Centauri Dreams
Imagining and Planning Interstellar Exploration
Skyscraper in the Clouds
Analemma seems the perfect name for the proposed ‘floating’ space tower being discussed by the Clouds Architecture Office, an imaginative New York firm whose unusual designs include a Martian habitat made of ice and a concept study of flight into deep space using comets for resources. An analemma is a diagram that traces the movement of the Sun in the sky as seen from a particular location on Earth. Over time, the position changes because of orbital eccentricity and our planet’s axial tilt, so that a slim figure-eight is the result.
That’s relevant to the Analemma tower because it is conceived as a huge construction tethered to an asteroid that would be moved into what the firm describes as ‘an eccentric geosynchronous orbit’ over Earth. The orbit allows the structure to move between the northern and southern hemispheres, tracing out a figure-eight over the surface. With the slowest speed over the ground at the top and bottom of the figure-eight, Clouds Architecture Office suggests that occupants could move back and forth, interacting with ground resources at these points. New York City is suggested as the location for one of the slow parts of the preferred orbit.
Image: Analemma inverts the traditional diagram of an earth-based foundation, instead depending on a space-based supporting foundation from which the tower is suspended. This system is referred to as the Universal Orbital Support System (UOSS). By placing a large asteroid into orbit over earth, a high strength cable can be lowered towards the surface of earth from which a super tall tower can be suspended. Since this new tower typology is suspended in the air, it can be constructed anywhere in the world and transported to its final location. The proposal calls for Analemma to be constructed over Dubai, which has proven to be a specialist in tall building construction at one fifth the cost of New York City construction. Credit (text and image): Clouds Architecture Office.
A skyscraper without a foundation on Earth? The concept makes for a great science fiction backdrop, if nothing else, and calls up images of a building so tall that occupants of its highest floors would look out upon the world from 32000 meters, with near vacuum conditions outside. Solar panels at the top would supply power, with water being supplied and replenished from clouds and rainwater. A transfer station would allow people and goods to move between the orbiting tower and the surface. As for the cost of building such a tower, one prefabricated unit at a time, plugging modules into an extendable core, the firm sees clear skies:
Harnessing the power of planetary design thinking, it taps into the desire for extreme height, seclusion and constant mobility. If the recent boom in residential towers proves that sales price per square foot rises with floor elevation, then Analemma Tower will command record prices, justifying its high cost of construction.
Want to get in on the ground floor? Not me. There’s plenty to question about this operation even if a market did emerge to support it, but the visuals are breathtaking, with spectacular vistas from residential windows aboard the building (craft?) and the futuristic imagery of a skyscraper emerging slowly out of the clouds for those below. Clouds Architecture Office trades off recent space news like the European Space Agency’s study of and landing on Comet 67P/Churyumov–Gerasimenko and NASA efforts at asteroid retrieval, which involve relocating a tiny asteroid to a new orbit. We’re already landing on comets and talking about moving asteroids, right, so why not stretch out the notion that much further? But Analemma Tower is a long, long stretch indeed.
Image: The Analemma Tower as it might appear over New York City. Credit: Clouds Architecture Office.
If it were ever built, the tower would certainly be an unusual place to live. The bottom of the structure is seen as a shopping and dining area. The firm talks about business being conducted above this zone in the lower and middle sections of the tower, with the residential quarters approximately ? of the way up. The sizes and shapes of the windows in the building would change with height because of the pressure and temperature differentials Analemma would experience. Those at the top of the tower would receive an additional 40 minutes of daylight per day because of the curvature of the Earth, says the Clouds Architecture Office website.
Image: Views through the windows of the Analemma tower, their shapes adapting to conditions outside. Credit: Clouds Architecture Office.
Because Dubai is the home of the world’s tallest building, Burj Khalifa, it is the firm’s choice for construction of the modules that would make up the tower. Exactly how building components would be delivered from the ground to the gradually growing core is not clear. Images of people parachuting past the tower’s windows suggest one quick way to the surface, though hardly one the average business person would be likely to take.
Can we create cables to support such a structure? And what about moving a sizeable asteroid into Earth orbit? NASA’s own plans in its Asteroid Redirect Mission have called for nudging a multi-ton boulder and pushing it into a stable orbit around the Moon, where it could be the destination of a manned mission that would bring samples back to Earth. That’s a $1.25 billion project in itself. The size of the asteroid Clouds Architecture Office is talking about isn’t specified, though it would far exceed the small rock NASA hopes to maneuver.
And consider the point that Sara Chodosh brought up in Popular Science in a piece appropriately titled This building hanging from an asteroid is absurd – but let’s take it seriously for a second. What happens to residents in a building this tall when they need an elevator?
The fastest elevators in the world are in The Shanghai Tower and move at 20.5 meters per second, so a straight shot to the top of the Analemma would take you just over 20 minutes. And that’s assuming there is a single elevator that goes all the way to the top. One of the logistically challenging things about building very tall structures is how you fit in enough elevators… The easiest way around that is to not require all of the elevators to go all the way to the ground (STRIKETHROUGH), er, first floor. You can stack elevator shafts on top of each other and simply have sky lobbies. So maybe you take an elevator from floor 1 to floor 50, then get off and take another from 50 to 100, and so on. Keep in mind that you’ll have to wait for the elevator to come each time, and with thousands upon thousands of residents you’ll probably be waiting a while.
Indeed. But there’s something to be said for black sky dreaming, and I love the visuals here. So while I’m not expecting to see Analemma appearing in the sky over New York any time soon, I do get some of the same buzz from the concept that I find in good science fiction. In fact, Analemma reminds me of some of Alastair Reynolds’ creations, particularly the world he portrays in novels like Chasm City (in his Revelation Space sequence). Chasm City is packed with mile-high skyscrapers, its planet surrounded by the Glitter Band, thousands of orbital habitats, all of which are threatened in the novel.
The people behind Analemma Tower have to be science fiction buffs. So why keep this idea on Earth? Geoffrey Landis has pointed out that at 50 kilometers above the surface of Venus, the pressure is about 1 bar and temperatures range between 0° and 50° Celsius. Here we wouldn’t need to be tethered to an asteroid, but could set up floating colonies in the Venusian clouds, where a human breathable air mixture is itself a lifting gas. Michael McCollum has likewise envisioned floating cities in the atmospheres of the outer gas giants. A skyscraper emerging out of the clouds may indeed be in our future, but I’m not sure Earth will be where we build it.
A note to the readers: Please help me out with further science fiction references on floating buildings/cities. I’m struggling to remember one recent depiction of a floating structure that moved between planet-bound cities — was it in Stephenson’s Seveneves, in the far future section of the novel? If so, I can’t find it. Your help on this is appreciated.
A Retrograde Asteroid Sharing Jupiter’s Orbit
We recently looked at JAXA’s planned solar sail mission to Jupiter (see JAXA Sail to Jupiter’s Trojan Asteroids), but I want to come back around to the Trojans this morning in light of a discovery announced today. The more we learn about the Trojans, the better. Most appear to be class D asteroids, dark with reddish hues and probably covered in tholins, organic polymers that result from the solar irradiation of organic compounds. Tholins show up all over the place in the outer system’s icy objects, adding to the view that the Jupiter Trojans were probably captured into their present orbits during the early days of Solar System formation.
Asteroid 2015 BZ509, discovered by the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) in 2015, turns out to be a Trojan with a difference. Revealed in the current issue of Nature by discoverers Paul Wiegert (Western University, London), Martin Connors (Athabasca University, CA) and Christian Veillet (Large Binocular Telescope Observatory, Tucson), the object is a Trojan moving in a retrograde orbit. [Addendum: See the comments below, where some writers question my use of the term ‘trojan’ to describe this object].
We think of Trojans as moving in the same direction as Jupiter, though offset from the giant planet by 60° ahead or behind (at the L4 or L5 Lagrangian points), so this object is a surprise. While a few non-Trojan asteroids do orbit the Sun in retrograde orbits, 2015 BZ509 is in a tight relationship with Jupiter, tracking its orbit in reverse in a way that allows it to weave out of the planet’s path each time they approach each other.
Image: The co-orbital asteroids of Jupiter, also known as the ‘Trojan asteroids’. The prograde asteroids are shown in white, and 2015 BZ509 (with a trail, shown in green) appears later. The planets and asteroids have been enlarged for visibility. Credit: Wiegert et al.
The situation is stable as 2015 BZ509 passes once inside and once outside Jupiter each time they orbit the Sun, with Jupiter’s gravitational tugs on the planet canceling out to maintain the relationship. The condition is called a retrograde co-orbital resonance, a topic explored by Anthony Dobrovolskis (NASA Ames) in 2012, and followed up on by Helena Morais (Universidade de Estadual Paulista, Brazil) and Fathi Namouni (Université de Nice-Sophia Antipolis, France) in the years following. Their work established the theoretical existence of such objects, but 2015 BZ509 marks the first time an actual asteroid has been found in this kind of resonance. How it got there remains a mystery.
Image: Images of 2015 BZ509 obtained at the Large Binocular Telescope Observatory (LBTO) that established its retrograde co-orbital nature. The LBTO has two 8.4 meter-wide main mirrors side-by-side, hence the two images. The bright stars and the asteroid (circled in yellow) appear black and the sky white in this negative image. The white dots, spots and stripes are imaging artifacts in these raw images. Credit: Wiegert et al.
Although 2015 BZ509 never gets closer to Jupiter than 176 million kilometers, it is Jupiter’s gravity that controls its movements and keeps it from what would otherwise be a collision with that planet. We may be looking at an inactive comet nucleus here, although there is no sign as yet of cometary outgassing or the formation of a tail, which in any case might not form because of the asteroid’s distance from the Sun.
While its retrograde motion sets it apart, 2015 BZ509 joins the ranks of co-orbital asteroids that share a planet’s orbit, a list that includes Earth’s co-orbital 3753 Cruithne, 2010 SO16 and 2002 AA29. All of these are in a 1:1 mean motion resonance with our planet, meaning that they orbit the Sun in the same average amount of time that the Earth does.
The paper is Wiegert et al., “A retrograde co-orbital asteroid of Jupiter,” Nature 543 (30 March 2017), 687-689 (abstract).
The Challenges of Przybylski’s Star
About 370 light years away in the constellation Centaurus is a variable star whose spectrum continues to raise eyebrows. The star is laced with oddball elements like europium, gadolinium, terbium and holmium. Moreover, while iron and nickel appear in unusually low abundances, we get short-lived ultra-heavy elements, actinides like actinium, plutonium, americium and einsteinium. Hence the mystery: How can such short-lived elements persist in the atmosphere of a star? Discovered in 1961 by the Polish-American astronomer Antoni Przybylski, these traits have firmly placed Przybylski’s Star in the Ap class of chemically peculiar stars. Its very name is a cause of continuing conversation.
PRZYBYLSKI'S STAR (HD 101065) Blue dwarf with a peculiar spectrum showing an almost complete absence of vowels.
— FSVO (@FSVO) November 22, 2012
Well, true enough. If Przybylski’s Star is a challenge to understand, it’s also a challenge to pronounce. Charles Cowley (University of Michigan), who offers a detailed analysis of the star online, met Przybylski in 1964, asking him how to say his name. “He obliged me,” Cowley writes, “and I thought I detected a slight “puff” at the beginning of the sound, which Mike Bessel writes is like “jebilskee”, with the “je” as if it were in French. The initial “P” gets minimal sound.” I think we can go with that, although I’ve seen a variety of pronunciations online.
What we’d like to know once we can say the star’s name is how the heavy elements observed here have come about. A neutron star is one solution, a companion object whose outflow of particles could create heavy elements in Przybylski’s Star, and keep them replenished. The solution seems to work theoretically, but no neutron star is found anywhere near the star. In a new paper, Vladimir Dzuba (University of New South Wales) and colleagues suggest that the actinides in Przybylski’s Star are evidence of the slow decay of superheavy elements.
The idea is that there may be a so-called island of stability involving elements with 114 or more protons in their nuclei, super-heavy elements that nonetheless are long-lived. If these exist, then the short-lived plutonium, einsteinium and the rest found in the star would simply be decay products. We may be, in other words, about to discover a new isotope not produced as a fleeting sample in an experiment but as an element observed in nature. That in itself is not unusual: Penn State’s Jason Wright reminds us that helium was first found in the Sun.
Wright has written a four-part discussion of Przybylski’s Star that begins with Przybylski’s Star I: What’s that? and continues through internal links. As with everything Wright does, it’s informative and also hugely entertaining. Moreover, it takes us however briefly into SETI terrain as Wright raises the point that advanced civilizations might use stars to store nuclear waste, a notion broached by Daniel Whitmire and David Wright as far back as 1980, and considered as well by Carl Sagan and Iosif Shklovskii in their Intelligent Life in the Universe (Holden-Day, 1966). Whitmire and Wright even opined that the most likely stars in which we would find such pollution were late A stars like Przybylski’s Star.
Image: Antoni Przybylski in the early 1960’s. Credit: Mike Bessell (via Charles Cowley’s site).
So we can note that this unusual star could fit into the artifact SETI category even as we continue to figure out the natural reasons why it should show the spectrum that it does. Wright adds this interesting note about the entire field of so-called Dysonian SETI, which scours our astronomical data for anomalies that could be the signs of the workings of an advanced civilization. As we continue to see in the controversy over Boyajian’s Star, this area of study can be frustrating, especially when one is in the business of working out what we might find:
This just goes to show that artifact SETI is hard. When people stick their necks out and make bold, silly-sounding predictions about unambiguous technosignatures like this (or like megastructures), I suspect they usually don’t actually expect them to come true. And then when they do come true (as in Przybylski’s Star, KIC 12557548, or Boyajian’s Star) not only are their prediction papers rarely cited (which is, I think, inappropriate), but there’s always immediately a flurry of perfectly natural explanations that arrive to explain things without aliens (which is, I think, totally appropriate).
We keep looking nonetheless because no matter what their origin, stars like Przybylski’s Star are fascinating in their own right. Meanwhile, Dzuba and team hope to pursue the island of stability elements and the idea that the actinides in Przybylski’s Star are the result of their slow and replenishing decay:
We hope that this work provides a motivation for a further progress in the measurements of the transition frequencies for superheavy elements, calculations of the isotope shifts and search for the corresponding transitions in astrophysical spectra.
Dzuba believes the next step should be a search for five elements with atomic numbers of 102 or more: nobelium (102), lawrencium (103), nihonium (113) and flerovium (114), as recounted in this New Scientist report on their work. The challenge is daunting, because while such elements could be part of the radioactive decay chain of stable super-heavy elements on the ‘island of stability,’ their half-lives are so short that their spectra are not well defined.
The original paper on Przybylski’s Star is Przybylski, “HD 101065-a G0 Star with High Metal Content,” Nature Vol. 189, Issue 4766 (1961), pp. 739 ff. (abstract). Amusingly enough, the paper was hidden in plain sight on the Internet for some time, invisible to ADS searches because the author’s name had been misspelled. That has now been fixed. The Dzuba paper is “Isotope shift and search for metastable superheavy elements in astrophysical data,” (preprint).
Looking for Our Sun’s ‘Super-Earth’
An obscure instrument called a blink comparator became world famous following Clyde Tombaugh’s discovery of Pluto in 1930. It was by rapidly switching between astronomical photographs that the young Tombaugh was able to compare objects in the field of view where ‘Planet X’ was presumed to hide. Pluto turned out to be a good deal smaller than Percival Lowell had imagined, leading to thoughts of still more distant planets, but for a time the new planet was best known as a faint dot on a series of plates, moving against a fixed field of stars.
Image: Clyde Tombaugh at the Blink Comparator five years after the Pluto discovery. Credit: Lowell Observatory Archives.
All of this is wonderfully told in Michael Byers’ 2010 novel Percival’s Planet (Henry Holt and Co.), which draws on Tombaugh’s story and depicts the entire Lowell Observatory scene in his time there (see A Tour de Force of Planetary Discovery for my review of the book). Or if you want the inside view, Tombaugh’s own Out of the Darkness: The Planet Pluto takes on his Pluto work and the entire field of planet hunting.
These days we can mimic what a blink comparator did with a computer, but in an era when charge-coupled devices (CCDs) have replaced photographic plates, we’re more prone to use computer software to tease out the motion of distant objects against the background stars, so that the nearby, moving object appears fixed amidst what appear to be the tracks of stars.
We’ll see what technique is to be employed in a new crowdsourced effort to find Planet X, still out there (or so many believe) and referred to by many astronomers as Planet Nine. Brad Tucker (ANU), who leads the project, explains that the new planet is predicted to be a super-Earth, perhaps 10 times the mass of Earth and as much as four times its size.
An object like this roughly 800 AU out would help to explain the unusual orbital characteristics of Sedna and five other ‘extreme trans-Neptunian objects’ (ETNOs) whose orbits are sharply tilted when compared to the plane in which the planets move. Chadwick Trujillo and Scott Sheppard (Carnegie Institution for Science, Washington) made a strong case for this scenario in a 2015 paper, followed up by work from Mike Brown and Konstantin Batygin (Caltech). You can check the Search for Planet Nine site for background on Brown and Batygin’s thoughts.
At ANU, the data in question will come from the Australian National University’s robotic 1.35-meter SkyMapper telescope at Siding Spring, the only telescope that maps the entire southern sky. The instrument will serve up hundreds of thousands of images for public assessment. As with the citizen science effort to study old astronomical prints we discussed on Friday, the ANU search will involve volunteers scanning the SkyMapper images online, in this case to look for any signs of a planet. The incidental discovery of comets or dwarf planets seems likely.
Image: The SkyMapper telescope at sunset. Credit: Steve Chapman.
The project is to be launched by Brian Cox during the BBC’s Stargazing Live broadcast from Siding Spring Observatory, joining other crowdsourced efforts on the Zooniverse site when it becomes available. Bear in mind that we already have Backyard Worlds, a project (likewise on Zooniverse) that uses data from WISE (Wide-field Infrared Survey Explorer) to pursue a similar search, though not targeted as tightly on a single planet. The latter’s animated ‘flipbooks’ are the modern day equivalent of what Tombaugh used for Pluto.
I don’t know what kind of interface the SkyMapper effort will come up with, but it’s pleasing to think of a modern-day analog of the famous Tombaugh search, now spread out over a global network of users. The Stargazing Live series begins on the BBC on March 28 at 2000 UTC. An Australian version of the show will also be broadcast from Siding Spring Observatory next week on ABC TV. Stargazing Live has its own blog page from which to learn more, and ANU Twitter feed @scienceANU is also tracking the rollout.
Astronomy Rewind: Keeping Our Data Alive
When I was growing up, there was a small outbuilding between my house and the stand of woods behind our property. The previous owner had built it as a little house in its own right, everything on a miniature scale, so that while it looked like an actual house — with front door, nice windows, even a porch and small deck on the back — it was comprised of only one room inside. This man’s kids had used it as a playhouse, but when I got my hands on it, I turned it into what a young boy thought of as his ‘lab,’ with microscope, chemistry set and telescope.
On the walls I put photographs I had bought at Chicago’s Adler Planetarium, and I can still see those blurry images of Saturn, Jupiter and the Milky Way, all taken at the Palomar Observatory, and almost as breathtaking for what they didn’t reveal as what they did. I gradually augmented these photos with sky charts and other imagery, and would use these to plan my observing sessions with the 3-inch reflector I would take out into the yard.
Image: A classic photo (though not one of my Palomar images). This is M31, then known as the Great Andromeda Nebula, its nature as a separate galaxy not being known when the photograph was taken in 1888 by Sir Isaac Roberts. Reproduced in A Selection of Photographs of Stars, Star-clusters and Nebulae, Volume II (The Universal Press, London, 1899), this is the image whose long exposure time first.revealed M31’s spiral structure. Photos like these may look quaint compared to the brilliant detail and color of today’s work, but studying the sky over long periods of time may tease out new information, making even our older datasets useful tools for exploration. Credit: Isaac Roberts.
You would think old astronomical photographs would have a place only in memories like these, but I’m reminded of the vigorous debate that broke out not long ago over the anomalous star KIC 8462852, and Bradley Schaefer’s contention that, on the basis of archival imagery, it could be shown to have undergone a long-term dimming (see KIC 8462852: A Century Long Fade? for more on this — there are likewise numerous articles in the archive).
Schaefer was using a collection of some 500,000 sky photographs in the archives of Harvard College Observatory, covering the period from 1890 to 1989. A program called Digital Access to a Sky Century@Harvard (DASCH) has been digitizing the observatory’s archives, offering a way for astronomers to re-examine historical imagery. DASCH has only digitized a fraction of the archives but it’s a work in progress. What else can we do to reinvigorate such material?
One answer is a project called Astronomy Rewind, whose aim is to restore tens of thousands of astronomical images — photographs, radio maps and other sky-related material — from a wide variety of sources, placing them into context in digital sky atlases and catalogs. The project is part of the Zooniverse platform that gave us Galaxy Zoo a decade ago and now includes ‘citizen science’ projects in a variety of disciplines. Here the idea is to turn our attention to the contents of scientific journals and collate their imagery over time.
American Astronomical Society journals go back to the 19th Century and became accessible electronically in the 1990s. The volunteers will catalog the types of images, separating photographs with and without sky coordinates, maps of planets with or without latitude and longitude grids, graphs and diagrams, focusing on labeled images or those with sufficient detail to make a clear determination of orientation and sky position. Other images will be sent to Astrometry.net, which identifies areas of sky by comparing photos to star catalogs.
The project depends upon human judgement and pattern recognition at a large scale:
“You simply couldn’t do a project like this in any reasonable amount of time without ‘crowdsourcing,'” says Julie Steffen, AAS Director of Publishing. “Astronomy Rewind will breathe new life into old journal articles and put long-lost images of the night sky back into circulation, and that’s exciting. But what’s more exciting is what happens when a volunteer on Zooniverse looks at one of our journal pages and goes, ‘Hmm, that’s odd!’ That’ll be the first step toward learning something new about the universe.”
Image: An early view of Orion. This is a digital print of a photographic plate from the Ritchey 60-inch telescope at Mount Wilson Observatory, made in 1908. Credit: Mt. Wilson Observatory.
The journals involved at present are The Astronomical Journal, Astrophysical Journal, Astrophysical Journal Letters and the ApJ Supplement Series. Images are to be annotated and extracted into digital files that will end up in data repositories as well as becoming part of the Astronomy Image Explorer and becoming viewable in the data visualization tool and sky atlas WorldWide Telescope.
Once up to speed, Astronomy Rewind hopes to process 1,000 journal pages daily, with each page examined by at least five different people to produce consensus. This number is based upon other projects at Zooniverse, where 1.6 million volunteers have classified 4 billion images over the last ten years. Peer-reviewed publications, over 100 of them, have flowed from the Zooniverse work, and as the KIC 8462852 story shows, the potential for discovery is here.
We have to remember as we look into old astronomical materials that we continue to accumulate data at a faster and faster rate. We’re learning how to sift through older material as we build the database from which details that may have escaped our attention decades ago can come to light, perhaps to be reinterpreted in the context of subsequent findings. An earlier effort, the ADS All-Sky Survey, was originally set up to analyze imagery from old astronomy papers, but using computers for the job wasn’t always effective. Now we turn volunteer eyes on the cosmos as we bring digital technologies and older printed journals together.
Ceres: Axial Tilt and Surface Ice
Earth’s axial tilt (its obliquity) is 23.5 degrees, a significant fact for those of us who enjoy seasonal change. The ’tilt’ is the angle between our planet’s rotational axis and its orbital axis. If we look at Earth’s obliquity over time, we find a 41,000 year cycle that oscillates between 22.1 and 24.5 degrees. Here the Moon becomes useful, with recent studies showing that without it, Earth’s obliquity could vary by 25° (some earlier analyses took this number much higher).
Now we have new data from the Dawn spacecraft at Ceres relating the dwarf planet’s axial tilt to the locations where frozen water can be found on its surface. This is interesting stuff, because it depends upon the spacecraft’s ability to measure the world it orbits.
“We cannot directly observe the changes in Ceres’ orientation over time, so we used the Dawn spacecraft’s measurements of shape and gravity to precisely reconstruct what turned out to be a dynamic history,” says Erwan Mazarico, a co-author of a paper on this work based at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
Image: This animation shows how the illumination of Ceres’ northern hemisphere varies with the dwarf planet’s axial tilt, or obliquity. Shadowed regions are highlighted for tilts of 2 degrees, 12 degrees and 20 degrees. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.
What we learn from the paper just published in Geophysical Research Letters is that in the last three million years, Ceres’ axial tilt has ranged from 2° to 20°. The last time of maximum obliquity of 19° was about 14,000 years ago, while its current tilt is just 4°, meaning seasonal effects over the course of a current Cerean year (4.6 Earth years) will be slight.
Charting Ceres’ obliquity allows researchers to examine which areas remain most deeply shadowed even during times of maximum tilt, and the current work, led by JPL’s Anton Ermakov, reports that craters that are shadowed during times of maximum obliquity show bright deposits that are most likely water ice. Ceres’ surface temperatures range from 130 to 200 Kelvin (-143° C to -73° C), but regions that rarely see sunlight are more likely to have ice deposits than sunlit areas where ice can sublimate directly into vapor.
Deeply shadowed areas at the poles never receive direct sunlight when Ceres’ axial tilt is as low as it is today — this is an area of about 2,000 square kilometers — but increasing obliquity reduces the shadow region to as little as 1 to 10 square kilometers. The researchers call craters with areas that stay in shadow over long periods of time ‘cold traps’ because volatiles that readily vaporize cannot escape once deposited there. We’ve already learned from Dawn that 10 such craters contain bright material, and one is already known to contain ice.
The northern and southern hemispheres have two persistently shadowed regions each at 20° tilt, and so far we have found bright deposits in three of the four. All of this should call up thoughts of the polar regions of the Moon, a body that has little variability in its tilt because of the influence of the Earth. Mercury, too, stabilized by its proximity to the Sun, shows little axial tilt, and on both objects, we are finding evidence of water ice in shadowed craters at the poles. As with Mercury, the Moon’s ice surely comes from the impact of asteroids and comets, whereas what we find on Ceres may, at least in part, come from the dwarf planet itself.
Remember that the European Space Agency’s Herschel Space Observatory found a tenuous atmosphere on Ceres several years ago, a possible source of water molecules that can accumulate in the cold traps. Meanwhile, note that Ceres’ axial tilt varies on a cycle of about 24,500 years, a figure researchers consider to be a surprisingly short time given the size of the variation. Ceres’ surface ice, then, gives us insight into its geological history as we continue to probe the question of whether the small body continues to give off water vapor.
The paper is Ermakov et al., “Ceres’s obliquity history and its implications for the permanently shadowed regions,” published online by Geophysical Research Letters 22 March 2017 (abstract).
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