by Paul Gilster | Aug 11, 2009 | Exoplanetary Science |
It doesn’t take much observation to realize that the early Solar System was a violent place. Mercury seems stripped of its outer crust, doubtless the result of a massive impact, while Uranus was knocked to one side at some point in its history, aligning its spin axis with the plane of the ecliptic. Venus was hit so violently that it rotates clockwise as seen from above, opposite to the other planets. At least, a collision is one of several theories that may explain Venus’ retrograde rotation, and it’s more than plausible.
100 light years from Earth is a place that reminds us of the impact that produced Earth’s own moon billions of years ago. HD 172555 is a young star in the southern constellation Pavo (the Peacock), its twelve-million year old system in its infancy. Spectral analysis from the Spitzer Space Telescope shows clear evidence of a collision much like that between the Earth and that early, Mars-sized object that once struck it.
Image: This artist’s concept shows a celestial body about the size of our moon slamming at great speed into a body the size of Mercury. NASA’s Spitzer Space Telescope found evidence that a high-speed collision of this sort occurred a few thousand years ago around a young star, called HD 172555, still in the early stages of planet formation. The star is about 100 light-years from Earth. Credit: NASA/JPL-Caltech.
The evidence? Amorphous silica in great quantity, common on Earth in obsidian rocks and tektites — the latter are evidently formed by the impact of meteorites on Earth’s surface, while the former is found around volcanoes. Silicon monoxide gas is also plentiful, the result of vaporizing rock, and all of this exists amidst the kind of rocky rubble you would expect from a major collision, including two populations of dust grains. From the paper:
The most likely cause of the unusual mineralogy is a hypervelocity (> 10 km sec-1) impact between two rocky planetesimals, similar to the ones which created the Moon, melted and differentiated Vesta, and stripped the surface crustal material off of Mercury’s surface in the Solar System, with the latter scenario being the most understood and likely. We hypothesize that the large, blackbody-like dust grains are the macroscopic fragments left over from the hypervelocity collision, while the small grains are the material recondensed (and recondensing) from the melt and SiO vapor created by the impact.
A slower-paced collision probably wouldn’t have left the debris of melted and vaporized rock found here, adding weight to the hypothesis that this was a high-speed event, one that destroyed the smaller body. The objects in question had a combined mass more than twice that of our Moon. Will the outcome be a small planet with a large moon? Geoff Bryden (JPL) is a co-author of the upcoming paper on this work:
“The collision that formed our moon would have been tremendous, enough to melt the surface of Earth. Debris from the collision most likely settled into a disk around Earth that eventually coalesced to make the moon. This is about the same scale of impact we’re seeing with Spitzer — we don’t know if a moon will form or not, but we know a large rocky body’s surface was red hot, warped and melted.”
The paper is Lisse et al., “Abundant Circumstellar Silica Dust and SiO Gas Created by a Giant Hypervelocity Collision in the ~12 Myr HD172555 System,” Astrophysical Journal 701 (August 20, 2009), pp. 2019-2032 (abstract). A preprint is also available.
by Paul Gilster | Aug 10, 2009 | Astrobiology and SETI |
An exotic planetary environment right here in the Solar System may be a useful test for answering the key question of how common life is in the universe. So argues Jonathan Lunine (University of Arizona) in an upcoming paper. Lunine believes there is a plausible case for life to form on Titan, and that if we were to find it there, its very dissimilarity from Earth would make it a test-case for life in other extreme environments of the sort that may be common in the cosmos.
We’d like to answer this question locally because it may be some time before we can answer it around other stars. After all, the best spectral signatures we can hope to get from the atmospheres of Earth-analogues elsewhere are quite possibly going to be ambiguous. Molecular oxygen can be a sign of photosynthesis but also of the abiotic escape of water from the upper atmosphere. Methane in the same atmosphere makes biology more likely but may be, Lunine thinks, difficult to detect from Earth.
Image: One way to search for life on Titan. A Montgolfière, or hot air balloon, floats high above a methane-ethane lake on the distant world. A power source provides heat for buoyancy, which in turn is regulated by computer-controlled opening and closing of flaps on the balloon. Credit: Tibor Balint, JPL/Caltech, J. Lunine (UA).
Thus the utility of finding a second origin of life right here in the Solar System. And it’s interesting that Europa may not be as viable a candidate for this as Titan. Although Richard Greenberg in his book Unmasking Europa (Springer, 2008) has made a strong case for finding biological materials in the crustal ice and even on the surface through upwellings of oceanic water as the surface ice shifts, Lunine is skeptical:
…this assumes that organisms would be preserved in a recoverable fashion, and that the ice layers where such organisms exist would be cycled near enough to the surface to be sampled without being so close that the prodigious particle radiation would destroy the organic remains. Even so, delivery and operation of a vehicle designed for the surface of Europa, given the intense particle radiation flux from the Jovian magnetosphere, would be a challenge.
Enceladus, then? As with Europa, Lunine worries about the danger, however faint, of cross-contamination from Earth, even though radiation presents much less of a problem. But why take even the smallest chance of contamination via impacts from space debris (possibly carrying life from Earth, or even Mars) when Titan is available? There water is frozen out, to the point that even if there is a layer of liquid water beneath Titan’s ice crust, it is probably more than fifty kilometers deep.
Like Europa, Titan doubtless receives debris from hypervelocity impacts on Earth, but any surviving terrestrial life form should quickly perish in the cryogenic conditions there. But Titan’s lakes and seas of liquid ethane and methane could provide an interesting context for the development of an altogether different kind of life:
Methane and ethane have their own problems, including their non-polarity,
which means that as liquids they provide no support for molecular structures that depend on interaction with the liquid for their stability. But small amounts of polar molecules might exist in the Titan seas. Furthermore, an interesting “bio”chemistry might be built around the dominance of hydrogen bonding between organic molecules immersed in the non-hydrogen-bonding ethane and methane…, and in such a biochemistry, the low temperatures and consequent slow reaction rates are not necessarily a disadvantage…
Here Lunine is drawing on work by Steven Benner (University of Florida) and colleagues and goes on to summarize their conclusions:
While nothing like a complete, theoretical biochemistry in liquid methane and ethane has been constructed, there is no particular property or set of properties of liquid methane and ethane that could lead one to a priori rule out in such a medium a kind of self-sustaining, replicating, catalytic organic chemistry that might be called life.
Finding life of independent origin in our Solar System would radically revise our view of life elsewhere. Ponder this: While a planet around an M-dwarf at a distance that would allow liquid water would face problems like solar flares and tidal lock in developing its own forms of life, a planet 1 AU from such a star would be much like Titan, at a distance where liquid methane and ethane could survive on the surface. Were we to find life on Titan, we would extend the concept of a habitable zone to include that kind of environment, and because M-dwarfs outnumber stars like the Sun by a huge amount, we would have found the prospects for life that much more likely.
It could be argued, of course, that the planets around an M-dwarf might easily mimic Mars or Europa as much as Titan, but Lunine’s point is that in our system, only Titan offers both (relatively) easy access to the habitable environment of interest and an assurance that any life found there would by necessity be of an origin independent of Earth life. He calls Titan ‘a fishing license to broaden the search for planets around other stars’ from Earth-like worlds to those like Saturn’s huge moon.
The paper is Lunine, “Saturn’s Titan: A Strict Test for Life’s Cosmic Ubiquity,” accepted for publication in Proceedings of the American Philosophical Society and available online.
by Paul Gilster | Aug 7, 2009 | Exoplanetary Science |
Unlike the Cassini Saturn orbiter, which we looked at yesterday in the context of cryovolcanism on Titan, the Kepler spacecraft has but a single scientific instrument. It’s a photometer based on a Schmidt telescope design with a 95 cm aperture and a field of view larger than 100 square degrees. Measuring brightness variations for over 100,000 stars, Kepler is the first mission that should be able to detect Earth-size planets in the habitable zones of their stars.
That made yesterday’s news conference an eagerly anticipated event, but we have to remember that it’s going to be a while before we start talking about terrestrial planet detections. It takes multiple transits and much data analysis to make that possible, and a transiting world at roughly Earth-like distance from its star will demand several years of work. Kepler’s baseline mission is three and a half years, more than enough to make such detections, and the good news is that the instrument works.
Image: Magnified Kepler measurements of the planet HAT-P-7b showing transits and occultations. Credit: NASA.
The lightcurve of the planet HAT-P-7b shown at the news conference yesterday was dramatic proof. It was based on a mere ten days of test data collected during Kepler’s commissioning period, before science operations officially began. And even before the instrument has been fully calibrated and its data analysis software fine-tuned, it was able to detect HAT-P-7b’s atmosphere. The level of exactitude in these measurements has everyone talking. Here’s William Borucki, Kepler principal science investigator:
“When the light curves from tens of thousands of stars were shown to the Kepler science team, everyone was awed; no one had ever seen such exquisitely detailed measurements of the light variations of so many different types of stars.”
The paper on HAT-P-7b is being published in Science today, describing work on a planet that is roughly a thousand light years from Earth. It was a useful early target for calibration given that this gas giant orbits in a mere 2.2 days, a ‘hot Jupiter’ some 26 times closer to its star than Earth is to the Sun. That makes for numerous transits in short order, and in this case offers observations of a planet that is hot enough to be glowing like the burner of a stove (more in this news release).
Both initial transit and occultation were clearly visible as the planet first passed in front of, then behind the star as seen from Kepler’s vantage point. What we learn is that HAT-P-7b’s atmosphere has a dayside temperature of more than 2350 degrees Celsius (4310 degrees Fahrenheit). And here’s the key: The observed brightness variation is a mere one and a half times what would be expected from a terrestrial planet transit.
The prospects for terrestrial planet detections, in other words, have never looked better. The NASA image above gives us an idea how the story will play out. Here are planets graphed by mass and orbital distance, with plentiful representation at the high end and no planets of Earth mass or lower yet detected in the habitable zones of their stars. We should be able to offer a significantly different chart within just a few years.
The paper is Borucki et al., “Kepler’s Optical Phase Curve of the Exoplanet HAT-P-7b,” Science Vol. 325. no. 5941 (7 August 2009), p. 709 (abstract).
by Paul Gilster | Aug 6, 2009 | Culture and Society |
Propulsion Book Discussion Available
Give a look, and then a listen, to David Livingston’s August 3rd Space Show. Livingston talked to Tau Zero founder Marc Millis and Eric Davis (Institute for Advanced Studies at Austin) about the recently published Frontiers of Propulsion Science, calling it “the ultimate research and reference book to have for advanced and out-of-the-box space propulsion science” and adding:
“As you will hear me say over and over again, this is a must own and a must read book. It is also a very valuable research and reference book for anyone wanting to know propulsion and physics facts regarding space travel and related issues.”
Knowing how much time and effort Marc and Eric spent coordinating the many contributions from leading authorities that went into this book, it’s a pleasure to see Frontiers of Propulsion Science achieving this kind of acclaim. At 739 pages and stuffed with technical and scientific papers aimed at scientists and university students, the book is an exhaustive treatment of where we stand today and where we’re going.
Cryovolcanism on Titan?
We’ve used radar imaging to get a good look at about a third of Titan’s surface, thanks to the phenomenal Cassini orbiter, examining a geologically young surface with numerous lakes of liquid hydrocarbons (such as methane and ethane) in the northern latitudes, few impact craters and chains of mountains. But word at the IAU General Assembly, meeting in Rio de Janeiro, is that another instrument aboard Cassini — the Visual and Infrared Mapping Spectrometer (VIMS) — has detected evidence of volcanic activity in the form of cryovolcanoes on the distant moon.
The focus is on an area called the Hotei Regio, a region whose variable infrared signature suggests a cycle of ammonia frost appearing and disappearing. The implication is that ammonia from Titan’s interior is being delivered to the surface and subsequently dissipating or being covered. Radar imaging of the area shows structures that resemble terrestrial volcanoes, the apparent mechanism for the ammonia deposition (more in this IAU news release).
Here’s Robert Nelson (JPL), who discussed the matter in Rio:
“These new results are the next advance in this exploration process. The images provide further evidence suggesting that cryovolcanism has deposited ammonia onto Titan’s surface. It has not escaped our attention that ammonia, in association with methane and nitrogen, the principal species of Titan’s atmosphere, closely replicates the environment at the time that life first emerged on Earth. One exciting question is whether Titan’s chemical processes today support a prebiotic chemistry similar to that under which life evolved on Earth?”
Related: Imagine what a robotic rover could do on Titan. Giancarlo Genta, in the second of his presentations at the recent Aosta conference, discussed the design parameters of such a rover, one that included the capability of crossing terrain as well as sailing across ethane lakes. Two papers by Dr. Genta on this topic should be appearing in Acta Astronautica in coming months, both now in Proceedings of the Sixth IAA Symposium on Realistic Near-Term Advanced Scientific Space Missions, Aosta Italy (2009). They are “A Small Robotic Rover for Titan Exploration: Part 1: General Layout” (pp. 105-110 in the proceedings), and “Part 2: Strategies for Trajectory Control,” pp. 111-116.
Update from Denver Propulsion Conference
Another report from the 45th Joint Propulsion Conference & Exhibit in Denver focuses on beamed energy propulsion, in this case ground-based laser systems that can heat fuels like hydrogen and lighten the mass of spacecraft. Leik Myrabo (Rensselaer Polytechnic Institute) has been working this turf for some time. The founder of Lightcraft Technologies is working under a grant from the US Air Force to use laser methods for satellite launches. Conveniently in terms of the conference, he’s in Brazil testing these ideas in collaboration with the Brazilian air force.
Myrabo thinks advances in energy-beaming technology in recent years have brought these systems closer to reality, but the article gives a nod to the skeptics:
Kevin Johnson, a space exploration and spacecraft propulsion manager at Lockheed Martin Space Systems in Denver, for example, expresses concern about the potential for atmospheric interference with the beam. Greg McAllister, a senior staff propulsion engineer also at Lockheed Martin, agrees and says that an energy source powerful enough to propel a rocket could also burn it up. (McAllister is presenting a paper at the conference on testing the pulse throttle thrusters used for the Mars Phoenix mission.)
Johnson says that while the system could generate enough power from a ground-based station and reduce costs, it is “20-plus years” from being feasible.
Twittering in Case of Emergency
Some of you are aware of the software problems that caused Monday’s Centauri Dreams post to be delayed for a day. Until I could fix the problem, I had no way to post on the site itself, which made Twitter a useful venue. In the event of future glitches, you may want to check the Centauri Dreams Twitter feed, where I’ll update problematic situations like Monday’s. I had no idea when Monday started that I would spend most of the day researching a glitch that came out of the blue. Suffice it to say that repairing the site internals on a rush basis can play havoc with anyone’s schedule. Let’s hope it doesn’t happen again, but knowing the Net, anything’s possible.
Addendum: Well that’s aggravating. Just after I published this nod to Twitter, I tried to use it to comment on the incoming Kepler news. You can follow the news conference (it’s now 1425 EDT) here. The Twitter site is up but it won’t take new posts…
by Paul Gilster | Aug 5, 2009 | Advanced Rocketry |
A story on MIT’s Technology Review site looks at ion propulsion, and specifically at improvements made in the technology at Glenn Research Center. Comparing the recent work to the engines used in the Deep Space 1 and Dawn missions, the story quotes GRC’s Michael Patterson as saying, “We made it physically bigger, but lighter, reduced the system’s complexity to extend its lifetime, and, overall, improved its efficiency.”
That’s good news, of course, and Patterson presented it to the AIAA’s Joint Propulsion Conference & Exhibit this week in Denver. With sessions on everything from Electric Propulsion Thruster Wear and Life Assessment to Advanced Propulsion Concepts, Denver was clearly the place to be for propulsion mavens. An entire session was devoted to the new ion thrust work, which goes under the name NASA’s Evolutionary Xenon Thruster (NEXT).
Quoting from an abstract of one of the talks:
The NASA NEXT thruster is engineered to be extremely flexible in terms of input power and specific impulse, while maintaining acceptable efficiency, and embodies a number of technological advances over previous ion engine systems.
Image: Follow the blue light. The xenon ion engine in this photograph was being tested in a vacuum chamber at the Jet Propulsion Laboratory. Credit: NASA/JPL.
Deep Space 1, launched in 1998, used an ion propulsion system that proved workable even if such engines produce only a tiny thrust. A continuous small push adds up, so one proof of concept was to keep Deep Space 1’s engine burning, which it did for 677 days (a spare of the same engine would later run for almost five years in NASA tests). The system used xenon gas released into a thrust chamber surrounded by magnets. The gas was ionized and expelled from the back of the chamber by electrically charged grids, demonstrating a method that, compared to chemical engines, yields ten times the thrust for the same amount of fuel.
So an ion engine is efficient, but the thrust is small. Used properly, however, the advantages of these systems are evident. Here’s an excerpt from a NASA description of the basic ion system, not yet amended to include the most recent NEXT work:
Modern ion thrusters are capable of propelling a spacecraft up to 90,000 meters per second (over 200,000 miles per hour (mph). To put that into perspective, the space shuttle is capable of a top speed of around 18,000 mph. The tradeoff for this high top speed is low thrust (or low acceleration). Thrust is the force that the thruster applies to the spacecraft. Modern ion thrusters can deliver up to 0.5 Newtons (0.1 pounds) of thrust, which is equivalent to the force you would feel by holding nine U.S. quarters in your hand. To compensate for low thrust, the ion thruster must be operated for a long time for the spacecraft to reach its top speed.
Deep Space 1 was only one of many ion thrust missions; in fact, testing on the basic concepts was begun in the 1970s. Since then, we’ve seen missions like Artemis (ESA), Hayabusa (an asteroid rendezvous mission by JAXA, the Japanese space agency), and Smart 1 (an ESA moon mission). The Dawn mission uses ion thrusters in its mission to the asteroids, while several other ion missions are now in progress.
Like Deep Space 1’s basic design, the improved NEXT engine also works with xenon gas. Here’s Brittany Sauser’s description from the Technology Review article:
The new ion engine builds upon the electric propulsion systems used by both DS1 and Dawn… It uses the same method to achieve thrust: xenon gas flows into a reaction chamber inside the engine and is ionized by electrons; electromagnets positioned around this chamber enhance the efficiency of ionization. Electrodes positioned near the engine’s thrusters (known as ion optics) are then used to accelerate the ions electrostatically and shoot them out of the exhaust to push the spacecraft forward.
Compared to earlier iterations, however, NEXT offers a larger throttling dynamic range, meaning it can operate for longer periods of time by adjusting power levels along the way. But the choice of destination says much about the problems these technologies face. Solar energy can power up an ion engine in the inner system, but it will take nuclear sources to keep one operational at the distance of the outer planets. The efficiencies involved in ion engines compared to the chemical alternative make resolving nuclear safety concerns an imperative.
