≡ Menu

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.



Probing a ‘Hot Saturn’

When researchers talk about ‘hot Saturns,’ it’s natural to imagine a ringed planet in a close orbit to its star, rings being Saturn’s most prominent feature. But WASP-39b hardly fits this picture. Some 700 light years from Earth in the constellation Virgo, this is a tidally locked world that is 20 times closer to its star than the Earth is to the Sun. WASP-39 itself is a G-class star of about 90 percent of the Sun’s mass. We have no evidence of planetary rings here, but we do see a planet whose temperature reaches 776 degrees Celsius, with a nightside not much cooler.

What keeps this world from being called a ‘hot Jupiter’ is its low density coupled with a large radius, some 1.27 times that of Jupiter (its density is about 0.28 times that of Jupiter). ‘Puffy’ planets like this show density levels far more like Saturn, and they orbit close to their stars, accounting for their extended atmospheres. WASP-39b’s atmosphere appears free of high-altitude clouds, allowing detailed study of its composition. Now we have word that the planet shows the signature of water vapor, three times as much as found on Saturn.

Image: Using Hubble and Spitzer, astronomers analyzed the atmosphere of the “hot Saturn” exoplanet WASP-39b, and they captured the most complete spectrum of an exoplanet’s atmosphere possible with present-day technology. By dissecting starlight filtering through the planet’s atmosphere into its component colors, the team found clear evidence for water vapor. Although the researchers predicted they would see water, they were surprised by how much water they found – three times as much water as Saturn has. Credit: NASA, ESA, G. Bacon and A. Feild (STScI), and H. Wakeford (STScI/Univ. of Exeter).

Transmission spectroscopy is the method here, analyzing light from the host star as it filters through the planet’s atmosphere during a transit. Hannah Wakeford (STscI and University of Exeter) is lead investigator on the WASP-39b work. Retrieved with the Hubble Space Telescope and the Spitzer instrument, her team’s data provide about as complete a spectrum of an exoplanetary atmosphere as current technology will allow. Says Wakeford: “This spectrum is thus far the most beautiful example we have of what a clear exoplanet atmosphere looks like.”

The formation history of worlds like these is suggested by the unexpectedly high values for water vapor. The implication is that the planet must have formed far from the star, beyond the snowline in a region where icy materials are abundant. That would mean a planetary migration into the inner system, perhaps creating havoc among smaller planets in inner orbits.

“WASP-39b shows exoplanets can have much different compositions than those of our solar system,” said co-author David Sing of the University of Exeter. “Hopefully, this diversity we see in exoplanets will give us clues in figuring out all the different ways a planet can form and evolve.”

The next move in the WASP-39b story will be to subject it to the James Webb Space Telescope, assuming a successful launch in 2019 (or perhaps later, given the recent report from the Government Accountability Office — see Marina Koren’s excellent analysis in The Atlantic for more on the problem). Once JWST is available, a more complete spectrum including atmospheric carbon will become possible, fleshing out our understanding of this unusual planet’s formation.



A Plausible Path for Life on Enceladus

Cassini has shown us that the plumes of Enceladus are laden not just with ammonia and carbon dioxide but also traces of methane. Scientists at the University of Vienna (Austria) are not claiming this finding as evidence for life, but they have produced laboratory work showing that at least one kind of microbe could survive in conditions like those within the moon. Couple this with the presence of molecular hydrogen (H2), also found within the plumes, and the existence of microorganisms deep within Enceladus appears at least plausible. Some of the methane found in the Enceladus plumes may turn out to be produced by methanogens.

The microorganism in question is Methanothermococcus okinawensis, which can be found around sea vents in the Okinawa Trough off Japan. In conditions like these, methanogenic archaea can sustain themselves by the chemical nutrients found around hydrothermal vents, a scenario that could likewise exist beneath the Enceladus ice.

Simon Rittmann, working with colleagues in Austria and Germany, put the microbe into Enceladus-like conditions in a series of laboratory tests, varying the amount of molecular hydrogen. Demonstrating the survival of the archaea involved introducing different values for pressure and pH, assuming abundant carbon dioxide and molecular hydrogen’s store of energy. M. okinawensis demonstrated that it could survive, producing methane as a by-product.

Image: Plumes erupting off the surface of Enceladus, an icy moon. Credit: NASA/JPL/SSI.

At the floor of the Enceladus ocean, temperatures above 0 degrees Celsius are likely to exist in a region abundant in rock and minerals. Enough molecular hydrogen could be produced by reactions involving the mineral olivine to sustain these lifeforms. The process is called serpentinization, involving interactions between seawater and rocks in the moon’s mantle that can also produce methane (CH4) and hydrogen sulfide (H2S). The experimental work shows that serpentinization reactions can support a rate of molecular hydrogen production high enough to sustain this kind of organism. As the paper notes:

When simultaneously applying putative gaseous (Table 4) and liquid inhibitors (Supplementary Table 3) under high-pressure conditions, we reproducibly demonstrated that M. okinawensis was able to perform H2 /CO2 conversion and CH4 production under Enceladus-like conditions.

Thus the microorganism survives under the conditions Rittmann and team introduced into the laboratory, producing methane as it grows, a possible source of the methane found in Cassini observations. At this point in the investigation, we can’t rule out abiotic methane either.

Temperature is an interesting variable, as the paper goes on to show:

The mean temperature in the subsurface ocean of Enceladus might be just above 0 °C except for the areas where hydrothermal activity is assumed to occur. In these hydrothermal settings temperatures higher than 90 °C are supposedly possible, and are therefore the most likely sites for higher biological activity on Enceladus. Although methanogens are found over a wide temperature range on Earth, including temperatures around 0 °C, growth of these organisms at low temperatures is observed to be slow.

With enough molecular hydrogen produced through serpentinization to support methane production, the case for searching for methanogenic biosignatures is clear, an investigation explored briefly in the paper. We may be able to detect lipids and hydrocarbons as well as carbon isotope ratios that flag the presence of living organisms. This laboratory work makes the case for the kind of mission that will be needed to study possible Enceladus-based life in situ to learn whether methanogenic organisms are more than an extrapolation.

The Washington Post quotes Rittmann’s caveat:

“We tried to be as broad as possible with our assumptions,” Rittmann said. There are no direct measurements for what exists beneath Enceladus’s ice crust. “No one will be able to tell if these conditions are really occurring on Enceladus,” he said. “However, we did our best to be as careful as possible.”

The paper is Taubner et al., “Biological methane production under putative Enceladus-like conditions,” Nature Communications Vol. 9, 748 (2018). Full text.



Voyager at Pluto? Alternative Histories

With New Horizons in hibernation as it pushes on toward MU69, it’s worth remembering how recently our knowledge of the Kuiper Belt has developed. Gerard Kuiper did not predict the belt’s existence, though he did believe that small planets or comets should have formed in the region beyond the orbit of Neptune (he also thought they would have been cleared by gravitational interactions long ago). And I always like to mention Kenneth Edgeworth’s work in a 1943 issue of the Journal of the British Astronomical Association, discussing the likelihood of small objects in the region. We could easily be calling the area the Edgeworth/Kuiper Belt, as I occasionally do in these pages.

Which takes me back to the Voyager days. It wasn’t until 1992 that astronomers discovered 15760 Albion, the first trans-Neptunian object detected after Pluto and Charon. Back in 1980, when controllers were deciding on adjustments to the trajectory of Voyager 1, Pluto was an option, as New Horizons PI Alan Stern has pointed out. The spacecraft could have reached Pluto in the spring of 1986, not long after Voyager 2’s flyby of Uranus in January of that year. That spectacular double-header was ruled out when the Voyager team chose to study Titan instead.

Image: The Voyagers’ paths through the planetary system. What if Voyager 1 had been pointed toward Pluto? Credit: NASA.

The choice to proceed with the Titan close pass occurred at a time when we simply didn’t have any information about the extent of what would soon be called the Kuiper Belt, or realize that Pluto itself could be considered a Kuiper Belt object. Interestingly, Pluto was almost exactly the same distance from the Sun in 1986 as Neptune was when Voyager 2 flew by it in 1989. Had it been sent Pluto’s way, Voyager 1’s encounter would probably have been a success.

Voyager’s ultraviolet spectrometer, Stern tells us, was not a match for the far more sophisticated Alice instrument carried on New Horizons, and the latter also brought a dust impact detector to bear, along with more powerful radio science equipment to study atmospheric temperature and pressure, and greatly improved mapping cameras. But Voyager would have brought a magnetometer, a wider range of plasma instruments, and an ability to send back data at a rate 10 times that of New Horizons. It would also have been looking at Pluto then orbiting equator-on to the Sun, as opposed to the high-latitude illumination New Horizons dealt with in its close flyby of 2015.

‘What if’s’ are always fun, and Stern looks at another in his latest PI Perspective, asking this time whether Voyager might have been able to explore the Kuiper Belt as New Horizons is now doing. Here the answer is more definitive. Without a target list, it’s difficult to see how Voyager could have made a flyby — we knew next to nothing about objects beyond Pluto as Voyager entered the region. When 15760 Albion was discovered, Voyager 1 had all but crossed the Kuiper Belt, while Voyager 2 was deep inside it. We were pulling data from within the Kuiper Belt, but lacked the ability to locate a specific KBO for a potential encounter. Writes Stern:

…with very few known KBOs at the time, and certainly no small ones known close to Voyager’s trajectory, it would have been impossible to put together a Kuiper Belt target observing list. But even had the team been able to somehow craft such a list, Voyager’s cameras used older-technology Vidicon detectors instead of the charge-coupled devices (CCDs) that LORRI [New Horizons’ Long Range Reconnaissance Imager] uses (and are found in most digital cameras). As a result, Voyager’s imagers were not anywhere near as sensitive as those aboard New Horizons, and they could not have detected faint KBOs like the telescopic LORRI can.

Image: New Horizons is the fifth spacecraft to traverse the Kuiper Belt. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Magda Saina.

Moreover, the Hubble telescope was at that time lacking the advanced wide-field camera that would later allow it to detect KBOs as small as MU69, so we simply would have had no target to aim for. Stern goes on to point out that Voyager 1 traveled well above the plane of the Solar System, while Voyager 2 was well below it, meaning the spacecraft were outside the great bulk of the KBO population. While Pluto was a Voyager possibility, a small KBO was definitely not.

Voyager’s spectrometers, magnetometers and charged-particle detectors have given us priceless data about the heliosphere and the Kuiper Belt, but New Horizons is now studying the area more intensively, viewing many KBOs from a distance, taking Kuiper Belt plasma and dust measurements with its SWAP, PEPSSI and SDC instruments, and homing in on MU69, which it will reach on January 1, 2019. The road ahead is exciting, but it’s essential that we keep working on a New Horizons successor, a craft designed from the outset to study the local interstellar medium. What surprises will this next generation of spacecraft convey?



What We Are Trying to Find

What is it we are looking for when we probe nearby planetary systems? Certainly the search for life elsewhere compels us to find planets like our own around stars much like the Sun. But surely our goal isn’t restricted to finding duplicate Earths, if indeed they exist. A larger goal would be to find life on planets unlike the Earth — perhaps around stars much different from the Sun — which would give us some idea how common living systems are in the galaxy.

And beyond that? The ultimate goal is simply to find out what is out there. That takes in outcomes as different as widespread microbial life, perhaps leading to more complex forms, and barren worlds in which life never emerged. A galaxy filled with life vs. a galaxy in which life is rare offers us two striking outcomes. We ignore preconceptions to find out which is true.

Flaring Red Stars

Let’s try to put Proxima Centauri’s recent flare, discussed yesterday, in context. Events like this highlight our doubts about the viability of red dwarf systems in developing life. Flares are not uncommon on ultra-cool dwarf stars, meaning nearby planets in any habitable zone (as defined by liquid water at the surface) would be bathed in ultraviolet and X-rays. We face the distinct possibility that such planets might have never become habitable in the first place, their oceans vaporized, their atmospheres hopelessly compromised.

I’m interested to see how Amaury Triaud and Michaël Gillon see things. It was their team at the University of Liège in Belgium and the University of Cambridge that did the initial work on TRAPPIST-1, a red dwarf around which ultimately seven planets were found to orbit. As the scientists point out in a recent essay for Big Think, three of the planets here are intriguing from the standpoint of habitability, and we’ve recently seen new work on their potential atmospheres.

Image: Exoplanet hunter Michaël Gillon (University of Liège, Belgium).

This is what is so exciting about red dwarfs. While we continue to look for rocky worlds around G-class stars, we’re putting the tools for studying red dwarf planets to use right now. Consider what we’ve learned already: TRAPPIST-1 d, e and f do not show signs of puffy atmospheres rich in hydrogen, which goes a long way toward eliminating them as Neptune-like worlds, and leaves open the issue of more compact atmospheres that could sustain life. We’ve also had a fine-tuning of mass estimates (see TRAPPIST-1: Planets Likely Rich in Volatiles). Thus a star producing a scant 0.05 percent as much light as the Sun is beginning to yield its secrets.

Triaud and Gillon look at the path forward, pointing to atmospheric work with the James Webb Space Telescope. It’s clear they have little patience with those content to rule out habitability without a great deal of further evidence. The arguments they examine are familiar: Planets around such stars may be tidally locked, and indeed, in systems like TRAPPIST-1, so tightly packed that they would produce major instabilities. They acknowledge the flare problem Proxima Centauri has so vividly demonstrated.

But what we are dealing with is a set of unknowns, and the exciting thing is that we are closing in on the ability to produce answers to many of these objections. From the essay:

Far from holding us back, those arguments motivated us. Now we can assess the actual conditions, and explore counter-arguments that Earth-sized planets around stars such as TRAPPIST-1 might in fact be hospitable to life. Oceans and thick atmospheres could mitigate the temperature contrast between day and night sides. Tidal interaction between close-orbiting planets might provide energy for biology. Some models suggest that planets forming around ultra-cool dwarfs start out with much more water than Earth has. Ultraviolet radiation could help to produce biologically relevant compounds… We are optimistic.

It’s the nature of that optimism that we need to explore. For Triaud and Gillon are not overlooking the problems of astrobiology here to insist on a result they want to see. Instead, they’re pointing to a demonstrable fact: In our rush to find a ‘twin’ of the Earth, we sometimes forget that our planet is not necessarily the template for life elsewhere. Indeed, searching for an Earth twin is a highly conservative approach. “Research should be about finding what we don’t already know,” they argue, and in the case of exoplanets, that certainly encompasses the nature of the 75 percent of the galaxy comprised of red dwarfs down to tiny ultra-cool stars.

Image: This artist’s impression shows two Earth-sized worlds passing in front of their parent red dwarf star, which is much smaller and cooler than our Sun. Astronomers using the Hubble instrument recently used transmission spectroscopy, in which starlight passes through potential atmospheres of planets, to look for evidence of extended hydrogen atmospheres around several TRAPPIST-1 planets. The lack of such atmospheres makes it unlikely that these worlds are Neptune-like. Image credit: ESA/Hubble.

If we’re looking for life elsewhere in the universe, we have only to consider the range of planetary systems thus far discovered to know that we need to study systems utterly unlike our own. The goal then, as the authors put it, is to measure the total frequency of biology. That makes red dwarfs an obvious target, particularly when, like TRAPPIST-1, nearby examples offer such deep transits, so much more amenable to current study than Earth-sized worlds transiting G-class stars like our Sun. Indeed, a TRAPPIST-1 transit can be 80 times more prominent, with like gains in the visibility of atmospheric chemistry.

So while we continue to search for a true Earth analog around a star like the Sun, we can put nearby red dwarf planets under immediate investigation, with TRAPPIST-1 transits happening in a matter of days or weeks rather than once a year. The nature of their atmospheres is the first priority, helping us decide what surface conditions may be like. The search for biosignatures of biologically produced gases follows, with the added advantage at TRAPPIST-1 in having so many planets that can be directly compared to each other.

The TRAPPIST (Transiting Planets and Planetesimals Small Telescopes) facility we used to find the TRAPPIST-1 planets was just the prototype of a more ambitious planet survey called SPECULOOS (Search for habitable Planets Eclipsing Ultra-Cool Stars), which has already begun operations. We expect to find many more Earth-sized, rocky planets around dwarf stars within the next five years. With this sample in hand, we will explore the many climates of such worlds. The solar system contains two: Venus and Earth. How many different types of environments will we discover?

If red dwarf planets near us are shown to be devoid of life, we may eventually learn that life is rarer than we thought. But I return to my earlier sentiment — at heart, our goal in studying the universe isn’t to find life, but rather to find what is out there. If we were to learn that life is vanishingly rare, that would be a finding of immense significance for our stewardship of our own planet, teaching us how unusual it may in fact be. In any case, while we all have ideas about what we hope to find, the universe will surely keep forcing us to adjust our expectations.



Proxima Flare May Force Rethinking of Dust Belts

News of a major stellar flare from Proxima Centauri is interesting because flares like these are problematic for habitability. Moreover, this one may tell us something about the nature of the planetary system around this star, making us rethink previous evidence for dust belts there.

But back to the habitability question. Can red dwarf stars sustain life in a habitable zone much closer to the primary than in our own Solar System, when they are subject to such violent outbursts? What we learn in a new paper from Meredith MacGregor and Alycia Weinberger (Carnegie Institution for Science) is that the flare at its peak on March 24, 2017 was 10 times brighter than the largest flares our G-class Sun produces at similar wavelengths (1.3 mm).

Image: The brightness of Proxima Centauri as observed by ALMA over the two minutes of the event on March 24, 2017. The massive stellar flare is shown in red, with the smaller earlier flare in orange, and the enhanced emission surrounding the flare that could mimic a disk in blue. At its peak, the flare increased Proxima Centauri’s brightness by 1,000 times. The shaded area represents uncertainty. Credit: Meredith MacGregor.

Lasting less than two minutes, the flare was preceded by a smaller flare, as shown above, revealing the interactions of accelerated electrons with Proxima Centauri’s charged plasma. We already knew that Proxima produced regular X-ray flares (recent studies have pegged the rate at one large event every few days), though these are much smaller than the flare just observed. The effects of such flaring on Proxima b could be profound, according to MacGregor:

“It’s likely that Proxima b was blasted by high energy radiation during this flare. Over the billions of years since Proxima b formed, flares like this one could have evaporated any atmosphere or ocean and sterilized the surface, suggesting that habitability may involve more than just being the right distance from the host star to have liquid water.”

The issue has significance far beyond Proxima because M-class stars are the most common in the galaxy. They’re also given to pre-main sequence periods marked by frequent changes in luminosity, and prone to a high degree of stellar activity throughout their lifetimes. Proxima Centauri, spectral class M5.5V, has long been known to be a flare star, leading to the current interest in determining the effects of its variability on the single known planet.

MacGregor and Weinberger worked with data from the ALMA 12-m array and the Atacama Compact Array (ACA). The datasets from these observations were examined by Guillem Anglada (Instituto de Astrofísica de Andalucía, Granada, Spain) in 2017, whose team found signs of a dust belt of about 1/100th of Earth’s mass in the 1-4 AU range, with the possibility of another outer belt. These two structures seemed to parallel the asteroid and Kuiper belts we find in our own Solar System, and it was thought that the inner belt might help us constrain the inclination of the Proxima Centauri system, while giving us an idea of its complexity.

I should pause to note that Guillem Anglada is not Guillem Anglada-Escudé, who led the work that discovered Proxima b — the similarity in names is striking but a coincidence. Making this even more confusing is the fact that Guillem Anglada-Escudé is a co-author on the dust paper on which Guillem Anglada was lead author. If we can get the names straight, we can go on to note that the MacGregor/Weinerger results question whether the dust belts are really there.

For MacGregor and Weinberger looked at the ALMA data as a function of observing time, noticing the transient nature of the radiation from the star. From the paper:

The quiescent emission detected by the sensitive 12-m array observations lies below the detection threshold of the ACA observations, and the only ACA detection of Proxima Centauri is during a series of small flares followed by a stronger flare of ∼ 1 minute duration. Due to the clear transient nature of this event, we conclude that there is no need to invoke the presence of an inner dust belt at 1 − 4 AU. It is also likely that the slight excess above the expected photosphere observed in the 12-m observations is due to coronal heating from continual smaller flares, as is seen for AU Mic, another active M dwarf that hosts a well-resolved debris disk. If that is the case, then the need to include warm dust emission at ∼ 0.4 AU is removed. Although the detection of a flare does not immediately impact the claim of an outer belt at ∼ 30 AU, the significant number of background sources expected in the image and known high level of background cirrus suggest that caution should be used in over-interpreting this marginal result.

So we still have the possibility of an outer belt, though not one that can be claimed with any degree of certainty, and we have removed the need for the inner belt. Proxima Centauri may indeed have other planets and a more complex system than we currently know, but the data from ALMA and ACA now appear to be indicative only of the known flaring phenomenon.

“There is now no reason to think that there is a substantial amount of dust around Proxima Cen,” says Weinberger. “Nor is there any information yet that indicates the star has a rich planetary system like ours.”

The paper is MacGregor et al., “Detection of a Millimeter Flare From Proxima Centauri,” accepted at Astrophysical Journal Letters (preprint). The paper on dust belts is Anglada et al., “ALMA Discovery of Dust Belts Around Proxima Centauri,” accepted at Astrophysical Journal Letters (preprint).