Centauri Dreams
Imagining and Planning Interstellar Exploration
A Near-Term Read on Life in the Galaxy
Although he doesn’t post nearly as often as some of us would like, Caleb Scharf’s Life, Unbounded site is always worth reading. Scharf, author of the textbook Extrasolar Planets and Astrobiology (University Science Books, 2008) is the director of the Columbia University Astrobiology Center. As such, he’s positioned to offer valuable insights into our investigations of the forms life might take on other worlds. Not long ago he wrote a fascinating post for Scientific American on a statistical approach to astrobiology, a timely idea as we discuss ongoing missions like Kepler and proposed space telescopes like WFIRST.
Natural Selection on a Galactic Scale
Scharf’s latest is a quick take on panspermia, the idea that viable organisms may be exchanged between planets as various early impacts spread debris through a planetary system. We know that surface material moves continually between the rocky moons and planets of our own system, and we’ve also come to understand that microbial organisms of great hardiness might survive such extreme journeys, even though they involve millions of years of exposure to interplanetary and even interstellar space. Life may indeed be seeded on a galactic level.
But if this is the case, what about the role of natural selection? Scharf writes:
Although it’s a complex problem, it seems likely that life driven by cosmic dispersal will end up being completely dominated by the super-hardy, spore-forming, radiation resistant, rock-eating (endolithic) type of critters. There will be no advantage to a particularly diverse gene pool. Billions of years of galactic transferral will have whittled it down to only the most indelicate and non-fussy microbes – super efficient, super persistent, and ubiquitous – the galactic top dogs.
All of this would fit with what we see on Earth, for we know about numerous organisms in extreme environments here that do indeed survive under conditions most living things would consider hostile. Scharf’s point, though, is that if panspermia is true on a galactic level, then tough organisms like these should be just about everywhere. As our robotic probes grow in sophistication, they should start finding life’s tenacious foothold throughout the Solar System, from the ancient seabeds of Mars to the smog-choked surface of Titan. A galactic panspermia would know no favorites, and it has had billions of years to work.
Galactic panspermia, in other words, is going to make itself apparent in the not distant future. If we find that this is not the case, that life doesn’t pop up just about everywhere we look, then the case for panspermia at this level is vastly weakened, although we can still see a role for panspermia between planets. The larger question of life around other stars, in that case, will remain as intractable as it does today, and will require our most advanced instrumentation to detect in the form of atmospheric biomarkers on Earth-like planets near enough to study.
From Sagan to Drake
All of which reminds me of a recent interview with Seth Shostak. Asked about Carl Sagan’s estimate that there might be one million intelligent species in the galaxy and Frank Drake’s speculation that the number was closer to 10,000, Shostak comes down on the side of Drake, noting that if 3 percent of the solar systems in the Milky Way have Earth-like planets, then 10 billion such planets must exist. Assume just one in a million to have intelligent life and you still end up with 10,000 civilizations. Kepler will let us tighten these numbers within a few years.
Shostak takes note of the debates he has had with Rare Earth author Peter Ward, who argues in his book with Donald Brownlee that intelligent life must be scarce due to the huge number of factors — Jupiter-like planets, a nearby moon, the tight parameters of habitable zones — that would make it possible. Shostak:
I’m not at all convinced that moons are needed to support life. Without the moon, the tides would be different and the poles would migrate every so often. But that wouldn’t wipe out life. Regarding gravity, there are already two planets with earthlike gravity in our solar system, and even Mars could probably have supported life earlier in its history (and maybe even today). And we are now fairly certain that Jupiter sized planets are commonplace – we have already located hundreds of them. So I just don’t find the argument that complex life must be rare in the galaxy to be compelling.
The encouraging thing about these discussions is that we are dealing with issues about which we will have preliminary answers within a matter of years. Kepler will be able to give us statistical answers about the prevalence of Earth-like worlds in the galaxy, and using Scharf’s reasoning, we can draw extrapolations on the likelihood of panspermia based on what we find in our own system fairly soon, just as long as it takes to get complex robotics to environments like Europa.
True terrestrial planet hunter spacecraft — the kind that can make spectroscopic analyses of exoplanet atmospheres on worlds this small — are at least a decade and perhaps much more away depending on funding issues, but they represent logical extrapolations of near-term technology. In 25 years, we will have not just a philosophical view of life in the universe but a practical knowledge based on data that can tell us whether we’re likely to be alone or simply one among many galactic species.
Star Wars? Not at NASA
I had started today’s entry — on dark energy — only to be sidetracked by a short piece in Space.com that almost had me spewing my morning coffee all over my keyboard. Here’s a quote from the story, which focuses on a Star Wars convention in Florida held last weekend:
“‘Star Wars’ filmmakers and fans asked NASA representatives to develop a hyperdrive that can transport astronauts through space at light speed. And to make it snappy.”
In response, the story quotes NASA’s Joseph Tellado, a logistics manager for the International Space Station, who says this:
“We need better propulsion systems. Right now I’d say that would be the one invention that would really help us out a lot. It’d be great if our astronauts could go at hyperspeed…. I believe ‘Star Wars’ and NASA have a lot in common. We’re looking to the future. NASA is like the first stepping stone to ultimately get to that ‘Star Wars’ level.”
And the story adds this:
The inspiration works both ways, with NASA and “Star Wars” inspiring each other to stretch out and envision the future and then fill in details of what that future might look like.
NASA in the Hunt for Breakthroughs?
Astounding. Here’s why I burned my tongue on a cup of Sumatra Mandheling this morning: Despite what convention-goers may now believe, NASA has no involvement whatsoever in the kind of technologies these people are talking about. True, the agency once funded the Breakthrough Propulsion Physics project, run out of Glenn Research Center by Marc Millis. BPP’s charter was to investigate the kind of technologies that might one day lead to deep space and interstellar flight, among them so-called ‘warp drive’ and other possibilities. But the agency stopped funding BPP in 2002.
NASA’s Institute for Advanced Concepts, not as ‘breakthrough’ oriented as BPP but a potent force for showcasing new ideas, was cut off from its funding in 2007. In short, the idea that NASA is conducting serious research on any aspect of advanced propulsion — I am talking here about the kind of concepts this convention glories in — is completely false. That work is now off the table. Marc Millis himself has left NASA and works on breakthrough concepts through the Tau Zero Foundation he founded, for which I toil on a daily basis in writing these posts. TZF has no NASA connection whatsoever and proceeds through private funding. The relevant links on the home page here give you the background on TZF.
So while I agree with NASA’s Joseph Tellado that hyperspeed is a desirable outcome, it should be added that it’s not one that NASA is engaged in studying. This is not to say that potential near-term technologies like solar sails may not be revived within the agency — the NASA solar sail is up to a Technological Readiness Level of 6 and a demonstrator sail like NanoSail-D should be launched within a year. But if you’re talking futuristic concepts like warp drive and the study of potential breakthroughs, NASA is no longer the place to be.
Pushing into Dark Territory
With that off my chest, let’s proceed to dark energy, which I want to discuss because utterly unexpected scientific results may offer us useful clues to future technologies. We’re only beginning to learn about dark energy, but the notion that the universe should be expanding at an accelerating rate has flowed out of supernova observations that have been supported by later studies like the Supernova Legacy Survey. Using the Hubble Space Telescope, the Higher-Z team concludes that dark energy has been a factor for at least nine billion years.
All of which is strange and wondrous, as it implies that a hugely important component of our universe only became known within the last twelve years, when the first supernova work was reported. If there is a factor that causes space to expand — one that seems to make up about 72 percent of the mass-energy of the universe — it must exert a strong negative pressure to account for its effects. The fact that it is so hard to detect and is not thought to interact with the fundamental forces other than gravity means that studying it in the laboratory is, to say the least, problematic.
And as far as harnessing its powers for future propulsion systems, the idea is far-fetched in the extreme — at our current technology level. We can’t, however, rule it out for the far future. And that’s the thing about the future. It plays out according to the inputs we give it, meaning that if in some future century our science progresses to the point that what we now know as dark energy becomes something we can engineer, it will be because a long line of scientists, starting now, have put in the groundwork to get us to that destination.
That’s why I keep an eye on dark energy studies in these pages, suggesting only that the more unexplained things we gradually master in the universe, the more likely we are to make genuine breakthroughs. The Breakthrough Propulsion Physics project used to make this sort of thing its bread and butter, but private organizations like the Tau Zero Foundation now have to continue that work without help from government agencies.
The Geometry of Spacetime
And there is some interesting news about dark energy as we continue to pursue this odd effect. One reason that dark energy work is so absorbing is that it tackles the very geometry of the universe. Findings from a team including Yale University physicist Priyamvada Natarajan, reported in the August 20 issue of Science, are based on gravitational lensing of 34 extremely distant galaxies situated behind the massive galactic cluster Abell 1689. Astronomers can study how the images are distorted by intervening mass. Says Natarajan:
“The content, geometry and fate of the universe are linked, so if you can constrain two of those things, you learn something about the third.”
Image: The massive gravitational force of the dark matter (shown in blue) in giant galaxy cluster Abell 1689 bends the light from distant background galaxies, giving astronomers clues to the nature of dark energy. Credit: NASA, ESA, Eric Jullo/JPL, Priyamvada Natarajan/Yale University, Jean-Paul Kneib/Universite de Provence.
As I said, we’re in early days when it comes to the study of dark energy, and if there are actually ways to harness it, such developments may well be centuries away. But it’s useful to know that Natarajan and team have been able to narrow the range of current estimates about dark energy’s effect on the universe — denoted by the value w — by some thirty percent. They did this by combining the gravitational lensing studies with new data from supernovae, X-ray galaxy clusters and related data from the Wilkinson Microwave Anisotropy Probe (WMAP).
We learn from all this that the dark energy work thus far confirms previous findings that we do indeed live in a flat universe, one in which the expansion will continue to accelerate and the universe will expand forever. Assuming, that is, that dark energy’s effects remain constant over cosmological time scales. We have much to do to understand how dark energy works, but with NASA out of the hunt when it comes to examining it and other components of far future propulsion engineering, we shouldn’t expect that hyperdrive any time soon.
The paper is Natarajan et al., “Cosmological Constraints from Strong Gravitational Lensing in Clusters of Galaxies,” Science Vol. 329, No. 5994 (20 August 2010), pp. 924-927 (abstract).
Decadal Survey Pushes WFIRST Telescope
What do you get if you combine the insights of nine expert panels, six study groups and a broad survey of the astronomy and astrophysics community? If you’re lucky and have the right committee, you wind up with useful analyses of the readiness and costs of science projects for the future, both major and minor. And as the National Research Council has done in its new report, you then create a decadal survey, in this case the sixth produced by the NRC, that identifies where the US should go next in answering ‘profound questions about the cosmos.’
A prepublication copy of New Worlds, New Horizons in Astronomy and Astrophysics is available online. To understand the needs of space science in the next ten years, though, be prepared for new acronyms. The most significant for Centauri Dreams readers will probably be WFIRST, the 1.5-meter Wide-Field Infrared Survey Telescope, which could launch as early as 2020 as part of our ongoing search for terrestrial exoplanets. In terms of the panel’s priorities, WFIRST comes in first in the category of space projects costing over $1 billion.
Image: WFIRST is an infrared telescope with a three?mirror design. It will have HgCdTe detectors with 144 megapixels in total and angular resolution of 200 milliarcseconds. The sensitivity should be about 200 nJy or 26th magnitude, enabling shape measurements and photometric redshifts to a depth of 100,000 galaxies per square degree over half the sky. Spectroscopy will be achieved with a grism or prism and will rely mainly on measurement of H alpha out to a redshift of about 1.8 Credit: JDEM Project, NASA?GSFC.
According to the survey, WFIRST will cost about $1.6 billion, and is conceived as “…an observatory designed to settle essential questions in both exoplanet and dark energy research… which will advance topics ranging from galaxy evolution to the study of objects within our own galaxy.” The telescope would be deployed at the L2 Lagrangian point, from which it could refine our estimates of the number of Earth-like planets in our galaxy and map the distribution of two billion other galaxies. These studies, complemented by weak lensing, would help us understand how dark energy has shaped the universe.
The latter component of the telescope’s mission highlights the difference between it and the James Webb Space Telescope, scheduled for a 2014 launch. From the report:
As a telescope capable of imaging a large area of the sky, WFIRST will complement the targeted infrared observations of the James Webb Space Telescope (JWST). The small field of view of JWST would render it incapable of carrying out the prime WFIRST program of dark energy and exoplanet studies, even if it were used exclusively for this task.
The five-year baseline mission could be stretched to a ten year extended mission, the report notes, and potential collaboration with the European Space Agency and a proposed mission called Euclid is urged in the report. It’s interesting to see that in its look at WFIRST’s role in exoplanet research, the panel focuses on gravitational microlensing, whereby we infer the presence of planets by their effect on light from background stars:
A survey for such events is one of the two main tasks of the proposed WFIRST satellite. As microlensing is sensitive to planets of all masses having orbits larger than about half of Earth’s, WFIRST would be able to complement and complete the statistical task underway with Kepler, resulting in an unbiased survey of the properties of distant planetary systems. The results from this survey will constrain theoretical models of the formation of planetary systems, enabling extrapolation of current understanding to systems that will still remain below the threshold of detectability.
The report sees our hunt for terrestrial worlds as having both space-based and terrestrial components. The panel pictures the process working like this:
• Carry out a focused program of computation and theory to understand the architectures of planets and disks.
• Use the Kepler transit survey to measure the probability that a solar?type star has a massive terrestrial companion, and that a red star harbors an Earth?like planet.
• Perform a microlensing survey from space using the recommended WFIRST to characterize in detail the statistical properties of habitable terrestrial planets.
• Improve radial velocity measurements on existing ground?based telescopes to discover planets within a few times the mass of Earth as potential targets for future space?based direct?detection missions.
• Use ground?based telescopes, including ALMA , AO?equipped optical/infrared telescopes such as GSMT, and mid?infrared interferometry, or space?based Explorers, to characterize the dust environment around stars like the Sun, so as to gauge the ability of future missions to directly detect Earth?size planets in orbits like that of our own Earth.
• Locate the prime targets for hosting habitable, terrestrial planets among our closest stellar neighbors.
• Use JWST to characterize the atmospheric or surface composition of planets within a few times the size of Earth, orbiting the coolest red stars. These are the planets that might be discovered by ground? and possibly space?based surveys.
• Follow up nearby systems discovered by Kepler.
• Assess habitability by using IXO to characterize the frequency and intensity of flares on host stars.
• Use ALMA and CCAT to seek biogenic molecules thought to be precursors to life.
• Develop the technology for an ambitious space mission to study nearby Earth?like planets.
WFIRST’s dark energy component comes in direct descent from the proposed Joint Dark Energy Mission — JDEM was used as the basis for the cost and technical evaluation assessment produced by the committee, and the report notes that JDEM’s capabilities are ‘essentially identical to those envisaged for WFIRST.’ Here the report zeroes in on the dark energy work:
To measure the properties of dark energy, WFIRST will employ three different techniques: it will image about 2 billion galaxies and carry out a detailed study of weak lensing that will provide distance and rate-of-growth information; it will measure spectra of about 200 million galaxies in order to monitor distances and expansion rate using baryon acoustic oscillations; and finally, it will detect about 2,000 distant supernova explosions, which can be used to measure distances. WFIRST provides the space-unique measurements that, combined with those from LSST (the committee’s highest-priority ground-based project), are essential to advance understanding of the cause of cosmic acceleration.
Pairing the ambitious WFIRST mission with ESA’s Euclid seems a rational thing to do, given the overlapping science objectives of the two missions and the opportunity to lower costs via collaboration, but we lack a firm decision on whether either of these missions will actually fly. This BBC story notes that ESA’s infrared detectors for Euclid would need to be sourced from the US, but Europe currently has the more advanced design, one that is within a year or so of completing its design assessment and being chosen (or not) to go operational.
The advantages of a collaborative mission seem clear at this stage. And BBC writer Jonathan Amos correctly notes the fiscal realities that will shape discussions between NASA and ESA. The James Webb Space Telescope has run far over budget, an experience ESA would not want to repeat as a partner on WFIRST. NASA and ESA will have to sort these and other issues out directly, because the Decadal Survey, while influential, is at heart only a set of recommendations, if exciting and well-informed ones at that.
IBEX: From System’s Edge to Nearby Space
When the Project Daedalus team went to work to design a starship back in the 1970s, they contemplated using the atmosphere of Jupiter as their source for helium-3, an isotope needed in vast quantity for Daedalus’ fusion engines. More recently, though, attention has turned to the lunar surface as a possible source. Now the IBEX spacecraft, normally charged with studying the interactions between the heliosphere and what lies beyond, has been used to examine a useful recycling process as particles hit the Moon, pushed there by the Sun’s 450 kilometer per second solar wind.
A Glow of Energetic Neutral Atoms
The process is straightforward — lacking a magnetosphere, the Moon takes the full force of the solar wind, absorbing most of its particles into lunar dust. But the IBEX team, led by David McComas (Southwest Research Institute), has been able to show that about ten percent of the solar wind particles escape back to space in the form of energetic neutral atoms, or ENA’s, detectable by the spacecraft. IBEX, traveling in an eight-day orbit around the Earth, sees this ‘glow’ in its ENA detectors.
What IBEX detects are enough solar wind particles bouncing off the lunar surface as ENA’s to account for about 150 tons of hydrogen atoms per year, the rest remaining behind, some doubtless in the form of surface helium-3, whose measurement will one day help us calculate how useful a source it may become. But IBEX (Interstellar Boundary Explorer) is primarily focused on a much more distant venue. Its real mission is to see what happens to those same solar wind protons when they encounter interstellar atoms at the edge of the heliosphere.
ENA’s from Deep Space
Energetic neutral atoms are created at system’s edge when solar wind protons draw electrons from interstellar atoms, making them electrically neutral and thus no longer controlled by magnetic fields. Those ENAs that bounce back in the direction of the Earth can be recorded by IBEX, which studies a section of the sky about seven degrees across, scanning overlapping strips that complete a 360-degree map of the sky every six months.
The IBEX surprise, announced last October, was the discovery that the expected variations in emissions from the interstellar boundary were not evident. Instead, IBEX found what McComas at the time called “a very narrow ribbon that is two to three times brighter than anything else in the sky.” It’s chastening to remember that the Voyager spacecraft totally missed this feature because, unlike their point source measurements, IBEX can use its detectors to build up a complete map.
Charting Earth’s Magnetopause
The spacecraft has also been used to observe Earth’s magnetosphere from the outside, using the same ENA detection methods to see the interactions between the solar wind and the magnetic bubble surrounding our planet. Here the parallel between the heliosphere and the magnetosphere is interesting. The magnetosphere protects the Earth’s surface, causing the solar wind to pile up along its outer boundary (the magnetopause) before being diverted to the side. The heliosphere’s interstellar boundary, in a similar way, protects the Solar System from the worst effects of galactic cosmic ray radiation.
Image: IBEX found that Energetic Neutral Atoms, or ENAs, are coming from a region just outside Earth’s magnetopause where nearly stationary protons from the solar wind interact with the tenuous cloud of hydrogen atoms in Earth’s exosphere. Credit: NASA/Goddard Space Flight Center.
The IBEX team worked closely with the European Space Agency’s Cluster 3 spacecraft in observations made in March and April of last year. The new maps thus created show the teardrop shape of the magnetopause as solar wind protons pull electrons from hydrogen atoms in the Earth’s outer atmosphere. This region, the outer exosphere, is now shown to be tenuous indeed, with about eight hydrogen atoms per cubic centimeter. Thus ENAs helps us notch another needed measurement of a region that has been tricky to study.
The Wind and the Sail
You can see, too, that we’re gradually building up a picture of the solar wind that will help us analyze whether propulsion options like magsails have potential. Is the solar wind stable enough to allow accurate navigation by a magnetic sail-enabled spacecraft? Certainly the potential of hitching a 450-kilometer per second ride to the outer Solar System has appeal, and the deployment issues involved in large solar sails disappear with a magsail. But let’s see what IBEX and other missions can tell us about the solar wind as it reacts to the interstellar medium at system’s edge and plays against the magnetosphere closer to home.
You can read more about IBEX in this NASA mission page. For more on the solar wind’s interactions with the Earth’s magnetosphere, see Fuselier et al., “Energetic neutral atoms from the Earth’s subsolar magnetopause,” Geophysical Research Letters Vol. 37 (8 July 2010), L13101 (abstract). For the lunar observations, see McComas et al., “Lunar backscatter and neutralization of the solar wind: First observations of neutral atoms from the Moon,” Geophysical Research Letters Vol. 36 (2009), L12104 (abstract).
Thoughts on Brown Dwarfs, Disks and Planets
Planetary systems around dim brown dwarfs are a fascinating thing to contemplate, and for a vivid imagining of future human activities on such planets, I’ll send you to Karl Schroeder’s Permanence. The 2002 novel posits ingenious engineering to sustain bases on such worlds, and even comes up with an interstellar propulsion method powered up by their energies that sustains an expanding starfaring culture. A brief sample of Schroeder’s universe (not enough to be a spoiler):
…the brown dwarfs each had their retinue of planets — the halo worlds, as they came to be called. And though they were not lit to the human eye, many of these planets were bathed in hot infrared radiation. Many were stretched and heated by tidal effects, like Io, a moon of Jupiter and the hottest place in the Solar System. And while Jupiter’s magnetic field was already strong enough to heat its moons through electrical induction, the magnetic field of a brown dwarf fifty times Jupiter’s mass radiated unimaginable power — power enough to heat worlds. Power enough to sustain a population of billions; enough to launch starships.
Speculative fiction has new wonders to mine as we learn more about brown dwarfs, and our discoveries are coming faster all the time. One reason is that we’re getting better at detecting them. In 2006, Katelyn Allers (University of Texas at Austin) and colleagues published a list of nineteen candidate brown dwarfs, all of them young and all but one now confirmed as either low-mass stars or sub-stellar objects. Allers’ team was able to use Spitzer data to search for infrared excesses, which is where the tale gets intriguing. The excess in the infrared is presumably due to circumstellar disks, leading us to wonder how small an object has to be to form with an accretion disk.
Current thinking has it that such disks probably don’t form around central objects smaller than a few Jupiter masses, but the fact that young brown dwarfs are more luminous, and hence easier to detect, than older ‘field’ brown dwarfs means we can find them by homing in on the places where stars are being born to study disk formation around these cool, low-mass objects.
This is what Paul Harvey, also at UT-Austin, has done, working with Allers and team to extend the early Spitzer results to luminosity levels that should allow the detection of objects as faint as two Jupiter masses. Their work used deeper Spitzer imaging of an area in the Ophiuchus star-forming region studied in the earlier work, where the youngest objects under investigation are evidently about one million years old.
The result: Eighteen new brown dwarf candidates with the near-infrared magnitudes and colors we would expect from such objects and the possible infrared excess that is consistent with a circum-object disk. From the paper:
Contamination by background field dwarfs in the brighter magnitude range of our sample and by extragalactic objects at the fainter magnitudes is likely to be significant. It is certainly possible that at least half of our candidates are such contaminants. Narrow-band filter photometry in progress, however, has shown that at least several of our candidates are likely to be low-mass BD’s with circum-object disks. It is likely that further candidates exist in our data set, though problems with diffuse 5.8 and 8μm emission in the region make it difficult to clearly confirm many more disk candidates.
If even the smallest brown dwarfs normally tend to form with accretion disks, the case for planets forming around these objects is strengthened, and given that brown dwarfs are found to be increasingly common (WISE will help us greatly in assessing their numbers), we may be looking at hosts of planetary systems of a kind we have only recently begun to imagine. The new work extends the earlier Allers study to luminosities a factor of ten below its limits, giving us new data about how smaller objects form with the accretion disks that can create companions.
The paper is Harvey et al., “A Spitzer Search for Planetary-Mass Brown Dwarfs with Circumstellar Disks: Candidate Selection,” available online. Thanks to Antonio Tavani for the tip on this paper.
A Continental Shift and Its Implications
Although it seems a long way from interstellar space, the early Earth is a fascinating laboratory for life’s development that should yield clues about how life takes hold elsewhere. Thus new work on the movements of the early continents catches the eye. In this case, the Gondwana supercontinent is found to have undergone a 60-degree rotation across Earth’s surface during a highly interesting period, the Early Cambrian. This is the fecund era when the major groups of complex animals appeared in relatively rapid succession.
Gondwana is what we can call the southern precursor supercontinent, a vast region that would eventually separate from Laurasia roughly 200 million years ago when the Pangaea supercontinent broke into two large areas. This Wikipedia article gives you the basics on Gondwana, noting that it included most of the landmass in today’s southern hemisphere, including Antarctica, South America, Africa, Madagascar, Australia, New Guinea and New Zealand, along with the Indian subcontinent and Arabia (although the latter two have, obviously, moved into the northern hemisphere).
Image: The paleomagnetic record from the Amadeus Basin in Australia (marked by the star) indicates a large shift in some parts of the Gondwana supercontinent relative to the South Pole. Credit: Ross Mitchell/Yale University.
The movement of the entire Gondwana landmass was relatively rapid, with some regions attaining a speed of at least 16 (+12/-8) centimeters per year about 525 million years ago. Compare that with the pace of today’s shifts, which are no higher than 4 centimeters per year. The intriguing question is whether the shift results from plate tectonics — the continental plates in motion with respect to each other — or ‘true polar wander,’ which involves the solid land mass down to the liquid outer core rotating together with respect to the planet’s rotational axis, changing the location of the geographic poles. More in this Yale University news release.
What study author Ross Mitchell (Yale University) found is that true polar wander is the most likely scenario, the rates of Gondwana’s motion exceeding those of plate tectonics of the past few hundred million years. But arguments about polar wander vs. plate tectonics are ongoing and have been for decades. Mitchell can only say “If true polar wander caused the shift, that makes sense. If the shift was due to plate tectonics, we’d have to come up with some pretty novel explanations.”
But back to the main issue, which is the effect such migration would have had on the environment and living creatures. In this model, Brazil shifted from close to the south pole toward the tropics, the kind of movement that would have affected carbon concentrations and ocean levels. Here’s what Mitchell has to say about the results:
“There were dramatic environmental changes taking place during the Early Cambrian, right at the same time as Gondwana was undergoing this massive shift. Apart from our understanding of plate tectonics and true polar wander, this could have had huge implications for the Cambrian explosion of animal life at that time.”
It will be fascinating to see how this work is followed up in other locales as we work to explain the shift’s effects. Mitchell and team did their work studying the magnetization of ancient rock in the Amadeus Basin of central Australia. The paper is Mitchell et al., “Rapid Early Cambrian Rotation of Gondwana,” in Geology Vol. 38, No. 8 (August, 2010), pp. 755-758 (abstract).
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