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Red Dwarfs: Their Impact on Biosignatures

We’re in the midst of a significant period defining the biosignatures life can produce and determining how we might identify them. Centauri Dreams regular Alex Tolley today looks at a paper offering a unique contribution to this effort. The work of Sarah Rugheimer and Lisa Kaltenegger, the paper looks at how exoplanet spectra change for different types of host star and different epochs of planetary evolution. As Alex points out, the effects are profound, especially given the fact that red dwarfs will be our testbed for biosignature detection as we probe planetary atmospheres during transits around nearby stars. How stellar class affects our analysis will affect our strategies especially as we probe early Earth atmosphere equivalents. What will we find, for example, at TRAPPIST-1?

By Alex Tolley

As the search for life on exoplanets ramps up, the question arises as to which types of stars represent the best targets. Based on distribution, M-Dwarfs are very attractive as they represent 3/4 of all stars in our galaxy. Their long lifetimes offer abundant opportunities for life to evolve, and to resist extinction as their stars increase in luminosity. On Earth, terrestrial life might last another one billion years before the level of CO2 in the atmosphere is forced to be reduced below photosynthetic requirements for plants to survive. All but lithophilic life might be extinct within 1 ½ billion years. An additional advantage for astronomers is that spectra of exoplanet atmospheres will be easier to distinguish around low luminosity stars. [6, 7]

From a purely numbers game, M-Dwarfs are most attractive targets:

“Temperate terrestrial planets transiting M-dwarf stars are often touted as the poor-astronomer’s Earth analog, since they are easier to detect and characterize than a true Earth twin. Based on what we currently know, however, M-Dwarf planets are the most common habitable worlds“ [1]

Image: Gliese 581 from a planet in its HZ. Credit: David Hardy.

That M-Dwarf rocky worlds may be the most common habitable world is due to:

“1. rocky planets are much more common in the temperate zones of M-Dwarfs (…) than in the temperate zones of Sun-like Stars (…)

2. small stars are more common than big stars (…)

3. the tidally-locked nature of these planets is not a challenge to climate and may double the width of the habitable zone (…)

4. the red stellar radiation results in a weaker ice-albedo feedback
and hence stabler climate (…), and (…)

5. the slow main sequence evolution of M-Dwarfs means that a geological thermostat is not strictly necessary to maintain habitable conditions for billions of years (…). Studying temperate terrestrial planets around M-Dwarfs is our best shot at understanding habitability writ large.” [1]

There are negatives for life around M-Dwarfs too. The closeness of the habitable zone (HZ) to the star results in tidal locking that may impact the stability of the atmosphere, as well as the intense flares that may strip the atmospheres from these worlds. However, these negatives for habitability and hence life may be compensated by the ubiquity of such worlds and the relative ease of studying them remotely. For lithophilic life, surface conditions largely can be ignored.

After the lifeless Hadean, the Archean and Proterozoic eons had life that was purely prokaryotic. During this time photosynthesis evolved that eventually resulted in an atmosphere with O2 and very little CO2 and CH4. This phase of life’s history covers the long period when Earth’s atmosphere changed from a largely reducing one of N2, CO2, and some CH4, to one that becomes oxidizing. The Phanerozoic, starting around 500 mya encompasses the period when O2 pressures increased to the level they are today and terrestrial, multicellular life blossomed in diversity.

If Earth’s history is any guide, life in our galaxy will be mostly unicellular bacteria, living in a reducing atmosphere. If that is a correct hypothesis, then most life in the galaxy will be non-photosynthetic, perhaps with biologies similar to the Archaea. A biosignature of such microbial life will still require looking for a disequilibrium in gases, mainly CO2 and CH4, rather than O2 and CH4 [2, 3]. Archaea include the extremophiles living in a diverse array of environments, including the lithosphere. Such organisms may well survive the harsher conditions of a tidally locked world, especially regarding the impact of flares.

The question then arises, if we look for a biosignature around stars of different spectral types, will the star’s type have an impact on the planet’s atmosphere, detectable spectral markers, and any potential biosignatures?

This question is examined in a paper by Rugheimer and Kaltenegger [5]. The authors modeled the spectra of atmospheres to simulate Earth-like worlds – rocky worlds large enough to hold an atmosphere and presumably with a mix of ocean and continents, rather than water worlds – orbiting in the HZ of different star types F, G, K and M. Their simulations cover the state of evolution of those worlds as if they were an Earth relocated to other stars, so that the spectra for different gas mixtures could be modeled.

The light of an M-Dwarf is shifted so that the UV component is much diminished. This affects the reactions of the gases in the atmosphere. Photolysis is reduced, reducing the loss of H20, which in turn, as a greenhouse gas, warms the surface more than with a hotter star. CH4 in particular is not lost and may even result in a runaway accumulation in some cases. The increase in H2O increases the cloud cover in the troposphere, which in turn increases the planet’s albedo. The increased IR component of the M-Dwarf’s output increases the surface temperature as well and may well further increase cloud formation.

The photolysis of water and the oxidation of CH4 is shown below. UV is required which results in the reduced loss of H2O and CH4 on exoplanets around M-Dwarfs.

H2O + hv (ƛ < 200 nm) -> H + OH
CH4 + OH -> CH3 + H2O

Similarly, UV is required to split O2 allowing O3 formation.

O2 + hv (ƛ < 240 nm) -> O + O
O + O2 -> O3

Previously, Kaltenegger [4] had modeled the atmospheres of Earth-like worlds around different stars and constructed synthetic spectra to determine the visibility of different biosignature gases in the visible and near-infrared.

Following on, Rugenheimer et al modeled gases for 4 different periods – 3.9, 2.0, 0.8 and 0 Ga for the 4 star types. The initial gas mixes are shown in Table 1.

Table1. Gas mixing ratios for 4 eons. N2 not shown.

Because the stars age at different rates, the periods are standardized to Earth. As M-Dwarfs age far more slowly than our sun, the different luminosities are modeled as if their planets are further out from their star earlier in its history to simulate the lower luminosity.

The result of the simulations shows that some markers will be difficult to observe under different spectral types of stars.

The impact of the star type is shown in Figure 1. Temperature and 5 gases are profiled with altitude, The M-Dwarfs show clear differences from the hotter star types. Of particular note are the higher H2O and CH4 atmosphere ratios, particularly at higher altitudes.

Figure 1. Planetary temperature vs. altitude profiles and mixing ratio profiles for H2O, O3, CH4, OH, and N2O (left to right) for a planet orbiting the grid of FGKM stellar models with a prebiotic atmosphere corresponding to 3.9 Ga (first row), the early rise of oxygen at 2.0 Ga (second row), the start of multicellular life on Earth at 0.8 Ga (third row), and the modern atmosphere (fourth row). Source: Rugheimer & Kaltenegger 2017 [5]

Figure 2 shows the simulated spectra for the star types. Because of the loss of shorter wavelengths with M-Dwarf stars, the O2 signatures are largely lost. This means that even should a planet around an M-Dwarf evolve photosynthesis and create an oxidizing atmosphere, this may not be detectable around such a world.

Figure 2. Disk-integrated VIS/NIR spectra at a resolution of 800 at the TOA for an Earth-like planet for the grid of stellar and geological epoch models assuming 60% Earth-analogue cloud coverage. For individual features highlighting the O2, O3, and H2O/CH4 bands in the VIS spectrum. Source: Rugheimer & Kaltenegger 2017 [5] [TOA = Top of the atmosphere – AMT]

In contrast, the strong markers for CO2 and CH4 are well represented in the spectrum for M type stars. This creates a complication for a biosignature for early life comparable to the Archean and early Proterozoic periods on Earth. An atmosphere of CO2 and CH4 assumes that the CH4 is due to methanogens being the dominant source of CH4, far outstripping geologic sources. On the Hadean Earth, CH4 outgassing should be rapidly eliminated by UV. During the Archean, the biogenic production of CH4 maintains the CH4 and therefore the disequilibrium biosignature. But on an M-Dwarf world, this CH4 photolysis is largely absent, resulting in a CO2/CH4 biosignature that is a false positive.

If photosynthesis evolves, the O2 signal can be detected at the longer wavelength of 760 nm, but only if there is no cloud cover, as shown in figure 3. For an M-Dwarf planet, clouds mask the O2 signal, and we expect more cloud cover due to the increased H2O on such worlds.

Figure 3. Disk-integrated spectra (R = 800) of the O2 feature at 0.76 m for clear sky in relative reflectivity (left) and the detectable reflected emergent flux for clear sky (middle) and 60% cloud cover (right). Source: Rugheimer & Kaltenegger 2017 [5]. Note the loss of detectable O2 feature for M-type stars – AMT

Fortunately, ozone (O3) can be detected strongly in the IR around 9500 nm, so we can hope to detect photosynthetic life when the O2 partial pressure increases. Figure 4 shows that the O3 signature can be detected in the Archean in the Phanerozoic, but not the Proterozoic.

Figure 4. Smoothed, disk-integrated IR spectra at the TOA for an Earth-like planet for the grid of stellar and geological epoch models assuming 60% Earth-analogue cloud coverage. For individual features highlighting the O3, H2O/CH4, and CO2 bands in the IR spectrum see Figs. 9, 10, and 11, respectively. Source: Rugheimer & Kaltenegger 2017 [5]

While current instruments cannot resolve spectra in sufficient detail to detect the needed signatures of gases, the authors conclude

“These spectra can be a useful input to design instruments and to optimize the observation strategy for direct imaging or secondary eclipse observations with EELT or JWST as well as other future mission design concepts such as LUVOIR/HDST.”

To conclude, the type of star complicates biosignature detection, especially the co-presence of CO2 and CH4 in Archean and early Proterozoic eons that dominate the history of life on Earth. Not only is the star’s light shifted, hiding shorter wavelength signals, but the light itself impacts the equilibrium composition of atmospheric gases which can lead to biosignature ambiguity.

While the ubiquity of M-Dwarf stars and the longevity of low O2 atmospheres due to the time to evolve photosynthesis on Earth and the delay before the atmosphere builds up its O2 partial pressure, favors M-Dwarf stars as targets for looking for early life, the potential of false positives for the Archaean and early Proterozoic equivalent eons complicates the search for life on these worlds using expected biosignatures for worlds around sol-like stars. There is still work to be done to resolve these issues.


1. N. B. Cowan et al “Characterizing Transiting Planet Atmospheres through 2025”
2015 PASP 127 311. DOI: https://doi.org/10.1086/680855

2. Tolley, A, “Detecting Early Life on Exoplanets”, 02/23/2018. https://www.centauri-dreams.org/2018/02/23/detecting-early-life-on-exoplanets/

3. Krissansen-Totton et al “Disequilibrium biosignatures over Earth history and implications for detecting exoplanet life” 2018 Science Advances Vol. 4, no. 1. DOI: 10.1126/sciadv.aao5747

4. Kaltenegger et al “Spectral Evolution of an Earth-like Planet”, The Astrophysical Journal, 658:598Y616, 2007 March 20 (abstract).

5. Rugheimer, Kaltenegger “Spectra of Earth-like Planets Through Geological Evolution Around FGKM Stars”, The Astrophysical Journal 854(1). DOI: 10.3847/1538-4357/aaa47a

6. Burrows, A. S ”Spectra as windows into exoplanet atmospheres,” 2014, PNAS, 111, 12601 (abstract)

7. Ehrenreich D “Transmission spectra of exoplanet atmospheres” 2011 http://www-astro.physik.tu-berlin.de/plato-2011/talks/PLATO_SC2011_S03T06_Ehrenreich.pdf



Maxing Out Kepler

What happens to a spacecraft at the end of its mission depends on where it’s located. We sent Galileo into Jupiter on September 21, 2003 not so much to gather data but because the spacecraft had not been sterilized before launch. A crash into one of the Galilean moons could potentially have compromised our future searches for life there, but a plunge into Jupiter’s atmosphere eliminated the problem.

Cassini met a similar fate at Saturn, and in both cases, the need to keep a fuel reserve available for that final maneuver was paramount. Now we face a different kind of problem with Kepler, a doughty spacecraft that has more than lived up to its promise despite numerous setbacks, but one that is getting perilously low on fuel. With no nearby world to compromise, Kepler’s challenge is to keep enough fuel in reserve to maximize its scientific potential before its thrusters fail, thus making it impossible for the spacecraft to be aimed at Earth for data transfer.

In an Earth-trailing orbit 151 million kilometers from Earth, Kepler’s fuel tank is expected to run dry within a few months, according to this news release from NASA Ames. The balancing act for its final observing run will be to reserve as much fuel as needed to aim the spacecraft, while gathering as much data as possible before the final maneuver takes place. Timing this will involve keeping a close eye on the fuel tank’s pressure and the performance of the Kepler thrusters, looking for signs that the end is near.

Image: K2 at work, in this image from NASA Ames.

Meanwhile, as we await the April launch of the Transiting Exoplanet Survey Satellite (TESS), we can reflect on Kepler’s longevity. The failure of its second reaction wheel ended the primary mission in 2013, but as we’ve discussed here on many occasions, the use of photon momentum to maintain its pointing meant that the craft could be reborn as K2, an extended mission that shifted its field of view to different portions of the sky on a roughly three-month basis.

As the mission team had assumed that Kepler was capable of about 10 of these observing campaigns, the fact that the mission is now on its 17th is another Kepler surprise. The current campaign, entered this month, will presumably be its last, but if we’ve learned anything about this spacecraft, it’s that we shouldn’t count it out. Let’s see how long the fuel will last.



Antimatter: The Heat Problem

My family has had a closer call with ALS than I would ever have wished for, so the news of Stephen Hawking’s death stays with me as I write this morning. I want to finish up my thoughts on antimatter from the last few days, but I have to preface that by noting how stunning Hawking’s non-scientific accomplishment was. In my family’s case, the ALS diagnosis turned out to be mistaken, but there was no doubt about Hawking’s affliction. How on Earth did he live so long with an illness that should have taken him mere years after it was identified?

Hawking’s name will, of course, continue to resonate in these pages — he was simply too major a figure not to be a continuing part of our discussions. With that in mind, and in a ruminative mood anyway, let me turn back to the 1950s, as I did yesterday in our look at Eugen Sänger’s attempt to create the design for an antimatter rocket. Because even as Sänger labored over the idea, one he had been pursuing since the 1930s, Les Shepherd was looking at the antimatter prospect, and coming up with aspects of the problem not previously identified.

Getting a Starship Up to Speed

Shepherd isn’t as well known as he should be to the public, but within the aerospace community he is something of a legend. A specialist in nuclear fusion, his activities within the International Academy of Astronautics (he was a founder) and the International Astronautical Federation (he was its president) were legion, but this morning I turn to “Interstellar Flight,” a Shepherd paper from 1952. This was published just a year before Sänger explained his antimatter rocket ideas to the 4th International Astronautical Congress in Zurich, later published in Space-Flight Problems (1953).

Remember that neither of these scientists knew about the antiproton as anything other than a theoretical construct, which meant that a ‘photon rocket’ in the Sänger mode just wasn’t going to work. But Shepherd saw that even if it could be made to function, antimatter propulsion ran into other difficulties. Producing and storing antimatter were known problems even then, but it was Shepherd who saw that “The most serious factor restricting journeys to the stars, indeed, is not likely to be the limitation on velocity but rather limitation on acceleration.”

This stems from the fact that the matter/antimatter annihilation is so mind-bogglingly powerful. Let me quote Shepherd on this, as the problem is serious:

…a photon rocket accelerating at 1 g would require to dissipate power in the exhaust beam at the fantastic rate of 3 million Megawatts/tonne. If we suppose that the photons take the form of black-body radiation and that there is 1 sq metre of radiating surface available per tonne of vehicle mass then we can obtain the necessary surface temperature from the Stefan-Boltzmann law…

Shepherd worked this out as:

5.7 x 10-8 T4 = 3 x 1012 watts/metre2

with T expressed in degrees Kelvin. So the crux of the problem is that we are producing an emitting surface with a temperature in the range of 100,000 K. The problem with huge temperatures is that we have to find some way of dissipating them. We’d like to get our rocket operating at 1 g acceleration so we could tour the galaxy, using relativistic time dilation to send a crew to the galactic center, for example, within a human lifetime. But we have to dispose of waste heat from the extraordinarily hot emitting surfaces of our spacecraft, because with numbers like these, even the most efficient engine is still going to produce waste heat.

Image: What I liked about the ‘Venture Star’ from James Cameron’s film Avatar was that the design included radiators, clearly visible in this image. How often have we seen the heat problem addressed in any Hollywood offering? Nice work.

Now we can look at Robert Frisbee’s design — an antimatter ’beamed-core’ starship forced by its nature to be thousands of kilometers long and, compared to its length, incredibly thin. Frisbee’s craft assumes, as I mentioned, a beamed-core design, with pions from the annihilation of protons and antiprotons being shaped into a stream of thrust by a magnetic nozzle; i.e., a superconducting magnet. The spacecraft has to be protected against the gamma rays produced in the annihilation process and it needs radiators to bleed off all the heat generated by the engine.

We also need system radiators for the refrigeration systems. Never forget that we’re storing antimatter within a fraction of a degree of absolute zero (-273 C), then levitating it using a magnetic field that takes advantage of the paramagnetism of frozen hydrogen. Thus:

…the width of the main radiator is fixed by the diameter of the superconductor magnet loop. This results in a very long main radiator (e.g., hundreds of km in length), but it does serve to minimize the radiation and dust shields by keeping the overall vehicle long and thin.

Frisbee wryly notes the need to consider the propellant feed in systems like this. After all, we’re trying to send antimatter pellets magnetically down a tube at least hundreds of kilometers long. The pellets are frozen at 1 K, but we’re doing this in an environment where our propellant feed is sitting next to a 1500 K radiator! Frisbee tries to get around this by converting the anti-hydrogen into antiprotons, feeding these down to the engine in the form of a particle beam.

Frisbee’s 40 light-year mission with a duration of 100 years is set up as a four-stage antimatter rocket massing millions of tons, with radiator length for the first stage climbing as high as 7500 kilometers, and computed radiator lengths for the later stages still in the hundreds of kilometers. Frisbee points out that the 123,000 TW of first-stage engine ‘jet’ power demands the dumping of 207,000 TW of 200 MeV gamma rays. Radiator technology will need an extreme upgrade.

And to drop just briefly back to antimatter production, check this out:

The full 4-stage vehicle requires a total antiproton propellant load of 39,300,000 MT. The annihilation (MC2) energy of this much antimatter (plus an equal amount of matter) corresponds to ~17.7 million years of current Human energy output. At current production efficiencies (10-9), the energy required to produce the antiprotons corresponds to ~17.7 quadrillion [1015] years of current Human energy output. For comparison, this is “only” 590 years of the total energy output of sun. Even at the maximum predicted energy efficiency of antiproton production (0.01%), we would need 177 billion years of current Human energy output for production. In terms of production rate, we only need about 4×1021 times the current annual antiproton production rate.

Impossible to build, I’m sure. But papers like these are immensely useful. They illustrate the consequences of taking known theory into the realm of engineering to see what is demanded. We need to know where the showstoppers are to continue exploring, hoping that at some point we find ways to mitigate them. Frisbee’s paper is available online, and repays a close reading. We could use the mind of a future Hawking to attack such intractable problems.

The Les Shepherd paper cited above is “Interstellar Flight,” JBIS, Vol. 11, 149-167, July 1952. The Frisbee paper is “How to Build an Antimatter Rocket for Interstellar Missions,” 39th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, 20-23 July 2003 (full text).



Stephen Hawking (1942-2018)

The Tau Zero Foundation expresses it deepest sympathies to the family, friends and colleagues of Stephen Hawking. His death is a loss to the the world, to our scientific communities, and to all who value courage in the face of extreme odds.



Harnessing Antimatter for Propulsion

Antimatter’s staggering energy potential always catches the eye, as I mentioned in yesterday’s post. The problem is how to harness it. Eugen Sänger’s ‘photon rocket’ was an attempt to do just that, but the concept was flawed because when he was developing it early in the 1950s, the only form of antimatter known was the positron, the antimatter equivalent of the electron. The antiproton would not be confirmed until 1955. A Sänger photon rocket would rely on the annihilation of positrons and electrons, and therein lies a problem.

Sänger wanted to jack up his rocket’s exhaust velocity to the speed of light, creating a specific impulse of a mind-boggling 3 X 107 seconds. Specific impulse is a broad measure of engine efficiency, so that the higher the specific impulse, the more thrust for a given amount of propellant. Antimatter annihilation could create the exhaust velocity he needed by producing gamma rays, but positron/electron annihilation was essentially a gamma ray bomb, pumping out gamma rays in random directions.

Image: Austrian rocket scientist Eugen Sänger, whose early work on antimatter rockets identified the problems with positron/electron annihilation for propulsion.

What Sänger needed was thrust. His idea of an ‘electron gas’ to channel the gamma rays his photon rocket would produce never bore fruit; in fact, Adam Crowl has pointed out in these pages that the 0.511 MeV gamma rays generated in the antimatter annihilation would demand an electron gas involving densities seen only in white dwarf stars (see Re-thinking the Antimatter Rocket). No wonder Sänger was forced to abandon the idea.

The discovery of the antiproton opened up a different range of possibilities. When protons and antiprotons annihilate each other, they produce gamma rays and, usefully, particles called pi-mesons, or pions. I’m drawing on Greg Matloff’s The Starflight Handbook (Wiley, 1989) in citing the breakdown: Each proton/antiproton annihilation produces an average of 1.5 positively charged pions, 1.5 negatively charged pions and 2 neutral pions.

Note the charge. We can use this to deflect some of these pions, because while the neutral ones decay quickly, the charged pions take a bit longer before they decay into gamma rays and neutrinos. In this interval, Robert Forward saw, we can use a magnetic nozzle created through superconducting coils to shape a charged pion exhaust stream. The charged pions will decay, but by the time they do, they will be far behind the rocket. We thus have useful momentum from this fleeting interaction or, as Matloff points out, we could also use the pions to heat an inert propellant — hydrogen, water, methane — to produce a channeled thrust.

But while we now have a theoretical way to produce thrust with an antimatter reaction, we still have nowhere near the specific impulse Sänger hoped for, because our ‘beamed core’ antimatter rocket can’t harness all the neutral pions produced by the matter/antimatter annihilation. My friend Giovanni Vulpetti analyzed the problem in the 1980s, concluding that we can expect a pion rocket to achieve a specific impulse equivalent to 0.58c. He summed the matter up in a paper in the Journal of the British Interplanetary Society in 1999:

In the case of proton-antiproton, annihilation generates photons, massive leptons and mesons that decay by chain; some of their final products are neutrinos. In addition, a considerable fraction of the high-energy photons cannot be utilised as jet energy. Both carry off about one third of the initial hadronic mass. Thus, it is not possible to control such amount of energy.

Image: Italian physicist Giovanni Vulpetti, a major figure in antimatter studies through papers in Acta Astronautica, JBIS and elsewhere.

We’re also plagued by inefficiencies in the magnetic nozzle, a further limitation on exhaust velocity. But we do have, in the pion rocket, a way to produce thrust if we can get around antimatter’s other problems.

In the comments to yesterday’s post, several readers asked about creating anti-hydrogen (a positron orbiting an antiproton), a feat that has already been accomplished at CERN. In fact, Gerald Jackson and Steve Howe (Hbar Technologies) created an unusual storage solution for anti-hydrogen in their ‘antimatter sail’ concept for NIAC, which you can see described in their final NIAC report. In more recent work, Jackson has suggested the possibility of using anti-lithium rather than anti-hydrogen.

The idea is to store the frozen anti-hydrogen in a chip much like the integrated circuit chips we use every day in our electronic devices. A series of tunnels on the chip (think of the etching techniques we already use with electronics) lead to periodic wells where the anti-hydrogen pellets are stored, with voltage changes moving them from one well to another. The anti-hydrogen storage bottle draws on methods Robert Millikan and Harvey Fletcher used in the early 20th Century to measure the charge of the electron to produce a portable storage device.

The paramagnetism of frozen anti-hydrogen makes this possible, paramagnetism being the weak attraction of certain materials to an externally applied magnetic field. Innovative approaches like these are changing the way we look at antimatter storage. Let me quote Adam Crowl, from the Centauri Dreams essay I cited earlier:

The old concept of storing [antimatter] as plasma is presently seen as too power intensive and too low in density. Newer understanding of the stability of frozen hydrogen and its paramagnetic properties has led to the concept of magnetically levitating snowballs of anti-hydrogen at the phenomenally low 0.01 K. This should mean a near-zero vapour pressure and minimal loses to annihilation of the frozen antimatter.

But out of this comes work like that of JPL’s Robert Frisbee, who has produced an antimatter rocket design that is thousands of kilometers long, the result of the need to store antimatter as well as to maximize the surface area of the radiators needed to keep the craft functional. In Frisbee’s craft, antimatter is stored within a fraction of a degree of absolute zero (-273 C) and then levitated in a magnetic field. Imagine the refrigeration demands on the spacecraft in sustaining antimatter storage while also incorporating radiators to channel off waste heat.

Image: An antimatter rocket as examined by Robert Frisbee. This is Figure 6 from the paper cited below. Caption: Conceptual Systems for an Antimatter Propulsion System.

Radiators? I’m running out of space this morning, so we’ll return to antimatter tomorrow, when I want to acknowledge Les Shepherd’s early contributions to the antimatter rocket concept.

The paper by Giovanni Vulpetti I quoted above is “Problems and Perspectives in Interstellar Exploration,” JBIS Vol. 52, No. 9/10, available on Vulpetti’s website. For Frisbee’s work, see for example “How to Build an Antimatter Rocket for Interstellar Missions,” 39th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, 20-23 July 2003 (full text).



Antimatter in Motion

Antimatter will never lose its allure when we’re talking about interstellar propulsion, even if the breakthroughs needed to harness it are legion. After all, a kilogram of antimatter, annihilating itself in contact with normal matter, yields roughly ten billion times the amount of energy released when a kilogram of TNT explodes. Per kilogram of fuel, we’re talking about 1,000 times more energy than nuclear fission, and 100 times the energy available through nuclear fusion.

Or we could put this into terms more suited for space. A single gram of antimatter, according to Frank Close’s book Antimatter (Oxford, 2010), could through its annihilation produce as much energy as the fuel from the tanks of two dozen Space Shuttles.

The catalog of energy comparisons could go on, each as marvelous as the last, but the reality is that antimatter is not only extremely difficult to produce in any quantity but even more challenging to store. Cram enough positrons or antiprotons into a magnetic bottle and the repulsive forces between them overcome the containing fields, creating a leak that in turn destroys the antimatter. How to store antimatter for propulsion remains a huge problem.

Here’s Close on the issue:

…`like charges repel’, so in order to contain the electric charge in a gram of pure antiprotons or of positrons, you would have to build a force field so powerful that were you to disrupt it, the explosive force as the charged particles flew apart would exceed anything that would have resulted from their annihilation.

As with so many issues regarding deep space, though, we tackle these things one step at a time. Thus recent news out of CERN draws my attention this morning. Bear in mind that between CERN and Fermilab we’re still talking about antimatter production levels that essentially have enough energy to light a single electric bulb for no more than a few minutes. But assuming we find ways to increase our production, perhaps through harvesting of naturally occurring antimatter, we’re learning some things about storage through a project called PUMA.

The acronym stands for ‘antiProton Unstable Matter Annihilation.’ The goal: To trap a record one billion antiprotons at CERN’s Extra Low ENergy Antiproton (ELENA) facility, a deceleration ring that works with CERN’s Antiproton Decelerator to slow antiprotons, reducing their energy by a factor of 50, from 5.3 MeV to just 0.1 MeV. ELENA should allow the number of antiprotons trapped to be increased by a factor of 10 to 100, a major increase in efficiency.

Image: The ELENA ring prior to the start of first beam in 2016. Credit: CERN.

The PUMA project aims to keep the antiprotons in storage for several weeks, allowing them to be loaded into a van and moved to a nearby ion-beam facility called ISOLDE (Isotope mass Separator On-Line), where they will be collided with radioactive ions as a way of examining exotic nuclear phenomena. The nature of the investigations is interesting — CERN has two experiments underway to study the effects of gravity on antimatter, for example — but it’s the issue of storage that draws my attention. How will CERN manage the feat?

This update from CERN lays out the essentials:

To trap the antiprotons for long enough for them to be transported and used at ISOLDE, PUMA plans to use a 70-cm-long “double-zone” trap inside a one-tonne superconducting solenoid magnet and keep it under an extremely high vacuum (10-17 mbar) and at cryogenic temperature (4 K). The so-called storage zone of the trap will confine the antiprotons, while the second zone will host collisions between the antiprotons and radioactive nuclei that are produced at ISOLDE but decay too rapidly to be transported and studied elsewhere.

Thus ELENA produces the antiprotons, while ISOLDE supplies the short-lived nuclei that CERN scientists intend to study, looking for new quantum phenomena that may emerge in the interactions between antiprotons and the nuclei. I’m taken with how Alexandre Obertelli (Darmstadt Technical University), who leads this work, describes it. “This project,” says the physicist, “might lead to the democratisation of the use of antimatter.” A striking concept, drawing on the fact that antimatter will be transported between two facilities.

Antiprotons traveling aboard a van to a separate site are welcome news. In today’s world, low-energy antiprotons are only being produced at CERN, but we’re improving our storage in ways that may make antimatter experimentation in other venues more practical. Bear in mind, too, that an experiment called BASE (Baryon Antibaryon Symmetry Experiment), also at CERN, has already proven that antiprotons can be kept in a storage reservoir for over a year.

Image: A potential future use for trapped antimatter. Here, a cloud of anti-hydrogen drifts towards a uranium-infused sail. Credit: Hbar Technologies, LLC/Elizabeth Lagana.

We’re a long way from propulsion, here, but I always point to the work of Gerald Jackson and Steve Howe (Hbar Technologies), who attack the problem from the other end. With antimatter scarce, how can we find ways to use it as a spark plug rather than a fuel, an idea the duo have explored in work for NASA’s Institute for Advanced Concepts. Here, milligrams of antimatter are released from a spacecraft onto a uranium-enriched five-meter sail. For all its challenges, antimatter’s promise is such that innovative concepts like these will continue to evolve. Have a look at Antimatter and the Sail for one of a number of my discussions of this concept.



Lab Work on ‘Super-Earth’ Atmospheres

How we do laboratory work on exoplanet atmospheres is an interesting challenge. We’ve worked up models of the early Earth’s atmosphere and conducted well-known experiments on them. Still within our own system, we’ve looked at worlds like Mars and Titan and, with a good read on their atmospheric chemistry, can reproduce an atmosphere within the laboratory with a fair degree of accuracy.

In the realm of exoplanets, we’re in the early stages of atmosphere characterization. We’re getting good results from transmission spectroscopy, which analyzes the light from a star as it filters through a planetary atmosphere during a transit. But thus far, the method has mostly been applied to gas giants. Getting down to the realm of rocky worlds is the next step, one that will be aided by space-based assets like the James Webb Space Telescope. Can lab work also help?

Probing the Atmosphere of a ‘Super-Earth’

Worlds smaller than gas giants are plentiful. Indeed, ‘super-Earths’ are the most common planets we’ve found outside our own Solar System. Larger than the Earth but smaller than Neptune, they present us with a challenge because we have no nearby examples to help us project what we might find. That leaves us with computer modeling to simulate possible targets of observation and, in the lab, experimentation to see which mixture produces what result.

At Johns Hopkins University, Sarah Hörst has been conducting experimental work that varies possible exoplanet atmospheres, working with different levels of carbon dioxide, hydrogen and water vapor, along with helium, carbon monoxide, methane and nitrogen. Hörst and team adjust the percentages of these gases, which they mix in a chamber and heat. The gaseous mixture is passed through a plasma discharge that initiates chemical reactions within the chamber.

The research team used JHU’s Planetary Haze Research chamber (PHAZER) to conduct the experiments. A key issue is how to choose atmospheric compositions that would be likely to be found on super-Earths, as the paper on this work explains:

Atmospheres in chemical equilibrium under a variety of expected super-Earth and mini-Neptune conditions can contain abundant H2O, CO, CO2, N2, H2 and/or CH4, various combinations of which may have a distinct complement of photochemically produced hazes, such as ‘tholins’ and complex organics in the low-temperature, H2-rich cases, and sulphuric acid in the high-metallicity, CO2/H2O-rich cases. Warm atmospheres outgassed from a silicate composition can also be dominated by H2O and CO2. We therefore chose to focus on a representative sample of gas mixtures that are based on equilibrium compositions for 100×, 1,000× and 10,000× solar metallicity over a range of temperatures from 300–600 K at an atmospheric pressure of 1 mbar.

Image: This is Figure 2 in the paper. Caption: Due to the large variety of gases used for the experiments, this schematic provides a general idea of the setup. The details varied depending on the gases used, with attention paid to the solubility of gases in liquid water, condensation temperatures and gas purity. Credit: Sarah Hörst/JHU.

At issue is the question of haze, solid particles suspended in gas that can make it difficult to gauge the spectral fingerprints that identify individual gases. You might recall the clear upper atmosphere scientists found at the ‘hot Saturn’ WASP-39b (see Probing a ‘Hot Saturn’). Using transmission spectroscopy on this world, much larger than a super-Earth, Hannah Wakeford’s team at STscI found clear evidence of water vapor, and a surprising amount of it.

It was the fact that WASP-39b’s upper atmosphere is apparently free of clouds that allowed such detailed study of the atmospheric constituents. When we’re dealing with planets with haze, our ability to read these signs is more problematic. Learning more about the kinds of atmospheres likely to be hazy should help us refine our target list for future observatories.

Hörst’s laboratory work probes the production of haze, as the scientist explains:

“The energy breaks up the gas molecules that we start with. They react with each other and make new things and sometimes they’ll make a solid particle [creating haze] and sometimes they won’t,” Hörst said. “The fundamental question for this paper was: Which of these gas mixtures – which of these atmospheres – will we expect to be hazy?”

Two of the atmospheres in which water was dominant turned out to produce a large amount of haze, an indication that haze is not solely the result of interactions in methane chemistry. From the paper:

The two experiments with the highest production rates had the two highest CH4 concentrations, but the one with the third highest production rate (10,000× at 600 K) had no CH4 at all, demonstrating that there are multiple pathways for organic haze formation and that CH4 is not necessarily required. In the case of the experiment with no CH4, the gas mixture had CO, which provided a source of carbon in place of CH4. However, it is important to note that the production rates are not simply a function of carbon abundance, C/O, C/H or C/N ratios in the initial gas mixtures. This result also demonstrates the need for experimental investigations to develop a robust theory of haze formation in planetary atmospheres.

The researchers found a wide variation in particle color as a function of metallicity. The color of particles produced in the haze turns out to have an effect on the amount of heat it traps. Such findings may have implications for astrobiology, when we consider that primitive layers of haze could shield life in its early stages, preventing energetic photons from reaching the surface.

This work is in its early stages, as the paper makes clear:

Although models of atmospheric photochemistry and haze optical properties provide good first estimates, they are incomplete and biased due to the relatively small phase space spanned by the Solar System atmospheres on which they are based. Laboratory production of exoplanet hazes is a crucial next step in our ability to properly characterize these planetary atmospheres. These experimental simulations of atmospheric chemistry and haze formation relevant to super-Earth and mini-Neptune atmospheres show that atmospheric characterization efforts for cool (T <  800 K) super-Earth- and mini-Neptune-type exoplanets will encounter planets with a wide variety of haze production rates.

The paper also reminds us that hazes will have an effect on reflected light, which will have a bearing on future direct imaging of exoplanets. Lab work like this is part of building the toolsets we’ll need for probing rocky worlds around nearby stars in search of biosignatures. My assumption is that in the early going, we are going to see a lot of ambiguous results, with atmospheres with potential biosignatures being likewise capable of interpretation through abiotic means. Homing in on the most likely targets and understanding the chemistry at play will give us the best chance for success when looking at worlds so unlike any in our own system.

The paper is Hörst et al., “Haze production rates in super-Earth and mini-Neptune atmosphere experiments,” Nature Astronomy 5 March 2018 (abstract).



Juno’s View of Jupiter’s Turbulent Poles

The imagery we’re getting of Jupiter’s polar regions is extraordinary. Juno’s Jovian Infrared Auroral Mapper instrument (JIRAM) works at infrared wavelengths, showing us a vivid picture of a massive central cyclone at the north pole and eight additional cyclones around it. In the image below, we’re looking at colors representing radiant heat, with yellow being thinner clouds at about -13 degrees Celsius, and dark red representing the thickest clouds, at about -118 degrees Celsius. JIRAM can probe down to 70 kilometers below the cloud tops.

Image: This composite image, derived from data collected by the Jovian Infrared Auroral Mapper (JIRAM) instrument aboard NASA’s Juno mission to Jupiter, shows the central cyclone at the planet’s north pole and the eight cyclones that encircle it. Credit: NASA/JPL-Caltech/SwRI/ASI/INAF/JIRAM.

This is hardly the orange, white and saffron belted world we are familiar with from telescope views of the lower latitudes. The scale of these storms is, as you would expect with Jupiter, quite impressive. Alberto Adriani is a Juno co-investigator based at the Institute for Space Astrophysics and Planetology in Rome:

“Prior to Juno we did not know what the weather was like near Jupiter’s poles. Now, we have been able to observe the polar weather up-close every two months. Each one of the northern cyclones is almost as wide as the distance between Naples, Italy and New York City — and the southern ones are even larger than that. They have very violent winds, reaching, in some cases, speeds as great as 350 kph. Finally, and perhaps most remarkably, they are very close together and enduring. There is nothing else like it that we know of in the solar system.”

Adriani’s work on the Jovian polar regions is part of a four-paper set of Juno findings just published in Nature (citations below). We also learn that the planet’s south pole likewise contains a central cyclone, surrounded by five other cyclones with diameters ranging from 5,600 to 7,000 kilometers (the eight northern circumpolar cyclones have diameters between 4,000 and 4,600 kilometers). As Adriani tellingly asks, “…why do they not merge?”

Contrast this situation with Saturn, which houses a single cyclonic vortex at each pole, and it becomes clear that the differences between gas giants can be striking. We also see evidence at Jupiter that the winds dominating its zones and belts run deep, a phenomenon put on display by gravity measurements Juno has collected during its close flybys. “Juno’s measurement of Jupiter’s gravity field indicates a north-south asymmetry, similar to the asymmetry observed in its zones and belts,” said Luciano Iess, Juno co-investigator from Sapienza University of Rome, and lead author on a Nature paper on Jupiter’s gravity field.

That such asymmetries in gravitational measurements exist — and the visible eastward and westward jet streams are likewise shown to be asymmetric — tells us a great deal about how deep these powerful flows extend. This JPL news release explains that the deeper the jets flow, the more massive they are, creating a stronger signal in the gravity field. Juno’s gravity asymmetries thus become a marker for how far down these weather patterns extend.

The massive Jovian weather layer, east-west flows extending to a depth on the order of 3,000 kilometers, contains about one percent of the planet’s mass. Yohai Kaspi, lead author of another of the recent papers in Nature explaining the result, says that seeing the depth of these weather jets and their structure takes us from a two- to a three-dimensional view, adding: “The fact that Jupiter has such a massive region rotating in separate east-west bands is definitely a surprise.” We have much work ahead to determine what drives these jet streams; their gravity signature is entangled with that of Jupiter’s core.

On that score, the surprises seem likely to continue. For a final Juno result now being released suggests that the planet rotates below its massive weather layer as a rigid body.

“This is really an amazing result, and future measurements by Juno will help us understand how the transition works between the weather layer and the rigid body below,” said Tristan Guillot, a Juno co-investigator from the Université Côte d’Azur, Nice, France, and lead author of the paper on Jupiter’s deep interior. “Juno’s discovery has implications for other worlds in our solar system and beyond. Our results imply that the outer differentially-rotating region should be at least three times deeper in Saturn and shallower in massive giant planets and brown dwarf stars.”

Let’s close with a Juno image of Jupiter’s south pole as processed from JunoCam imager data by citizen scientist Gerald Eichstädt.

Image: This image captures the swirling cloud formations around the south pole of Jupiter, looking up toward the equatorial region. NASA’s Juno spacecraft took the color-enhanced image during its eleventh close flyby of the gas giant planet on Feb. 7 at 1011 EST (1411 UTC). At the time, the spacecraft was 120,533 kilometers from the tops of Jupiter’s clouds at 84.9 degrees south latitude. Credit: NASA/JPL-Caltech/SwRI/MSSS/Gerald Eichstadt.

All four papers are in Nature 555 (8 March 2018). They are: Adriani et al., “Clusters of cyclones encircling Jupiter’s poles,” 216-219 (abstract); Iess et al., “Measurement of Jupiter’s asymmetric gravity field,” 220-222 (abstract); Kaspi et al., “Jupiter’s atmospheric jet streams extend thousands of kilometres deep,” 223-226 (abstract); and Guillot et al., “A suppression of differential rotation in Jupiter’s deep interior,” 227-230 (abstract).



Extracting Exoplanet Topography from Transit Data

How do we go from seeing an exoplanet as a dip on a light curve or even a single pixel on an image to a richly textured world, with oceans, continents and, perhaps, life? We’ve got a long way to go in this effort, but we’re already having success at studying exoplanet atmospheres, with the real prospect of delving into planets as small as the Earth around nearby red dwarfs in the near future. Atmospheric detection and analysis can help us in the search for biosignatures.

But I was surprised when reading a recent paper to realize just how many proposals are out there to analyze planetary surfaces pending the development of next-generation technologies. Back in 2010, for example, I wrote about Tyler Robinson (University of Washington), who was working on how we might detect the glint of exo-oceans (see Light Off Distant Oceans for more on Robinson’s work). And Robinson’s ideas are joined by numerous other approaches. I won’t go into detail on any of these, but l do want to illustrate the range of possibilities here:

  • Exomoon detection (see Sartoretti & Schneider, 1999, or the Hunt for Exomoons with Kepler and papers from David Kipping);
  • Planetary oblateness — i.e., having an equatorial diameter greater than the distance between poles (Seager & Hui 2002; Carter & Winn 2010);
  • Light from alien cities (Loeb & Turner 2012);
  • Plant pigments (Berdyugina et al 2016);
  • Industrial pollution (Lin, Gonzalez Abad, & Loeb 2014);
  • Circumplanetary rings (Arnold & Schneider 2006).

I’ve pulled this list with references out of a paper suggesting yet another target, the surface topography of exoplanets. The work of graduate student Moiya McTier and David Kipping (Columbia University), the paper points out that while many of these effects are beyond the reach of current equipment, they are nonetheless valuable in pushing the limits of exoplanet characterization and helping us understand what technologies we will need going forward.

So is it really possible to detect surface features like mountains, trenches and craters on a distant exoplanet? McTier and Kipping make the case that we can draw conclusions about a planetary surface through what they call its ‘bumpiness,’ which should show up in a planetary transit as a scattering in the light curve produced as its silhouette gradually changes (assuming, of course, that we are dealing with a rotating planet in transit). We would obtain not the image of a specific mountain or other surface feature but a general analysis of overall topography.

The paper’s method is to model planetary transits for known bodies — the Earth, the Moon, Mars, Venus, Mercury — to see what it would take to tease out such a signature. We have ample elevation data for rocky planets in our Solar System. Using this information, we can model what would happen if one of them transited a nearby white dwarf. The researchers used thse values to find a general relationship between bumpiness and transit depth scatter.

In terms of bumpiness, the paper argues:

…the definition should encode the planet’s radius. An Everest-sized mountain on an otherwise featureless Mercury provides more contrast to the average planet radius than an Everest on an otherwise featureless Earth, and should result in a higher bumpiness value.

What we are after here is what the paper calls “an assessment of global average features,” one that incorporates the largest feature on a planet (an enormous mountain, for example) but also includes the contribution to the lightcurve scatter produced by all the planet’s features.

Mars, because of its small size and low surface gravity, turns out to be the bumpiest of these planets. A Mars-sized planet orbiting a white dwarf in its habitable zone proves to be an optimal situation for detecting bumpiness. Why white dwarfs? We learn that even huge ground-based telescopes planned for future decades such as the Extremely Large Telescope and Colossus would be unable to detect bumpiness on planets around stars like the Sun or M-dwarfs because of astrophysical noise and the limitations of the instruments. False positives through pulsations on the star’s surface, for example, can likewise appear as extra scatter in the light curve.

White dwarfs, on the other hand, appear unlikely to have convective star spots, but even if they do occur, McTier and Kipping argue that they can be detected and filtered out. Orbiting moons could similarly cause variations in the transit depth that could be mistaken for topography, but here the signature of the exomoons shows up just outside the ingress and egress points in the transit curve, unlike the topographical signature, which appears only in the in-transit data.

It turns out that the largest of our next generation of big telescopes would be able to work with a white dwarf planet, which the paper models as orbiting at 0.01 AU in the center of the star’s habitable zone. If we assume a mass typical of such stars (0.6 M), we get an orbital period of just over 11 hours. 20 hours of observing time covering some 400 transits with a telescope like the 74-meter combined aperture of Colossus should be able to detect topography.

Image: This is Figure 8 from the paper, showing an oceanless Earth transiting a white dwarf. The caption: Top: Transits of a dry Earth with features (in red) and an idealized spherical Earth (in black) in front of a .01R white dwarf with noise of 20 ppm added (20σ detection). The exaggerated silhouettes of Earth at different rotational phases are shown in brown. Middle: Zoomed-in frame of the bottom of the light curve in the top panel. Bottom: Residual plot showing the difference between the realistic and idealized transits. Grey shadows show the error bars on the residuals equal to 50 ppm. Dashed lines are to illustrate that residuals deviate from 0ppm only inside the transit. Credit: McTier & Kipping.

Surface features should tell us a good deal about a planet’s composition. From the paper:

…a detection of bumpiness could lead to constraints on a planet’s internal processes. Mountain ranges like the Himalayas on Earth form from the movement and collision of tectonic plates (Allen 2008). Large volcanoes like Olympic Mons on Mars form from the uninterrupted buildup of lava from internal heating sources. A high-bumpiness planet is likely to have such internal processes, with the highest bumpiness values resulting from a combination of low surface gravity, volcanism, and a lack of tectonic plate movement. Truly low-bumpiness planets are less likely to have these internal processes. On such planets, surface features are likely caused by external factors like asteroid bombardment.

I like the phrase the authors use in closing the paper, referring to their mission “of adding texture to worlds outside our own.” Texture indeed, for we are beginning to move into the realm of deeper planetary analysis, like a painter gradually applying detail to the roughest of sketches. Because of the magnitude of the challenge, we are coming at the question of exoplanet characterization from numerous different directions, as the list at the beginning of this post suggests. Synergies between their methods will be key to exoplanet surface discoveries.

The paper is McTier and Kipping, “Finding Mountains with Molehills: The Detectability of Exotopography,” accepted at Monthly Notices of the Royal Astronomical Society (preprint).



A New Theory of Lunar Formation

Simon Lock and Sarah Stewart are intent upon revising our views on how the Moon was formed. Lock is a Harvard graduate student who last year, in company with Stewart (UC-Davis) presented interesting work on what the duo are calling a ‘synestia,’ which is the kind of ‘structure’ resulting from the collision of huge objects. Current thinking about the Moon is that it formed following the collision of a Mars-sized object with the Earth, two huge objects indeed.

What Lock and Stewart asked is whether this formation scenario can produce the result we see today. What it calls for is the ejection of material that forms into a disk and, through processes of accretion, gradually becomes the Moon. The problem with it, says Lock, is that it’s a very hard trick to pull off:

“Getting enough mass into orbit in the canonical scenario is actually very difficult, and there’s a very narrow range of collisions that might be able to do it. There’s only a couple-of-degree window of impact angles and a very narrow range of sizes … and even then some impacts still don’t work.”

Perhaps we’ve misunderstood the original, massive collision. An adjusted formation scenario could explain why some volatile elements like potassium, sodium and copper are less abundant on the Moon than the Earth, and why isotope ratios for the Earth and the Moon are nearly identical. The ‘synestia’ hypothesis works like this: We still begin with an impact, but the assumed disk of raw materials never forms. Instead, the angular momentum of both colliding bodies is added together, creating a vast, indented disk much bigger than either object.

I’m going to drop back to an earlier Lock and Stewart paper for an illustration here.

Image: The structure of a planet, a planet with a disk and a synestia, all of the same mass. Credit: Simon Lock and Sarah Stewart.

The 2018 paper describes a synestia as:

…an impact-generated structure with Earth-mass and composition that exceeds the corotation limit (CoRoL). Synestias are formed by a range of high-energy, high-AM [angular momentum] collisions during the giant impact stage of planet formation (Lock and Stewart, 2017, hereafter LS17). A synestia is a distinct dynamical structure compared to a planet with a condensate-dominated circumplanetary disk, and, as a result, different processes dominate the early evolution of a synestia.

So the synestia we get from major collisions — and these should be frequent in young planetary systems — is a rapidly rotating, partially vaporized object, molten or gaseous material expanding in volume, an object in the shape of a squashed doughnut without any solid surface. The synestia cannot rotate like a solid body because of variations in rotational rate and thermal energy, so we get an inner region rotating one way and an outer region moving at orbital speeds. Perhaps 10 percent of the Earth’s rock is vaporized, while the rest becomes liquid.

When Lock and Stewart set up simulations of cooling synestias and examine them with dynamic, thermodynamic and geochemical calculations, they find that a ‘seed’ forms within the synestia, a gathering of liquid rock that forms off-center and grows as the structure cools, with vaporized rock condensing and falling toward the center of the synestia. As some of this material strikes the ‘seed’ that will become the Moon, it begins to grow. The Moon eventually emerges from the vapor of the synestia as condensation continues and the synestia recedes within the lunar orbit, with the remainder of the spinning debris coalescing into the Earth.

From the paper:

Most high-energy, high-AM giant impacts can produce synestias. The formation of the Moon within a terrestrial synestia can potentially reproduce the lunar bulk composition, the isotopic similarity between Earth and the Moon, and the large mass of the Moon. If the post-impact body also had high obliquity, the same giant impact may trigger a tidal evolution sequence that explains the present day lunar inclination and the AM of the Earth-Moon system…

Image: Part of the paper’s Figure 18, illustrating Moon formation within a terrestrial synestia. Credit: Lock & Stewart.

The next image, likewise part of Figure 18 in the paper, shows the emergence of the Moon within the synestia as the latter contracts. Credit: Lock & Stewart.

The impact scenario for lunar formation thus shifts to a study of the properties of the synestia that produced the Moon. The similarity in isotopes between the Earth and the Moon is an issue because simulations of giant impacts under the older model produce a lunar disk made primarily of material from the impacting body. But isotope ratios vary among the planets. We would expect differences within these ratios if the Moon formed largely from the impactor’s materials.

Under the synestia model, Earth and Moon emerge from the same cloud of vaporized rock, explaining the isotopic similarity. In this scenario, a planetary satellite forms inside the planet it will orbit. Lock and Stewart explain the Moon’s lack of volatile elements by the same formation story, with the forming Moon surrounded by high-temperature material from the synestia. The paper adds:

The MVEs [moderately volatile elements] that are not incorporated into the Moon remain in the synestia. As the synestia cools and contracts within the lunar orbit, the remaining MVEs are destined to be incorporated into the bulk silicate Earth.

This is a complicated model, but Stewart points out in this Harvard news release that it replicates features of the Moon’s composition that are otherwise hard to explain. Further exploration of synestias will be useful as we model what happens in early exoplanet systems, where collisions on a similar colossal scale should be a feature of planet formation.

The paper is Lock et al., “The Origin of the Moon within a Terrestrial Synestia,” Journal of Geographysical Research: Planets 28 February 2018 (abstract / preprint). The 2017 paper is Lock & Stewart, “The structure of terrestrial bodies: Impact heating, corotation limits, and synestias,” Journal of Geophysical Research: Planets 122 (2017). Abstract / preprint.