Centauri Dreams
Imagining and Planning Interstellar Exploration
Volcanism and Astrobiology
A question in a grad school astrobiology seminar at the University of Washington prompted Amit Misra to go to work on plate tectonics. The movement of huge blocks of a planetary surface is beneficial to life because it prompts recycling, as materials move back and forth between the inside of the planet and the atmosphere. We’ve learned a lot about plate tectonics on Earth, but the seminar question stuck with Misra. How could we detect plate tectonics on an exoplanet?
The result is a paper in press at Astriobiology. Misra and colleagues make the case that transient sulfate aerosols produced by volcanic outgassing could provide just the signature scientists need. Explosive volcanic events produced by subduction at the edges of tectonic plates inject such aerosols directly into the atmosphere, where they can persist over periods of months to years. The paper argues that future instruments like the James Webb Space Telescope or the European Extremely Large Telescope (E-ELT) will be able to detect eruptions via the atmospheric spectra of planets in transit across their star.
“I came up with the idea of looking at explosive volcanic eruptions as a proxy, or stand-in, for plate tectonics,” says Misra. “I had done some work modeling aerosols produced by volcanic eruptions for other projects, so I started looking into how we might detect an eruption and what it would tell us.”
Image: Explosive volcanic activity on the edge of tectonic plates leaves a detectable signature in the atmosphere, one that may prove useful for characterizing conditions on an exoplanet. Credit: Kevin West/Liaison/Getty Images.
Geological activity is thought to be favorable for the emergence of life on planets in the habitable zone. Volcanic and hydrothermal activity during the Hadean and early Archaean eras, from the time of Earth’s formation to about 2.5 billion years ago, produced the energy necessary for early life and fostered the creation of organic carbon molecules. As for life’s survival, geological activity helps to stabilize long-term climates through the carbonate-silicate cycle, which continues to regulate the concentration of CO2 in Earth’s atmosphere.
So if we can detect explosive volcanism that is produced by tectonic action, we have another way to evaluate the potential habitability of terrestrial-class exoplanets. It’s true that we can also imagine habitats such as subsurface oceans on icy moons that persist without geological activity, but I think Misra has it right when he and his team note that for now, astrobiology focuses on biospheres at the largest scale, affecting the surface and atmosphere. These, after all, will be the first we will have the ability to detect and characterize.
From the paper:
The most readily detectable exoplanet biospheres will be large-biomass, high metabolism surface biospheres that persist for geologically significant timescales. This is because small biospheres or biospheres not in contact with the atmosphere will have minimal influence on atmospheric composition, and because short-lived biospheres are unlikely to temporally overlap with our observations… {W]e argue geological activity is necessary for the genesis and maintenance of these detectable biospheres. We propose that transient sulfate aerosols could be a spectral signature for volcanic activity and that the detection of transient sulfate aerosols would help inform habitability evaluations.
The researchers used transit transmission spectra with Earth as a model to test the detectability of transient sulfate aerosols in the stratosphere, with Earth-like assumptions for radius, mass and atmospheric composition. The models included Earth analogs around a Sun-like star as well as an M5 dwarf, in both cases assuming planets in the habitable zone. Aerosol data from Earth-observing satellites was used with different assumptions on cloud cover and altitudes.
The result: Evidence of volcanism is detectable using transit transmission spectra for planets within about 10 parsecs of the Sun. The trick will be to rule out false positives that could be the result of dust storms or impacts. And while transient sulfate aerosols are injected by explosive volcanism on Earth, occurring largely at converging plate boundaries, the paper notes the need for caution: “…even a tentative claim of plate tectonics would require a detection of transient sulfate aerosols coupled with other system/planetary observations and modeling.”
Detecting both transient aerosols and atmospheric oxygen together would be an even more interesting biosignature than a detection of oxygen by itself. The paper adds:
In the absence of surface fluxes of reduced gases, oxygen can potentially build up in an atmosphere even without the presence of life, making O2 alone an ambiguous biosignature. However the detection of volcanism would strongly suggest a source of oxygen-consuming reduced gases and would thus strengthen the case for biogenic oxygen.
The paper is Misra et al., “Transient Sulfate Aerosols as a Signature of Exoplanet Volcanism,”
in press at Astrobiology (preprint). A University of Washington news release is available.
Sail in View
The main post for today will be online around 1230 EDT (1630 UTC), but first I have to publish this image from LightSail, along with Jason Davis’ description. Nice work!
“The Planetary Society’s LightSail test mission successfully completed its primary objective of deploying a solar sail in low-Earth orbit, mission managers said today [June 9]. During a ground station pass over Cal Poly San Luis Obispo that began at 1:26 p.m. EDT (17:26 UTC), the final pieces of an image showcasing LightSail’s deployed solar sails were received on Earth. The image confirms the sails have unfurled, which was the final milestone of a shakedown mission designed to pave the way for a full-fledged solar sail flight in 2016.”
A second image may include a view of the Earth, according to Davis. What may happen next is a further tensioning, or ‘walking out,’ of the sail booms, which should further flatten the sail. Davis notes, too, that the ‘fish-eye’ lens of the camera produces a bit of distortion in the image.
Addendum: Bill Nye’s statement on LightSail’s success.
“I’m very proud to say that our LightSail test mission was a success. We saw again that space is hard. It’s a test flight, and sure enough our little spacecraft tested us. I’ve got to congratulate our remarkable team. They solved some unexpected big problems up there with nothing but short radio signals sent from down here. This LightSail test taught us a lot, just as we hoped it would, and so we’re ready to do some real solar sailing with LightSail’s 2016 mission. Let me finish by reminding everyone that this mission and next year’s flight are funded entirely by our supporters and especially our members— people of Earth, who want to participate in space exploration. We’re changing the way humankind explores space. Today is a big day for The Society and for space explorers everywhere.”
Mission Updates Far and Near
The Planetary Society’s Emily Lakdawalla tells us (via Twitter) that she has a history with jigsaw puzzles, one that finally paid off in the image below. You’re looking at her work on a partially de-scrambled image from LightSail, fragmentary because the entire image was not downloaded during a Cal Poly (San Luis Obispo) overflight on the afternoon of the 8th. The complete image should be downloaded later today, and perhaps shown at an upcoming press conference with LightSail engineering team leaders scheduled for Wednesday June 10 at 1730 UTC (1330 EDT).
At any rate, LightSail’s deployed sails are in view. Bill Nye, CEO of The Planetary Society, was on National Public Radio yesterday (audio here) in a brief spot in which he described the unfurling of the solar sail as a ‘sail Mary pass,’ a longshot required by circumstance as the spacecraft continued to tumble. If the phrase ‘sail Mary pass’ is inscrutable to you, you may not be familiar with American football, where ‘hail Mary pass’ has become a routine way of describing desperate plays made under pressure (this exhausts my knowledge of football — I know only baseball — and I will send you to the Wikipedia for details). At any rate, sail deployment was the purpose of this first LightSail mission, and sail deployment we had [although I’m now hearing we have only ‘partial’ deployment – stay tuned].
Ceres in Motion
Meanwhile, spectacular views from ongoing space missions continue to flow in. The next few weeks in particular are going to be rich in imagery as New Horizons closes on Pluto/Charon and Dawn continues its work at Ceres. The latest video animation from Ceres is something to enjoy more than once with a cup of coffee. If this doesn’t start your day right, what will?
Eighty images were combined to create the video, with the vertical dimension exaggerated by a factor of two. The views are from Dawn’s first mapping orbit at 13,600 kilometers and include later navigation images from 5100 kilometers, providing a detailed view of the terrain. I’m using the copy of the JPL video that io9 kindly posted to YouTube.
Dawn’s second mapping orbit began on June 3, with the rest of the month slated for observations about 4400 kilometers above the surface. Of the video, Dawn team member Ralf Jaumann (DLR, Berlin) says: “We used a three-dimensional terrain model that we had produced based on the images acquired so far. They will become increasingly detailed as the mission progresses — with each additional orbit bringing us closer to the surface.”
A Restive Comet
The Ma’at region at comet 67P/Churyumov-Gerasimenko’s ‘head’ clearly stays active even after it falls into shadow, as we can see from the latest imagery from the Rosetta spacecraft. Here we’re looking at jets of dust escaping into space, the result, researchers believe, of the increased heating of the comet as it moves closer to the Sun. Perihelion is coming up in mid-August, and this image was taken with a scant 270 million kilometers separating Sun and comet. “Only recently have we begun to observe dust jets persisting even after sunset”, says OSIRIS principal investigator Holger Sierks (Max Planck Institute for Solar System Research).
Image: This image of Rosetta’s comet taken on 25 April, 2015 from a distance of approximately 93 kilometers shows clearly distinguishable jets of dust after nightfall. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA.
OSIRIS scientist Xian Shi (MPS) adds that although the dusty cometary surface cools rapidly after sunset, deeper layers of the comet — evidently containing frozen gases — retain their heat, a phenomenon that has also been observed on comet 81P/Wild 2 and Deep Impact comet 9P/Tempel 1. We’ll learn a good deal more about how comets awaken as Rosetta follows 67P/Churyumov-Gerasimenko into the summer in the long fall toward perihelion.
LightSail Deployment Apparently Successful
After a nerve-wracking week in which contact was repeatedly lost and then regained, The Planetary Society’s LightSail has successfully charged its batteries and deployed its solar sail. Deployment began at 1947 UTC (1547 EDT) June 7, just off the coast of Baja California, with telemetry showing climbing motor counts and power levels consistent with ground testing. In a late afternoon update, Jason Davis also noted that the spacecraft’s cameras were on (see Deployment! LightSail Boom Motor Whirrs to Life).
If you’re following this mission closely, you’ll want to know about Ted Molczan’s page LightSail-A: Estimated Post-Sail Deployment Orbital Elements, with early predictions on
orbital decay with the sails extended. Bonnie Link (hflink.com) produced a map showing Monday’s LightSail passes over North America that you can see below. Here the white boxes are UTC times. The green arcs are sunlit, the blue in shadow and thus not visible.
Further confirmation of sail deployment came in a tweet from Davis noting that early post-deployment telemetry showed the spacecraft’s tumble rate had slowed dramatically. So a tiny spacecraft the size of a shoebox paid for by private donors has apparently managed to achieve its primary goal despite early questions about the effectiveness of its solar arrays and battery and a series of communications breakdowns caused by software issues. We’ll learn more later today, with the first team teleconference scheduled for 1330 UTC.
Images of full sail deployment should become available at some point today and I’ll update this post then. LightSail is likely to remain in space no more than about three days, given the drag it’s already experiencing. Remember that this was expected, as the first LightSail mission wasn’t designed as a test of true solar sailing, but rather a way to shake out deployment issues (and plenty of them, obviously, turned up, especially with regard to attitude control).
Cees Bassa reported the first visual observations from Dwingeloo (NOTE: In The Netherlands, not Australia, as I previously wrote) post-deployment, noting the increase in the rate of orbital decay:
LightSail [40661/15025L] was with the video camera! At about 15 deg elevation it first was invisible, but then gave a long flare up to about 4th or 5th magnitude. It was running about 10s early on the last JSpOC elset, suggesting the sail has deployed, increasing the drag significantly. Congratulations to the Planetary Society!
A subsequent observation (via Ron Dantowitz, Clay Center Observatory) was of about 4th magnitude and stable in brightness. As Ted Molczan points out, you can go to the Heavens Above page, enter your location, and view a chart of visible passes of LightSail. Or check the LightSail viewing tips Jason Davis gives in the article referenced above. Flyovers around dawn and dusk are the best time to see a spacecraft still illuminated by sunlight.
The View from Outside the Galaxy
The Russian Federal Space Agency (Roscosmos) has recently released a video (viewable here on YouTube) showing how a number of celestial objects might look if they were substantially closer to Earth than they are. The image of the Andromeda galaxy and its trillion stars projected against an apparent Earthscape is below. Unfortunately, this seems to be an astronomical image inserted into a view that purports to show what we would see in visible light. What we would actually see if we were standing in such a location is much different. After all, astronomical images are teased out of lengthy exposures in carefully chosen wavelengths.
In reality the Andromeda galaxy is gigantic even when viewed from 2.5 million light years, but I doubt the average person has any idea where it is in the sky. Although considerably wider than the Moon as seen from Earth, M31 is visually faint, a fact that reminds us of the importance of photographs and charged coupled devices (CCDs) in light gathering as we probe the universe. The human eye is a very limited instrument. As it happens, I’m currently reading Alan Hirshfeld’s Starlight Detectives: How Astronomers, Inventors, and Eccentrics Discovered the Modern Universe (2014), which examines the transition between sketches of visual observations and the steady advances in photography in the 19th Century that brought so much more of the galaxy into view.
About a year ago I looked at Poul Anderson’s novel World Without Stars, which tells of a starship crew dispatched to a planet far outside the Milky Way, a place where from 200,000 light years, galaxy-rise is a rather muted event (see The Milky Way from a Distance). Anderson depicts a galaxy filling 22 degrees along its major axis, but one that appears ‘ghostly pale across seventy thousand parsecs.’ In the same issue of Analog (June, 1966) in which the novel was originally serialized, John Campbell published Anderson’s letter describing the calculations that went into that description, and explaining why the galaxy from outside would be so dim.
One point came up which may interest you. Though the galaxy would be a huge object in the sky, covering some 20? of arc, it would not be bright. In fact, I make its luminosity, as far as this planet is concerned, somewhere between 1% and 0.1% of the total sky-glow (stars, zodiacal light, and permanent aurora) on a clear moonless Earth night. Sure, there are a lot of stars there — but they’re an awfully long ways off!
But it’s not just the distance. Galaxies, as UC-Santa Cruz astrophysicist Greg Laughlin has explained on his systemic site, are mostly empty space, or as he puts it, “To zeroth, to first, to second approximation, a galaxy is nothing at all.”
Building the Galactic Map
Interesting, then, to think about recent work on mapping the galaxy. The WISE mission (Wide-Field Infrared Survey Explorer) produced the data used in this effort to improve our understanding of the Milky Way’s spiral arm structure. Precisely because we don’t have the kind of overview of the galactic disk referred to above, we’re trying to sense the shape of our galaxy from within a disk obscured by dust and seen from our vantage two-thirds of the way out from the galaxy’s center. The new work supports a four-arm model for the Milky Way.
To derive this result, Denilso Camargo (Federal University of Rio Grande do Sul, Brazil) and team worked with WISE data revealing over 400 embedded star clusters, stellar nurseries of the kind that form in the dust- and gas-packed spiral arms, where most stars originate. Usefully for the purposes of the study, young clusters like these have not had the chance to drift out of the arms, thus providing a powerful tool for visualizing spiral arm structure.
In the paper, Camargo and team investigated 18 embedded clusters, seven of which are newly discovered in the WISE images — this complements a list of 437 new clusters Camargo recently published. The work supports the hypothesis that embedded clusters are predominantly found in the galactic thin disk and along its spiral arms. The Perseus and Scutum-Centaurus arms are the most prominent, while the Sagittarius and Outer arms show fewer stars but appear to have the same amount of gas as the other two arms.
Image: This illustration shows where WISE data revealed clusters of young stars shrouded in dust, called embedded clusters, which are known to reside in spiral arms. The bars represent uncertainties in the data. The nearly 100 clusters shown here were found in the arms called Perseus, Sagittarius-Carina, and Outer — three of the galaxy’s four proposed primary arms. Credit: NASA/JPL-Caltech/Federal University of Rio Grande do Sul.
Our Sun is in a minor arm called Orion Cygnus, marked on the image. You can see what a mapping challenge it is to work out, from within the disk itself, what kind of spiral arm structure exists. What we wouldn’t give for a perspective like the one above…
Supernovae in the Deep
Backing out to see the galaxy whole would take us far into intergalactic space, where we’re also learning about three supernovae found between galaxies in large galactic clusters. Melissa Graham, a UC-Berkeley postdoc, is the leader of a study of these objects. She also turns out to be a science fiction fan, one whose reference to the intergalactic deep isn’t the Anderson story I cited above, but Iain Banks’ novel Against a Dark Background (Orbit, 2009). There, the planet Golter lies a million light years from the nearest star.
We can imagine Anderson’s pale galaxy in an otherwise starless night sky, and Banks’ as well. Planets around the three supernovae in Graham’s study would have been destroyed in the event, but if there were any before, their night skies would likewise have been depleted of stars. “It would have been a fairly dark background indeed,” Golter adds, “populated only by the occasional faint and fuzzy blobs of the nearest and brightest cluster galaxies.”
Graham’s work using Hubble Space Telescope imaging confirms the earlier discovery (at the Canada-France-Hawaii Telescope on Mauna Kea) of the three supernovae and shows that they reside in a population of solitary stars in regions where the density of stars is about a million times less than we see from Earth. Gravitational interactions in massive galactic clusters can sometimes fling as many as 15 percent of a single galaxy’s stars out of the main disk, though the stars remain gravitationally bound within the cluster itself. Stars like these are going to be all but invisible unless they explode as Type Ia supernovae. Their explosions thus prove useful as a way to study the broader population of intracluster stars.
And this is intriguing: A fourth exploding star was found by the same observatory, one that may well be inside a globular cluster. If this is the case, we have an unusual event, the first time that a supernova has been found inside a globular cluster (GC). From the paper:
We have shown that the SN Ia in Abell 399 was very likely hosted by a faint red point-like source that has a magnitude and color consistent with both dwarf red sequence galaxies and red GCs. Our statistical analysis of the expected surface densities has shown that a dwarf galaxy is less likely at that location than a GC, due to the presence of a nearby elliptical galaxy. We have demonstrated that the rate enhancements in dwarfs or GCs implied by this new faint host are plausible under current observational constraints, and we do not reject either hypothesis.
Image: One of the four supernovae (top, 2009) may be part of a dwarf galaxy or globular cluster visible on the 2013 HST image (bottom). Credit: Melissa Graham, CFHT and HST.
The supernovae paper is Graham et al., “Confirmation of Hostless Type Ia Supernovae Using Hubble Space Telescope Imaging,” accepted at The Astrophysical Journal (preprint). The paper on galactic mapping is Camargo et al., “Tracing the Galactic spiral structure with embedded clusters,” Monthly Notices of the Royal Society Vol. 450, Issue 4 (20 May, 2015), pp. 4150-4160 (full text).
Science Fiction: An Updated Solar System
Having written yesterday about the constellation of missions now returning data from deep space, I found Geoffrey Landis’ essay “Spaceflight and Science Fiction” timely. The essay is freely available in the inaugural issue of The Journal of Astrosociology, the publication of the Astrosociology Research Institute (downloadable here). And while it covers some familiar ground — Jules Verne’s moon cannon, Frau im Monde, etc. — it also highlights Landis’ insights into the relationship between the space program and the genre that helped inspire it.
My friend Al Jackson has written in various comments here (and in a number of back-channel emails) about Wernher von Braun’s ideas and their relation to science fiction. As Landis notes, von Braun was himself a science fiction reader who credited an 1897 novel called Auf Zwei Planeten (Two Planets) by Kurd Lasswitz with inspiring his interest in rocketry. So, by the way, did Walter Hohmann, the German engineer who helped develop the area of orbital dynamics and demonstrated fuel-efficient ways to move between two orbits.
Although there have been numerous editions of the Lasswitz book since its original publication, it would not be until 1971 that an English translation (badly abridged) was published. The story depicts the discovery of a Martian base at the Earth’s north pole, with humans being taken back to Mars for a look at its canals. Lasswitz followed Schiaparelli and Percival Lowell in his fascination with a thriving, fertile Mars and the ancient race that lived there. The science fiction historian Everett F. Bleiler believes Lasswitz was a major influence on Hugo Gernsback and hence on the shape of science fiction in the 1920s and ’30s.
Image: Wernher von Braun with Walt Disney, with whom he collaborated on a series of three films. Credit: Wikimedia Commons.
But back to von Braun, who lived in a Germany in which Fritz Lang made the 1929 film Frau im Mond (‘Woman in the Moon’) with the help of rocket scientists Hermann Oberth and Willy Ley, who were hired to build a real rocket to launch in synch with the film’s opening. That stunt didn’t happen, but Oberth, Ley and von Braun would have worked together in the early 1930s as part of the Verein für Raumschiffahrt, a rocket club created by amateurs that would go on to influence the development of the deadly V-2.
In his early years in the United States, von Braun wrote a short science fiction novel in German about a trip to Mars, one that describes intelligent Martians in the context of a carefully designed mission. This is where things get tricky for the bibliographer. The technical appendix for this novel was published as Das Marsproject in 1952, appearing as The Mars Project the following year. The novel that had included it was not published until a 2006 edition from a Canadian publisher, who offered it as something of a historical curiosity (available as Project Mars: A Technical Tale, from Collector’s Guide Publishing).
Science fiction, meanwhile, had entered a robust post-war era in which spaceflight seized the public imagination. Landis comments:
The V-2 brought the reality of rockets public in a highly visible way; rockets were no longer comic-strip stuff, but real and highly-visible tools of warfare and, presumably, spaceflight. Following the end of the war, the rockets on science fiction magazine and covers now all looked remarkably like the V-2, and science fiction entered a golden age, with spaceflight stories written by a number of classic writers such as Robert Heinlein, Arthur C. Clarke (who was also noted for inventing the concept of a geosynchronous communications satellite), Isaac Asimov, and Andre Norton reaching new audiences.
It was in this same era that Collier’s ran its highly popular series of articles on von Braun’s ideas, with eight issues illustrated by Chesley Bonestell and other artists between 1952 and 1954. Soon von Braun was a household name thanks not only to Collier’s but also Walt Disney’s TV programs, on which he appeared three times. By 1956, von Braun had scaled down his Mars mission and published his later thinking in The Exploration of Mars, written with co-author Willy Ley.
What effect did the space program von Braun did so much to launch have on the science fiction of its day? It’s an interesting question, and one that Landis is ambivalent about, for as we began to probe the planets, we learned that they differed sharply from what writers had imagined:
In some respects the space program was a disappointment to science fiction. Spaceflight has not become as simple and ubiquitous (nor as cheap) as science fiction predicted. The cratered Mars revealed by the Mariner and Viking missions was not nearly as colorful a setting for science fiction as the Mars of Percival Lowell, with its canals and ancient, dying civilization; the furnace of the Venus surface revealed by Russian and American probes was not nearly as picturesque a setting for science fiction as the earlier swampy or even ocean-covered Venus hypothesized by astronomers when all that could be seen were clouds. Even the moon, dry and grey and mostly lacking in resources, was a disappointment.
Image: The April 30, 1954 issue of Collier’s, part of a series that explored von Braun’s ideas.
What grows out of this is a turn in the field’s direction. If the Solar System was the great venue of exploration for the science fiction of the Gernsback and later Campbell eras, by the mid-1960s many of its destinations had been revealed as barren places (think Mariner 4 and its images of a cratered, evidently lifeless Mars). Interstellar destinations, Landis notes, became the new terra incognita, but so did an entirely different kind of exploration into social and psychological realms (Bradbury becomes an interesting bridge between these two worlds). Landis doesn’t say it but I assume he’s thinking that trends like science fiction’s ‘New Wave’ grew directly out of this impulse.
Think back, then, to some of science fiction’s precursors. Johannes Kepler could write in his Somnium (1609) about space travel by non-technological means in a book that used the Moon as a place where basic ideas of astronomy could be discussed. Edgar Allen Poe would develop his 1835 story “The Unparalleled Adventure of One Hans Pfaall” with a lunar trip using actual technology, in his case a balloon, finding ways to make Earth’s atmosphere extend high enough for a balloon filled with a new kind of gas to get there.
From Voltaire (Micromegas, 1752) to Verne, science fiction shaped spaceflight around technologies available at the time, like Verne’s 274-meter cannon driven by 200 tons of gun cotton. What could be envisioned drove the narrative, but so did the desire for an exotic destination on which humans could walk. It’s interesting that we’re again seeing Solar System destinations as revealed by the space program as settings for modern SF — I think of tales like Gerald Nordley’s “Into the Miranda Rift” as a classic in this vein — but the interstellar impulse has never been stronger as SF continues its quest for alien, habitable worlds.
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