Getting Water into the Inner Solar System

Water delivery to the inner Solar System is crucial for life to develop, for worlds like our own must have formed dry, well within the ‘snowline.’ We need a mechanism to bring volatiles from the ice-rich regions beyond 3 AU or so, and while much attention has been paid to comets, we’ve been learning more about asteroids as a second delivery option, for isotopic measurements have shown that Earth’s water has similarities to water bound up in carbonaceous asteroids.

Focusing on asteroid delivery, Pete Schultz (Brown University) and colleague Terik Daly, a postdoctoral researcher at Johns Hopkins University, have confronted the issues raised by early system impacts in a series of experiments. The results appear in the journal Science Advances. Says Schultz:

“Impact models tell us that impactors should completely devolatilize at many of the impact speeds common in the solar system, meaning all the water they contain just boils off in the heat of the impact. But nature has a tendency to be more interesting than our models, which is why we need to do experiments.”

Daly and Schultz found the equipment they needed to study volatile delivery at the Vertical Gun Range at NASA Ames. Their methodology was to fire marble-sized projectiles similar in composition to water-rich carbonaceous chondrite asteroids at a dry target made of pumice power. The speed at impact is some 5 kilometers per second, producing debris that can be analyzed in search of water traces.

Image: Hypervelocity impact experiments, like the one shown here, reveal key clues about how impacts deliver water to asteroids, moons, and planets. In this experiment, a water-rich impactor collides with a bone-dry pumice target at around 18,000 kilometers per hour. The target was designed to rupture partway through the experiment in order to capture materials for analysis. This high-speed video, taken at 130,000 frames per second, slows down the action, which in real time is over in less than a second. Credit: Schultz Lab / Brown University.

The results are a useful window into water delivery. The heat of the impact destroys much of the impactor, while a vapor plume then forms that contains water that was inside the impactor. Inside the plume itself, melted materials and breccias — particles of shattered rock re-formed within a fine-grained matrix — contain some of the original water in recaptured form.The original impactor may be gone, in other words, but a portion of its internal water can survive.

The implications for the early Solar System are clear, as the paper notes:

The fact that the amorphous, glassy component—not projectile survivors—constitutes the primary reservoir for impact-delivered water is critical for extrapolating these experiments. Impact melt production increases with impact speed. If impact melt derived primarily from the target successfully traps water during collisions among planetary bodies (as it does in experiments), then higher-speed impacts may still deliver significant quantities of water.

Image: Samples of impact glasses created during an impact experiment. In impact experiments, these glasses capture surprisingly large amounts of water delivered by water-rich, asteroid-like impactors. Credit: Schultz Lab / Brown University.

The authors calculate that carbonaceous chondrite impactors should be able to deliver up to 30 percent of their internal water to silicate bodies under conditions of impact speeds and angles that we would expect during the early phases of planet formation. Impacts at velocities high enough to vaporize the volatiles still allow for the recapture of those volatiles through impact melts and breccias, so water can be incorporated into the growing planetesimals.

“[T]hese new experiments raise the possibility that growing terrestrial planets trap water in their interiors as they grow, which would profoundly affect their geodynamical evolution,” the authors write. It’s a finding that also helps us explain water distribution later on in the system, such as water ice found on the Moon’s surface in the rays of the Tycho crater, or asteroid-derived water that could account for ice deposits in the polar regions of Mercury.

“The point is that this gives us a mechanism for how water can stick around after these asteroid impacts,” Schultz adds. “And it shows why experiments are so important because this is something that models have missed.”

The paper is Daly & Schultz, “The delivery of water by impacts from planetary accretion to present,” Science Advances Vol. 4, No. 4 (25 April 2018). Full text.


Exoplanets: The Interplanetary Dust Factor

I usually get up while it’s still dark and take a walk. The idea is to shake the night’s dreams out of my head, listen to the birds waking up and pull in a lot of fresh air, all conducive to thinking about what I want to write that day. Last fall I kept noticing the glow before morning twilight that marked the zodiacal cloud, faint enough to be lost in moonlight and challenging to see when competing with city lights. But catch the right conditions and its diffuse glow is apparent, as in the photograph below, a striking example of zodiacal light’s effect.

Image: Sometimes mistaken for light pollution, zodiacal light is sunlight that is reflected by zodiacal dust. It is most visible several hours after sunset on dark, cloudless nights surrounding the spring and fall equinoxes, when the Earth’s equator is aligned with the plane of the solar system. Credit: Malcol.

What we’re seeing, especially at times when the ecliptic is at its largest angle to the horizon, hence autumn and spring, is the light of the Sun reflecting off dust in the Solar System. Most of the material in this interplanetary dust cloud is concentrated along the plane of the system, and as you would expect, similar dust clouds are to be found around other stars. The Spitzer Space Telescope, for instance, has found evidence for a strong dust cloud around the star HD 69830, presumably the result of collisions within the system. No planet has yet been found there.

We often talk about interstellar gas and dust as serious issues for spacecraft moving at a substantial percentage of the speed of light. But what about interplanetary dust? The complications it poses involve how we see exoplanets, particularly those in the habitable zone.

Imagine zodiacal light perhaps a thousand times brighter than our own, enough to outshine the Milky Way. The challenge such light presents to astronomers are potentially serious but not well quantified, which is one reason why researchers using the Large Binocular Telescope Interferometer (LBTI) on Arizona’s Mt. Graham has been at work in a program called HOSTS — the Hunt for Observable Signatures of Terrestrial Systems. The team’s paper in the Astrophysical Journal gives us a look at the survey’s early results.

“There is dust in our own solar system,” says Philip Hinz, the lead for the HOSTS Survey team and associate professor of astronomy at the University of Arizona. “We want to characterize stars that are similar to our own solar system, because that’s our best guess as to what other planetary systems might have life.”

Image: This artist’s concept illustrates what the night sky might look like from a hypothetical alien planet in a star system with an asteroid belt 25 times as massive as the one in our own solar system (alien system above, ours below. Credit: NASA/JPL-Caltech.

Steve Ertel (University of Arizona) is lead author of the paper, which delves into the question of how much dust within a stellar system can affect our ability to see planets within it. All of this goes into planning for future space telescopes, with the HOSTS survey examining the issue for 30 nearby stars. What we learn from the paper is that exozodiacal dust in the stars surveyed is typically less than 15 times the amount found in our own Solar System’s habitable zone.

But planets with larger dust volumes become seriously problematic. Epsilon Eridani, long of interest because of its proximity to the Sun (10.5 light years) is one of these. Says Ertel:

“It is very nearby. It’s a star very similar to our sun. It would be a very nice target to look at, but we figured out that it would not be a good idea. You would not be able to see an Earth-like planet around it.”

Even so, Epsilon Eridani offers us a useful study in planet formation, albeit one with serious challenges for observers. This is from the paper, referring to a previously studied dust clump in the system, which could indicate:

…local dust production in the known asteroid belt and potential shepherding by a planet interior to the belt which could also be creating the clump. There is a long history of planet claims for ?Eri, but radial velocity detection is complicated by stellar activity induced jitter. The existence of the planet claimed by Hatzes et al. (2000) and Benedict et al. (2006) has been debated in the literature (Anglada-Escudé & Butler 2012; Howard & Fulton 2016), it is possible that a planet of period 6.8 – 7.3 yr and mass 0.6 – 1.55 MJup does orbit the star. Attempts to infer the presence of outer planets based on the ring structure are problematic due to the uncertain nature of the intrinsic disk morphology.

This is interesting stuff, although as the paper is at pains to note, we are very early in the study of dust distribution at this level. The issue can tell us something about the possibility of planets within a star system. Given the standard model — that dust is formed during asteroid collisions and spirals inward so that it is distributed throughout the entire system — the survey turned up at least one surprising result. We’ve known for some time that the star Vega has a large belt of cold dust in about the same relation to Vega as the Kuiper Belt is to our system. There is also a disk of hot dust very close to the star. From the paper:

A most puzzling result is our non-detection of warm dust around Vega, for which massive asteroid belt and Kuiper belt analogs have been detected in the mIR to fIR and a large amount of hot dust has been detected in the nIR. This raises the question of what mechanism clears the region between ?0.5 AU and ?5 AU from the star of dust.

We have as yet to detect planets around Vega, but the lack of habitable zone dust may be telling us about a massive planet whose gravitational influence could be clearing this area or, as Ertel notes, several Earth-mass planets. Several other stars in the survey showed, unlike Vega, no dust belts close to the star or far from it, but large amounts of warm dust in the habitable zone. A massive asteroid belt producing numerous collisions could be the culprit in such cases.

Overall, the HOSTS survey to this point has been able to make four new detections of habitable zone dust among its 30 stars; among these, three are the first to be found around Sun-like stars, and two occur around stars without any previous detections of circumstellar dust. The paper notes that the survey’s sensitivity is five to ten times better than previous results. Future exo-imaging attempts will be well served by extending the survey to a larger sample of stars, so we’ll know how the quantity of dust in a given system affects our ability to see HZ planets.

The paper is Ertel et al., “The HOSTS survey – Exozodiacal dust measurements for 30 stars,” Astrophysical Journal Vol. 155, No. 5 (17 April 2018). Abstract / preprint.


Gaia: Data Release 2 Announced

Back in the late 1970s I didn’t know anything about star catalogs. I suppose that, if asked, I would have assumed they were out there — how otherwise could astronomers do their jobs? But the first catalog of stars that came into my life emerged when I was writing an article about SETI, a field I developed an intense interest in and at that time knew very little about. For the article I needed to identify the closest stars, and thus I stumbled upon the Gliese Catalog of Nearby Stars, and over the course of time became absorbed by the idea of exoplanets.

In many ways my first dip into the Gliese catalog began the journey that continues here, because that first SETI article was the forerunner of the kind of writing I have been doing since the turn of the century. In that time our catalogs have grown more and more interesting to me, but none can match today’s, the all-sky view of almost 1.7 billion stars that is the result of the European Space Agency’s Gaia mission. The second Gaia data release became available on April 25 and includes parallax, proper motion and color data for more than 1.3 billion of these stars.

Image: Gaia’s all-sky view of our Milky Way Galaxy and neighbouring galaxies, based on measurements of nearly 1.7 billion stars. The map shows the total brightness and colour of stars observed by the ESA satellite in each portion of the sky between July 2014 and May 2016. Brighter regions indicate denser concentrations of especially bright stars, while darker regions correspond to patches of the sky where fewer bright stars are observed. The colour representation is obtained by combining the total amount of light with the amount of blue and red light recorded by Gaia in each patch of the sky. The bright horizontal structure that dominates the image is the Galactic plane, the flattened disc that hosts most of the stars in our home Galaxy. In the middle of the image, the Galactic centre appears vivid and teeming with stars. Credit: ESA.

You might note the two bright objects at lower right. These are the Large and Small Magellanic clouds, two of the dwarf galaxies that orbit the Milky Way. Note as well the darker regions along the galactic plane, clouds of interstellar dust that hide the light of stars behind and within them, some serving as the breeding ground for young stars.

This is the richest star catalog yet produced, based on 22 months of observation. Gaia, says ESA director of science Günther Hasinger, is “redefining the foundations of astronomy.” And he adds:

“Gaia is an ambitious mission that relies on a huge human collaboration to make sense of a large volume of highly complex data. It demonstrates the need for long-term projects to guarantee progress in space science and technology and to implement even more daring scientific missions of the coming decades.”

Image: The all-sky map of median velocities of stars towards or away from the Sun. The large scale pattern caused by rotation of our Galaxy is evident. Credit: DPAC/ESA.

We’ve had quite a jump here, moving from the first data release, published in 2016, and containing the distances and motions of 2 million stars, to today’s 1.7 billion. ESA is saying that the new release identifies the positions of some of the brightest stars in the field to the same level of precision as Earth observers would need to spot a coin on the surface of the Moon. We achieve estimated distances to individual stars for about ten percent of the total catalog.

A news release from the Science & Technology Facilities Council (UK) surveys what’s available:

This second data release allows progress in all these studies by providing not only distances and apparent motions across the sky for 1.3 billion sources, but also very precise measurements of brightness and colour for an even larger catalogue of 1.7 billion sources. Seven million stars have their line of sight velocities measured, providing full 6-dimensional – three space positions, 3 space motions – information, determining full orbits for those stars in the Milky Way. This is the information needed to weigh the Galaxy, and determine the distribution – and perhaps the properties – of Dark Matter, the mysterious substance which dominates the mass of the Galaxy and the Universe. Credit: Science & Technology Facilities Council’s (STFC) Rutherford Appleton Laboratory, UK.

Out of Gaia comes a refined version of the Hertzsprung-Russell diagram, an essential tool relating stellar intrinsic brightness to color that helps us make sense of the evolution of stars. Gaia offers data on four million stars within 5,000 light years of the Sun, revealing fine-grained details in the traditional H-R plot, including differing signatures for white dwarfs with hydrogen-rich and helium-rich cores, and the ability to distinguish between disk and halo stars. The orbits of 75 globular clusters and 12 dwarf galaxies around the Milky Way can likewise be derived.

“The new Gaia data are so powerful that exciting results are just jumping at us,” says Antonella Vallenari from the Istituto Nazionale di Astrofisica (INAF) and the Astronomical Observatory of Padua, Italy, deputy chair of the data processing consortium executive board. [W]e have built the most detailed Hertzsprung-Russell diagram of stars ever made on the full sky and we can already spot some interesting trends. It feels like we are inaugurating a new era of Galactic archaeology.”

Image: Named after the two astronomers who devised it in the early twentieth century, the Hertzsprung-Russell diagram compares the intrinsic brightness of stars with their colour and is a fundamental tool to study populations of stars and their evolution. Credit: ESA.

Papers on the second Gaia data release appear in a special issue of Astronomy & Astrophysics, while numerous video and virtual reality resources are available here. As we look forward to abundant discoveries from the current release, it’s worth remembering that the final Gaia catalog will not be published until the 2020s. The five-year Gaia mission has been approved for extension until the end of 2020. Gaia’s 3-dimensional map of our galaxy shows 600 times more stars than previously available and covers a volume 1,000 times larger than the first Gaia data release, with a precision 100 times larger. It is hard to imagine any area of astrophysical research that will not be advanced by the availability of these data.

And I have to add this coda: Star catalogs, after all these years, still astound me. I think that’s because the sheer scale of things is so astonishing. After all, the massive Gaia haul at this point reaches 1.7 billion stars, out of a galaxy made up of perhaps 200 billion, in a universe of galaxies whose true extent we are still trying to fathom. Our cataloging species has only begun its immense task.


More News from the ‘Planet of Doubt’

The detection of hydrogen sulfide just above the upper cloud deck of Uranus has received the nods you might expect to rotten eggs, H2S having the odor of such unappetizing objects. But this corrosive, flammable gas is quite an interesting find even if it makes a whiff of Uranian air more off-putting than it already was. Not that you’d live long enough to notice the scent if you happened to be there, as Patrick Irwin (University of Oxford, UK) is quick to note:

“If an unfortunate human were ever to descend through Uranus’s clouds, they would be met with very unpleasant and odiferous conditions. Suffocation and exposure in the negative 200 degrees Celsius atmosphere made of mostly hydrogen, helium, and methane would take its toll long before the smell.”

We can leave that excruciating end to the imagination of science fiction writers, among whom I want to mention my two favorite stories about this planet, Geoff Landis’ “Into the Blue Abyss” (2001) and Gerald Nordley’s “Into the Miranda Rift” (1993). And I always give Stanley Weinbaum full credit for the best Uranus story title of all time: “The Planet of Doubt” (1935).

Patrick Irwin is lead author on the paper discussing the H2S discovery, which is a significant one because it highlights the differences between the cloud decks on the two closer gas giants — Jupiter and Saturn — and the outer ice giants Uranus and Neptune. The former show no trace of hydrogen sulfide above the clouds, while ammonia is clearly present. In fact, most of Jupiter and Saturn’s upper clouds are laden with ammonia ice, a difference that can tell us much about the formation of the respective planets and their subsequent development.

Image: This image of a crescent Uranus, taken by Voyager 2 on January 24th, 1986, reveals its icy blue atmosphere. Despite Voyager 2’s close flyby, the composition of the atmosphere remained a mystery until now. Credit: NASA/JPL-Caltech.

Leigh Fletcher, a member of the research team from the University of Leicester in the UK, notes that the balance between nitrogen and sulfur, and thus between ammonia and hydrogen sulfide, would have depended on the temperature and the location of the planet when it formed. These differences, in other words, are the signature of the gas giants’ formation history, adding to the evidence that the giant planets migrated from the position of their original formation.

Fletcher adds that when a cloud deck forms by condensation, most of the gas forming the cloud becomes embedded in a deep internal reservoir, putting it out of the view of our telescopes:

“Only a tiny amount remains above the clouds as a saturated vapour,” said Fletcher. “And this is why it is so challenging to capture the signatures of ammonia and hydrogen sulfide above cloud decks of Uranus. The superior capabilities of Gemini finally gave us that lucky break.”

Indeed. It took the 8-meter Gemini North telescope at Mauna Kea (Hawaii) to make the find, which the researchers achieved through spectroscopic analysis using the Near-Infrared Integral Field Spectrometer (NIFS), which sampled reflected light from a region just above the main visible cloud layer in the atmosphere of Uranus. Irwin describes these lines as being “just barely there,” at the outer limits of detection, but finding them in the Gemini data has solved a mystery of this planet’s atmospheric composition that persisted even through the Voyager flyby.

Image: Hydrogen sulfide is hardly the only interesting thing about Uranus. Near-infrared views of the planet reveal its otherwise faint ring system, highlighting the extent to which it is tilted. Credit: Lawrence Sromovsky (University of Wisconsin – Madison) / Keck Observatory.

Thus we finally identify a component of the Uranian clouds that it probably shares with Neptune, and the ‘planet of doubt’ takes us a little closer to revealing the secrets of the ice giants. The paper is Irwin et al., “Detection of hydrogen sulfide above the clouds in Uranus’s atmosphere,” published online by Nature Astronomy 23 April 2018 (abstract).


NEOWISE: New Data Release, Implications

The Wide-field Infrared Survey Explorer (WISE) has been featured often in these pages, usually in terms of brown dwarfs and the possibility of uncovering a small star or brown dwarf closer than Proxima Centauri. But while we still have no evidence of such, we do have abundant data on brown dwarfs, as well as a useful compendium of objects that come close to the Earth.

For WISE, launched in 2009 and placed into hibernation in 2011 upon completion of its primary mission, was reactivated in 2013 as NEOWISE. The goal is now the observation of asteroids and comets both near and far by way of characterizing their size and composition.

Amy Mainzer (JPL), NEOWISE principal investigator, points to the mission’s success:

“NEOWISE continues to expand our catalog and knowledge of these elusive and important objects. In total, NEOWISE has now characterized sizes and reflectivities of over 1,300 near-Earth objects since the spacecraft was launched, offering an invaluable resource for understanding the physical properties of this population, and studying what they are made of and where they have come from.”

The WISE/NEOWISE archive is publicly available through IRSA, the NASA/IPAC Infrared Science Archive. The animation below shows detections during NEOWISE’s four years of operation under the current mission parameters. All told, 2.5 million infrared images were collected during the fourth year and are now combined with the prior three years of NEOWISE data in the archive. 10.3 million sets of images are available, and a database of more than 76 billion source detections extracted from those images. The April, 2018 NEOWISE data release can be found here.

Image: This movie shows the progression of NASA’s Near-Earth Object Wide-field Survey Explorer (NEOWISE) investigation for the mission’s first four years following its restart in December 2013. Green dots represent near-Earth objects. Gray dots represent all other asteroids which are mainly in the main asteroid belt between Mars and Jupiter. Yellow squares represent comets. Credit: NASA/JPL-Caltech/PSI.

Obtaining measurements of the diameters and albedo of near-Earth objects through infrared observation, NEOWISE has, in its four years of tracking asteroids and comets, scanned the skies a total of eight times, observing 29,375 objects, a total that includes 788 near-Earth objects and 136 comets since the WISE to NEOWISE transition. Ten of these objects have been classified as PHAs, or potentially hazardous asteroids, based on both their size and the proximity of their approach to Earth’s orbit. In fact, the first PHA found by WISE was 2013 YP139, discovered a mere six days after observations resumed in December, 2013.

We’re now well into the ninth sky coverage period for NEOWISE, with the mission extended through June of 2018. Although potentially hazardous asteroids obviously get greater attention, not all of the mission’s news has been made among this population. In mid-2017, for example, we learned that mission scientists had found about seven times more long-period comets measuring at least one kilometer across than had previously been predicted. Long-period comets take more than 200 years to complete a single revolution of the Sun.

That was a finding with interesting implications:

“The number of comets speaks to the amount of material left over from the solar system’s formation,” said James Bauer, lead author of the study and now a research professor at the University of Maryland, College Park. “We now know that there are more relatively large chunks of ancient material coming from the Oort Cloud than we thought.”

Although discovered in the NEOWISE era, the paper on this work actually draws on data produced during the original WISE mission, an example of how the spacecraft continues to let us examine objects both near and far, including those that have been perturbed out of their orbits in the Oort Cloud, pristine material from the Solar System’s era of formation. That there are so many more long-period comets than predicted would seem to reinforce the idea that cometary delivery of icy materials from the outer Solar System must have been common.

The paper on the long-period cometary population is Bauer et al., “Debiasing the NEOWISE Cryogenic Mission Comet Populations,” Astronomical Journal Vol. 154, No. 2 (July 2017). Abstract available.