by Paul Gilster | Jan 7, 2008 | Outer Solar System |
The outer planets turn out to be far livelier places than we ever expected in those distant pre-Voyager days. Io set the tone, but look at all the activity we’ve found from Enceladus to Triton, and now we’ve got continuing Cassini revelations as well as new interpretations of what the Galileo probe found around Jupiter. Some of this is striking indeed, as witness a paper called to my attention by Larry Klaes and Adam Crowl discussing what may be happening on Europa in terms of energy.
The apparent presence of that sub-ice ocean on Europa has made it of great interest for astrobiology. The problem has been the availability of energy. Christopher Chyba (Princeton University), working with Kevin Hand (Princeton) and Robert Carlson (Caltech) have used data from Galileo’s Near-Infrared Mapping Spectrometer to determine that the Jovian moon could maintain an oxidized ocean. I should have the full text of this one shortly (right now I’m working solely from the abstract), but the gist of the matter is here:
We analyzed chemical sources and sinks and concluded that the radiolytically processed surface of Europa could serve to maintain an oxidized ocean even if the surface oxidants…are delivered only once every 0.5 Gyr. If delivery periods are comparable to the observed surface age (30-70 Myr), then Europa’s ocean could reach O2 concentrations comparable to those found in terrestrial surface waters, even if ~109 moles yr-1 of hydrothermally delivered reductants consume most of the oxidant flux. Such an ocean would be energetically hospitable for terrestrial marine macrofauna. The availability of reductants could be the limiting factor for biologically useful chemical energy on Europa.
If the term ‘macrofauna’ doesn’t get your attention, nothing will. The paper is Hand, Carlson and Chyba, “Energy, Chemical Disequilibrium, and Geological Constraints on Europa,” Astrobiology Vol. 7 (6) (2007), pp. 1006-1022 (abstract). I have two other articles this team published in the same journal last summer but I’m going to hold on those until I can dig into the full text of this latest paper, hoping to place the earlier work in a broader context. So we’ll return to Europa before long for more on this encouraging study.
Meanwhile, we continue to learn more about the changeable surface of Titan, harking back to the announcement of sand dunes on that moon in 2006 and later work examining Titan’s methane lakes. Jani Radebaugh (Brigham Young University) and team, who produced these earlier finds, now look at Cassini radar images showing mountains on Titan. The researchers used light and shadow in the radar images to calculate the slope of the mountains and thus determine their height.
The study indicates that Titan’s mountains are made of water ice, the largest being a scant two kilometers from base to peak. The small scale is interesting in its own right because it suggests erosion. We have a surface whose topography, rather than being the result of impact structures, seems to offer up lively geological processes. Which is not to say that impact craters have not played their own role in some places, but that a more likely explanation for what we are seeing is crustal activity, either through compression or separation of crustal materials.
The Titan paper is Radebaugh et al., “Mountains on Titan observed by Cassini Radar,” Icarus Vol. 192, Issue 1 (December, 2007), pp. 77-91 (abstract).
by Paul Gilster | Jan 5, 2008 | Culture and Society |
Studying people on Earth is one way to learn how long-duration space missions may affect the human body. The Human Test Subject Facility at the Johnson Space Center is looking for test subjects in a series of ‘bed rest’ studies that will be conducted over the next ten years. A head-down tilt bed is used, with extended periods in which the participants stay in bed with their body tilted slightly downward (a minus six degrees incline). The longest stretch involves 90 days lying in bed, hard to imagine, but those interested in volunteering for these compensated studies should have a look at the Human Test Subject Facility background page.
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Both of Saturn’s poles — and not just the southern one — seem to be home to a set of hot cyclonic vortices. That’s a bit of a surprise given the earlier belief that sunlit conditions contributed to the south pole hot spot. But the north pole has been out of sunlight since 1995. Just what are these apparently long-lasting vortices? Leigh Fletcher (University of Oxford) says this:
“The hot spots are the result of air moving polewards, being compressed and heated up as it descends over the poles into the depths of Saturn. The driving forces behind the motion, and indeed the global motion of Saturn’s atmosphere, still need to be understood.”
Indeed. Remember that Saturn’s north pole is also home to the odd polar hexagon, which frames the newly discovered vortex. The hexagon seems to be higher than previous studies indicated, extending to the top of the troposphere. The Juno mission, scheduled for a 2011 launch and 2016 arrival, will eventually tell us more. See Fletcher et al., “Temperature and Composition of Saturn’s Polar Hot Spots and Hexagon,” in Science Vol. 319, No. 5859 (January 4 2008), pp. 79-81 (abstract).
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Those of you who have been running SETI@home are part of a group of some five million volunteers who have signed up since the project began eight years ago. At present, according to this UC-Berkeley news release, the actual number running the software seems to be 170,000, working with 320,000 computers. But new receivers at Arecibo now generate 500 times more data for the project than before. The new data amounts to 300 gigabytes per day, which is why SETI@home could use more volunteers.
Says chief project scientist Dan Werthimer:
“Earthlings are just getting started looking at the frequencies in the sky; we’re looking only at the cosmically brightest sources, hoping we are scanning the right radio channels. The good news is, we’re entering an era when we will be able to scan billions of channels. Arecibo is now optimized for this kind of search, so if there are signals out there, we or our volunteers will find them.”
For more information and software, visit the SETI@home site.
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The 35th Carnival of Space is now up at Music of the Spheres. Particularly useful this time around is a compendium of space books published in the last year, gathered by the CollectSPACE site. My problem with such lists is that keeping up with Centauri Dreams leaves me little room for lengthy titles, so immersed do I find myself in much shorter papers and news releases. But there’s plenty of good reading here, particularly the titles by Ed Belbruno, Michael Michaud, Giancarlo Genta, and the excellent Living Off the Land in Space book by Gregory Matloff, Les Johnson and C Bangs.
by Paul Gilster | Jan 4, 2008 | Exoplanetary Science |
Another youthful star makes the news today, the eight million year old HR 4796A in Centaurus, some 220 light years from Earth. As we saw yesterday, we have much to learn about how planets form around young stars. This one hasn’t yielded a planet, but its dust disk, discovered in 1991, seems to derive from a planetary system in formation, the evident product of collisions between small bodies called planetesimals.
The latest work on HR 4796A draws on observations made by the Near-Infrared Multi-Object Spectrometer aboard the Hubble Space Telescope. The spectra that John Debes and Alycia Weinberger (Carnegie Institution, Washington) studied in visible and infrared light scattered by the star’s disk look red and imply the existence of the large organic carbon molecules called tholins. These are organic aerosols, complex molecules that, on Titan at any rate, remain suspended in the atmosphere and may contain chemical precursors to life.
Image: Red and near infrared wavelengths from the dust disk surrounding the star HR 4796A (masked in false-color image to make fainter disk visible) suggest the presence of complex organic molecules. The inner “hole” of the ring-shaped disk is big enough to fit our entire solar system and may have been swept clean of dust by orbiting planets. Credit: John Debes.
How sure are we of the tholin identification? The authors are understandably cautious, saying:
Longer wavelength scattered light observations will further constrain the grain models we have used, particularly around 3.8-4µm where a large absorption feature is seen for different grain sizes of tholins. This would help to directly confirm whether Titan tholins are an adequate proxy for the material in orbit around HR 4796A. Additionally, measuring the optical properties of organic materials in meteorites and from samples of the Stardust mission will provide further tests of our model grains.
If ‘tholins’ ring a bell, it may be because we’ve looked at them before as a factor on Titan, just one place in the Solar System they have been detected. But until now (and assuming these findings are confirmed) tholins have not been found outside the vicinity of our Sun. That makes this result interesting in itself, but even more so when placed in context. HR 4796A is twice as massive as the Sun and twenty times more luminous. Learning how planets form and looking for possible ways for life to evolve in systems sharply different from our own may teach us much about the broader mechanisms at work around other stars.
The paper is Debes, Weinberger et al., “Complex Organic Materials in the Circumstellar Disk of HR 4796A,” to be published in Astrophysical Journal Letters and available in preprint form online.
by Paul Gilster | Jan 3, 2008 | Exoplanetary Science |
People seem to be getting younger all the time. I’m told this is a common perception as you get older. In any case, it wasn’t so long ago that I met the son of an acquaintance at an informal gathering. He looked to me to be about fourteen years old, but something warned me not to assume this. I said “What do you do? Are you in school?” His reply: “No, I’ve got my own dental practice downtown.” I don’t know how old you have to be to become a dentist, but I do know it’s a lot older than fourteen!
Exoplanets and the stars they circle, on the other hand, seem to be mostly of a certain age, the denizens of relatively mature systems. Which is why TW Hydrae is so interesting. It’s an infant in stellar terms, at eight to ten million years old only a fraction of the Sun’s age. Like other stars in its age group, it is surrounded by a circumstellar disk of gas and dust, the sort of place where planets can form. And indeed, what seems to be the youngest planet yet detected has now been located within a gap in that disk. TW Hydrae b is about ten times as massive as Jupiter, orbiting in 3.56 days at a distance of some six million kilometers, or 0.04 AU.
Image: The newly discovered giant planet orbits around its young and active host star inside the inner hole of a dusty circumstellar disk (artist view). Credit: Max Planck Institute for Astronomy.
This work comes out of the Max Planck Institute for Astronomy (MPIA) in Heidelberg, where a team led by Johny Setiawan used European Southern Observatory equipment at La Silla (Chile) to make the find. TW Hydrae b turns out to be quite a catch. Starspots analogous to the sunspots on our own star can distort radial velocity readings, a particular problem with young stars whose surface is still relatively unstable. But MPIA’s Ralf Launhardt seems sure of the result, saying:
“To exclude any misinterpretation of our data, we have investigated all activity indicators of TW Hydrae in detail. But their characteristics are significantly different from those of the main radial velocity variation. They are less regular and have shorter periods.”
Bear in mind that none of the known extrasolar planets have, until now, been found around stars young enough to still have their circumstellar disks. Here again we’re looking at a limitation in our methods, younger stars having been excluded from many searches because of the difficulty of measurement caused by the above mentioned solar activity. Now we’re seeing some constraints on planetary formation, learning that a planet can form within a ten million year timeframe and, presumably, migrate inward as it interacts with the circumstellar disk to its present position.
Discoveries like this one add to our knowledge of planetary formation. Is this how all ‘hot Jupiters’ form? Things we need to learn more about include the average lifetime of a circumstellar disk, now thought to be somewhere between ten and thirty million years. And the core accretion model we’re talking about here is still challenged by the gravitational instability alternative, which theoretically allows much faster formation of such giant worlds. The new planet becomes a helpful test case in which to simulate both scenarios as we look for still younger planets.
The paper is Setiawan, Henning et al., “A young massive planet in a star-disk system,” Nature 451 (3 January 2008), pp. 38-41 (abstract). New Scientist also offers an article on TW Hydrae b.
by Paul Gilster | Jan 2, 2008 | Astrobiology and SETI |
Our recent discussions of active SETI, otherwise known as METI (Messaging to Extraterrestrial Intelligence), highlighted many of the key issues involved while demonstrating just how controversial the topic has become. But is there a way to look at METI experiments more objectively?
The San Marino Scale has been widely suggested as a method for assessing the risks we incur with deliberate transmissions from the Earth to other stars. Introduced by Iván Almár in 2005, the Scale is a work in progress that draws on the model of the Richter Scale, which quantifies the severity of earthquakes. The IAA SETI Permanent Study Group continues to work on it, hoping to measure “…the potential exposure of employing electromagnetic communications technology to announce Earth’s presence to our cosmic companions, or replying to a successful SETI detection.” More on the background of the Scale here.
Hungarian theorist Tibor Pacher has been calling my attention to the San Marino Scale for some time, and recently provided the link to a paper by Almár and Paul Shuch that looks at past METI transmissions. Note that this is a simple ordinal scale ranging from a risk factor of 1 (insignificant) and climbing to 10 (extraordinary). Almár and Shuch apply it to several widely known experiments, looking at them in terms of duration, directionality, information content and transmitter power, before assigning them a value on the scale.
The experiments are these:
- Evpatoria Planetary Radar Telescope: The source of three METI attempts, beginning with the Cosmic Call transmissions of 1999 and 2003, and including the so-called Teen Age Message to the Stars in 2001. The Evpatoria experiments targeted specific stars; their signal amplitude exceeded that of the quiet sun (see below) by about four orders of magnitude. Almár and Shuch assign them a San Marino Scale value of 7, which ranks as ‘high’ in significance.
- Arecibo Message: This was the first METI attempt ever made, sent in November of 1974 and targeting M13, a star cluster some 25,000 light years from Earth. By the paper’s calculations, the Arecibo message outshone the Sun by a factor of 105. The authors assign it a San Marino score of 8, ranking it as ‘far-reaching’ on the scale.
- Planetary Radar: This one is quite interesting. Planetary radars like that at Arecibo are routinely used (when funding is present!) to study near-Earth objects like asteroids and comets. Such transmissions are not intended as communications signals, but Alexander Zaitsev at Evpatoria has examined their potential for detectability over interstellar distances, and Almár and Shuch consider them inadvertent but de facto METI signals. Here we have a powerful signal but little information content, registering at a 6 on the San Marino Scale for ‘noteworthy.’
- Finally, and perhaps still more curious, is Allen Tough’s continuing work with the Invitation to ETI Initiative. Here we have no targeted interstellar transmissions but an attempt to reach out via the Internet. Thus even with the most powerful uplink signals to satellites, we are many orders of magnitude below the solar flux, so the intensity of the message can be discarded. On the other hand, the information content at the Invitation site is high, creating a combined San Marino score of 4, or ‘moderate.’ The Invitation strategy assumes an ETI civilization advanced enough to monitor terrestrial networks, and thus the presence of an ETI probe nearby.
A key in all such grading is the character of the information content in a message and the intensity of the transmitted signal. Almár and Shuch examine the latter in relation to the solar flux. Modeling a ‘quiet sun’ baseline (i.e., conditions of minimum solar noise), the researchers compare the outgoing signal to that flux. Obviously, the actual flux varies significantly, but the method is carefully chosen. From the paper:
By quantifying our transmissions relative to minimum solar flux, we are perhaps overstating the SNR [signal to noise ratio] which a given terrestrial transmission might impart on extraterrestrial receivers. This approach ensures that our resulting Intensity term, which contributes to the overall San Marino Scale value, is a best-case number as far as signal detectability is concerned. Since it is the potential negative consequences of transmission which we seek to quantify, we believe this conservative approach, which may slightly overstate signal impact, is appropriate to the function the San Marino Scale was intended to serve.
How useful are the San Marino Scale’s gradations? Are the distinctions between METI experiments helpful as we look to future possibilities? I’m glad to see an attempt being made to quantify the risks involved, but the Scale seems to be only a beginning given the complexity of the issues involved. And I agree with Tibor Pacher, who said this in a recent comment here:
I think it would be useful to devote some really interdisciplinary effort to investigate the role of such a measure like the San Marino Scale. Informatics, social sciences, law, etc. could say a lot about this; so I hope that people knowledgeable in these fields will pick up the issue.
‘Interdisciplinary’ is the key word for a topic with potential ramifications for our entire species. More details in the paper, which is Shuch and Almár, “Quantifying Past Transmissions Using the San Marino Scale,” available online. See also the same authors’ “The San Marino Scale: A New Analytical Tool for Assessing Transmission Risk,” Acta Astronautica 60 (January 2007), pp. 57-59, available here.
