Centauri Dreams
Imagining and Planning Interstellar Exploration
‘Oumuamua: A Hydrogen Iceberg?
Studies of interstellar interloper ‘Oumuamua move at lightning pace, to judge from a recent exchange on hydrogen ice. A study by Greg Laughlin and Darryl Seligman (both at Yale) just published in June, has now met a response from Thiem Hoang (Korea University of Science and Technology, Daejeon) and Harvard’s Avi Loeb. The issue is significant because if, as Laughlin and Seligman argued, ‘Oumuamua were made of hydrogen ice, then the outgassing that drove its slight acceleration would not have been detectable. At least one mystery solved.
Or was it? One reason the 0.2km radius object didn’t fit the description of a comet was that there was no explanation for its tiny change in velocity. Hoang and Loeb have examined the hydrogen ice concept and found it wanting. Says Hoang:
“The proposal by Seligman and Laughlin appeared promising because it might explain the extreme elongated shape of ‘Oumuamua as well as the non-gravitational acceleration. However, their theory is based on an assumption that H2 ice could form in dense molecular clouds. If this is true, H2 ice objects could be abundant in the universe, and thus would have far-reaching implications. H2 ice was also proposed to explain dark matter, a mystery of modern astrophysics.”
Which sounds interesting in itself because anything bearing on dark matter is worth a look, given our frustration at understanding the matter behind the hypothesis. Laughlin and Seligman had suggested a giant molecular cloud [GMC] as the origin for ‘Oumuamua, but Hoang and Loeb argue that the earlier paper, while considering the destruction of H2 ice in the interstellar medium through evaporation, did not take into account the improbability of such ices forming within a GMC, or the effects of that environment upon their later growth. From the paper:
Assuming that H2 objects could be formed in GMCs by some mechanisms (Füglistaler & Pfenniger 2016; Füglistaler & Pfenniger 2018; Seligman & Laughlin 2020), we quantify their destruction and determine the minimum size of an H2 object that can reach the solar system. We assume that the H2 objects are ejected from GMCs into the ISM by some dynamical mechanism such as tidal disruption of bigger objects or collisions (see Raymond et al. 2018; Rice & Laughlin 2019).
Image: An artist’s rendering of ‘Oumuamua, a visitor from outside the Solar System. Credit: The international Gemini Observatory/NOIRLab/NSF/AURA artwork by J. Pollard.
Hoang and Loeb’s calculations show that H2 icebergs are unlikely to grow to large size because of collisional heating not just from dust but from gas within the birth cloud, meaning that ‘Oumuamua likely wasn’t a hydrogen iceberg (and, along the way, taking out the ancillary proposition that dark matter may be accounted for by H2 snowballs). Micron sized grains in regions where the density of gas is high will cause the hydrogen on the grains to sublimate.
Assuming that H2 objects could somehow form in the densest regions of GMCs, we found that sublimation by collisional heating inside the GMC would destroy the objects before their escape into the ISM [interstellar medium]. We also studied various destruction mechanisms of H2 ice in the ISM. In particular, we found that H2 objects are heated by the average interstellar radiation, so that they cannot survive beyond a sublimation time of tsub ~ 10 Myr for R = 300 m (see Figure 1). Only H2 objects larger than 5 km could survive.
While giant molecular clouds like (GMC) W51, one of the closest to Earth at roughly 17,000 light years, could be a point of origin for the object, the authors argue that even this close GMC is simply too far away. Moreover, it might be hard for a hydrogen iceberg even to exit the giant molecular cloud in the first place. Collisional heating within a GMC would destroy objects like this through thermal sublimation long before they reached a distant stellar system. The paper finds that objects below 200 meters in radius would be destroyed within the parent GMC.
We also have to find a way to get ‘Oumuamua all the way from its birthplace to our Solar System. What Hoang and Loeb point out is that an iceberg made of hydrogen would be unlikely to survive an interstellar journey that would probably take hundreds of millions of years. An object like this is going to begin to evaporate. Their paper goes to work out the survivability of H2 ice from interstellar radiation given thermal sublimation and photodesorption along the way.
Many other factors come into play that cause problems for a hydrogen iceberg. It has to stand up to cosmic rays as well as impacts with matter in the interstellar medium. And we have to throw in what can happen when an object like ‘Oumuamua enters the Solar System, where solar radiation becomes an issue. The calculations presented here show the significance of thermal sublimation due to starlight, while going beyond this to reveal the effects of cosmic rays and impacts with interstellar matter, which turn out to be less significant.
Image: This is Figure 1 from the paper. Caption: Comparison of various destruction timescales (slanted colored lines) as a function of the object radius (in meters) to the travel time from a GMC at a distance of 5.2 kpc, assuming a characteristic speed of 30 km s?1 (horizontal black line). Credit: Hoang & Loeb.
The authors calculate a minimum radius of H2 objects in the range of 5 kilometers for survival in a journey that would have to take in formation in giant molecular clouds and movement through the interstellar medium. 10 million years would wreak havoc on an object the size of ‘Oumuamua.
We’re in a period of energetic debate, a time when the unresolved questions about ‘Oumuamua remain in play. It seems clear that we need a larger population of interstellar objects to place the current work in context, and Loeb has pointed out that we won’t have long to wait:
“If ‘Oumuamua is a member of a population of similar objects on random trajectories, then the Vera C. Rubin Observatory (VRO), which is scheduled to have its first light next year, should detect roughly one ‘Oumuamua-like object per month. We will all wait with anticipation to see what it will find.”
The paper is Hoang & Loeb, “Destruction of Molecular Hydrogen Ice and Implications for 1I/2017 U1 (‘Oumuamua),” Astrophysical Journal Letters Vol. 899, No. 2 (17 August 2020). (Abstract). The Seligman & Laughlin study that argued for “‘Oumuamua as a hydrogen iceberg is “Evidence that 1I/2017 U1 (‘Oumuamua) was Composed of Molecular Hydrogen Ice,” Astrophysical Journal Letters Vol. 896, No. 1 (9 June 2020). Abstract.
A Fast Inflatable Sail Using Desorption
The first laboratory work on pushing a space sail with microwaves was performed by Jim and Greg Benford at the Jet Propulsion Laboratory back in 1999, with the results presented the following year at a European conference. Leik Myrabo (then at Rensselaer Polytechnic Institute) was, at about the same time, performing experiments with lasers at Wright-Patterson Air Force Base in Ohio. When you think about the problems of laboratory work on these matters, consider the fact of gravity, meaning that you are working in a 1 g gravity well with diaphanous materials whose acceleration depends on how hot you can allow them to become.
Advances in materials and in particular in lightweight carbon structures allowed the Benfords’ experiments to succeed, with the help of a 10-kilowatt microwave beam that produced significant acceleration on the test object. But I’m reminded by looking at a new paper on sail technologies using no beam at all that the Benfords also demonstrated something else. Molecules of CO2, hydrocarbons and hydrogen that had been incorporated in the lattice of their material at manufacture emerged under the high temperatures involved. Thus another form of acceleration came into play through the phenomenon called desorption.
Image: Carbon disk sail lifting off of truncated rectangular waveguide under 10 kW microwave power (four frames, 30 ms interval, first at top). Credit: James and Gregory Benford.
Just how useful an effect may desorption turn out to be? It seems to offer an acceleration dividend. Roman Kezerashvili (City University of New York), working with colleagues at Samara National Research University in Russia and the State University of New York at Buffalo, has now analyzed the effect of desorption on an inflated, torus-shaped sail. I had the pleasure of talking with Dr. Kezerashvili in Italy at the Aosta conference in 2009, where he discussed relativistic effects that have to be taken into account in the navigation of a space sail close to the Sun. The current work follows up an earlier paper on desorption he presented in 2016.
We have the prospect of incorporating compounds into a sail that become a kind of propulsive shell that is triggered either by microwave beam or, in the case of a close Solar pass, by the Sun itself. Think of desorption as involving a kind of propulsive ‘paint.’ In terms of beaming, it is necessary to understand that desorption can take place under either microwave or laser beam, but microwaves do not damage sail materials and so heat them far less destructively. The Benfords’ work, as Kezerashvili points out, shows that microwave beaming produces more efficient absorption in the sail’s coating materials, and high specific impulse can result from desorption.
In fact, in the Benford experiments on ultra-light carbon sails, photon pressure can account for no more than 30 percent of the observed acceleration. The rest comes from desorption.
Various materials can undergo desorption at different temperatures — the sail analyzed by Kezerashvili in 2016 uses carbon, whose properties of acceleration are there analyzed. If low mass atoms or molecules can be blown out of the sail lattice at known temperatures, the mission concept Kezerashvili suggested in that 2016 paper can emerge, one in which the sail reaches desorption temperature at a particular point in a close pass around the Sun. In this scenario, no microwave or laser beam is needed:
It is of particular interest to consider an inflatable torus-shaped solar sail as both propellant-less and propellant-based system. It is a propellant-based and a propellant-less system which create thrust by the sun-driven ejection of a flux of particles of non-zero rest mass due to the desorption of coating and solar radiation pressure, while it performs as a propellant-less conventional solar sail after the thermal desorption ends.
As far as I know, this new paper is the first analysis of desorption on an inflatable sail, but inflatable structures have a long history due to their flexibility and low volume at launch. Among the earliest inflatables to be used in space were the Echo balloons launched in the 1960s, but inflatable structures have been developed extensively in the decades since. I’ve written in the past on a hydrogen-inflated sail using a molybdenum reflector that became the basis for an early orbiting radio telescope design, a concept Greg Matloff extended and investigated along with Kezerashvili and Italian physicist Giancarlo Genta for interstellar flight purposes.
This is also the first paper that considers thermal desorption in the context of solar heating alone, which leads to an interesting class of ‘sundiver’ missions with close solar passes. Thus a conventional solar sail coated with desorptive paint is heated by the space environment.
Image: From the Aosta conference of 2009, a fine memory: A snapshot taken in the Italian Alps. Left to right: Giovanni Vulpetti, Roman Kezerashvili, and Justin Vazquez-Poritz.
On deployment matters: Most solar and beamed sails discussed in the literature assume systems of electromagnetic actuation devices, often involving guide rollers and booms. All of this, of course, adds to the mass of the resulting structure as well as its complexity. Deployment through a series of inflatable booms, as assumed in the inflatable sail concept, simplifies an otherwise intricate procedure. The paper also mentions the advances in flexible polymers and high-strength fibers that allow more mass packed into the launch vehicle at lower cost.
So inflatable sails make sense. The new Kezerashvili paper looks at the dynamics of an inflatable sail, conceived as “a thin reflective membrane attached to an inflatable torus-shaped rim.” The round, flat membrane is coated with heat sensitive materials that make the transition from solid state to gas. Pressure introduced into the rim allows sail deployment from the initial stowed configuration, with deployment occurring at the chosen heliocentric distance.
At this point the sail membrane is extended into its final flat shape, with thermal desorption occurring at a specific temperature. For the purposes of these calculations, the authors cite an acceleration time for the torus-shaped solar sail due to thermal desorption of about 1500 seconds, with the mass of the coating material pegged at 1.5 kg and a desorption rate of 1 g/s.
Image: This is Figure 1 from the paper. Credit: Kezerashvili et al.
And here is a figure from one of Dr. Kezerashvili’s presentations on the inflated sail.
Thus we wind up with two types of acceleration at perihelion, the first being the expected solar radiation pressure, the second being that caused by thermal desorption from the sail itself. The authors assume a beryllium-coated solar sail and analyze the membrane mass of the sail as well as the toroidal rim and coating material, along with the desorption rate, under varying molecular hydrogen gas fills and a temperature range varying with the perihelion approach considered. The different gas fills change the tensile stress on the sail membrane. The paper analyzes the structural strength required in the inflatable torus to support the circular membrane of the sail and considers the deflection of the membrane due to acceleration.
A ‘sundiver’ maneuver is a mission scenario we’ve discussed often in these pages — only this one occurs with an inflatable sail, and it is a mission in which the sail is deployed just as the appropriate temperature is reached. The additional kick provided by desorption produces a substantial boost in performance over a sail driven by solar radiation alone:
The present study reveals that the inflation deployed torus-shaped solar sail accelerated via thermal desorption of coating results in high post-perihelion heliocentric solar sail velocities. With the speed 20-40 AU/year, post-perihelion travel times to the vicinity of Kuiper Belt Objects (KBO) will be less than 1-3 years, while the Sun’s gravity focus at 547 AU can be reached in 13-25 years.
What interested me in particular was the idea of extending the concept into small sails and opting for a ‘fleet’ concept:
The suggested configuration of the torus-shaped solar sail fits the cube-scale size configurations. Recent research reveals that much smaller sails could be incorporated with highly miniaturized chip-scale spacecraft. It is quite possible that a single dedicated interplanetary ”bus” could deploy many cube-scale sails at perihelion. Sequential deployment of a fleet of solar sails could be timed to allow exploration of many small KBOs from a single launch. The natural continuation of this work can be extended in the following directions: i. Detailed research on materials for thermal desorption at temperature suitable for solar sailing; ii. consideration of the Sun as an extended source of radiation; iii. study the influence of solar sail surface oscillations on the motion of a spacecraft performing an interplanetary flight.
Interesting issues arise with a sail like this. The coating on the surface of the sail must be uniform, and in fact one would expect that slight variations will occur that can lead to trajectory deviations during the desorption process. Remember that the time of desorption acceleration is small, around 1500 seconds, in the configuration discussed here. The authors assume there would be a need for periodic corrections in the sail’s tilting angle as these deviations occur. You can also see how neat a fit this work makes with Kezerashvili’s earlier studies on navigation issues deep in a gravity well. An error in relativistic calculations close to the Sun could lead to substantial variations in the final trajectory.
The paper is Kezerashvili et al., “A torus-shaped solar sail accelerated via thermal desorption of coating,” in press at Advances in Space Research (abstract). The original 2016 paper is Ancona and Kezerashvili, “Orbital dynamics of a solar sail accelerated by thermal desorption of coatings,” Proceedings of 67th International Astronautical Congress (IAC 2016), Guadalajara, Mexico, 26-30 September 2016. Paper IAC-16-C1.6.7.32480 (preprint). The comprehensive paper on extrasolar exploration by a solar sail accelerated via thermal desorption of a coating that grew out of this is published in Advances in Space Research 63, pp. 2021-2034 (2019).
Across the ‘Jupiter Gap’
A great part of the excitement of scientific discovery is not knowing what will emerge when you take data. Our space missions have proven that time and again, and I have no doubt that as we tighten the resolution on future telescopes, we’ll find things that defy many an accepted theory. NASA’s Stardust mission reflects the phenomenon. Designed as a comet sample return, Stardust is now providing information about the migration of materials in the primordial Solar System, which may point toward a phenomenon more widespread than earlier believed.
Thus the work of Devin Schrader and Jemma Davidson (University of Arizona Center for Meteorite Studies). Working with colleagues at the Smithsonian Institution’s National Museum of Natural History, the University of Hawai?i at M?noa, Washington University in St. Louis, and Harvard University, the duo have produced evidence that at least fragmentary materials in the inner Solar System crossed what is often called the ‘Jupiter Gap’ and moved much further from the Sun. The Stardust samples produced evidence for the kind of migration that the authors then followed up by analyzing numerous samples of early chondrite meteorites.
Says Schrader:
“This research provides new information about the dynamics of the early Solar System. Our research shows that these two reservoirs were not completely isolated from one another.”
Image: Atacama Large Millimeter Array image of the protoplanetary disk around HL Tauri. The dark rings are gaps in the dust and gas-rich protoplanetary disk, likely due to the formation of planets. These gaps may be similar to the disk gap thought to be formed by the formation of Jupiter in our protoplanetary disk. Credit: ESO/ALMA.
The so-called Jupiter Gap should be examined within the context of current theories of gas giant formation. A gas giant core of between 5 and 10 Earth masses likely formed about 1 million years into the growth of the new system, soon drawing in a dense atmospheric envelope. The gravitational influence of this world would be sufficient to create a gap in the disk of gas and dust around the Sun out of which the planets we see today emerged. The idea that material might somehow cross this divide to move from one disk reservoir to another seemed unlikely.
But the matter demanded exploration because the degree to which such mixing occurs in protoplanetary disks is unresolved, and the nature of the chemical and physical structure of the resulting system depends on it. It’s a complicated analysis, one in which scientists must plug in the conservation of angular momentum as they study the migration of tiny particles, the role played by the pressure of the gases involved and the drag within them, and the effects of whatever planetesimals and larger objects begin to emerge from the mix.
The idea of a Jupiter Gap emerged out of modeling of these factors in various studies in the literature, and it combines with analyses of the calcium- and aluminum-rich inclusions in chondrites, with the prediction that while smaller particles (less than 300 ?m) could pass through the gap, larger particles would be blocked. And thus we would have differences in the isotopes found in meteorites formed in the inner system (non-carbonaceous) and those that formed in the outer system (carbonaceous chondrites) up to 4 million years after the system’s birth.
Image: Associate Research Professor Devin Schrader uses the ion mass spectrometer at the University of Hawai?i at M?noa to determine the isotopic composition of individual minerals in meteorites. Photo credit: Devin Schrader/ASU.
Samples of both are fortunately abundant at the various institutions involved in Schrader and Davidson’s work. The scientists deployed electron probe microanalyzers for imagery and analysis of individual minerals and a secondary ion mass spectrometer to analyze the isotopic composition of the samples. The round grains known as chondrules found in chondrites as part of their earliest formation, particularly those that were only partially melted during formation (‘relict grains’) provide evidence of chondrite formation and early transport within the disk.
The population of chondrites is fortunately varied enough to make fine distinctions among them by measuring the chemical and oxygen isotope compositions of relict grains in their chondrules, allowing the authors to differentiate between their origins and discuss disk transport within the first 4 million years of the Solar System’s formation. The mixing between inner and outer Solar System, occurring despite the Jupiter Gap, seems to have been complex and widespread. It will be interesting to see what analysis of asteroid samples from Hayabusa2 and OSIRIS-REx, both expected within the next several years, may yield in support or refutation of this theory.
The paper is Schrader et al., “Outward migration of chondrule fragments in the early Solar System: O-isotopic evidence for rocky material crossing the Jupiter Gap?” Geochimica et Cosmochimica Acta Volume 282 (1 August 2020), pp. 133-155 (abstract).
Ceres: The Lesson of Occator Crater
We learned some time ago from the Dawn mission just how interesting a place Ceres is. If you’re wanting to dig into the latest research on the dwarf planet, as it is now termed, be aware that a collection of papers has appeared in Nature Astronomy, Nature Geoscience and Nature Communications, all published on August 10. These analyze data gathered during Dawn’s second extended mission (XM2) phase, which closed with a series of low orbits as close as 35 kilometers from the surface. Rather than listing these papers separately, I’ll just offer this link to the entire collection at nature.com.
The upshot is that we’re continuing to learn about a small world that remains surprisingly active. Let’s home in on cryovolcanism, which leverages the temperature differential between a frozen world’s interior water and its frigid surface to produce ejections. These are becoming almost common — think Enceladus, for example, and then remember what Voyager saw at Triton. The thinking has been that some kind of cryovolcanism makes sense in the outer Solar System because the gas and ice giants place gravitational stresses on their moons that warm their interior.
But Ceres? Not only is it the only dwarf planet inside Neptune’s orbit, but it’s also the largest of the main belt asteroids. You would think that orbits between Mars and Jupiter, which are obviously not associated with the tidal forces of a closely orbited gas giant, would produce inactive bodies. The Dawn data and the papers it has spawned indicate otherwise. Dawn, studying Ceres from 2015 to 2018, shows us a world of 950 kilometers diameter that is active and possessed of its own form of cryovolcanism. At the core of these studies is Occator Crater, about 92 kilometers in diameter and possessed of a remarkably bright, white coloration.
Image: Through eruptions of brine from the interior of dwarf planet Ceres, Occator Crater received its current shape over millions of years. Credit: © Nathues et al., Nature Astronomy.
The evidence of Occator Crater argues for the remains of a global, salty ocean, a briny mixture that remains liquid and may still be escaping from the interior. We know that the European Space Agency’s Herschel Space Observatory produced evidence of a sporadic exosphere, thin but containing water. A thin haze was spotted during the Dawn mission over Occator Crater, though only intermittently, and apparently young deposits of salt compounds containing water turn up in analysis of the Dawn spectrometer data, as presented in a paper from the Dawn spectrometer team led by the Istituto di Astrofisica Spaziale e Fisica Cosmica in Italy.
“We assume that Ceres is still occasionally cryovolcanically active,” says Andreas Nathues (Max Planck Institute for Solar System Research, Germany), who led a team evaluating high-resolution imagery of Ceres during the final phase of the Dawn mission. Indeed, the bright white coloration found at Occator Crater was picked up during the approach to Ceres, and yielded to subsequent analysis. “On closer inspection, Occator Crater has a very complex structure with elevations, a large central depression, deposits, cracks, and furrows,” Nathues continues. “In all its details, this became clear only during the final phase of the mission.”
Without a source of internal heating, the deep dive into Occator Crater found in the new papers tells us that this is nonetheless a world rich in water, one containing a brine reservoir believed to be about 40 kilometers deep and several hundred kilometers in width. This would explain Occator’s bright areas, which are found to be deposits made largely of sodium carbonate that would have moved up to the surface only to evaporate, leaving the reflective salt crust behind. It’s the deep reservoir of brine found by studying Ceres’ gravity that helps us parse its interior structure. Significantly, crustal density here increases with depth well beyond the effects of pressure.
Image; NASA’s Dawn spacecraft captured pictures in visible and infrared wavelengths, which were combined to create this false-color view of a region in 92-kilometer-wide Occator Crater on the dwarf planet Ceres (in the main asteroid belt between Mars and Jupiter). Here, recently exposed brine, or salty liquids, in the center of the crater were pushed up from a deep reservoir below Ceres’ crust. In this view, they appear reddish. In the foreground is Cerealia Facula (“facula” means bright area), a 15-kilometer-wide region with a composition dominated by salts. The central dome, Cerealia Tholus, is about 3 kilometers across at its base and 340 meters tall. The dome is inside a central depression about 900 meters deep. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.
As Dawn examined the surface from a range of altitudes, it captured images of two areas within Occator Crater — Cerealia Facula and Vinalia Faculae — whose very brightness was unusual, an indication that they were youthful enough not to be darkened by micrometeorite debris. Subsequent research analyzed in this collection has shown that these faculae (bright areas) are less than 2 million years old. Moreover, the salt compounds of Cerealia Facula — sodium chloride bound with water and ammonium chloride — reached the surface only recently.
“For the large deposit at Cerealia Facula, the bulk of the salts were supplied from a slushy area just beneath the surface that was melted by the heat of the impact that formed the crater about 20 million years ago,” said Dawn Principal Investigator Carol Raymond. “The impact heat subsided after a few million years; however, the impact also created large fractures that could reach the deep, long-lived reservoir, allowing brine to continue percolating to the surface.”
So we have two paths that allow liquids to reach the surface. Again, this is happening in the absence of gravitational interactions with planets, an indication that if Ceres is active, so too may be other ice-rich bodies in the outer Solar System. Small conical hills on Ceres analyzed in this work are another avenue showing the effect of frozen groundwater.
Image; This mosaic of Ceres’ Occator Crater is composed of images NASA’s Dawn mission captured on its second extended mission, in 2018. Bright pits and mounds (foreground) were formed by salty liquid released as Occator’s water-rich floor froze after the crater-forming impact about 20 million years ago. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/USRA/LPI.
So we have brine from below the surface pushed upward, the water evaporating, and bright salty deposits left behind, with the remains of a once global ocean surviving and producing a form of cryovolcanism that is most likely still ongoing. The impact that formed Occator Crater occurred perhaps 22 million years ago, with the subsequent subsiding of the inner part of the crater resulting from the loss of material from the interior.
But what stands out in these papers is that brine continued to reach the surface for millions of years afterwards, and may be part of a process that still occurs, as opposed to being the result of a one-time event caused by meltwater from the initial impact. Another dwarf planet turns out to be surprisingly active, as the once enigmatic Occator Crater demonstrates.
Lunar Eclipse: A Proxy for Exoplanet Observation
When it comes to detecting life on planets around other stars, my guess is that what will initially appear to be a life signature will quickly become controversial. We might, for example, find ozone in an exoplanet atmosphere with a space telescope like HabEX (Habitable Exoplanet Observatory). That would lead to hyperbolic news stories, to be sure, but ozone can happen when nitrogen and oxygen are exposed to ultraviolet light. The presence of ozone makes no definitive statement about life.
In fact, definitive statements about life may take more than a few decades to achieve. If ozone seems like a good catch, that’s because it implies oxygen, which makes us think of photosynthesis, but oxygen itself is hardly infallible as a biosignature. Oxygen-rich atmospheres can be completely abiotic, with UV from the host star breaking down carbon dioxide. For that matter, an atmosphere rich in water vapor can produce oxygen and hydrogen through the effects of UV radiation.
Better, then, to look for a mix of things. Methane and oxygen detected simultaneously would be interesting because the two would need constant replenishment to appear together, indicating a lack of chemical equilibrium, and surely that’s a life signature. Or is it? Volcanoes can produce methane [although see Brig Klyce’s comment below], and so can infalling carbon-rich debris entering an atmosphere.
No, given the significance of the find and the difficulties in the measurement, I don’t expect anything but ambiguity when we do get the right instrumentation to study terrestrial-class worlds transiting their stars. We’ll be looking at starlight screened through a thin layer of atmosphere, and I thInk we can expect a battle royal among astronomers as we try to decide among the possible causes of the chemical signatures we do find.
And then there’s the question of just where a particular planet is in its development. Our planet when young was rich in hydrogen and helium, but later volcanic eruptions would supply carbon dioxide, water vapor, sulphur. No oxygen, yet, but if we find these signatures in an exoplanet atmosphere, we may be looking at a world that will one day experience an oxygenation event like the one that changed everything on Earth two and a half billion years ago.
How tricky atmospheric biosignatures turn out to be. All this gets more complicated still because of the fact that we need an observing campaign at different wavelengths to make a detection of the different gases involved.
But ozone seems like a logical place to start. Giada Arney (NASA GSFC) is co-author of a paper that addresses biosignatures in a more local way, as I’ll explain after her quote:
“Astronomers will also have to take the developmental stage of the planet into account when looking at younger stars with young planets. If you wanted to detect oxygen or ozone from a planet similar to the early Earth, when there was less oxygen in our atmosphere, the spectral features in optical and infrared light aren’t strong enough. We think Earth had low concentrations of ozone before the mid-Proterozoic geological period (between roughly 2.0 billion to 0.7 billion years ago) when photosynthesis contributed to the build up of oxygen and ozone in the atmosphere to the levels we see today. But because the ultraviolet-light signature of ozone features is very strong, you would have a hope of detecting small amounts of ozone. The ultraviolet may therefore be the best wavelength for detecting photosynthetic life on low-oxygen exoplanets.”
Arney, working with Allison Youngblood (Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder) and colleagues, has explored ozone detection close at hand through new Hubble observations. The idea here is to use light reflected off the Moon during a lunar eclipse to try to detect biosignatures in Earth’s own atmosphere. Consider it a test case for the kind of atmospheric characterization we hope to be doing with much more distant worlds in coming decades. Have a look at how things lined up for this observation.
Image: This diagram explains the geometry of the lunar eclipse. When the Moon is entirely in the Earth’s umbra (known as a total lunar eclipse or umbral eclipse), all sunlight reaching the lunar surface has been refracted or scattered through Earth’s atmosphere. When the Moon is in Earth’s penumbra (known as a penumbral eclipse), illumination comes from both direct sunlight and sunlight refracted and scattered through the planet’s atmosphere. This process is similar to an exoplanet transit observation. Credit: M. Kornmesser (ESA/Hubble), NASA, and ESA.
As you can see, the Moon is reflecting sunlight that has passed through Earth’s atmosphere before being reflected back to Hubble. Make no mistake, we’ve done this kind of thing before — Earthshine has its uses — but this is the first time a total lunar eclipse has been captured at ultraviolet wavelengths and from a space telescope, so it’s a decent proxy for what we’ll do later with HabEX and other instruments. Hubble found a strong ozone signature, a photochemical byproduct of molecular oxygen we’ll hope to be finding on rocky exoplanets.
The strength of the ozone signature demonstrates the problem with working with ground-based observations at these wavelengths, for Earth’s atmosphere absorbs ultraviolet light. This work, along with other observations — ground-based but at different wavelengths — of the January 20-21, 2019 lunar eclipse, helps to develop spectral models for atmospheric characterization. The weakness of the lunar eclipse as a proxy is also clear, as the paper notes:
…observing the moon with HST has different challenges, namely lower spectral resolution and pointing instability. The moon is not a homogeneous surface, and pointing instability does not guarantee that the overall albedo and reflectivity spectrum of the lunar surfaces observed in-transit and out-of-transit are identical. Creating transmission spectra with only Earth’s spectral signatures relies on the in-transit and out-of-transit solar and lunar features being identical. We find evidence for overall albedo variations of the moon between our out-of-eclipse spectra, but defer a thorough analysis to a future paper.
This, too, is helpful, though, for previous papers in the literature have analyzed problems of refraction and transit geometries as they affect the exoplanet spectroscopic signature. The models for future work are only now being tested through events like these.
Image: This ground-based telescopic image of the Moon highlights the general region where astronomers used NASA’s Hubble Space Telescope to measure the amount of ozone in Earth’s atmosphere. This method serves as a proxy for how they will observe Earth-like planets around other stars in search of life. Credit: M. Kornmesser (ESA/Hubble), NASA, and ESA.
The paper is Youngblood et al., “The Hubble Space Telescope’s Near-UV and Optical Transmission Spectrum of Earth as an Exoplanet,” Astronomical Journal Vol. 160, No. 3 (6 August 2020). Abstract / Preprint.
A Dense Sub-Neptune Challenges Formation Theories
The exoplanet K2-25b, a young world in the Hyades cluster orbiting an M-dwarf star, raises intriguing questions. We’d like to know how it formed, for K2-25b is much more dense than we would expect for a world slightly smaller than Neptune. Planets in a range between Earth and Neptune seem to be common around other stars, although we have none in our Solar System unless we make an interesting discovery about putative Planet 9.
But let lead author Gudmundur Stefánsson (Princeton University) point out the unusual nature of K2-25b::
“The planet is dense for its size and age, in contrast to other young, sub-Neptune-sized planets that orbit close to their host star. Usually these worlds are observed to have low densities — and some even have extended evaporating atmospheres. K2-25b, with the measurements in hand, seems to have a dense core, either rocky or water-rich, with a thin envelope.”
Image: New detailed observations with NSF’s NOIRLab facilities reveal a young exoplanet, orbiting a young star in the Hyades cluster, that is unusually dense for its size and age. Slightly smaller than Neptune, K2-25b orbits an M-dwarf star — the most common type of star in the galaxy — in 3.5 days. Credit: NOIRLab/NSF/AURA/J. Pollard.
Let’s put this world into the context of giant planet formation because of its unusual density. A widely studied scenario would have gas giants forming through the accretion of disk materials into a core of ice and rock some 5 to 10 times the mass of Earth. A massive gas envelope hundreds of times the mass of Earth is then drawn in through gravitational interaction with the still young disk. K2-25b’s mass — some 25 times that of Earth, and thus possessed of robust gravitational pull — has emerged without accumulating more than a tenuous gas envelope.
This planet is a Kepler discovery and a world producing a large transit depth, one mentioned in the literature as a candidate for atmospheric characterization by the James Webb Space Telescope. Particularly useful is the fact that its mass, radius and even the tilt of its orbit have been determined with the help of the Habitable-zone Planet Finder (HPF), a spectrograph out of Penn State installed on the 10-meter Hobby-Eberly Telescope at McDonald Observatory in Texas.
The size measurement was tightened with observations at Kitt Peak’s WIYN 0.9-meter telescope and the 3.5-meter telescope at Apache Point Observatory in New Mexico. I mention both because Stefánsson subjected their data to an engineered diffuser, a beam shaper that effectively spreads the light from the star to allow more accurate measurements of the planetary transit. This in turn allows transit prediction for K2-25b to be reduced from a window of 30-40 minutes to 20 seconds in a technique Stefánsson developed in his doctoral thesis.
Here you may remember Stefánsson’s name in connection with the exoplanet G 9-40b, where he used diffuser-assisted photometry at Apache Point to tighten transit timing (see G 9-40b: Confirming a Planet Candidate). Tightening the shape of the transit can be invaluable, says Jayadev Rajagopal (National Optical-Infrared Astronomy Research Laboratory), a co-author of the paper:
“The innovative diffuser allowed us to better define the shape of the transit and thereby further constrain the size, density and composition of the planet. Smaller aperture telescopes, when equipped with state-of-the-art, but inexpensive, equipment can be platforms for high impact science programs. Very accurate photometry will be in demand for exploring host stars and planets in tandem with space missions and larger apertures from the ground, and this is an illustration of the role that a modest-sized 0.9-meter telescope can play in that effort.”
The authors of the paper on this work consider K2-25b to be a ‘useful laboratory’ for examining planet formation. If so, it’s a laboratory with unexplained secrets. Why didn’t K2-25b experience runaway gas accretion to produce a gas giant? The paper offers one possible solution:
…we surmise that K2-25b could be the product of planet merging events of smaller planetary cores to produce a more massive planet. Such a dynamical environment could have excited K2-25b into an eccentric orbit, and K2-25b could be in the process of migrating to a shorter period orbit through tidal interactions with the host star. To explain K2-25b’s current moderate eccentricity from tidal circularization theory, we place a lower limit on the tidal quality factor of Qtp ? 105, corresponding to a circularization timescale consistent with the age of the system. This tidal quality factor is higher than the tidal quality factor for Neptune, which suggest that K2-25b’s internal structure could be different than that of the small gas-giants (Uranus, Neptune) in the Solar System.
Image: Wide-field view of the Hyades star cluster created from images forming part of the Digitized Sky Survey 2. Credit: NOIRLab/NSF/AURA/Digitized Sky Survey 2.
Follow-up observations will include transit spectroscopy on data from the Transiting Exoplanet Survey Satellite (TESS) mission, whose target list includes K2-25b. Also useful will be NEID, an extreme precision radial velocity spectrometer installed at Kitt Peak’s WIYN telescope. A pertinent astronomical reference is TOI 849b, a possible exposed planetary core — see An Exposed Planetary Core at TOI-849 for more on this one and its similar issues re planet formation.
The paper is Stefánsson et al., “The Habitable-zone Planet Finder Reveals A High Mass and a Low Obliquity for the Young Neptune K2-25b,” accepted at the Astronomical Journal (preprint).
Follow with RSS or E-Mail
