by Paul Gilster | Apr 15, 2011 | Deep Sky Astronomy & Telescopes |
Data from the first 57 percent of the sky surveyed by the WISE mission (Wide-Field Infrared Survey Explorer) are now available and accessible through the online archive. You can dig into the archive hunting for WISE imagery right now, as I did this morning to retrieve this Alpha Centauri image. The WISE team has put up a help page on the image data service with useful information about how to find and work with color images. The method is straightforward: You enter a name or set of coordinates for stars, nebulae or galaxies, choose the size of the image to retrieve (the defaults bring you images from each of the four WISE detectors), and select three of the four WISE bands to pull up a color image that combines their results.
Play around with the help page a bit and you’ll quickly become familiar with the setup, and there is further help available within the archive itself. We have much more to come from WISE, with the complete survey, including improved data processing, scheduled for release about a year from now. But thus far the early count from WISE is rich, including nearby objects ranging from twenty comets to over 33,000 asteroids between Mars and Jupiter and 133 Near-Earth Objects. And in the deep sky, the haul is huge. Says Fengchuan Liu, project manager for WISE at the Jet Propulsion Laboratory:
“We are excited that the preliminary data contain millions of newfound objects. But the mission is not yet over — the real treasure is the final catalog available a year from now, which will have twice as many sources, covering the entire sky and reaching even deeper into the universe than today’s release.”
We now move toward making these objects available to astronomers for follow-up study, meaning that, as principal investigator Ned Wright puts it, “Starting today thousands of new eyes will be looking at WISE data, and I expect many surprises.” Leaning about new brown dwarf discoveries in the neighborhood of the Sun is certainly high on my list of interests, as is resolving the issue of whether a large object in the inner Oort Cloud could be disrupting cometary orbits there. The useful thing about surveys is that they turn up surprises because they’re open to the entire sky, rather than focusing on a single set of targets. You can be sure that the most intriguing WISE discoveries will be fodder for quick follow-up observations by other observatories.
Image: A map of the portion of the sky covered by the preliminary release of WISE data. The fuzzy line down the middle is our Milky Way galaxy, and bright clouds are star-forming complexes. The gray regions are the part of the sky not available in the preliminary WISE data release. For the regions with data, the colors used are representational: blue and cyan (blue-green) represent data from the 3.4- and 4.6-micron detectors aboard WISE, and green and red represent data from the 12- and 22-micron detectors. The blue and cyan reveals mostly light from stars, while the green and red come from mostly warm dust. Credit: NASA/JPL-Caltech/UCLA.
WISE scanned the entire sky while collecting images taken at four infrared wavelengths of light accumulating a total of over 2.7 million images. And although its hydrogen coolant ran out in October of last year, two of its four infrared channels remained operational for an extended mission covering four more months, sweeping for asteroids and comets in the main asteroid belt. The satellite is now in hibernation, but astronomers here on Earth will be busy for years looking for the hidden gems WISE has uncovered. And as they go to work on new data, be aware that the first six of eighteen segments forming the James Webb Space Telescope’s primary mirror will begin final cryogenic testing this week. Things can seem to move at a glacial pace when we’re anticipating new discoveries, but step by step we’re getting there.
by Paul Gilster | Apr 14, 2011 | Deep Sky Astronomy & Telescopes |
We’ve got to come up with a better name that ‘Milkomeda’ to describe what’s going to eventually happen when the Milky Way and Andromeda merge. Remember that Andromeda is one of the galaxies with a blueshift, showing that it is moving toward us. That the merger will probably happen — in about five billion years — appears inevitable, and it’s fascinating to speculate on the evolution of the elliptical galaxy that should result from all this. In fact, Avi Loeb (Harvard-Smithsonian Center for Astrophysics) and colleague T.J. Cox have run computer simulations showing a faint possibility that our Solar System will be pulled into a ‘tidal tail’ of orphan stars and eventually, before the final merger, wind up in the Andromeda galaxy.
But after a series of close passes, the galaxies will most likely begin to intermingle. Loeb is the one behind the Milkomeda coinage, but I’ve also heard the even worse ‘Milkymeda’ and the at least acceptable ‘Andromeda Way.’ There’s plenty of time to work this out, so I put it to Centauri Dreams readers to ponder a poetic and inspirational name for the ultimate elliptical galaxy. By the time it has fully merged, our Sun will be entering its red giant stage, and our Solar System most likely pushed out to 100,000 light years from the new galactic center. That’s four times the current 25,000 light year distance, a move way out beyond the familiar galactic suburbs.
Image: A near galactic-collision between NGC 2207 (left) and IC 2163 captured by the Hubble Space Telescope. Scientists predict the Milky Way will merge with its neighbor Andromeda in about 5 billion years. Credit: NASA and The Hubble Heritage Team (STScI).
Will the descendants of the human race, however constituted, still be around to study the stars? It’s impossible to know, but it’s clear that any astronomers living in the merged galaxy era will have a far different night sky than ours to work with. And that view will hardly be static. Let’s run the process forward as if we had an H.G. Wells-style time machine (or a Loeb-style computer). 100 billion years from now the Sun and many of the stars we are familiar with will have burned out. Moreover, the accelerated expansion of the universe will have pushed many galaxies out past our cosmic horizon, while many of those that can be seen will only grow dimmer.
Loeb talked about that scenario back in 2001, noting that in this era, 100 billion years from now, an astronomer’s view will be reduced to about a thousand members of the local Virgo Cluster and surrounding areas. When a remote galaxy crosses our ‘horizon,’ the light it emits after that point will not be able to reach us. The galaxy will simply be moving too fast for us to see it. As Loeb says, “This process is analogous to what you see if you watch a light source fall into a black hole. As an object crosses the black hole’s event horizon, its image seems to freeze and fade away because you can’t see the light it emits after that point.”
Trillion Year Spree
Galaxies will slowly disappear, their image frozen and fading. It’s a chilling prospect, but Loeb’s latest paper takes us into an even more remote scenario, fully one trillion years from now, when the universe is 100 times older than it is today. By then the photons of the Cosmic Microwave Background will have a wavelength longer than the visible universe, and all other galaxies will be lost to our view. But Loeb believes that the astronomers of this era will still be able to figure out the Big Bang and the existence of dark matter by studying hypervelocity stars flung out from the galaxy.
Flung out, that is, from the center of the inelegantly named Milkomeda. Here’s the scenario: When a binary star system gets too close to the black hole at galactic center, one star falls into the black hole while the other is thrown outward at speeds high enough to cause it to be ejected from the galaxy. This occurs roughly every 100,000 years, and sharp-eyed future astronomers will be able to use these hypervelocity stars to infer the accelerated expansion of the universe as the stars move beyond the galaxy’s gravitational pull. Advanced technologies measuring that acceleration should make it possible to infer an expanding universe and, in Loeb’s view, calculate the age of the universe and key parameters like the cosmological constant.
“We used to think that observational cosmology wouldn’t be feasible a trillion years from now,” says Loeb. “Now we know this won’t be the case. Hypervelocity stars will allow Milkomeda residents to learn about the cosmic expansion and reconstruct the past… Astronomers of the future won’t have to take the Big Bang on faith. With careful measurements and clever analysis, they can find the subtle evidence outlining the history of the universe.”
Beyond the evidence afforded by hypervelocity stars, other possibilities come to mind, as Loeb outlines in his new paper. Consider all the possible sources of information:
The existence of an early radiation-dominated epoch could be inferred by measuring the abundance of light elements in metal-poor stars and interpreting it with a theory of Big Bang nucleosynthesis. The mass fraction of baryons within Milkomeda could be assumed to be representative of the mean cosmic value at early times. The nucleosynthesis theory can then be used to find the necessary radiation temperature T? ? a?1 , such that the correct light element abundances would be produced. This would lead to an estimate of the time when matter and radiation had the same energy densities. Since density perturbations grew mainly after that time, it will be possible to estimate the amplitude of the initial density fluctuation on the mass scale of Mtot that was required for making Milkomeda at a time (t ? t? ) ? Hv-1 after the Big Bang. Without a radiation-dominated epoch, this amplitude could have been arbitrarily low at arbitrarily early times. Future astronomers may already have cosmology texts available to them, but even if they do not, we have outlined a methodology by which they will be able to arrive at, and empirically verify, the standard cosmological model.
Loeb sketches out a remote futurity indeed, but it’s a comforting thought that science will still be able to unlock cosmological mysteries even when the compelling view of other galaxies is long gone. You can follow this up in Loeb’s paper “Cosmology with Hypervelocity Stars,” accepted by the Journal of Cosmology and Astroparticle Physics and available as a preprint. And please, for the good of our remote descendants, give some thought to a name more elegant than ‘Milkomeda.’
by Paul Gilster | Apr 13, 2011 | Exotic Physics |
What on Earth — or off it — could inspire a physicist with the credentials of Caltech’s Kip Thorne to say “I’ve never before coauthored a paper where essentially everything is new. But that’s the case here.” Yet if Thorne couldn’t say that about some of his earlier work with wormholes (!), he feels safe in saying it about the new tools for visualizing warped space and time that are discussed in a paper he and his colleagues have just published. Imagine space and time undulating in hitherto unfathomable patterns as objects like black holes run into each other.
How do we visualize such effects in a credible way? The new tools help us do just that. They are the result of powerful computer simulations that bring to visual life the complex equations of black hole mergers and other extreme events, and they should help us with problems like this one: Manuela Campanelli (University of Texas in Brownsville) and team used simulations a few years ago to show that colliding black holes produce a direct burst of gravitational waves. The result is that the black hole itself seems to recoil, with a force strong enough that the newly merged object can be thrown entirely out of its own galaxy. When this work was done in 2007, nobody could explain how a directional burst of gravitational waves could be produced.
Thorne’s team can now produce an explanation by working with spacetime analogues to the electric and magnetic field lines that describe those two forces. A tendex line describes the stretching force that warped spacetime exerts, while a vortex line describes the twisting of space. Run enough tendex lines together and you create a region — a tendex — of strong stretching. Merge a bundle of vortex lines and the result is a whirling region of space — a vortex.
Image: Two doughnut-shaped vortexes ejected by a pulsating black hole. Also shown at the center are two red and two blue vortex lines attached to the hole, which will be ejected as a third doughnut-shaped vortex in the next pulsation. Credit: The Caltech/Cornell SXS Collaboration.
Let me quote the Caltech news release on the result:
Using these tools, [the researchers] have discovered that black-hole collisions can produce vortex lines that form a doughnut-shaped pattern, flying away from the merged black hole like smoke rings. The researchers also found that these bundles of vortex lines—called vortexes—can spiral out of the black hole like water from a rotating sprinkler.
The computer tools now show how these distortions of spacetime are produced, and can explain things as complex as the black hole collisions and ejection discussed by Campanelli. So what does account for that gravitational kick experienced by the merged black hole at galactic center? The unidirectional force comes from gravitational waves from spiraling vortexes added together with waves from spiraling tendexes, while on the other side, the vortex and tendex waves are canceled out. The newly merged black hole experiences a recoil. The new conceptual tools are useful not just for a black hole scenario, but a wide range of possibilities:
“Though we’ve developed these tools for black-hole collisions, they can be applied wherever space-time is warped,” says Dr. Geoffrey Lovelace, a member of the team from Cornell. “For instance, I expect that people will apply vortex and tendex lines to cosmology, to black holes ripping stars apart, and to the singularities that live inside black holes. They’ll become standard tools throughout general relativity.”
Various black hole merger situations suggest themselves, including two spinning black holes colliding head on or spiraling toward each other before the merger. Each of these scenarios can be explained through the use of tendexes and vortices, but it’s also important to note that in either case, outward-moving vortexes and tendexes become gravitational waves. Usefully, that’s just the kind of waves that the Laser Interferometer Gravitational Wave Observatory (LIGO) has been created to detect. LIGO, which began its search in 2002, looks for gravitational wave emissions from collisions of neutron stars or black holes as well as supernovae. Tendexes and vortexes may help researchers predict the waveforms LIGO is looking for.
The paper is Owen et al., “Frame-dragging vortexes and tidal tendexes attached to colliding black holes: Visualizing the curvature of spacetime,” Physical Review Letters 106, 151101 (2011). Abstract / Preprint.
by Paul Gilster | Apr 12, 2011 | Outer Solar System |
These days funding for missions to some of the most interesting places in the Solar System is much in question. But sooner or later we’re going into the outer system to investigate the possibilities for life on worlds like Europa, Enceladus or Titan. The case for Europa seems particularly compelling, but we have to be careful about our assumptions. When the Europa Orbiter Science Definition Team developed a strategy for Europan exploration in 1999, it was generally believed that any Europan ocean would be covered by a thick and impermeable layer of ice. Life, then, might exist around deep sub-oceanic volcanic vents if it existed at all.
Thus the strategy for Europan exploration that evolved: Three missions, beginning with an orbiter, followed up by a lander and, finally, a third mission that would drill down through the presumably many kilometers of surface ice to explore whatever lay beneath. Even in more financially optimistic times, that strategy didn’t get us into Europa’s ocean until well after 2030, and today we struggle to come up with a date for the Europa Jupiter System Mission (surely later than 2030 just for an orbiter), with a timetable for ocean exploration that is without doubt set back by decades.
Re-examining Europan Exploration
Why not then, says Richard Greenberg, take time to re-evaluate our entire Europan strategy, factoring in all the work that has been done sine the late 1990s? Greenberg (Lunar and Planetary Laboratory, University of Arizona) is the author of Unmasking Europa (Springer, 2008), which sharply critiques the ‘thick ice’ assumption by pointing to many instances of Galileo imagery showing what appears to be young and constantly resurfacing ice. Cracking and melting of a thinner ice sheet could actually expose the ocean, and make the job of studying it far easier, while demanding considerable caution in terms of possible contamination from terrestrial organisms.
Image: A representation of possible subsurface structures, prepared by the Jet Propulsion Laboratory for the recent Europa Orbiter Science Definition Team (SDT-2010), shows a very different picture from the version of a decade earlier. According to SDT-2010, ”The NASA Jupiter Europa Orbiter will address the fundamental issue of whether Europa’s ice shell is ~few km (left) or >30 km (right), with different implications for processes and habitability.” The thick ice as shown on the right extends tens of kilometers down toward the rocky interior. The model with thinner, permeable ice is now considered on par with the earlier canonical model of thick, impermeable ice. Credit: Richard Greenberg/Astrobiology.
In a new paper, Greenberg notes that the recent Joint Jupiter Science Definition Team identified the thick vs. thin ice question in 2010 as a key objective of Europa exploration. The thick ice model, in other words, is no longer the only game in town, raising real questions about how we proceed in long-term planning. Greenberg reviews the case for thin ice, especially in places like Conamara Chaos, where rafts of displaced crust can be seen lodged in what appears to be lumpy, refrozen ice, with new cracks changing the terrain yet again and suggesting melt-through. Unmasking Europa has that story in detail, but the paper offers a helpful summary to get you up to speed.
I think Greenberg’s case is strong, but I want to focus on the implications of thin ice rather than the case for it. For if we do determine that the Europan ocean is accessible, our mission focus shifts to exploiting the terrain to find the best place for surface operations. From the paper:
Rather than focusing on the daunting, perhaps impossible, task of drilling down to the ocean, we should consider how to take advantage of the biosphere’s natural accessibility. With the rapid resurfacing, almost any europan landing site might provide oceanic samples; the issue will be how to find the freshest ones. When chaotic terrain forms, it replaces a section of crust with frozen ocean. Ridge formation squeezes out ocean material. Fresh oceanic material may be exposed at the surface as gaps open up and create the dilational bands. Any of these processes could be laying out biological samples on the surface.
Thus the need, says Greenberg, to ‘land smart,’ picking the optimum landing site to avoid the need for drilling through the ice in the first place. Reassessing a Europa orbiter, then, should involve a key objective: Identifying the most likely sites where the underlying ocean may have been recently exposed. A lander integrated with the orbiter mission would have the chance of finding extraterrestrial life near or even on the surface, as the frozen remnants of materials that have been briefly exposed through surface shifting of the ice. Greenberg thinks such a strategy could give us an answer on Europan life within the lifetime of many adults living today, as opposed to pursuing what is essentially a holding strategy as we develop a thick ice drilling model.
Ice and Contamination
An overriding concern is protecting Europa from life that has hitched a ride from Earth, a problem that looms large with the thin ice model, whereas with thick ice and an isolated ocean, the chances of contamination seemed more remote. The NRC Task Group on the Forward Contamination of Europa (2000) adopted a standard for protection that says the probability of contaminating a Europan ocean with a viable terrestrial organism must be less than 10-4 per mission, referencing a 1964 resolution that Carl Sagan had a hand in fashioning with regard to the exploration of Mars.
But the Sagan formulation, developed with Sidney Coleman, is deeply flawed. Here is part of Greenberg’s critique:
The basis of the calculation was that the probability of contaminating Mars during these missions should be very small for at least the duration of the specific envisioned exploration campaign. In other words, the underlying premise of that study was that the purpose of planetary protection was to protect the interests of then currently active scientists, rather than future generations of scientists or of alien organisms themselves. They arbitrarily selected a value of 0.1% as the maximum acceptable probability of contamination. On that basis, they calculated that each spacecraft launched to Mars had to be sterilized enough so that there was less than a 0.01% chance of having any organism on board. This calculation depended on specific assumptions about unknown conditions relevant to Mars and on guesses about the future exploration program.
The international Committee on Space Research (COSPAR) accepted the Sagan/Coleman recommendation in 1964, even though its selection of a level of underlying risk was arbitrary and relied on assumptions about the survival of terrestrial organisms in space that were, in Greenberg’s view, crude. Its ethical premises were also shaky, with the assumption that we need to preserve a biosphere only long enough for the current scientific community to finish exploration. What about future generations of researchers, and what about extraterrestrial life forms themselves? This COSPAR resolution was a key source for the 2000 NRC task group report on Europa at a time when a thick ice model featuring an isolated ocean was still favored.
We need to reconsider such questions, and thankfully, a new National Research Council study on planetary protection has been commissioned by NASA, although at the moment its mission is to develop contamination standards for icy moons that are still based on Sagan/Coleman. Thus Greenberg’s call for a new analysis. And the scientist has an interesting suggestion involving what he has previously called a ‘natural contamination standard.’ It goes like this:
…exploration would be acceptable if the probability of humans infecting other planets with terrestrial microbes is smaller than the probability that interplanetary contamination happens naturally. Such a foundation principle would be ethically defensible and could be translated into specific, research-based, quantitative standards.
Greenberg also thinks the NRC panel should come up with practical sterilization criteria so that NASA can draw on an independent analysis as the basis of building Europan exploration craft. The NRC 2000 report left such matters in the hands of NASA and its contractors. The questions this raises are too significant to go unanswered. If Europa does have permeable ice, there is a distinct possibility that its biosphere extends near to the surface, raising the odds for contamination. This calls for a new look at planetary protection whatever the time frame involved, and a tightening of standards that need to move beyond those of Sagan/Coleman.
The Greenberg formulation, based as it is on a natural contamination standard, would depend on the rate of spacecraft arrivals on Europa because it would be keyed to the natural rate of interplanetary transport. It is, at least, one way of looking at contamination that takes in current data, which include the distinct possibility that we are dealing with thin and permeable ice on the distant moon. And there is a philosophical issue that trumps the proceedings. Do we have an ethical imperative to protect indigenous life forms from contamination wherever we go in the universe, and should this imperative be factored in to every mission concept we create?
If so, we need to think long and hard about the potential for thin ice on Europa, because getting the contamination question wrong would compromise both our scientific and moral objectives. The paper is Greenberg, “Exploration and Protection of Europa’s Biosphere: Implications of Permeable Ice,” Astrobiology Vol. 11, No. 2 (2011). Abstract available.
by Paul Gilster | Apr 11, 2011 | Outer Solar System |
Back in December, scientists from the Cassini team presented evidence for ice volcanoes on Titan, looking at a region called Sotra Facula, which bears some resemblance to volcanoes on Earth like Mt. Etna in Italy and Laki in Iceland. An ice volcano, also known as a cryovolcano, would draw on geological activity beneath the surface that warms and melts parts of the interior and sends icy materials through a surface opening. Sotra Facula’s two 1000-meter peaks combine what appear to be deep volcanic craters with finger-like flows of material, a kind of surface sculpting that could explain some of the processes occurring on other ice-rich moons.
But work like this is part of an ongoing dialogue testing various hypotheses, and the latest round takes us in a sharply different direction. In a new paper in Icarus, Jeff Moore (NASA Ames) and Robert Pappalardo (JPL) argue that Titan is in fact much less geologically active than some have thought. A cool and dormant interior would be incapable of producing active ice volcanoes:
“It would be fantastic to find strong evidence that clearly shows Titan has an internal heat source that causes ice volcanoes and lava flows to form,” adds Moore. “But we find that the evidence presented to date is unconvincing, and recent studies of Titan’s interior conducted by geophysicists and gravity experts also weaken the possibility of volcanoes there.”
The new work looks at Titan in light of what we see on Callisto, which Moore sees as analogous to Titan ‘if Callisto had weather.’ And indeed, the two moons are roughly the same size, with Callisto’s cratered surface solely the result of impact events rather than internal heating. In the new paper, Moore and Pappalardo see Titan’s surface as explicable entirely from external processes like wind, rain and impacts. These we see in profusion — lakes of liquid methane and ethane, valleys carved by rivers, craters — through infrared and radar instrumentation, but the debate now moves to whether all surface features can be explained in the same way.
Image: These images compare surface features observed by NASA’s Cassini spacecraft at the Xanadu region on Saturn’s moon Titan (left), and features observed by NASA’s Galileo spacecraft on Jupiter’s cratered moon Callisto (right). The Cassini radar image, obtained on a Titan flyby April 30, 2006, is centered on 10 degrees south latitude and 85 degrees west longitude. The Galileo camera image, obtained on June 25, 1997, is centered on 6 degrees south latitude and 7 degrees west longitude. Titan may originally have had a cratered landscape similar to Callisto that has since been eroded by rainfall and runoff. There are many large circular features in Titan’s Xanadu region that have some of the characteristics of impact craters — such as central peaks and inward-facing circular cliffs — which make scientists think they are, in fact, eroded impact craters. Credit: NASA/JPL.
Titan’s atmosphere may remain the focus of debate between those who believe the moon is geologically dormant and the ice volcano theorists. The atmosphere is primarily nitrogen, with a few percent methane, and the Sotra Facula analysis, presented at the American Geophysical Union meeting in San Francisco last December, focused partially on that mix. Thus Linda Spilker (JPL):
“Cryovolcanoes help explain the geological forces sculpting some of these exotic places in our solar system. At Titan, for instance, they explain how methane can be continually replenished in the atmosphere when the sun is constantly breaking that molecule down.”
But are ice volcanoes the only explanation for replenished methane in this atmosphere? Both sides in the debate would agree that there is no evidence of current activity at Sotra Facula even if the topography there is suggestive of a volcanic origin. Further Cassini studies will advance the argument, but it’s helpful that Pappalardo and Moore have injected what they call ‘a necessary level of caution into the discussion’ as we wait for more definitive results.
The paper is Moore and Pappalardo, “Titan: An exogenic world?” Icarus Volume 212, Issue 2 (April, 2011), p. 790-806 (abstract).
