Jupiter in the Public Eye

by Paul Gilster on January 23, 2017

Have a look at Jupiter as seen by the Juno spacecraft on its third close pass. A view as complex as the one below reminds us how images can be manipulated to bring out detail. This happens so frequently in astronomical images that it’s easy to forget this view is not necessarily what the human eye would see, and we always have to check to find out how a given image was processed. In this case, we’re looking at the work of a ‘citizen scientist,’ one Eric Jorgensen, who enhanced a JunoCam image to highlight the cloud movement.

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Image: This amateur-processed image was taken on Dec. 11, 2016, at 1227 EST (1727 UTC), as NASA’s Juno spacecraft performed its third close flyby of Jupiter. At the time the image was taken, the spacecraft was about 24,400 kilometers from the gas giant planet. Credit: NASA/JPL-Caltech/SwRI/MSSS/Eric Jorgensen.

The image shows a region of Jupiter southeast of what is known as the ‘pearl,’ one of eight rotating storms at 40 degrees south latitude on the planet, a region of vast and roiling turbulence. Citizen science efforts like Planet Hunters, SETI@Home and Galaxy Zoo have brought private individuals into contact with scientific data and fostered interest in a wide range of sciences, with Planet Hunters rising to particular visibility thanks to its work with Boyajian’s Star and the still mysterious light curves observed there.

The Juno mission is delving into this realm with the announcement that on the spacecraft’s February 2 pass of Jupiter, the public will have had a voice in the selection of targets for the imaging team. As JPL notes in this news release, JunoCam will begin taking pictures as Juno approaches Jupiter’s north pole. Scientists have to keep an eye on onboard storage limitations as they consider which images to collect with JunoCam. Each close pass (‘perijove’) happens in a 2-hour window as the spacecraft goes from the north pole of the giant planet to the south pole, with JunoCam imaging a circumscribed strip of territory.

The voting for the February 2 flyby is still open, but the process repeats: Each orbit will have a voting page, and each perijove on Juno’s 53-day orbit will have space for two polar images within which the public can participate in prioritizing particular points of interest, in accordance with the science goals the mission is trying to meet. Several pages at the voting site will be devoted to unique points of interest that will be within range of JunoCam’s field of view during the next close approach. Raw images will then be made available for processing.

“The pictures JunoCam can take depict a narrow swath of territory the spacecraft flies over, so the points of interest imaged can provide a great amount of detail,” said Juno co-investigator Candy Hansen, (Planetary Science Institute). “They play a vital role in helping the Juno science team establish what is going on in Jupiter’s atmosphere at any moment. We are looking forward to seeing what people from outside the science team think is important.”

Bear in mind that JunoCam was included on the mission because, working in color and visible light, it could offer a wide field of view that would, among other things, spur public interest and involvement. So it’s not surprising to see this citizen science angle being brought forward, offering engagement not just from amateur scientists but students worldwide. Building public support is also a key component in keeping up the pressure for better space funding.

The February 2 flyby makes its closest approach to Jupiter at 0758 EST (1258 UTC), with the spacecraft about 4300 kilometers above the cloud tops. We’ll see Jupiter up close once again through a spacecraft’s lens, translated for us into images that mimic what we would see with our own eyes before we get to work processing them. If you’re interested in having a say on future JunoCam targets, click here for information on how to get involved.

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Image: Jupiter’s south pole as seen during perijove 3, in an image processed by Julien Potier (Planetario Silvia Torres Castilleja, Ags, Mexico), rotated, cropped to get rid of yellowish band, processed with RGB levels, brightness, contrast and HDR Toning.

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A Vision to Bootstrap the Solar System Economy

by Paul Gilster on January 20, 2017

Early probes are one thing, but can we build a continuing presence among the stars, human or robotic? An evolutionary treatment of starflight sees it growing from a steadily expanding presence right here in our Solar System, the kind of infrastructure Alex Tolley examines in the essay below. How we get to a system-wide infrastructure is the challenge, one analyzed by a paper that sees artificial intelligence and 3D printing as key drivers leading to a rapidly expanding space economy. The subject is a natural for Tolley, who is co-author (with Brian McConnell) of A Design for a Reusable Water-Based Spacecraft Known as the Spacecoach (Springer, 2016). An ingenious solution to cheap transportation among the planets, the Spacecoach could readily be part of the equation as we bring assets available off-planet into our economy and deploy them for even deeper explorations. Alex is a lecturer in biology at the University of California, and has been a Centauri Dreams regular for as long as I can remember, one whose insights are often a touchstone for my own thinking.

by Alex Tolley

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Crewed starflight is going to be expensive, really expensive. All the various proposed methods from slow world ships to faster fusion vessels require huge resources to build and fuel. Even at Apollo levels of funding in the 1960’s, an economy growing at a fast clip of 3% per year is estimated to need about half a millennium of sustained growth to afford the first flights to the stars. It is unlikely that planet Earth can sustain such a sizable economy that is millions of times larger than today’s. The energy use alone would be impossible to manage. The implication is that such a large economy will likely be solar system wide, exploiting the material and energy resources of the system with extensive industrialization.

Economies grow by both productivity improvements and population increases. We are fairly confident that Earth is likely nearing its carrying capacity and certainly cannot increase its population even 10-fold. This implies that such a solar system wide economy will need huge human populations living in space. The vision has been illustrated by countless SciFi stories and perhaps popularized by Gerry O’Neill who suggested that space colonies were the natural home of a space faring species. John Lewis showed that the solar system has immense resources to exploit that could sustain human populations in the trillions.

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Image credit: John Frassanito & Associates

But now we run into a problem. Even with the most optimistic estimates of reduced launch costs, and assuming people want to go and live off planet probably permanently, the difficulties and resources needed to develop this economy will make the US colonization by Europeans seem like a walk in the park by comparison. No doubt it can be done, but our industrial civilization is little more than a quarter of a millennium old. Can we sustain the sort of growth we have had on Earth for another 500 years, especially when it means leaving behind our home world to achieve it? Does this mean that our hopes of vastly larger economies, richer lives for our descendents and an interstellar future for humans is just a pipe dream, or at best a slow grind that might get us there if we are lucky?

Well, there may be another path to that future. Philip Metzger and colleagues have suggested that such a large economy can be developed. More extraordinary, that such an economy can be built quickly and without huge Earth spending, starting and quickly ending with very modest space launched resources. Their suggestion is that the technologies of AI and 3D printing will drive a robotic economy that will bootstrap itself quickly to industrialize the solar system. Quickly means that in a few decades, the total mass of space industrial assets will be in the millions of tonnes and expanding at rates far in excess of our Earth-based economies.

The authors ask, can we solve the launch cost problem by using mostly self-replicating machines instead? This should remind you of the von Neumann replicating probe concept. Their idea is to launch seed factories of almost self-replicating robots to the Moon. The initial payload is a mere 8 tonnes. The robots will not need to be fully autonomous at this stage as they can be teleoperated from Earth due to the short 2.5 second communication delay. They are not fully self-replicating at this stage as need for microelectronics is best met with shipments from Earth. Almost complete self-replication has already been demonstrated with fabs, and 3D printing promises to extend the power of this approach.

The authors assume that initial replication will neither be fully complete, nor high fidelity. They foresee the need for Earth to ship the microelectronics to the Moon as the task of building fabs is too difficult. In addition, the materials for new robots will be much cruder than the technology earth can currently deliver, so that the next few generations of robots and machinery will be of poorer technology than the initial generation. However the quality of replication will improve with each generation and by generation 4, a mere 8 years after starting, the robot technology will be at the initial level of quality, and the industrial base on the Moon should be large enough to support microelectronics fabs. From then on, replication closure is complete and Earth need ship no further resources to the Moon.

GenHuman/Robotic InteractionArtificial IntelligenceScale of IndustryMaterials ManufacturedSource of Electronics
1.0Teleoperated and/or locally operated by a human outpostInsect-likeImported, small-scale, limited diversityGases, water, crude alloys, ceramics, solar cellsImport fully integrated machines
2.0TeleoperatedLizard-likeCrude fabrication, inefficient, but greater throughput than 1.0(Same)Import electronics boxes
2.5TeleoperatedLizard-likeDiversifying processes, especially volatiles and metalsPlastics, rubbers, some chemicalsFabricate crude components plus import electronics boxes
3.0Teleoperated with experiments in autonomyLizard-likeLarger, more complex processing plantsDiversify chemicals, simple fabrics, eventually polymersLocally build PC cards, chassis and simple components, but import the chips
4.0Closely supervised autonomyMouse-likeLarge plants for chemicals, fabrics, metalsSandwiched and other advanced material processesBuilding larger assets such as lithography machines
5.0Loosely supervised autonomyMouse-likeLabs and factories for electronics and robotics. Shipyards to support main belt.Large scale productionMake chips locally. Make bots in situ for export to asteroid belt.
6.0Nearly full autonomyMonkey-likeLarge-scale, self-supporting industry, exporting industry to asteroid main beltMakes all necessary materials, increasing sophisticationMakes everything locally, increasing sophistication
X.0Autonomous robotics pervasive throughout Solar System enabling human presenceHuman-likeRobust exports/imports through zones of solar systemMaterial factories specialized by zone of the Solar SystemElectronics factories in various locations

Table 1. The development path for robotic space industrialization. The type of robots and the products created are shown. Each generation takes about 2 years to complete. Within a decade, chip fabrication is initiated. By generation 6, full autonomy is achieved.

AssetQty. per setMass minus Electronics (kg)Mass of Electronics (kg)Power (kW)Feedstock Input (kg'hr)Product Output (kg/hr)
Power Distrib & Backup12000-----------------
Excavators (swarming)570190.3020----
Chem Plant 1 - Gases1733305.5841.8
Chem Plant 2 - Solids1733305.58101.0
Metals Refinery110191910.00203.15
Solar Cell Manufacturer1169190.500.3----
3D Printer 1 - Small Parts4169195.000.50.5
3D Printer 2 - Large Parts4300195.000.50.5
Robonaut assemblers3135150.40--------
Total per Set~7.7 MT
launched to Moon
64.36 kW20 kg
regolith/hr
4 kg
parts/hr

Table 2. The products and resources needed to bootstrap the industrialization of the Moon with robots. Note the low mass needed to start, a capability already achievable with existing technology. For context, the Apollo Lunar Module had a gross mass of over 15 tonnes on landing.

The authors test their basic model with a number of assumptions. However the conclusions seem robust. Assets double every year, more than an order of magnitude faster than Earth economic growth.

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Figure 13 of the Metzger paper shows that within 6 generations, about 12 years, the industrial base off planet could potentially be pushing towards 100K MT.

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Figure 14 of the paper shows that with various scenarios for robots, the needed launch masses from Earth every 2 years is far less than 100 tonnes and possibly below 10 tonnes. This is quite low and well within the launch capabilities of either government or private industry.

Once robots become sophisticated enough, with sufficient AI and full self-replication, they can leave the Moon and start industrializing the asteroid belt. This could happen a decade after initiation of the project.

With the huge resources that we know to exist, robot industrialization would rapidly, within decades not centuries, create more manufactures by many orders of magnitude than Earth has. Putting this growth in context, after just 50 years of such growth, the assets in space would require 1% of the mass of the asteroid belt, with complete use within the following decade. Most importantly, those manufactures, outside of Earth’s gravity well, require no further costly launches to transmute into useful products in space. O’Neill colonies popped out like automobiles? Trivial. The authors suggest that one piece could be the manufacture of solar power satellites able to supply Earth with cheap, non-polluting power, in quantities suitable for environmental remediation and achieving a high standard of living for Earth’s population.

With such growth, seed factories travel to the stars and continue their operation there, just as von Neumann would predict with his self-replicating probes. Following behind will be humans in starships, with habitats already prepared by their robot emissaries. All this within a century, possibly within the lifetime of a Centauri Dreams reader today.

Is it viable? The authors believe the technology is available today. The use of telerobotics staves off autonomous robots for a decade. In the 4 years since the article was written, AI research has shown remarkable capabilities that might well increase the viability of this aspect of the project. It will certainly need to be ready once the robots leave the Moon to start extracting resources in the asteroid belt and beyond.

The vision of machines doing the work is probably comfortable. It is the fast exponential growth that is perhaps new. From a small factory launched from Earth, we end up with robots exploiting resources that dwarf the current human economy within a lifetime of the reader.

The logic of the model implies something the authors do not explore. Large human populations in space to use the industrial output of the robots in situ will need to be launched from Earth initially. This will remain expensive unless we are envisaging the birthing of humans in space, much as conceived for some approaches to colonizing the stars. Alternatively an emigrant population will need to be highly reproductive to fill the cities the robots have built. How long will that take? Probably far longer, centuries, rather than the decades of robotic expansion.

Another issue is that the authors envisage the robots migrating to the stars and continuing their industrialization there. Will humans have the technology to follow, and if so, will they continue to fall behind the rate at which robots expand? Will the local star systems be full of machines, industriously creating manufactures with only themselves to use them? And what of the development of AI towards AGI, or Artificial General Intelligence? Will that mean that our robots become the inevitable dominant form of agency in the galaxy?

The paper is Metzger, Muscatello, Mueller & Mantovani, “Affordable, Rapid Bootstrapping of the Space Industry and Solar System Civilization,” Journal of Aerospace Engineering Volume 26 Issue 1 (January 2013). Abstract / Preprint.

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A Possible Planet Hidden in the Data

by Paul Gilster on January 19, 2017

One of the great joys of science is taking something that seems beyond reach and figuring out a way to do it. We can use a coronagraph, for example, to screen out much of the light of a star to see planets around it, but coronagraphs can only do so much, as planets too near the star are still hidden from view. Now scientists have used an unusual observation to deduce information about one such hidden planet and its interactions with a circumstellar disk.

Announced at the recent meeting of the American Astronomical Society, the work involves 18 years of archival observations with the Hubble Space Telescope, which have yielded an intriguing shadow sweeping across the disk of the TW Hydrae system. We’re evidently looking at a young planetary system in formation, as the star — slightly less massive than the Sun and about 192 light years away in the constellation Hydra — is only about 8 million years old. Helpfully for our work, the TW Hydrae disk is seen face-on from our perspective.

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Image: These images, taken a year apart by NASA’s Hubble Space Telescope, reveal a shadow moving counterclockwise around a gas-and-dust disk encircling the young star TW Hydrae. The two images at the top, taken by the Space Telescope Imaging Spectrograph, show an uneven brightness across the disk. Through enhanced image processing (images at bottom), the darkening becomes even more apparent. These enhanced images allowed astronomers to determine the reason for the changes in brightness. The dimmer areas of the disk, at top left, are caused by a shadow spreading across the outer disk. The dotted lines approximate the shadow’s coverage. The long arrows show how far the shadow has moved in a year (from 2015-2016), which is roughly 20 degrees. Credit: NASA, ESA, and J. Debes (STScI).

We have no other circumstellar disk with an archival dataset this rich, allowing the effect to be studied in depth. John Debes (Space Telescope Science Institute, Baltimore) was able to put observations from Hubble’s Space Telescope Imaging Spectrograph (STIS) together with images from different observing runs, some of which included the Hubble Near Infrared Camera and Multi-Object Spectrometer (NICMOS).

The STIS observations use the instrument’s coronagraph to look close to the star, but the coronagraph still can’t reveal an image of the planet itself. The shadow, however, is another matter. It has swept around the disk counter-clockwise until returning in 2016 to the same position it was in in 2000. The disk’s slow rotation ruled out a feature that was itself a part of the disk, implicating a shadow caused by a tilt to the inner disk relative to the outer. Observations from the Atacama Large Millimeter Array (ALMA) at submillimeter wavelengths backed the idea.

Debes believes a hitherto unseen planet is the most likely cause of the twisted inner disk, pulling material out of the disk plane to block light from the star, thus producing the shadow sweeping across the outer disk. If this is the case, we are talking about a planet roughly 160 million kilometers from the star, too close to observe even with STIS, which can penetrate as close to the star as the orbit of Saturn (roughly 1.5 billion kilometers).

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Image: This diagram reveals the proposed structure of a gas-and-dust disk surrounding the nearby, young star TW Hydrae. The illustration shows an inner disk that is tilted due to the gravitational influence of an unseen companion, which is orbiting just outside the disk. Credit: NASA, ESA, and A. Feild (STScI).

To produce the effects Hubble has noted, the planet would need to be about the size of Jupiter. Recent work on TW Hydrae with ALMA has confirmed a gap in the disk about 14.5 million kilometers from the star, perhaps the signature of another planet clearing the inner disk. Thus we are working with a phenomenon that allows us to study early planet formation in an inner system that is otherwise inaccessible, simply by measuring these broad effects.

“What is surprising is that we can learn something about an unseen part of the disk by studying the disk’s outer region and by measuring the motion, location, and behavior of a shadow,” Debes said. “This study shows us that even these large disks, whose inner regions are unobservable, are still dynamic, or changing in detectable ways which we didn’t imagine.”

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A New Context for Complex Life

by Paul Gilster on January 18, 2017

We normally think of the appearance of oxygen on Earth in terms of a ‘great oxygenation event,’ sometimes referred to as the ‘oxygen catastrophe’ or ‘great oxidation.’ Here oxygen begins to emerge in the atmosphere about 2.3 billion years ago as oceanic cyanobacteria produce oxygen by photosynthesis. The actual oxygenation event would be the point when oxygen is not all chemically captured but becomes free to escape into the atmosphere.

It’s a straightforward picture — we move from a lack of oxygen to gradual production through photosynthesis and then a concentration strong enough to destroy many anaerobic organisms, an early and huge extinction event as life on our planet adjusted to the new balance. But a team of researchers led by Michael Kipp (University of Washington) has produced a paper showing a much more complicated emergence of oxygen, one that produced a surge in oxygenation that lasted a quarter of a billion years before easing.

Kipp and team studied oxygen in the Earth’s atmosphere between 2 and 2.4 billion years ago. Their work focuses on the element selenium and its isotopic ratios in sedimentary shale, using mass spectrometry techniques at the University of Washington Isotope Geochemistry Lab. The question: How have isotopic ratios been changed by the presence of oxygen? The reduction of oxidized selenium compounds causes a shift in these ratios which can be measured, and the abundance of selenium itself increases as oxygen levels climb.

What the team found is that oxygen levels were higher far earlier than we’ve believed. Indeed, these levels may have supported complex life, at least for a time. For instead of a gradual and continuing rise, these levels then drop. Roger Buick (UW Astrobiology Program) explains:

“There is fossil evidence of complex cells that go back maybe 1 ¾ billion years. But the oldest fossil is not necessarily the oldest one that ever lived – because the chances of getting preserved as a fossil are pretty low. This research shows that there was enough oxygen in the environment to have allowed complex cells to have evolved, and to have become ecologically important, before there was fossil evidence. That doesn’t mean that they did — but they could have.”

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Image: This is a 1.9-billion-year-old stromatolite — or mound made by microbes that lived in shallow water — called the Gunflint Formation in northern Minnesota. The environment of the oxygen “overshoot” described in research by Michael Kipp, Eva Stüeken and Roger Buick may have included this sort of oxygen-rich setting that is suitable for complex life. Credit: Eva Stüeken.

Thus shallow coastal waters may have held the oxygen needed for complex life hundreds of millions of years earlier than thought. The researchers call this event an ‘oxygen overshoot,’ a significant increase in atmospheric oxygen and in the surface ocean, but one that did not affect the deep ocean. Oxygen levels would have risen for a quarter of a billion years before sinking back. That makes the so-called ‘great oxygenation event’ a more complex process than we realized, with a sharp peak in oxygen before a drop to a lower, more stable level.

About this complex process there remain plenty of questions. What caused the elevation of oxygen levels in the first place, and what precipitated its decline? The researchers have no answer, but can only point to a selenium isotope record that clearly sets this period apart. The selenium technique is a potent way to analyze our own planet’s past, but it also reminds us of the need to be cautious in evaluating exoplanet habitability. Says lead author Kipp:

“The recognition of an interval in Earth’s distant past that may have had near-modern oxygen levels, but far different biological inhabitants, could mean that the remote detection of an oxygen-rich world is not necessarily proof of a complex biosphere.”

The paper is Kipp et al., “Selenium isotopes record extensive marine suboxia during the Great Oxidation Event,” published online by Proceedings of the National Academy of Sciences 18 January 2017 (abstract). This UW news release is also helpful.

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Galactic Interaction: Rivers of Stars

by Paul Gilster on January 16, 2017

Discovered as recently as 1994, the Sagittarius dwarf spheroidal galaxy is a satellite of the Milky Way, and one with an interesting history. One of the nearest of the dwarf galaxies, the Sagittarius dwarf lies 25 kiloparsecs (roughly 82,000 light years) from the center of the Milky Way, and has passed through the disk of the parent galaxy more than once. The result: We see what a new paper on this object calls a ‘stream of tidally stripped stars’ that wraps completely around the celestial sphere. Our own Sun, in fact, is close enough to the Sgr galaxy’s orbital plane that it lies within the width of what can be called the debris tail.

What astronomers would like to do is to reconstruct the orbital history of this interesting dwarf galaxy, something Marion Dierickx (Harvard-Smithsonian Center for Astrophysics), working with her PhD advisor Avi Loeb (Harvard) have now managed through computer simulations. Dierickx and Loeb simulated the movements of the Sgr dwarf for the past 8 billion years, varying factors like the initial velocity and angle of approach to the Milky Way. As expected, these variables have a major effect on the orbit and the resulting stellar stream. Each passage around the Milky Way has pulled the Sgr dwarf apart, costing it component stars.

Matching their models with current observations, the researchers have learned that over time, the dwarf galaxy, which begins the simulations with 10 billion times the mass of the Sun (about one percent of the mass of the Milky Way), loses a third of its stars and nine-tenths of its dark matter. The stripping of this material from the Sgr dwarf produced three streams of stars reaching as far as one million light years from the center of the Milky Way. These streams have become one of the largest structures observable on the sky.

From the paper:

The simulation presented here includes stellar overdensities at distances of up to ∼ 250 − 300 kpc, extending beyond the MW virial radius [the radius within which the density perturbation that will become a galaxy is collapsing]. We provide their predicted positions on the sky, heliocentric distances and line of sight velocities for possible future observational searches. The most distant known stars of the MW coincide with our predicted streams in both position and small radial velocities. If verified observationally, the distant branches of the Sgr stream would be the farthest-ranging stellar stream in the MW halo known to date.

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Image: In this computer-generated image, a red oval marks the disk of our Milky Way galaxy and a red dot shows the location of the Sagittarius dwarf galaxy. The yellow circles represent stars that have been ripped from the Sagittarius dwarf and flung far across space. Five of the 11 farthest known stars in our galaxy were probably stolen this way. Marion Dierickx / CfA.

The cosmological landscape that can emerge from work like this is striking. As the paper notes, we should be able to use the Panoramic Survey Telescope & Rapid Response System (Pan-STARRS) datasets to drill deeper into the location of these streams, and future data from the Large Synoptic Survey Telescope (LSST) will offer mapping of the outer galactic halo at visible wavelengths. The Wide Field Infrared Survey Telescope (WFIRST) will produce useful new data in the infrared. With all of these tools, we can probe the outer edges of the Milky Way, in conjunction with existing missions like Gaia. As the paper notes:

The Gaia mission will accurately map a large volume of the MW at smaller Galactocentric radii. With a complete picture of the MW mass distribution from the solar neighborhood to the outskirts of the halo, we will be able to place our Galaxy and the Local Group in a cosmological context.

Note that the 11 farthest known stars in the Milky Way are about 300,000 light years out, well beyond the galaxy’s spiral disk. About half of these are evidently stars captured from the Sagittarius dwarf galaxy, while the rest may have been pulled from a different dwarf galaxy. Says Dierickx, “The star streams that have been mapped so far are like creeks compared to the giant river of stars we predict will be observed eventually.”

The paper is Dierickx & Loeb, “Predicted Extension of the Sagittarius Stream to the Milky Way Virial Radius,” accepted at the Astrophysical Journal (preprint).

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Inconstant Moons: A New Lunar Origin Scenario

by Paul Gilster on January 13, 2017

A recent snowfall followed by warming temperatures produced a foggy night recently, one in which I was out for my usual walk and noticed a beautiful Moon trying to break through the fog layers. The scene was silvery, almost surreal, the kind of thing my wife would write a poem about. For my part, I was thinking about the effect of the Moon on life, and the theory that a large single moon might have an effect on our planet’s habitability. Perhaps its presence helps to keep Earth’s obliquity within tolerable grounds, allowing for a more stable climate.

But that assumes we’ve had a single moon all along, or at least since the ‘big whack’ the Earth sustained from a Mars-sized protoplanet that may have caused the Moon’s formation. Is it possible the Earth has had more than one moon in its past? It’s an intriguing question, as witness a new paper in Nature Geoscience from researchers at the Technion-Israel Institute of Technology and the Weizmann Institute of Science. The paper suggests the Moon we see today is the last of a series of moons that once orbited the Earth.

“Our model suggests that the ancient Earth once hosted a series of moons, each one formed from a different collision with the proto-Earth,” says co-author Assistant Prof. Perets (Technion). “It’s likely that such moonlets were later ejected, or collided with the Earth or with each other to form bigger moons.”

To explore alternatives to giant impact theories, the researchers have produced simulations of early Earth impacts, varying the values for the impactor’s velocity, mass, angle of impact and the initial rotation of the target. The process that emerges involves multiple impacts that would produce small moons, whose gravitational interactions would eventually cause collisions and mergers, to produce the Moon we see today. Here’s how the paper describes the process:

… we consider a multi-impact hypothesis for the Moon’s formation. In this scenario, the proto-Earth experiences a sequence of collisions by medium- to large-size bodies (0.01–0.1M). Small satellites form from the impact-generated disks and migrate outward controlled by tidal interactions, faster at first, and slower as the body retreats away from the proto-Earth. The slowing migration causes the satellites to enter their mutual Hill radii and eventually coalesce to form the final Moon. In this fashion, the Moon forms as a consequence of a variety of multiple impacts in contrast to a more precisely tuned single impact.

Here’s a graphic from the paper (listed as Figure 1) that shows the process at work:

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Image (click to enlarge): a,b, Moon- to Mars-sized bodies impact the proto-Earth (a) forming a debris disk (b). c, Due to tidal interaction, accreted moonlets migrate outward. d,e, Moonlets reach distant orbits before the next collision (d) and the subsequent debris disk generation (e). As the moonlet–proto-Earth distance grows, the tidal acceleration slows and moonlets enter their mutual Hill radii. f, The moonlet interactions can eventually lead to moonlet loss or merger. The timescale between these stages is estimated from previous works.

The Hill radius mentioned above describes the gravitational sphere of influence of an object; in this case, meshing Hill radii can produce interactions that sometimes lead to mergers. The paper notes that in head-on impacts, the rotation of the planet is important because the disk needs angular momentum resulting from the rotation to stay stable. With increased rates of rotation, the angular momentum of the disks increases. Moons like ours emerge from many of the simulations:

We find that debris disks resulting from medium- to large-size impactors (0.01–0.1M) have sufficient angular momentum and mass to accrete a sub-lunar-size moonlet. We performed 1,000 Monte Carlo simulations of sequences of N = 10, 20 and 30 impacts each, to estimate the ability of multiple impacts to produce a Moon-like satellite. The impact parameters were drawn from distributions previously found in terrestrial formation dynamical studies. With perfect accretionary mergers, approximately half the simulations result in a moon mass that grows to its present value after ~20 impacts.

If the multi-moon hypothesis proves credible, how would it affect the larger astrobiology question? In Ward and Brownlee’s Rare Earth (Copernicus, 2000), after a discussion of obliquity and the Moon’s effect on the Earth’s early history, the authors say this:

If the Earth’s formation could be replayed 100 times, how many times would it have such a large moon? If the great impactor had resulted in a retrograde orbit, it would have decayed. It has been suggested that this may have happened for Venus and may explain that planet’s slow rotation and lack of any moon. If the great impact had occurred at a later stage in Earth’s formation, the higher mass and gravity of the planet would not have allowed enough mass to be ejected to form a large moon. If the impact had occurred earlier, much of the debris would have been lost to space, and the resulting moon would have been too small to stabilize the obliquity of Earth’s spin axis. If the giant impact had not occurred at all, the Earth might have retained a much higher inventory of water, carbon and nitrogen, perhaps leading to a Runaway Greenhouse atmosphere.

The idea of a series of impacts eventually leading to a larger moon significantly muddies the waters here. It is true that in our Solar System, the inner planets are nearly devoid of moons, but we have no way of extending this situation to exoplanets without collecting the necessary data, which will begin with our first exomoon detections. Certainly if numerous collisions in an early planetary system can produce a large moon, as this paper argues, then we can expect similar collisional scenarios in many systems, making such moons a frequent outcome.

The paper is Rufu, Oharonson & Perets, “A Multiple Impact Hypothesis for Moon Formation,” published online by Nature Geoscience 9 January 2017 (abstract).

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A New Look at ‘Exocomets’

by Paul Gilster on January 12, 2017

Moving groups are collections of stars that share a common origin, useful to us because we can study a group of stars that are all close to each other in age. Among these, the Beta Pictoris moving group is turning out to be quite productive for the study of planet formation. These are young stars, aged in the tens of millions of years (Beta Pictoris itself is between 20 and 26 million years old). Within the moving group, we’ve detected planets around 51 Eridani and Beta Pictoris, while infalling, star-grazing objects have been found around Beta Pictoris.

Evidence of comet activity around another of these stars was discussed at the American Astronomical Society meeting in Texas. The star HD 172555, 23 million years old and about 95 light years from Earth, shows the presence of the vaporized remnants of cometary nuclei, marking the third extrasolar system where such activity has been traced. All the stars involved are under 40 million years old, giving us a glimpse of the kind of activity that happens during the era when young terrestrial planets have begun to emerge in their systems.

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Image: This illustration shows several comets speeding across a vast protoplanetary disk of gas and dust and heading straight for the youthful, central star. The comets will eventually plunge into the star and vaporize. The comets are too small to photograph, but their gaseous spectral “fingerprints” on the star’s light were detected by NASA’s Hubble Space Telescope. The gravitational influence of a suspected Jupiter-sized planet in the foreground may have catapulted the comets into the star. This star, called HD 172555, represents the third extrasolar system where astronomers have detected doomed, wayward comets. The star resides 95 light-years from Earth. Credit: NASA, ESA, and A. Feild and G. Bacon (STScI).

Carol Grady (Eureka Scientific/NASA GSFC) led the study reported on at the AAS. Her thoughts:

“Seeing these sun-grazing comets in our solar system and in three extrasolar systems means that this activity may be common in young star systems. This activity at its peak represents a star’s active teenage years. Watching these events gives us insight into what probably went on in the early days of our solar system, when comets were pelting the inner solar system bodies, including Earth. In fact, these star-grazing comets may make life possible, because they carry water and other life-forming elements, such as carbon, to terrestrial planets.”

The deflection of comets by the gravitational influence of a massive gas giant in an emerging planetary system is a vivid picture, one clarified by Grady and team’s work with the Hubble Space Telescope Imaging Spectrograph (STIS) and the Cosmic Origins Spectrograph (COS) in 2015. The team’s spectrographic analysis, using Hubble data collected from two observing runs separated by six days, detected carbon gas and silicon in the light of HD 172555 moving across the face of the star at a speed of 160 kilometers per second.

This work follows up a French study that first found exocomets transiting the same star in archival data from the HARPS spectrograph. That work detected signs of calcium. Grady and team have extended the analysis with a spectrographic analysis in ultraviolet light. They believe they are seeing gaseous debris left behind as comets disintegrated, vaporized materials that contain large chunks of the original comet. Helpfully, the disk around HD 172555 is seen almost edge-on from Earth, offering Hubble a clear view of the highly dispersed activity.

“As transiting features go, this vaporized material is easy to see because it contains very large structures,” Grady said. “This is in marked contrast to trying to find a small, transiting exoplanet, where you’re looking for tiny dips in the star’s light.”

To confirm that they are seeing the disintegration of icy comets as opposed to rocky asteroids, Grady’s researchers hope to use the STIS again to search for oxygen and hydrogen, a composition that would add further weight to these conclusions.

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Hubble Looks at Voyager’s Future

by Paul Gilster on January 11, 2017

Nothing built by humans has ever gotten as far from our planet as Voyager 1, which is now almost 21 billion kilometers from Earth. We’ve talked about the future of both Voyagers before in these pages — Voyager 1 passes within about 1.6 light years of the star Gliese 445 in some 40,000 years, its closest approach to a neighboring star. Voyager 2, which is now almost 17 billion kilometers out, closes to within 1.7 light years of Ross 248 in the same 40,000 years.

My case for doing what Carl Sagan once discussed, giving each Voyager a final kick with its remaining hydrazine, so that those closing distances could be reduced, can be found in Voyager to a Star. It would be a symbolic and philosophical act rather than a scientific one, as both Voyagers are losing their ability to transmit data and will be silent in about a decade. And nothing can reduce those huge timeframes, which means that any such symbolic statement would be made to the future, a way of saying we are learning to be a starfaring species.

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Image: In this artist’s conception, NASA’s Voyager 1 spacecraft has a bird’s-eye view of the solar system. The circles represent the orbits of the major outer planets: Jupiter, Saturn, Uranus, and Neptune. Launched in 1977, Voyager 1 visited the planets Jupiter and Saturn. The spacecraft is now 21 billion kilometers from Earth, making it the farthest and fastest-moving human-made object ever built. In fact, Voyager 1 is now zooming through interstellar space, the region between the stars that is filled with gas, dust, and material recycled from dying stars. Credit: NASA, ESA, and J. Zachary and S. Redfield (Wesleyan University); Artist’s Illustration Credit: NASA, ESA, and G. Bacon (STScI).

Meanwhile, we still have two viable spacecraft in the outer reaches of our Solar System, taking data on interstellar material, magnetic fields and cosmic ray hits and giving us a sense of what the local interstellar medium (LISM) is like. That’s crucial information, of course, for one day we hope to have not just a few but many spacecraft operating on the edge of interstellar space, and going beyond our system will require us to know the nature of the medium through which they move. On that score, the best book I know is Bruce Draine’s Physics of the Interstellar and Intergalactic Medium (Princeton, 2010). I enjoyed talking to Draine (Princeton University) at the latest Breakthrough Starshot sessions.

As you can imagine, learning more about the interstellar medium is a prerequisite if you’re thinking of pushing something up to 20 percent of lightspeed, as Breakthrough Starshot is, so the topic was a lively one at those meetings. At the recent American Astronomical Society meetings in Texas, we learned that astronomers have been using Hubble data to supplement what Voyager has been giving us, charting the hydrogen clouds and other elements of the LISM. Seth Redfield (Wesleyan University), who leads the study, offers this comment:

“This is a great opportunity to compare data from in situ measurements of the space environment by the Voyager spacecraft and telescopic measurements by Hubble. The Voyagers are sampling tiny regions as they plow through space at roughly 38,000 miles per hour [61,000 kph). But we have no idea if these small areas are typical or rare. The Hubble observations give us a broader view because the telescope is looking along a longer and wider path. So Hubble gives context to what each Voyager is passing through.”

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Image: In this illustration, NASA’s Hubble Space Telescope is looking along the paths of NASA’s Voyager 1 and 2 spacecraft as they journey through the solar system and into interstellar space. Hubble is gazing at two sight lines (the twin cone-shaped features) along each spacecraft’s path. The telescope’s goal is to help astronomers map interstellar structure along each spacecraft’s star-bound route. Each sight line stretches several light-years to nearby stars. Credit: NASA, ESA, and Z. Levy (STScI).

The Hubble work makes it clear that in two thousand years or so, Voyager 2 will move out of the interstellar cloud that surrounds the Solar System before moving into another cloud, in which it will remain for as much as 90,000 years. The astronomers find slight variations in the abundances of the chemical elements in these clouds, which could chart a history involving different paths to formation. We do know that as the solar wind pushes against the interstellar medium, the heliosphere can be compressed, only to expand again when the Sun moves through lower-density matter. For more, see this Hubblesite news release.

We still haven’t built the next generation LISM explorer, one crafted from the outset as an interstellar data gatherer. As much as the Voyagers continue to give us, we have to remember that they were designed as planetary probes, their survival to this point being an amazing and unexpected gift, but one that has to be adapted to the medium through which the spacecraft move. A spacecraft fine-tuned for exploration beyond the heliopause is a goal that continues to see its share of study (more on this soon), but when it will fly remains an open question.

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Upgraded Search for Alpha Centauri Planets

by Paul Gilster on January 10, 2017

Breakthrough Starshot, the research and engineering effort to lay the groundwork for the launch of nanocraft to Alpha Centauri within a generation, is now investing in an attempt to learn a great deal more about possible planets around these stars. We already know about Proxima b, the highly interesting world orbiting the red dwarf in the system, but we also have a K- and G-class star here, either of which might have planets of its own.

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Image: The Alpha Centauri system. The combined light of Centauri A (G-class) and Centauri B (K-class) appears here as a single overwhelmingly bright ‘star.’ Proxima Centauri can be seen circled at bottom right. Credit: European Southern Observatory.

To learn more, Breakthrough Initiatives is working with the European Southern Observatory on modifications to the VISIR instrument (VLT Imager and Spectrometer for mid-Infrared) mounted at ESO’s Very Large Telescope (VLT). Observing in the infrared has advantages for detecting an exoplanet because the contrast between the light of the star and the light of the planet is diminished at these wavelengths, although the star is still millions of times brighter.

To surmount the problem, VISIR will be fitted out for adaptive optics. In addition, Kampf Telescope Optics of Munich will deliver a wavefront sensor and calibration device, while the University of Liège (Belgium) and Uppsala University (Sweden) will jointly develop a coronagraph that will mask the light of the star enough to reveal terrestrial planets.

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Image: Paranal at sunset. This panoramic photograph captures the ESO Very Large Telescope (VLT) as twilight comes to Cerro Paranal. The enclosures of the VLT stand out in the picture as the telescopes in them are readied for the night. The VLT is the world’s most powerful advanced optical telescope, consisting of four Unit Telescopes with primary mirrors 8.2 metres in diameter and four movable 1.8-metre Auxiliary Telescopes (ATs), which can be seen in the left corner of the image. Credit: ESO.

According to the agreement signed by Breakthrough Initiatives executive director Pete Worden and European Southern Observatory director general Tim de Zeeuw, Breakthrough Initiatives will pay for a large part of the technology and development costs for the VISIR modifications. Meanwhile, the ESO will provide the necessary telescope time for a search program that will be conducted in 2019. The VISIR work, according to this ESO news release, should provide a proof of concept for the METIS instrument (Mid-infrared E-ELT Imager and Spectrograph), the third instrument on the upcoming European Extremely Large Telescope.

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Garnet World: Stellar Composition & Planetary Outcomes

by Paul Gilster on January 9, 2017

What effect does the composition of a star have on the planets that form around it? Enough of one that we need to take it into account as we assess exoplanets in terms of astrobiology. So says a study that was presented at the American Astronomical Society meeting in Texas last week, looking at ninety specific stars identified by Kepler as having evidence of rocky planets.

We know about the composition of these stars because they are part of the 200,000 star dataset compiled by APOGEE, the Apache Point Observatory Galactic Evolution Experiment spectrograph mounted on the 2.5m Sloan Foundation telescope in New Mexico. APOGEE allows us to examine the spectra of stellar atmospheres to identify their elements.

Modeling the formation of planets around these stars shows us the implications for astrobiology. Johana Teske (Carnegie Observatories) explains:

“Our study combines new observations of stars with new models of planetary interiors. We want to better understand the diversity of small, rocky exoplanet composition and structure — how likely are they to have plate tectonics or magnetic fields?”

At the AAS meeting, Teske described how the team of astronomers and geoscientists she is working with focused on Kepler 102 and Kepler 407, the former a star slightly less luminous than the Sun hosting five known planets, the latter hosting two planets orbiting a star of roughly the Sun’s mass. The APOGEE data show that in terms of chemical composition, Kepler 102 is similar to the Sun, while Kepler 407 is much richer in silicon.

Geophysicist Cayman Unterborn (Arizona State) ran computer simulations of planet formation incorporating the APOGEE data. The result:

“We took the star compositions found by APOGEE and modeled how the elements condensed into planets in our models. We found that the planet around Kepler 407, which we called ‘Janet,” would likely be rich in the mineral garnet. The planet around Kepler 102, which we called ‘Olive,’ is probably rich in olivine, like Earth.”

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Image: The picture shows what minerals are likely to occur at several different depths. Kepler 102 is Earth-like, dominated by olivine minerals, whereas Kepler 407 is dominated by garnet, so less likely to have plate tectonics. Credit: Robin Dienel, Carnegie DTM.

In Unterborn’s view, the difference is significant because garnet, a far stiffer mineral than olivine, flows more slowly, implying a garnet planet would be unlikely to have long-term plate tectonics. Like the Earth, the planet around Kepler 102 could sustain tectonics, which are thought to be essential for life because atmospheric recycling through geological processes like volcanoes and ocean ridge formation regulates the atmosphere’s composition. Without such geological processes, life would not necessarily have the chance to evolve.

Centauri Dreams’ take: The interplay of the two datasets — APOGEE and Kepler — is deeply productive, but we’re only at the beginning of the analysis. APOGEE’s 200,000 stars include others known to host small planets, so similar methods can now be put to work on the mineral content of these worlds. Those most Earth-like in their mineral content would rank higher on our list for further astrobiological study, helping us refine our targets for future observation.

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