Asteroids as Spacecraft

by Paul Gilster on June 14, 2016

Rama is a name that resonates with science fiction fans who remember Arthur C. Clarke’s wonderful Rendezvous with Rama (1973). The novel depicts a 50-kilometer starship that enters the Solar System and is intercepted by a human crew, finding remarkable and enigmatic things that I will leave undescribed for the pleasure of those who haven’t yet read the book. Suffice it to say that among Clarke’s many fine novels, Rendezvous with Rama is, along with The City and the Stars, a personal favorite.

What a company called Made in Space Inc. has in mind is something different than Clarke’s vision, though it too evokes names from the past, as we’ll shortly see. Based in Mountain View, CA the company is embarking on an attempt to turn asteroids into small spacecraft that can move themselves to new trajectories. RAMA in this case stands for Reconstituting Asteroids into Mechanical Automata, and it proceeds by putting ‘Seed Craft’ on asteroids that will use materials found on the surface. This is the kind of in situ resource utilization (ISRU) that Ian Crawford discussed in his essay in these pages last Friday.

A suitably modified asteroid could take itself to the nearest extraction point for human mining, while the seed craft could be sent on to another asteroid. Build the system right, Made in Space believes, and you can do away with at least some of the human control needed for space operations. 3-D printing plays a big role here, no surprise given the company’s background in providing the first such printer (to the ISS in 2014) that can function in zero-g. Our friend Jon Lomberg worked with Made in Space to create a ‘Golden Plate’ commemorating the first space manufacturing operation, now attached internally to the functioning space printer.

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That name from the past I mentioned above is Dandridge Cole, an aerospace engineer, former paratrooper and futurist whose death in 1965 at the young age of 44 cost the space community one of its true visionaries. Cole had plenty of ideas of his own on moving asteroids, but in his case, the idea involved more than robotic transfer into a new orbit. Much more. Why not, thought Cole, actually hollow out an asteroid to create an internal habitat? Here’s how Alex Michael Bonnici described Cole’s idea in a tribute written in 2007:

In 1963, Cole wrote Exploring the Secrets of Space: Astronautics for the Layman with I. M. Levitt. In this book they suggested hollowing out an ellipsoidal asteroid about 30 km long, and rotating it about its major axis to simulate gravity. By reflecting sunlight inside with mirrors, and creating, on its inner surface, a pastoral setting an asteroid could be transformed into a permanent space colony. Cole and Cox also envisioned that asteroids would provide the raw materials to form the basis of a spacefaring civilization. And, that asteroidal materials would also serve terrestrial needs. In their view these materials could be transported using mass drivers or linear motors. Cole’s work largely presages that of Gerard K. O’Neill by more than a decade.

Image: Aerospace engineer and futurist Dandridge Cole, who coined the term ‘macro-life’ to refer to human colonies in space and their evolution. Credit: Wikimedia Commons.

Hollow asteroids are an idea familiar to science fiction fans, who will have encountered the trope in various short stories and perhaps in George Zebrowski’s 1979 novel Macrolife. The name is carefully chosen because Cole used ‘macro-life’ to describe future human evolution within space habitats like these, a development he thought would involve a life-form incorporating technology and intimately synchronized with its environment. Putting large colonies of hollow asteroids into play would ensure our species’ survival while allowing us to progress, he believed, beyond dangers like nuclear proliferation and population pressure.

Here’s Cole in 1961, from an essay called “The Ultimate Human Society”:

This concept of a new life form which I call Macro Life and Isaac Asimov calls ‘multiorganismic life’ serves as a convenient shorthand whereby the whole collection of social, political, and biological problems facing the future space colonist may be represented with two-word symbols. It also communicates quickly an appreciation for the similar problems which are rapidly descending on the whole human race. Macro Life can be defined as ‘life squared per cell.’ Taking man as representative of multicelled life we can say that man is the mean proportional between Macro Life and the cell, or Macro Life is to man as man is to the cell. Macro Life is a new life form of gigantic size which has for its cells individual human beings, plants, animals and machines.”

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Arthur Clarke liked the notion enough to call Zebrowski’s novel ‘a worthy successor to Olaf Stapledon’s Star Maker,’ which had been a major influence on Clarke and most of his contemporaries. As to the notion of moving asteroids about, an early treatment was Robert Heinlein’s ‘Misfit,’ in which an asteroid is moved out of the main belt to an orbit between Mars and the Earth. This one made its appearance in the November 1939 issue of Astounding Science Fiction, and would hardly be the last asteroid-themed tale. A more modern take shows up in Larry Niven’s Known Space stories and the memorable ‘Belters.’

Image: An engineered asteroid from without and within. Illustrator Roy Scarfo worked with Cole on the 1965 book Beyond Tomorrow. Credit: Roy Scarfo.

We’ve come a long way from Made in Space and their plans to move asteroids through ‘seed craft’ and in situ resource utilization, but what I find exciting here is the synergy between some of these ideas from the past and the conceptual studies Made in Space is performing, with help from NASA’s Innovative Advanced Concepts Program. Asteroid mining gives us a route forward as we contemplate infrastructures within the Solar System, building, we can hope, toward a society comfortable working in deep space and continuing to explore.

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We get to the stars one step at a time, or as the ever insightful Lao Tzu put it long ago, ​”You accomplish the great task by a series of small acts.” Right now, of course, many of the necessary ‘acts’ seem anything but small, but as Ian Crawford explains below, they’re a necessary part of building up the kind of space economy that will result in a true infrastructure, one that can sustain the exploration of space at the outskirts of our own system and beyond. Dr. Crawford is Professor of Planetary Science and Astrobiology in the Department of Earth and Planetary Sciences, Birkbeck College, University of London. Today he brings us a report on a discussion of these matters at the Royal Astronomical Society earlier this year.

By Ian A. Crawford

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There is increasing interest in the possibility of using the energy and material resources of the solar system to build a space economy, and in recent years a number of private companies have been established with the stated aim of developing extraterrestrial resources with this aim in mind (see, for example, the websites of Planetary Resources, Deep Space Industries, Shackleton Energy, and Moon Express). Although many aspects of this economic activity will likely be pursued for purely commercial reasons (e.g. space tourism, and the mining of the Moon and asteroids for economically valuable materials), science will nevertheless be a major beneficiary.

The potential scientific benefits of utilising space resources were considered at a Specialist Discussion Meeting organised by the UK’s Royal Astronomical Society on 8 April. This meeting, which was attended by over 60 participants, demonstrated widespread interest in the potential scientific benefits of space resource utilisation. A report of the meeting has now been accepted for publication in the RAS journal Astronomy & Geophysics and videos of the talks are available on the RAS website.

The participants agreed that multiple (and non-mutually exclusive) scientific benefits will result from the development of a space economy, including:

  • Scientific discoveries made during prospecting for, and extraction of, space resources;
  • Using space resources to build, provision and maintain scientific instruments and outposts (i.e. in situ resource utilisation, or ISRU);
  • Leveraging economic wealth generated by commercial space activities to help pay for space science activities (e.g. by taxing profits from asteroid mining, space tourism, etc);
  • Scientific utilisation of the transportation and other infrastructure developed to support commercial space activities.

Specific examples of scientific activities that would be facilitated by the development of a space economy include the construction of large space telescopes to study planets orbiting other stars, ambitious space missions (including human missions) to the outer Solar System, and the establishment of scientific research stations on the Moon and Mars (and perhaps elsewhere).

In the more distant future, and of special interest to readers of Centauri Dreams, an important scientific application of a well-developed space infrastructure may be the construction of interstellar space probes for the exploration of planets around nearby stars. The history of planetary exploration clearly shows that in situ investigations by space probes are required if we are to learn about the interior structures, geological evolution, and possible habitability of the planets in our own solar system, and so it seems clear that spacecraft will eventually be needed for the investigation of other planetary systems as well.

For example, if future astronomical observations from the solar system (perhaps using large space telescopes themselves built and paid for using space resources) find evidence suggesting that life might exist on a planet orbiting a nearby star, in situ measurements will probably be required to get definitive proof of its existence and to learn more about its underlying biochemistry, ecology, and evolutionary history. This in turn will eventually require transporting sophisticated scientific instruments across interstellar space.

However, the scale of such an undertaking should not be underestimated. Although very low-mass laser-pushed nano-craft, such as are being considered by Project Starshot, could conceivably be launched directly from Earth, the scientific capabilities of such small payloads will surely be very limited. Initiatives like Starshot will certainly help to develop useful technology that will enable more capable interstellar missions later on, and are therefore greatly to be welcomed, but ultimately much more massive interstellar payloads will be required if detailed scientific studies of nearby exoplanet systems are to be conducted.

Even allowing for future progress in miniaturisation, a scientifically useful interstellar payload will probably need to have a mass of at least several tonnes, and perhaps much more (as I have discussed in this recent paper in the Journal of the British Interplanetary Society). Moreover, in order to get this to even the nearest stars within a scientifically useful timescale (say ≤100 years) then spacecraft velocities of order 10% of the speed of light will be required. This will likely require vehicles of such a size, with such highly energetic (and thus potentially dangerous) propulsion systems that their construction and launch will surely have to take place in space.

The potential long-term scientific benefits of an interstellar spacefaring capability are hard to exaggerate, but it seems certain that it is a capability that will only become possible in the context of a well-developed space economy with access to the material and energy resources of our own solar system.

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Hot Jupiters: The Missing Water Vapor

by Paul Gilster on June 9, 2016

In late 2015, an international team led by David Sing (University of Exeter, UK) studied ten ‘hot Jupiters’ to try to figure out why some of these planets have less water in their atmospheres than expected from earlier modeling. Sing and company were working with transmission spectroscopy, possible when a planet transits its star and starlight is filtered by the planet’s atmosphere. The team used data from the Hubble instrument as well as the Spitzer Space Telescope, covering wavelengths ranging from the optical into the infrared.

A cloudy planet appears larger in visible light than in infrared, the difference in radius at the two wavelengths being used to show whether the atmosphere is cloudy or clear. The result, published in Nature, concluded that there was a correlation between hazy and cloudy atmospheres and scant detection of water. In other words, clouds were simply hiding the expected water vapor, and dry hot Jupiters were ruled out. It’s an important finding because dry hot Jupiters imply planets that formed in an environment deprived of water.

As interesting as the Sing study was, it’s helpful to have its findings confirmed by new work using data from the Hubble Wide Field Camera 3. Before now, we had information from a dozen different studies using varying methods of analysis, looking at Hubble’s detection of water vapor in the atmospheres of 10 hot Jupiters, while nine others showed no water at all. The new work, led by Aishwarya Iyer, a JPL intern and graduate student, standardized the data by combining the datasets for all 19 hot Jupiters to create an average overall light spectrum.

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Image: Hot Jupiters, exoplanets around the same size as Jupiter that orbit very closely to their stars, often have cloud or haze layers in their atmospheres. This may prevent space telescopes from detecting atmospheric water that lies beneath the clouds, according to a study in the Astrophysical Journal. Credit: NASA/JPL-Caltech.

It turns out that for almost all these planets, haze or clouds were a factor, blocking on average half the water in their atmospheres and preventing our instruments from detecting substantial amounts of water vapor. Says Iyer:

“Clouds or haze seem to be on almost every planet we studied. You have to be careful to take clouds or haze into account, or else you could underestimate the amount of water in an exoplanet’s atmosphere by a factor of two… In some of these planets, you can see water peeking its head up above the clouds or haze, and there could still be more water below.”

What we still don’t know is the composition of these hazes and clouds, leaving much work for upcoming space observatories like the James Webb Space Telescope, scheduled for launch in 2018. Key to understanding hot Jupiters is to learn whether they formed in their current positions or migrated from much further out in their solar systems. The more we learn about the abundance of water on such worlds, the deeper we’ll be able to delve into their origin.

The paper is Iyer et al., “A Characteristic Transmission Spectrum Dominated by H2O Applies to the Majority of HST/WFC3 Exoplanet Observations,” Astrophysical Journal Vol. 823, No. 2 (abstract).

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In Search of Carbon Planets

by Paul Gilster on June 8, 2016

The first generation of stars in the universe began to shine in an era when chemical elements like carbon and oxygen were not available. It was the explosion of these early stars in supernovae that began the process of enrichment, with heavier elements fused in their cores now spreading into the cosmos. Lower-mass stars and planetary systems began to appear as heavier elements could form the needed dust grains to build planetary cores.

Avi Loeb (Harvard-Smithsonian Center for Astrophysics) and grad student Natalie Mashian have been looking at a particular class of ancient stars called carbon-enhanced metal-poor (CEMP) stars. Here the level of iron is about one hundred-thousandth as high as our Sun, a clear marker that these stars formed before heavy elements were widely distributed. These stars are interesting because despite their lack of iron and other heavy elements in comparison to the Sun, they are rich in carbon, an excess that leads to the possibility of planets forming around them out of clumping carbon dust grains.

The new paper on this work looks at the possibility of carbon planet formation, pointing to early work that has simulated such planets, and observational indications of planets with carbon-rich atmospheres (WASP-12b) and carbon-rich interiors (55 Cancri e). If they’re out there, finding such planets — made of graphite, carbides and diamond — around CEMP stars could be a productive exercise. “These stars are fossils from the young universe,” explains Loeb. “By studying them, we can look at how planets, and possibly life in the universe, got started.”

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Image: In this artist’s conception, a carbon planet orbits a sunlike star in the early universe. Young planetary systems lacking heavy chemical elements but relatively rich in carbon could form worlds made of graphite, carbides and diamond rather than Earth-like silicate rocks. Blue patches show where water has pooled on the planet’s surface, forming potential habitats for alien life. Credit: Christine Pulliam (CfA). Sun image: NASA/SDO.

Loeb and Mashian point out that the planetary system with the lowest metallicity we’ve yet detected is around the K-class star BD+20 24 57, which shows levels of metals below what was once considered the critical value for planets to form. While CEMP stars are extremely iron-deficient, their carbon abundances make the formation of solid carbon exoplanets a real possibility. Differentiating them from water or silicate worlds could be difficult, but the paper argues that spectral studies of planetary atmospheres could supply the needed markers:

At high temperatures (T ≳ 1000 K), the absorption spectra of massive (M ∼ 10 – 60 M) carbon planets are expected to be dominated by CO, in contrast with the H2O-dominated spectra of hot massive planets with solar-composition atmospheres (Kuchner & Seager 2005). The atmospheres of low-mass (M ≲ 10 M) carbon planets are also expected to be differentiable from their solar-composition counterparts due to their abundance of CO and CH4, and lack of oxygen-rich gases like CO2, O2, and O3 (Kuchner & Seager 2005).

So carbon monoxide and methane in the atmosphere could help us tell carbon worlds of similar mass and physical size apart from iron and silicate worlds like the Earth. Detecting carbon planets around ancient stars could provide us with a window into planet formation in the early universe, with implications for where life could form. The paper calls for an observational program using transit methods to search for planets around CEMP stars. Says Mashian:

“This work shows that even stars with a tiny fraction of the carbon in our solar system can host planets. We have good reason to believe that alien life will be carbon-based, like life on Earth, so this also bodes well for the possibility of life in the early universe.”

The paper is Mashian and Loeb, “CEMP stars: possible hosts to carbon planets in the early universe,” accepted at Monthly Notices of the Royal Astronomical Society (preprint).

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New Insights into Ceres’ Bright Spots

by Paul Gilster on June 7, 2016

One reason for catching up with recent planetary science here in the Solar System is the upcoming arrival of Juno, which enters into polar orbit around Jupiter on July 4. Juno’s arrival is a reminder that the past year has been packed with interesting news from places like Pluto/Charon (New Horizons), Comet 67P/Churyumov-Gerasimenko (Rosetta), and the topic of today’s post, the intriguing dwarf planet Ceres, as studied by the orbiting Dawn spacecraft.

But the recent Ceres news hasn’t just involved Dawn. Paolo Molaro (INAF-Trieste Astronomical Observatory) had led a study looking at the bright spots Dawn found upon approaching Ceres last year. The data Molaro and team drew on came from the European Southern Observatory’s 3.6-meter instrument at La Silla and its HARPS spectrograph, which have shown us not only the motion of the bright spots as Ceres rotates but also variations that indicate volatile material within them. The suggestion is that this material evaporates when exposed to sunlight.

Ceres’ nine-hour rotation produces a small but measurable Doppler effect, with the bright spots expected to affect the spectrum of the reflected light, producing what shows up as a radial velocity variation within the overall Doppler rotational measurement. But the resulting measurements were more complex than expected, indicating a change in the reflectivity of the bright features in Occator crater. Says co-author Antonino Lanza (INAF-Catania Astrophysical Observatory), a co-author of the study:

“The result was a surprise. We did find the expected changes to the spectrum from the rotation of Ceres, but with considerable other variations from night to night.”

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Image: The bright spots on Ceres as imaged by the Dawn spacecraft at an altitude of approximately 1500 kilometers. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.

The changes in albedo vary from night to night, showing up as random patterns on short and long timescales. If, as has been suggested, the reflective material in the bright spots is fresh water ice or hydrated magnesium sulfates, then these changes could be caused by evaporation, forming highly reflective plumes. Still unknown is the energy source that seems to be driving the leakage of material from deeper below.

The work plays interestingly off other recent observations, as the paper notes:

Perna et al. (2015) found variations in the slope of visible spectra at the level of 2-3 percent over 1000 Angstrom with a variation in the relative reflectivity of about 10 percent in the region between 500 and 800 nm. Herschel detected water vapor plumes erupting off the surface of Ceres, which may come from volcano-like ice geysers (Küppers et al. 2014). The recent Dawn observations suggest that the bright spots could provide some atmosphere in this particular region of Ceres confirming Herschel’s water vapor detection (Witze 2015). It has been noted that the spots appear bright at dawn on Ceres while they seem to fade by dusk. That could mean that sunlight plays an important role, for instance by heating up ice just beneath the surface and causing it to blast off some kind of plume or other feature.

This description would account for plumes quickly losing their reflectivity as sunlight conditions change, only to form again in a cycle of evaporation and freezing that is also reflected in the radial velocity measurements from HARPS. Subsequent analysis of data from Dawn has shown bright localized areas that are consistent with hydrated magnesium sulfates, including a bright pit on the floor of the Occator crater that shows probable sublimation of water ice. Haze clouds are thus produced, appearing and disappearing in a regular daily cycle.

This ‘diffuse haze,’ as the authors describe it, fills the floor of Occator and then disappears almost completely at dusk, providing a possible counterpart to the radial velocity variability that Molaro and team have detected. That’s useful, because if both are produced by the same cause, then we have a way to continue to monitor daily activity on Ceres even after the Dawn mission ends. Further radial velocity observations and analysis of the Dawn imagery could confirm this possibility.

The paper is Molaro et al., “Daily variability of Ceres’ Albedo detected by means of
radial velocities changes of the reflected sunlight,” published online by Monthly Notices of the Royal Astronomical Society 7 February 2016 (abstract / preprint).

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Looking Back: Pluto’s Twilight Landscape

by Paul Gilster on June 6, 2016

Friday’s look at the possible composition of Pluto’s Sputnik Planum took me into a deep enough dive on the two papers — Pluto gets my full attention! — that I ran out of time. I had planned to include the images below in that post, but we can do that this morning as a reminder that New Horizons shows no signs of running out of data. What caught my eye here was the possible presence of a cloud, which you can see at the top right of the left image, and in the top inset image.

The wispy structure is tens of kilometers across (the entire inset measures about 230 kilometers) and if it is a cloud, it’s the only one we’ve yet picked out of the New Horizons imagery. But if you consider the rest of the image, it would make sense that we could see a cloud here — notice how the haze layers are brightened by the sunlight that grazes Pluto’s surface at a low angle. Also in the top right inset, the southern parts of Sputnik Planum’s nitrogen ice fields show up (click the image to enlarge), along with peaks of the Norgay Montes.

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Image: Looking back at Pluto with images like this gives New Horizons scientists information about Pluto’s hazes and surface properties that they can’t get from images taken on approach. The image was obtained by New Horizons’ Ralph/Multispectral Visual Imaging Camera (MVIC) approximately 21,550 kilometers from Pluto, about 19 minutes after New Horizons’ closest approach. The image has a resolution of 430 meters per pixel. Pluto’s diameter is 2,374 kilometers. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

Here we’re at a high phase angle, meaning the Sun is on the other side of the dwarf planet as New Horizons leaves the system. The Planetary Society’s Emily Lakdawalla, who is an expert on spacecraft imagery, explains phase angle in typically clear and concise terms:

“Phase angle” means the angle from the Sun, to the target being observed, to the observer — it’s basically a number applied to the descriptive terms we use for lunar phases like crescent, half, gibbous, and full.”

Which means that a full moon is the equivalent of zero phase, while a half moon is a phase angle of 90 degrees. A high phase angle marks a new moon. Back in 2009, Emily wrote an outstanding post on the appearance of Saturn’s moons at different phase angles, as viewed of course by the Cassini spacecraft. This next bit is also worth quoting:

Cassini has returned well over 200,000 images from the Saturn system, but it hasn’t surveyed every moon at every latitude and longitude at every possible phase angle; we do the best we can with what we have, and always hope to get more data. If you’ve ever wondered what the point is in Cassini getting more and more images of the moons, this is why — Cassini will never fully sample every possible combination of latitude, longitude, and phase; more images will fill in gaps and make a more complete picture of the moon’s photometric behavior, which tells us what its surface is made of and how it varies from place to place.

How much more strongly those words apply to New Horizons, which while giving us a treasure trove of data, was unable in its fast flyby to depict large portions of Pluto’s surface in any detail. I know we’ll get back out there again, but as I mentioned at the end of Friday’s post, I’m impatient. I want to get to Eris, and Haumea, and Makemake, and make the kind of studies that will surely yield still more surprises among the ice worlds. Sputnik Planum is yet one more incentive: Possible bergs of water ice floating amidst a millennially elastic nitrogen sea.

Have another look at the Pluto imagery at the top of the post, where the inset at bottom right brings out further detail on the night side. The high phase angle in this case helps us tease out valleys and peaks that were not nearly as apparent in the days before New Horizons’ closest approach. The inset shows us a scene approximately 750 kilometers wide, a twilight view that only becomes apparent because of the relative positions of spacecraft, dwarf planet and Sun. And below is the view without insets, added here to provide a bit more of the ‘wow’ factor.

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Image: The view without inset imagery. An uninterrupted look at Pluto at high phase angle shows sunlight filtering through the dwarf world’s atmospheric hazes. All of this is a reminder of how much good science we can do after closest approach. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

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Explaining Sputnik Planum

by Paul Gilster on June 3, 2016

It’s been a week spent catching up with space mission news, focusing on Rosetta, Juno and today, New Horizons. Usually I ponder what I’m going to write each day on Centauri Dreams while I’m having breakfast, a quiet time to reflect on recent events. And if Jay Melosh (Purdue University) is to be believed, I might have taken inspiration from the dish of oatmeal sitting in front of me when it comes to Pluto. Because Melosh and grad student Alex Trowbridge led recent research that may explain what we see at Sputnik Planum.

A bit of background before I return to that bowl of oatmeal. We’ve seen that Sputnik Planum has an unusual appearance, visible in the photo below, that shows patterned polygons. One way of explaining this is to invoke icebergs floating on a sea of nitrogen ice. Melosh and Trowbridge believe the polygons could be what are called Rayleigh-Bénard convection cells, which flag convection that occurs in a fluid that is being heated from below. Says Melosh:

“Imagine oatmeal boiling on the stove; it doesn’t produce one bubble for the entire pot as the heated oatmeal rises to the surface and the cooler oatmeal is pushed down into the depths, this happens in small sections across the pot, creating a quilted pattern on the surface similar to what we see on Pluto. Of course, on Pluto this is not a fast process; the overturn within each unit happens at a rate of maybe 2 centimeters per year.”

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Image: Like a cosmic lava lamp, a large section of Pluto’s icy surface is being constantly renewed by a process called convection that replaces older surface ices with fresher material. Scientists from NASA’s New Horizons mission used state-of-the-art computer simulations to show that the surface of Pluto’s informally named Sputnik Planum is covered with churning ice “cells” that are geologically young and turning over due to a process called convection. The scene above, which is about 400 kilometers across, uses data from the New Horizons Ralph/Multispectral Visible Imaging Camera (MVIC), gathered July 14, 2015. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

We’re looking at a surface, then, that is under continual renewal as older surface ice is replaced by new material coming up from below. The nitrogen ice of Sputnik Planum appears in a basin on a planetary surface primarily composed of water ice. Structurally weak, nitrogen ice has a low enough viscosity to flow in an area where water ice, of much higher viscosity, forms hard mountains. Thus we see mountains of water ice at the edges of the polygon regions. The researchers believe these have floated as icebergs within the convection current.

Analysis of the New Horizons data shows that the polygons are between 16 and 48 kilometers across. Melosh and Trowbridge calculate that the nitrogen ice must be at least five kilometers deep for the water icebergs to appear where they do, and analysis of the width-to-depth ratio of the nitrogen convection cells suggests the figure is closer to 10 kilometers, quite a bit deeper than the minimum needed for convection to occur. Over millions of years, the moving nitrogen ‘blobs’ can merge, with ridges marking places where cooler nitrogen ice sinks back down.

Thus we have a way to explain a Sputnik Planum surface that appears to undergo frequent renewal, a surface that investigators have thought to be less than ten million years old. The new work drops that number to about one million years. That’s a reminder that on icy dwarf planets in the Kuiper Belt, materials we normally think of as gases can produce surface change, making them active in complex ways. Considering we’re dealing with a surface that averages -229° C (44 Kelvin), a surface this active emphasizes the need for dedicated missions to the outer Solar System. Only actual data from the mission target can refine or overturn earlier models.

A second paper appearing in the same issue of Nature reinforces the convection explanation, although it may take a probe on the surface to actually confirm it. This work comes from William McKinnon and colleagues (Washington University, St. Louis). Says McKinnon:

“For the first time, we can really determine what these strange welts of the icy surface of Pluto really are. We found evidence that even on a distant cold planet billions of miles from Earth, there is sufficient energy for vigorous geological activity, as long as you have ‘the right stuff,’ meaning something as soft and pliable as solid nitrogen.”

McKinnon’s computer modeling shows solid nitrogen being warmed by internal heat and rising before cooling sufficiently to sink again, beginning the cycle all over again. The work explains X- or Y-shaped features that appear at the junctions where multiple convection cells once met. Pluto’s atmosphere may well be renewed by this activity. Principal New Horizons investigator Alan Stern calls Sputnik Planum, an area bigger than Texas and Oklahoma combined, ‘one of the most amazing geological discoveries in fifty-plus years of planetary exploration.’

Will we find similar processes at work at similar timescales — a few centimeters a year — on other dwarf planets? The next target for New Horizons isn’t a dwarf planet, but 2014 MU69 may still come up with some surprises of its own, assuming NASA approves funding for the 2019 flyby as part of an extended mission. But wouldn’t it be fascinating to one day compare the surfaces of places like Eris and Makemake with what we’ve found at Pluto’s heart?

The papers are Trowbridge et al., “Vigorous convection as the explanation for Pluto’s polygonal terrain,” Nature 534 (2 June 2016), pp. 79-81 (abstract) and McKinnon et al., “Convection in a volatile nitrogen-ice-rich layer drives Pluto’s geological vigour,” Nature 534 (2 June 2016), pp. 82-85 (abstract).

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Radio Map of Jupiter Anticipates Juno Findings

by Paul Gilster on June 2, 2016

Interesting news about Jupiter this morning even as the Juno spacecraft crosses into the realm of Jupiter’s gravity. It was six days ago that Juno made the transition into Jupiter space, where the gravitational influence of Jupiter now dominates over all other celestial bodies. And it will be on July 4 of this year that Juno performs a 35-minute burn of its main engine, imparting a 542 meters per second mean change in velocity to the spacecraft for orbital insertion.

The spacecraft’s 37 flybys will close to within 5000 kilometers of the cloud tops. I only wish Poul Anderson could be alive to see some of the imagery. I always think of him in relation to Jupiter because of his stunning 1957 story “Call Me Joe,” describing the exploration of the planet by remote-controlled life forms (available in Anderson’s collection The Dark Between the Stars as well as various science fiction anthologies).

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Image: Launched in 2011, the Juno spacecraft will arrive at Jupiter in 2016 to study the giant planet from an elliptical, polar orbit. Juno will repeatedly dive between the planet and its intense belts of charged particle radiation, traveling from pole to pole in about an hour, and coming within 5,000 kilometers of the cloud tops at closest approach. Credit: NASA/JPL-Caltech.

Our view of Jupiter has changed a lot since 1957, and Anderson’s low temperature, high pressure surface conditions have been ruled out, but the tale still carries quite a punch. As to Jupiter itself, today we get news that data from the Very Large Array (New Mexico) have been used to create the most detailed radio map ever made of its atmosphere. The work allows researchers to probe about 100 kilometers below the cloud tops using radio emissions at wavelengths where the clouds themselves are transparent.

Recent upgrades to the VLA have improved the array’s sensitivity by a factor of 10, a fact made apparent by the new Jupiter maps. Working the entire frequency range between 4 and 18 gigahertz, the team from UC-Berkeley supplements the Juno mission, anticipating its arrival to create a map that can put the spacecraft’s findings into context. Because the thermal radio emissions are partially absorbed by ammonia, it’s possible to track flows of the gas that define cloud-top features like bands and spots at various depths within the atmosphere.

We’re learning how the interactions between internal heat sources and the atmosphere produce the global circulation and cloud formation we see in Jupiter and other gas giant planets. The three-dimensional view shows ammonium hydrosulfide clouds rising into the upper cloud layers along with ammonia ice clouds in colder regions, while ammonia-poor air sinks into the planet amidst ‘hotspots’ (bright in radio and thermal infrared) that are low in ammonia and circle the planet just north of its equator.

“With radio, we can peer through the clouds and see that those hotspots are interleaved with plumes of ammonia rising from deep in the planet, tracing the vertical undulations of an equatorial wave system,” said UC Berkeley research astronomer Michael Wong.

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Image: The VLA radio map of the region around the Great Red Spot in Jupiter’s atmosphere shows complex upwellings and downwellings of ammonia gas (upper map), that shape the colorful cloud layers seen in the approximately true-color Hubble map (lower map). Two radio wavelengths are shown in blue (2 cm) and gold (3 cm), probing depths of 30-90 kilometers below the clouds. Credit: Radio: Michael H. Wong, Imke de Pater (UC Berkeley), Robert J. Sault (Univ. Melbourne). Optical: NASA, ESA, A.A. Simon (GSFC), M.H. Wong (UC Berkeley), and G.S. Orton (JPL-Caltech).

Fine structure becomes visible in this work, especially in the areas near the Great Red Spot. The resolution is about 1300 kilometers, considered to be the best spatial resolution ever achieved in a radio map. “We now see high ammonia levels like those detected by Galileo from over 100 kilometers deep, where the pressure is about eight times Earth’s atmospheric pressure, all the way up to the cloud condensation levels,” says principal author Imke de Pater (UC-Berkeley). The work is reported in the June 3 issue of Science.

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Image: In this animated gif, optical images of the surface clouds encircling Jupiter’s equator –including the famous Great Red Spot — alternate with new detailed radio images of the deep atmosphere (up to 30 kilometers below the clouds). The radio map shows ammonia-rich gases rising to the surface (dark) intermixed with descending, ammonia-poor gases (bright). In the cold temperatures of the upper atmosphere (160 to 200 Kelvin, or -170 to -100 degrees Fahrenheit), the rising ammonia condenses into clouds, which are invisible in the radio region. Credit: Radio: Robert J. Sault (Univ. Melbourne), Imke de Pater and Michael H. Wong (UC Berkeley). Optical: Marco Vedovato, Christopher Go, Manos Kardasis, Ian Sharp, Imke de Pater.

Earlier VLA measurements of ammonia levels in Jupiter’s atmosphere had shown much less ammonia than what the Galileo probe found when it plunged into the atmosphere in 1995. The new work resolves the issue by applying a technique to remove the blurring in radio maps that occurs because of Jupiter’s fast rotation. The UC-Berkeley team reports that it can clearly distinguish upwelling and downwelling ammonia flows using the new methods, preventing the confusion between the two that had led to the earlier mis-estimates of ammonia levels.

The paper is de Pater et al., “Peering through Jupiter’s Clouds with Radio Spectral Imaging,” Science 3 June 2016 (abstract).

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Cometary Breakup and Reassembly

by Paul Gilster on June 1, 2016

Yesterday’s look at organic compounds on Comet 67P/Churyumov-Gerasimenko needs to be augmented today by a just released study of the comet with implications for how all comets evolve. But first, a renewed pointer to the Kickstarter campaign for KIC 8462852, the unusual star whose light curves continue to baffle astronomers. Please consider contributing to the project, which would raise enough money ($100,000) to support a year of observations.

We’re about halfway through the campaign but not yet at the halfway point in funds. Have a look at the information provided on the Kickstarter page, or in my essay A Kickstarter Campaign for KIC 8462852, which also has the relevant links. We know the light curves of ‘Tabby’s Star’ are not periodic, so we need continuous monitoring to gain more data on what may be happening there. If we can raise the funds, the Las Cumbres Observatory Global Telescope Network, already supporting the project, can give us the multi-wavelength observations we need.

A Comet’s Evolution

The rubber-duck shape of Comet 67P/Churyumov-Gerasimenko has long been noted. The ‘neck’ of the comet is what connects the two larger lobes, as is obvious in the image below. As a new study led by Masatoshi Hirabayashi (Purdue) and Daniel Scheeres (University of Colorado) points out, two large cracks appear on the neck connecting the two larger lobes. The team simulated rotation rates for the twin-lobed assembly different from its actual 12-hour spin.

The result: Two cracks similar enough to those on 67P to show just how much stress is imparted. The rotation rate is variable in an object like this one because flybys of the Sun or of Jupiter can produce a gravitational torque. And as also appears in the photo, cometary outgassing is a factor, with compounds like carbon dioxide and ammonia sublimating from the surface. A fast enough spin produced by these factors can cause the two lobes to separate. Seven hours per rotation is what it takes for the head of the ‘duck’ to break off.

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Image: Comet 67P’s distinctive shape tells us much about its history. Credit: ESA/Rosetta/NAVCAM, CC BY-SA IGO 3.0.

The researchers used numerical models that examined 1000 instances of 67P ‘clones’ under varying conditions over a 5000 year period. What Hirabayashi and Scheeres have learned is that the breakup and reassembly is an ongoing process as comets respond to these stresses. It’s also one that could last the lifetime of the comet. Says Scheeres:

“The head and body aren’t going to be able to escape from each other. They will begin orbiting each other, and in weeks, days or even hours they will come together again during a slow collision, creating a new comet nucleus configuration.”

As strange as it looks, Comet 67P may not be all that unusual. So far we have imaged seven comets at high resolution, five of which are bi-lobed. The researchers have learned that all of the bi-lobed comets have similar volume ratios between each lobe, an indication that the same cycle of disassembly and reassembly is happening in them as well. In some, there are similarities to what we find in a certain kind of asteroid. From the paper:

…bilobate nuclei observed by spacecraft encounters or ground-based radar have component volume ratios consistent with their nuclei being trapped in a similar cycle to that of 67P’s nucleus. For bilobate nuclei with a volume ratio between their lobes larger than about 0.2, the total energy of these systems will be negative after fission. This means that they are bounded in a similar way to some rubble pile asteroids; however additional sublimation effects could further erode or spin up the individual lobes before re-impact.

The process may be a major factor in cometary evolution, giving us insights into how these objects change over time:

Taking material density to be constant, we computed the volume ratios of the imaged bilobate nuclei of comets 1P/Halley, 8P/Tuttle, 19P/Borrelly, 67P and 100P/Hartley 2; we found that all of these nuclei had a volume ratio higher than 0.2… Observed nuclei with a single component might either be primordial, or have been part of a multi-component object, from which smaller parts are more easily shed.

Window into the Late Heavy Bombardment?

67P/Churyumov-Gerasimenko is a Jupiter-family comet orbiting the Sun every 6.5 years; such periodic comets are thought to originate in the Kuiper Belt, far beyond Neptune’s orbit. We learn that chaotic spin rate changes and the subsequent breaking into parts and reassembling probably caused the breakup of many ancient periodic comets originating at similar distances from the Sun. Enough erosion would have been produced by the continuing reconfiguration of their nuclei to reduce their ability to survive migration into the inner Solar System.

This could explain why comets were not a strong factor in the late heavy bombardment some four billion years ago, when numerous asteroids collided with the early terrestrial planets — two recent papers have made this case. “The reconfiguration cycles of short-period cometary nuclei,” the paper adds, “constitute a new evolutionary process that could affect their ability to survive during migration into the inner solar system.”

ESA’s mission to Comet 67P may, in other words, be giving us insights into the primordial bombardment that reshaped terrestrial worlds. The paper is Scheeres et al., “Fission and reconfiguration of bilobate comets as revealed by 67P/Churyumov–Gerasimenko,” Nature 1 June 2016 (abstract).

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Rosetta’s Comet: Ingredients for Life

by Paul Gilster on May 31, 2016

The thought that water and organic molecules might have arrived on the early Earth from the impacts of comets and asteroids has long been provocative, and our missions to nearby comets are now paying off with insights into the possibility. It was back in 2004 that the Stardust mission flew past Comet Wild 2, collecting dust samples that showed traces of the amino acid glycine. Possible contamination of the samples during their analysis left the question open, however.

Now we have news that the European Space Agency’s Rosetta mission has also found glycine — a significant organic compound that appears in proteins — at Comet 67P/Churyumov-Gerasimenko. The spacecraft’s ROSINA instrument (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) detected glycine in October of 2014, with later measurements taken during the August 2015 perihelion event, where cometary outgassing was at its peak.

Kathrin Altwegg (University of Bern), who led the study, calls this “…the first unambiguous detection of glycine in the thin atmosphere of a comet.” From the paper:

ROSINA’s double focusing mass spectrometer (DFMS) ionizes the incoming volatiles by electron impact ionization and detects the corresponding positively charged fragments. Unlike for meteorites or Stardust grains, there is no chemical sample preparation involved. Furthermore, the absence of a terrestrial source of glycine from the spacecraft is verified from observations before arrival at the comet. Therefore, glycine detected by DFMS has to be in this form already in the coma of the comet and is clearly not the result of contamination.

Although little glycine is released from the cometary surface, the detection of precursors to glycine formation, the organic molecules methylamine and ethylamine, is also significant. Glycine is the only amino acid known to be able to form without liquid water. Says Altwegg:

“We see a strong correlation of glycine to dust, suggesting that it is probably released from the grains’ icy mantles once they have warmed up in the coma, perhaps together with other volatiles. The simultaneous presence of methylamine and ethylamine, and the correlation between dust and glycine, also hints at how the glycine was formed.”

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Image (click to enlarge): An ESA infographic detailing the recent work on organic compounds found at Comet 67P/Churyumov-Gerasimenko.

We also get word, in the paper published by Science Advances, that Rosetta found phosphorus at Comet 67P/C-G. The researchers argue that the number of organic molecules ROSINA has detected at the comet, now including glycine and phosphorus, make a strong case for comets as a delivery mechanism for prebiotic chemistry. This marks the first time phosphorus, an ingredient in the framework of DNA and RNA, has been found at a comet.

The presence of glycine, phosphorus, and a multitude of organic molecules, including hydrogen sulfide (H2S) and hydrogen cyanide (HCN), seen in the coma of 67P/Churyumov-Gerasimenko supports the idea that comets delivered key molecules for prebiotic chemistry throughout the solar system and, in particular, to the early Earth, drastically increasing the concentration of life-related chemicals by impact on a closed water body. The simultaneous presence of methylamine and the correlation between dust and glycine also suggest that the pathways for glycine formation on dust grain ices, as described for the ISM or the protosolar nebula, could also account for the cometary glycine.

The paper is Altwegg et al., “Prebiotic chemicals – amino acid and phosphorus – in the coma of comet 67P/Churyumov-Gerasimenko,” Science Advances, 27 May 2016 (abstract / full text).

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