and the inner Solar System. All the planets found around TRAPPIST-1 orbit much closer to their star than Mercury is to the Sun, but as their star is far fainter, they are exposed to similar levels of irradiation as Venus, Earth and Mars in the Solar System. Credit: ESO/O. Furtak.
The parent star is an M8V red dwarf about 40 light years from the Sun. It would be intriguing indeed if we detected life here, especially given the star’s estimated age of well over 7 billion years. Any complex life would have had plenty of time to evolve into a technological phase, if this can be done in these conditions. But our first order of business is to find out whether these worlds have atmospheres. TRAPPIST-1 is a flare star, implying the possibility that any gaseous envelopes have long since been disrupted by such activity.
Thus the importance of the early work on TRAPPIST-1 b and c, the former examined by Webb’s Mid-Infrared Instrument (MIRI), with results presented in a paper in Nature. We learn here that the planet’s dayside temperature is in the range of 500 Kelvin, a remarkable find in itself given that this is the first time any form of light from a rocky exoplanet as small and cool as this has been detected. The planet’s infrared glow as it moved behind the star produced a striking result, explained by co-author Elsa Ducrot (French Alternative Energies and Atomic Energy Commission):
“We compared the results to computer models showing what the temperature should be in different scenarios. The results are almost perfectly consistent with a blackbody made of bare rock and no atmosphere to circulate the heat. We also didn’t see any signs of light being absorbed by carbon dioxide, which would be apparent in these measurements.”
The TRAPPIST-1 work is moving relatively swiftly, for already we have the results of a second JWST program, this one executed by the Max Planck Institute for Astronomy and explained in another Nature paper, this one by lead author Sebastian Zieba. Here the target is TRAPPIST-1 c, which is roughly the size of Venus and which, moreover, receives about the same amount of stellar radiation. That might imply the kind of thick atmosphere we see at Venus, rich in carbon dioxide, but no such result is found. Let me quote Zieba:
“Our results are consistent with the planet being a bare rock with no atmosphere, or the planet having a really thin CO2 atmosphere (thinner than on Earth or even Mars) with no clouds. If the planet had a thick CO2 atmosphere, we would have observed a really shallow secondary eclipse, or none at all. This is because the CO2 would be absorbing all of the 15-micron light, so we wouldn’t detect any coming from the planet.”
Image: This light curve shows the change in brightness of the TRAPPIST-1 system as the second planet, TRAPPIST-1 c, moves behind the star. This phenomenon is known as a secondary eclipse. Astronomers used Webb’s Mid-Infrared Instrument (MIRI) to measure the brightness of mid-infrared light. When the planet is beside the star, the light emitted by both the star and the dayside of the planet reach the telescope, and the system appears brighter. When the planet is behind the star, the light emitted by the planet is blocked and only the starlight reaches the telescope, causing the apparent brightness to decrease. Credits: NASA, ESA, CSA, Joseph Olmsted (STScI)
What JWST is measuring is the 15-micron mid-infrared light emitted by the planet, using the world’s secondary eclipse, the same technique used in the TRAPPIST-1 b work. The MIRI instrument observed four secondary eclipses as the planet moved behind the star. The comparison of brightness between starlight only and the combined light of star and planet allowed the calculation of the amount of mid-infrared given off by the dayside of the planet. This is remarkable work: The decrease in brightness during the secondary eclipse amounts to 0.04 percent, and all of this working with a target 40 light years out.
Image: This graph compares the measured brightness of TRAPPIST-1 c to simulated brightness data for three different scenarios. The measurement (red diamond) is consistent with a bare rocky surface with no atmosphere (green line) or a very thin carbon dioxide atmosphere with no clouds (blue line). A thick carbon dioxide-rich atmosphere with sulfuric acid clouds, similar to that of Venus (yellow line), is unlikely. Credit: NASA, ESA, CSA, Joseph Olmsted (STScI).
I should also mention that the paper on TRAPPIST-1 b points out the similarity of its results to earlier observations of two other M-dwarf stars and their inner planets, LHS 3844 b and GJ 1252 b, where the recorded dayside temperatures showed that heat was not being redistributed through an atmosphere and that there was no absorption of carbon dioxide, as one would expect from an atmosphere like that of Venus.
Thus the need to move further away from the star, as in the TRAPPIST-1 c work, and now, it appears, further still, to cooler worlds more likely to retain their atmospheres. As I said, things are moving swiftly. In the coming year for Webb is a follow-up investigation on both TRAPPIST-1 b and c, in the hands of the system’s discoverer, Michaël Gillon (Université de Liège) and team. With a thick atmosphere ruled out at planet c, we need to learn whether the still cooler planets further out in this system have atmospheres of their own. If not, that would imply formation with little water in the early circumstellar disk.
The paper is Zieba et al., “No thick carbon dioxide atmosphere on the rocky exoplanet TRAPPIST-1 c,” Nature 19 June 2023 (full text). The paper on TRAPPIST-1 b is Greene et al., “Thermal emission from the Earth-sized exoplanet TRAPPIST-1 b using JWST,” Nature 618 (2023), 39-42 (abstract).
Abundant Phosphorus in Enceladus Ocean Increases Habitability: But is Enceladus Inhabited?
Finding the right conditions for life off the Earth justifiably drives many a researcher’s work, but nailing down just what might make the environment beneath an icy moon’s surface benign isn’t easy. The recent wave of speculation about Enceladus revolves around the discovery of phosphorus, a key ingredient for the kind of life we are familiar with. But Alex Tolley speculates in the essay below that we really don’t have a handle on what this discovery means. There’s a long way between ‘habitable’ and ‘inhabited,’ and many data points remain to be analyzed, most of which we have yet to collect. Can we gain the knowledge we need from a future Enceladus plume mission?
by Alex Tolley
There has been abundant speculation about the possibility of life in the subsurface oceans of icy moons. Europa’s oceans with possible hydrothermal vents mimicking Earth’s abyssal oceans and the probable site of the origin of life, caught our attention now that Mars has no extant surface life. Arthur C Clarke had long suggested Europa as an inhabited moon in his novel 2010: Odyssey Two. (1982). While Europa’s hot vents are still speculative based on interpretations of the surface features of its icy crust, Saturn’s moon, Enceladus, showed visible aqueous plumes at the southern pole. These plumes ejected material that contributes to the E-Ring around Saturn as shown below.
While most searches for evidence for life focus on organic material, it has been noted that of the necessary elements for terrestrial life, Carbon, Hydrogen, Oxygen, Nitrogen, Sulfur, and Phosphorus (CHONSP), phosphorus is the least abundant cosmically. Phosphorus is a key component in terrestrial life, from energy management (ATP-ADP cycle) and information molecules DNA, and RNA, with their phosphorylated sugar backbones.
If phosphorus is absent, terrestrial biology cannot exist. Phosphorus is often the limiting factor for biomass on Earth, In freshwater environments phosphorus is the limiting nutrient [1]. Typically, algae require about 10x as much nitrogen as phosphorus. If the amount of available nitrogen is increased, the algae cannot use that extra nitrogen as the amount of available phosphorus now determines how large the algal population can grow. The biomass-to-phosphorus ratio is around 100:1. When phosphorus is the limiting nutrient, then the available phosphorus will limit the biomass of the local plants and therefore animals, regardless of the availability of other nutrients like nitrogen, and other factors such as the amount of sunlight, or water. Agriculture fertilizer runoff can cause algal blooms in aqueous environments and may result in dead zones as oxygen is depleted by respiration as phytoplankton blooms die or are consumed by bacteria.
While nitrogen can be fixed by bacteria from the atmosphere, phosphorus is derived from phosphate rocks, and rich sources of phosphorus for agriculture were historically gleaned from bird guano.
A recent paper in Nature about the detection of phosphorus in the grains from the E-ring by the Cassini probe’s Cosmic Dust Analyzer (CDA) suggested that phosphorus is very abundant. As these grains are probably sourced from Enceladus’ plumes, this implies that this moon’s subsurface ocean has high levels of dissolved phosphorus.
The authors of the paper have modeled, and experimentally confirmed the model, and make the claim that Enceladus’ ocean is very rich in phosphorus:
…phosphorus concentrations at least 100-fold higher in the moon’s plume-forming ocean waters than in Earth’s oceans.
around 100-fold greater than terrestrial phosphorus abundance levels. They show that the CDA spectrum [figure 1) is consistent with a solution of disodium phosphate (Na2HPO4) and trisodium phosphate (Na3PO4) (figure 2) The source of these salts on Enceladus is likely from the hot vents chemically releasing the material from the carbonaceous chondritic rocky core and the relatively alkaline ocean. Contrary to intuition, the greater CO2 concentrations in cold water with the hydroxyapatite-calcite and whitlockite-calcite buffer system maintain an alkaline solution that allows for the high phosphate abundance in the plume material that produces the grains in Saturn’s E-ring.
Figure 1. CDA cation spectrum co-added from nine baseline-corrected individual ice grain spectra. The mass lines signifying a high-salinity Type 3 spectrum are Na + (23 u) and (NaOH)Na + (63 u) with secondary Na-rich signatures of (H2O)Na + (41 u) and Na 2+ (46 u). Sodium phosphates are represented by phosphate-bearing Na-cluster cations, with (Na3 PO4)Na + (187 u) possessing the highest amplitude in each spectrum followed by (Na2HPO4 )Na + (165 u) and (NaPO3)Na + (125 u). The first two unlabelled peaks at the beginning of the spectrum are H + and C +, stemming from target contamination 3 (source nature paper). a.u., arbitrary units.
Figure 2. Spectrum from the LILBID analogue experiment reproducing the features in the CDA spectrum. An aqueous solution of 0.420 M Na2HPO4 and 0.038 M Na3PO4 was used. All major characteristics of the CDA spectrum of phosphate-rich grains (Fig. 1) are reproduced at the higher mass resolution of the laboratory mass spectrometer (roughly 700 m/?m). Note: this solution is not equivalent to the inferred ocean concentration. To derive the latter quantity, the concentration determined in these P-rich grains must be averaged over the entire dataset of salt-rich ice grains. (source Nature paper).
Fig. 3: Comparison of observed and calculated concentrations of ΣPO43– in fluids affected by water–rock reactions within Enceladus. a, Relation between ΣPO43– and ΣCO2 at a temperature of 0.1 °C for the hydroxyapatite-calcite buffer system (solid lines) and the whitlockite-calcite buffer system (dashed lines). Constraints on ΣCO2 obtained in previous studies are indicated by the blue shaded area. The area highlighted in pink represents the range of ΣPO43– constrained in this study from CDA data. b, Dependence of ΣPO43– on temperature for the hydroxyapatite-calcite buffer and different values of pH and ΣCO2. A similar diagram for the whitlockite-calcite buffer can be found in Extended Data Fig. 11.
The simple conclusion to draw from this is that phosphorus is very abundant in the Enceladan ocean and that any extant life could be very abundant too.
While the presence of phosphorus ensures that the necessary conditions of elements for habitability are present on Enceladus, it raises the question: “Does this imply Enceladus is also inhabited?”
On Earth, phosphorus is often, the limiting factor for local biomass. On Enceladus, if phosphorus was the limiting factor, then one would not expect it to be detected as inorganic phosphate, but rather in an organic form, bound with biomolecules.
But suppose Enceladus is inhabited, what might account for this finding?
1. Phosphorus is not limiting on Enceladus. Perhaps another element is limiting allowing phosphates to remain inorganic. In Earth’s oceans, where iron (Fe) can be the limiting factor, adding soluble Fe to ocean water can increase algal blooms for enhanced food production and possible CO2 sequestration. On Enceladus, the limiting factor might be another macro or micronutrient. [This may be an energy limitation as Enceladus does not have the high solar energy flux on Earth.]
2. Enceladan life may not use phosphorus. Some years ago Wolfe-Simon claimed that bacteria in Mono Lake used arsenic (As) as a phosphorus substitute. [2] This would have been a major discovery in the search for “shadow life” on Earth. However, it proved to be an experimental error. Arsenic is not a good substitute for phosphorus, especially for life already evolved using such a critical element, and as is well-known, arsenic is a poison for complex life.
3. The authors’ modeling assumptions are incorrect. Phosphorus exists in the Enceladan ocean, but it is mostly in organic form. The plume material is non-biological and is ejected before mixing in the ocean and being taken up by life. The authors may also have wildly overestimated the true abundance of phosphorus in the ocean.
Of these explanations allowing for Enceladus to be inhabited, all seem to be a stretch that life may be in the ocean despite the high inorganic phosphorus abundance. Enceladan biomass may be constrained by the energy derived from the moon’s geochemistry. On Earth, sunlight is the main source of energy maintaining the rich biosphere. In the abyssal darkness, life is very sparse, although it can huddle around the deep ocean’s hot vents.
However, if life is not extant, then the abundance of inorganic phosphorus salts is simply the result of chemical equilibria based on the composition of Enceladus rocky core and abundant frozen CO2 where it formed beyond the CO2 snow line.
While the popular press often conflate habitability with inhabited, the authors are careful to make no such claim, simply arguing that the presence of phosphorus completes the set of major elements required for life:
Regardless of these theoretical considerations, with the finding of phosphates the ocean of Enceladus is now known to satisfy what is generally considered to be the strictest requirement of habitability.
With this detection, it would seem Enceladus should be the highest priority candidate for a search for life in the outer solar system. Its plumes would likely contain evidence of life in the subsurface ocean and avoid the difficult task of drilling through many kilometers of ice crust to reach it. A mission to Enceladus with a suite of life-detecting instruments would be the best way to try to resolve whether life is extant on Enceladus.
The paper is Postberg, F., Sekine, Y., Klenner, F. et al. Detection of phosphates originating from Enceladus’s ocean. Nature 618, 489–493 (2023). https://doi.org/10.1038/s41586-023-05987-9
References
1. Smil, V (2000) Phosphorus in the Environment: Natural Flows and Human Interferences. Annual Review of Energy and the Environment Volume 25, 2000 Smil, pp 53-88
2. Wolfe-Simon F, et al (2010) “A Bacterium That Can Grow by Using Arsenic Instead of Phosphorus. SCIENCE 2 Dec 2010, Vol 332, Issue 6034 pp. 1163-1166
DOI:10.1126/science.1197258
Tightening our Understanding of Circumbinary Worlds
I’m collecting a number of documents on gravitational wave detection and unusual concepts regarding their use by advanced civilizations. It’s going to take a while for me to go through all these, but as I mentioned in the last post, I plan to zero in on the intriguing notion that civilizations with abilities far beyond our own might use gravitational waves rather than the electromagnetic spectrum to serve as the backbone of their communication system. It’s a science fictional concept for sure, though there may be ways it could be imagined for a sufficiently advanced culture.
For today, though, let’s look at a new survey that targets highly unusual planets. Binaries Escorted by Orbiting Planets has an acronym I can get into: BEBOP. It awakens the Charlie Parker in me; I can almost smell the smoky air of a mid-20th century jazz club and hear the clinking of glasses above Parker’s stunning alto work. I was thinking about the great sax player because I had just watched, for about the fifth time, Clint Eastwood’s superb 1988 film Bird, whose soundtrack is, of course, fabulous.
On the astronomy front, the BEBOP survey is a radial velocity sweep for circumbinary planets, those intriguing worlds, rare but definitely out there, that orbit around two stars in tight binary systems. Beginning in 2013, BEBOP targeted 47 eclipsing binaries, using data from the CORALIE spectrograph on the Swiss Euler Telescope at La Silla, Chile. This is intriguing because what we know about circumbinary planets has largely come from detections based on transit analysis. Radial velocity work has uncovered planets orbiting one star in a wide binary configuration but until now, not both.
Image: Artist’s visualization of a circumbinary planet. Credit: Ohio State University / Getty Images.
The new work adds data from the HARPS spectrograph at La Silla and the ESPRESSO spectrograph at Paranal to confirm one of two planets at TOI-1338/BEBOP-1. Thus we have radial velocity evidence for the gas giant BEBOP-1 c, massing in the range of 65 Earth masses, in an orbit around the binary of 215 days. A second world, referenced as TOI-1338 b because it shows up only in transit data from TESS, complements the RV find, making this only the second circumbinary system known to host multiple planets. TOI-1338 b is 21.8 times as massive as the Earth and as a transiting world could well be a candidate for atmospheric studies by the James Webb Space Telescope.
But BEBOP-1 c is the planet that stands out. I think I am safe in calling a co-author on this paper, David Martin (Ohio State University), a master of understatement when he describes the problems in extracting radial velocity data on a circumbinary world. After all, we’re relying on the tiniest gravitational effects flagged by minute changes in wavelength, and now we have to factor in multiple sets of stellar spectra. Here’s Martin:
“When a planet orbits two stars, it can be a bit more complicated to find because both of its stars are also moving through space. So how we can detect these stars’ exoplanets, and the way in which they are formed, are all quite different. Whereas people were previously able to find planets around single stars using radial velocities pretty easily, this technique was not being successfully used to search for binaries.”
Nice work indeed. Circumbinary planets are what the paper describes as ‘harsh environments’ for planet formation given the gravitational matrix in which such formation occurs, and thus we should be able to use the growing number of such systems (now 14 including this one) in the study of how planets form and also migrate. BEBOP should be a useful survey in providing accurate masses for planets in systems we’ve already discovered with the transit method.
Image: This is Figure 3 from the paper, offering an overview of the BEBOP-1 system. Caption: The BEBOP-1 system is shown along with the extent of the system’s habitable zone (HZ) calculated using the Multiple Star HZ website. The conservative habitable zone is shown by the dark green region, while the optimistic habitable zone is shown by the light green region. The binary stars are marked by the blue star symbols in the centre. The red shaded region denotes the instability region surrounding the binary stars as described by Holman and Wiegert. BEBOP-1 c’s orbit is shown by the red orbit models…shaded from the 50th to 99th percentiles. TOI-1338 b’s orbit is shown by the yellow models, and is also based on 500 random samples drawn from the posterior in its discovery paper. Credit: Standing et al.
Learning more about how planets in such perturbed environments emerge should advance the study of planet growth around single stars. It’s likely that the increased transit probabilities of circumbinary planets should play into our efforts to study planetary atmospheres as well. And while transits should provide the bulk of new discoveries in this space, radial velocity follow-ups should expand our knowledge of individual systems, being less dependent on orbital periods and inclination. BEBOP presages a productive use of these complementary observing methods.
The paper is Standing et al., “Radial-velocity discovery of a second planet in the TOI-1338/BEBOP-1 circumbinary system,” Nature Astronomy 12 June 2023 (full text). See also Martin et al., “The BEBOP radial-velocity survey for circumbinary planets I. Eight years of CORALIE observations of 47 single-line eclipsing binaries and abundance constraints on the masses of circumbinary planets,” Astronomy & Astrophysics Vol. 624, A68 (April 2019), 45 pp. Abstract.
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