by Paul Gilster | Dec 14, 2007 | Outer Solar System |
The accepted take on the formation of Saturn’s rings not so long ago was that they had emerged within the past 100 million years. The most likely driver: A comet that broke a larger moon into pieces, forming ring features seemingly consistent with what the Voyagers saw in the 1970s and the Hubble telescope has seen ever since. But leave it to Cassini to stir things up yet again with much more precise data suggesting that the rings did not form in a cataclysmic event but are continually recycled.
Thus Larry Esposito (Colarado University, Boulder), who is also principal investigator for Cassini’s Ultraviolet Imaging Spectrograph:
“The evidence is consistent with the picture that Saturn has had rings all through its history. We see extensive, rapid recycling of ring material, in which moons are continually shattered into ring particles, which then gather together and re-form moons.”
The findings received less media attention than Voyager 2’s crossing of the termination shock, discussed at the same American Geophysical Union meeting in San Francisco recently. But they’re interesting nonetheless. Because what Cassini is telling us is that Saturn’s ring system is probably a good deal larger than previously understood, which would explain why the rings aren’t darker. Incoming meteoric dust, it was once thought, should cause such a darkening, but the rings clearly appear bright when seen from Earth and, obviously, from Cassini.
Image: An artist concept of a close-up view of Saturn’s ring particles. The blue particles are composed mostly of ice and clump together to form elongated, curved aggregates, continually forming and dispersing. The space between the clumps is mostly empty. The largest individual particles shown are a few yards across. Image courtesy NASA/JPL/University of Colorado.
“The more mass there is in the rings, the more raw material there is for recycling, which essentially spreads this cosmic pollution around,” says Esposito. “If this pollution is being shared by a much larger volume of ring material, it becomes diluted and helps explain why the rings appear brighter and more pristine than we would have expected.”
We’ll get details in Esposito’s upcoming paper in Icarus. Clumps of icy debris form and break apart with regularity, as observed by Esposito’s team using stellar occultation, examining the rings’ effect on starlight passing through them. A far more ancient structure than once thought, then, but the whole system is also in a continuous state of change as these processes churn on across their vast expanse. A video animation of Saturn’s F-ring undergoing these changes can be viewed here.
by Paul Gilster | Dec 13, 2007 | Exoplanetary Science |
Gliese 581 is back in the news with a flourish. Astronomy & Astrophysics is publishing two independent studies of the system asserting that at least one of the inner planets is indeed located within the habitable zone of that star. Gliese 581 c and d are noteworthy every time they’re mentioned. Of five and eight Earth masses respectively, they are the first exoplanets ever considered serious candidates for habitability.
M dwarfs have inherent problems in terms of habitability, not the least of which is the tidal lock that planets in the HZ of such stars presumably experience, keeping one side perpetually dark. But models of atmospheric circulation exist that overcome that obstacle, and the intense magnetic activity of early M star life (producing dangerous flares) is no longer considered a necessary disqualifier for all forms of life. It’s going to be a while before we have any definitive answers, but current thinking is that habitable M dwarf planets are very much in the picture, with the inner two Gliese 581 worlds due for a look from future missions like Darwin.
So how big is the actual habitable zone around Gl 581? Franck Selsis (CRAL and LAB, France) and team look at the HZ in terms of liquid water at the surface, coping with the difficult problem of cloud modeling that makes precise determinations so difficult. Werner von Bloh (PIK, Germany) and team examine a more truncated HZ where photosynthesis could be possible. Interestingly, von Bloh’s work depends on the age of the planet, looking at the balance between sources of atmospheric CO2, released through ridges and volcanoes, and CO2 sinks, which consume the gas through weathering processes. Too old a planet might not release enough CO2 to keep the surface above the freezing point of water.
Here’s von Bloh’s conclusion, after ruling out Gliese 581c for habitability as being too close to its star. The team has looked at various climate models for Gl 581d, finding two of them promising:
A planet with eight Earth masses has more volatiles than an Earth size planet to build up such a dense atmosphere. This prevents the atmosphere from freezing out due to tidal locking. In case of an eccentric orbit of Gl 581d (e = 0.2), the planet is habitable for the entire luminosity range considered in this study, even if the maximum CO2 pressure is assumed as low as 5 bar. In conclusion, one might expect that life may have originated on Gl 581d. The appearance of complex life, however, is unlikely due to the rather adverse environmental conditions. To get an ultimate answer to the profound question of life on Gl 581d, we have to await future space missions such as the TPF/Darwin. They will allow for the first time to attempt the detection of biomarkers… in the atmospheres of the two super-Earths around Gl 581.
And this from the Selsis paper:
Darwin/TPF-I and TPF-C could eventually reveal what the actual properties of the atmosphere of Gl 581c and Gl 581d are. From their thermal light curves we could infer if a thick atmosphere is making the climate more or less uniform on both the day and night hemispheres of these planets, despite a (nearly?) synchronized rotation… Visible and mid-IR water vapor bands could be searched in the atmosphere of Gl 581d to confirm its habitability. Mid-IR spectra of this planet could also reveal other greenhouse gases at work. Spectral observations of Gl 581c could potentially distinguish between a Venus-like atmosphere dominated by CO2 or an H2O-rich atmosphere. The detection of O2 on this planet would generate a fascinating debate about its possible origin: as either a leftover of H2O photolysis and H escape or a biological release. There is certainly no doubt that Gl 581c and Gl 581d are prime targets for exoplanet characterization missions.
Image (click to enlarge): Illustration of the habitable zone (HZ) boundaries as obtained by the two teams. The upper part of the figure shows the HZ of the Sun (at its present age). The red curve shows only the most extreme outer limit of the HZ. The actual outer boundary is indeed located somewhere between 1.7 and 2.4 AU. The green limits show the boundaries of the photosynthetic zone as computed with the model by von Bloh et al. The middle part of the figure shows the limits of the HZ of Gliese 581 computed with the atmospheric models from Selsis et al. The lower part illustrates the boundaries of the photosynthetic zone computed with the geophysical models from von Bloh et al. The boundaries are shown for several possible ages (5, 7, and 9 Gyr-old) of the Gliese 581 planetary system. Following the latest estimation, Gliese 581 would be 7 Gyr-old. The purple bars surrounding planets Gliese 581 c and d illustrate the variable distance to the star caused by the eccentricity of the orbits. Credit: Astronomy & Astrophysics.
Note that the Selsis paper does not rule out GL 581c’s possible habitability, although noting that the world receives thirty percent more energy from its star than Venus does from the Sun. Uncertainties in cloud properties make the final call problematic. Like von Bloh’s team, however, Selsis leans toward the possibility of CO2 ice clouds creating much better conditions for life on GL 581d, with the potential for greenhouse gases to maintain habitable conditions over long periods of time.
The papers are von Bloh et al., “The habitability of super-Earths in Gliese 581” (abstract) and Selsis et al., “Habitable planets around the star Gliese 581?” (abstract), appearing in Astronomy & Astrophysics Vol 476-3 (2007), pp. 1365-1387. Note that a third paper, to be published in 2008 in the same journal, will examine the dynamical stability of the Gl 581 planetary system. That paper, by Hervé Beust (LAOG France) and team, will conclude that the GL 581 system appears dynamically stable, thus rendering the climate on both these worlds potentially stable as well.
by Paul Gilster | Dec 12, 2007 | Exoplanetary Science |
We’re learning more and more about HD 189733b, an extrasolar planet some 63 light years from Earth in the direction of the constellation Vulpecula. This transiting ‘hot Jupiter’ orbits once every two days about three million miles out from its primary. David Charbonneau (Harvard-Smithsonian Center for Astrophysics) and team recently measured an unusual spectrum from the planet’s atmosphere using the Spitzer Space Telescope. Looking for water, carbon dioxide and methane, they found instead a flat spectrum that Charbonneau thinks may indicate the presence of dark silicate clouds.
Now we have further work, this time using Hubble Space Telescope data, that points to the presence of haze in the atmosphere of HD 189733b. That’s an interesting finding to which we can add another result: Studying how light varies when the planet makes its transit indicates that this world has neither Earth-sized moons or a discernible ring system. Moreover, we’ve got a fairly good read on the temperature of its atmosphere, a toasty seven hundred degrees Celsius. That’s a lot to learn about a place we can’t image with the best equipment we possess.
Like Charbonneau’s team, the group studying the Hubble data, led by Frédéric Pont (Geneva University Observatory), knew what they were looking for. But there were no signatures characteristic of sodium, water or potassium. Weighing what they found in the entire planetary spectrum, the scientists infer that high level hazes about 1000 kilometers in altitude are present. Likely haze constituents are tiny particles of condensates of iron, silicates and aluminum oxide dust. Another finding: A starspot on HD 189733b’s surface thought to be over 80,000 kilometers across.
Pont points to the significance of the work in terms of future goals:
“One of the long-term goals of studying extrasolar planets is to measure the atmosphere of an Earth-like planet. This present result is a step in this direction. HD 189733b is the first extrasolar planet for which we are piecing together a complete idea of what it really looks like.”
True enough, but let’s be cautious. I can recall when we had a pretty good idea of what the Jovian moons looked like. Then Voyager changed everything, and we realized that the outer planets weren’t simply orbited by small worlds much like our Moon. The stunning variety that exists around Jupiter alone reminds us to hedge our bets when weighing data from so much further out. Still, this work, which studies starlight passing through the atmosphere of a giant planet in transit, is a marker of how we’re learning to use existing equipment to begin filling in planetary details. Bit by bit, HD 189733b is getting to be known. The unanswered question is, how many surprises does it hold in store for the future?
Addendum: The lack of detected sodium in the atmosphere of this world, as one reader has already pointed out (thanks, Luis!), seems to contradict the recent findings from the HET instrument in Texas, discussed in this earlier Centauri Dreams article. Comments?
by Paul Gilster | Dec 11, 2007 | Missions, Outer Solar System |
The recent Voyager news, reported from the American Geophysical Union conference in San Francisco and recently discussed here, has drawn attention to the apparent asymmetry of our Solar System. Voyager 1 crossed the termination shock — where the solar wind first encounters the thin gas of the interstellar medium — some three years ago. But that was a crossing with a difference. Voyager 1 went through the termination shock just once. Voyager 2 has apparently crossed it five times and may encounter it again. Ahead in a decade or so: The heliopause, where the Sun’s influence effectively ends.
Thus we have a glimpse of how the solar wind varies with changes in the Sun’s activity level, pulsating as the solar cycle swings from solar flares into quiet periods, pushing the shock area out a bit farther, then contracting it. And while Voyager 1’s plasma science instrument had stopped working when it encountered the termination shock, Voyager 2’s is working well and making detailed measurements.
John Richardson, principal investigator for the instrument, calls the area Voyager 2 is now in “…a different kind of shockwave than we’ve seen anywhere else.” Among the new Voyager findings is the existence of an unexpectedly strong magnetic field in the surrounding interstellar region, one that is distorting the bubble of outflowing gases from the Sun. A second surprise: The temperature just outside the termination shock, hotter than inside it, was nonetheless found to be fully ten times cooler than had been expected. This may be the result of limitations in the plasma instrument, though other ideas may emerge.
J. Randy Jokipii (University of Arizona) is already looking farther out:
“The Holy Grail for me will be when the spacecraft begin traveling in pure interstellar space. That’s maybe 10 years away. Scientists now can start thinking about what they want to look for when the Voyagers break through the last barriers to true interstellar space.”
Yes, providing we’re still getting good data from our doughty spacecraft, which will probably be the case. Our first look at actual interstellar space, the medium through which, let’s hope, our interstellar probes will one day fly, will come as a triumphant vindication for the Voyagers, both of them built for a five year mission. John Belcher and Mark Bessette at MIT have put together several nifty animations showing the solar wind interacting with the interstellar medium. Keep them in mind as the Voyagers push on through the heliosheath on the way to their final exit of the system.
by Paul Gilster | Dec 10, 2007 | Sail Concepts |
Some day we may be using solar sails to take payloads into an increasingly busy Solar System. Let’s hope that day isn’t far off, because the technology looks practical. But as we study solar sail methods, in which the sail is pushed by the momentum of photons, we also want to keep magnetic sail possibilities firmly in mind. A magsail could theoretically ride the ‘solar wind,’ that stream of charged particles pushing out from the Sun at speeds approaching 1.5 million kilometers per hour.
Dana Andrews (Andrew Space) considered the problem of drag posed by interstellar ramjet concepts back in the 1980s, and along with fellow engineer Robert Zubrin went on to ponder how enormous magnetic sails could take advantage of the solar wind. The beauty of the concept is that you do away with a material structure of the sort so tricky to deploy in large solar sail designs. Instead, you generate the magsail from within the spacecraft. Couple this with a particle beam and you may have an interstellar vehicle on your hands.
Robert Winglee (University of Washington) has been working with his own version of such a sail under a methodology called Mini-Magnetospheric Plasma Propulsion, or M2P2, for some time. A magsail to the heliopause would really move out, continuously gaining momentum from the flux of protons and electrons flowing outward from the Sun. Launched today, such a vehicle might cross the heliopause before either of our Voyagers. But of course it won’t be launched today, for we have much to learn not only about how magsails might function, but also about the nature of the solar wind itself.
Image: An X-ray jet launching plasma out into the solar system from the Sun’s north polar coronal hole. This image was taken Jan. 10, 2007, by Hinode’s X-ray telescope. Credit: Hinode/SAO/NASA/JAXA
Japan’s Hinode mission is helpful on that score. It’s clear from Hinode data recently reported in Science that magnetic waves are critical in driving the solar wind outward. So-called Alfvén waves, magnetic waves that transfer energy from the Sun up through its atmosphere, have been theorized as the cause of the solar wind, but it took Hinode to clarify the role they play. Thus Alexei Pevtsov, a Hinode program scientist:
“Until now, Alfvén waves have been impossible to observe because of limited resolution of available instruments. With the help of Hinode, we are now able to see direct evidence of Alfvén waves, which will help us unravel the mystery of how the solar wind is powered.”
Hinode carries a high resolution x-ray telescope, allowing researchers to observe x-ray jets low in the corona at the Sun’s poles. The upshot: Whereas before we only saw a few of these jets of hot plasma a day, Hinode saw an average of 240 a day. Alfvén waves and the plasma bursts of these jets are both formed when oppositely charged magnetic fields collide and release energy, a process called magnetic reconnection. The x-ray jets and the Alfvén waves within them are thus identified as major factors in the creation of the solar wind, a theory born out by another team’s findings that the solar chromosphere is riddled with Alfvén waves.
The more we learn about the power of the solar wind, the more interesting it becomes not only as a natural phenomenon but as a propulsion concept. The European Space Agency offers an overview of the recent Hinode findings, and the December 7 issue of Science carries ten papers on Hinode, among them Cirtain et al., “Evidence for Alfvén Waves in Solar X-ray Jets,” Science Vol. 318, No. 5856, pp. 1580-1582 (abstract).
