by Paul Gilster | Mar 8, 2013 | Exoplanetary Science |
Just what does it take to make a habitable world? Keith Cooper is editor of Astronomy Now, the British monthly whose first editor was the fabled Patrick Moore. An accomplished writer on astronautics and astronomy as well as a Centauri Dreams regular, Keith has recently become editor of Principium, the newsletter of the Institute for Interstellar Studies, whose third issue has just appeared. In this essay, Keith looks at our changing views of habitable zones in light of recent work, and takes us to two famous science fictional worlds where extreme climates challenge life but do not preclude it. How such worlds emerge and how life might cope on them are questions as timely as the latest exoplanet findings.
by Keith Cooper
Literally overnight, two habitable planets – tau Ceti f and HD 85512b – were rendered barren and lifeless. What was the cause of this cataclysm? A nearby supernova? Asteroid impacts? On the contrary, it was something far more mundane.
A dozen light years away, scientists at Penn State University were re-analysing the extent to which habitable zones penetrate the space around stars; in other words, at what distance liquid water could potentially exist on a planetary surface assuming an Earth-like atmosphere. The basics for habitable zone theory had been worked out in part by, among others, Penn State’s James Kasting in decades previous. Building on his work, Ravi Kumar Kopparapu and Ramses Ramirez discovered that habitable zones are found further from their stars than had been envisaged (see Habitable Zones: A Moving Target for more).
The result was bad news for our two exoplanets. Suddenly, as the habitable zone shifted imperceptibly around them, they found themselves on the wrong side of the inner habitable zone boundary, too close to their respective stars. Consequently the Planetary Habitability Laboratory at the University of Puerto Rico, Arecibo, declared them uninhabitable. Too bad for any life-forms living there.
Despite only knowing the scarcest of details about these worlds – mass, radius, density, the amount of heating from their stars – these two worlds have been cast into the obsolescence in a manner that seems shockingly final. We know so little about these planets, how can we possibly say whether they are habitable or not, especially when the only standard we are holding them to is habitability for human beings?
Key Factors for Habitability
Determination of habitability is based on worlds not necessarily having exactly the same atmosphere as Earth, but at least having water and carbon dioxide, which are abundant and vital for life, Dr Abel Mendez of the Planetary Habitability Laboratory at the University of Puerto Rico, Arecibo, tells me. “The problem of the inner edge is that once you evaporate more water you get into a runaway greenhouse effect that will make the planet lose all its water,” he says.
There are other factors that play a part though. Just because a planet is inside a habitable zone doesn’t mean it is automatically habitable. The presence of an atmosphere, water, a global magnetic field, plate tectonics and a not too heavy impact rate are all factors. For those worlds close to the edges of the habitable zone, the margins are even narrower.
For example, habitability of planets on the edge could be largely dependent upon cloud cover, says Mendez, which can increase a planet’s albedo, or reflectivity, preventing heat from reaching the surface, but if there’s no way to see clouds on a planet many light years away, how can we just write off worlds like tau Ceti f and HD 85512b? Mendez admits nothing is for certain. “The intention of the habitable zone is to determine the limits [at which habitable planets can exist from their stars], but I will not call them hard limits yet due to uncertainties such as the effects of clouds.”
A constrained, limited view of habitability that says only Earth-like conditions will do limits the number of worlds we think would look friendly. And there’s nothing wrong with this approach – we know that a planet like Earth is suitable for life, so that is what we look for, whereas we don’t know yet whether life could exist on worlds like Europa or Titan, for example. It’s not that planetary scientists are ignoring other kinds of worlds, either. “Many groups are considering the more exotic possibilities, such as tidal habitable zones, habitable planets around white dwarfs, etc,” says Mendez. “The problem is that the habitability of such conditions are harder to observe or interpret than known biosignatures, and observational astronomers need to measure things, but we will get there.”
Science Fiction at the Boundaries
Until we do, however, we’re left to speculate with our imaginations and where is that not done best but in science fiction? So let’s take a look at a few imaginary worlds that are different to Earth but which could exist on the boundaries of the habitable zone and see how they stack up in comparison. Could reality really be as strange as fiction?
One common science fiction trope is the planet with the same climatic conditions over its entire globe, for example the desert planet, the ice planet, the jungle planet. In reality things are more complex – you can have what seems like all four seasons in one day on parts of the Earth. We don’t expect the same climate at the equator as at the poles. Meanwhile the change of seasons see cycles of weather, not just on our planet but on Mars, Saturn and Titan to name but three. What then do we make of our first two science fiction choices, the desert world Arrakis from Frank Herbert’s Dune, and the ice planet Hoth from The Empire Strikes Back?
Arrakis first. Dry as a bone, it has no surface water and no precipitation. What little atmospheric moisture there is is harvested by wind-traps and the water then ferried by canal to underground reservoirs in anticipation of using it as part of the terraforming of the planet. In the novels, however, the planet is mostly sand dunes, inhabited of course by the fearsome sandworms, except for at the pole where a large slab of bedrock ringed by mountains provides a more habitable zone.
Image: A sandworm rears up out of the desert of Arrakis on the March, 1965 cover of Analog. I can never resist the chance to display the artwork of the remarkable John Schoenherr. What memories…
So how did Arrakis end up like this? In Dune, Frank Herbert described the world as having salt flats, indicating that it once had lakes. The introduction of the non-indigenous sandworms, in their protoform as ‘sand trout’, saw them sequester all of the water. Arrakis, described as orbiting the star Canopus, changed from a fertile world to a desert planet.
We have our own desert planet in the Solar System in the form of Mars, skirting the outer edge of the habitable zone. While there are no sandworms on the red planet, liquid water dried up on the surface long ago and today only exists in frozen ice caps, sub-surface ice or possibly in aquifers deep underground. Indeed, what happened to Mars’ water, and the truth behind the climatic history of the planet, are still something of mystery, but we can hazard a best guess.
We know that Mars once had running water on its surface, in the form of rivers, lakes and even a northern sea. They existed billions of years ago. Today we see only their long-lasting consequences on the Martian terrain: river channels, floodplains, a surface chemistry forever altered by the presence of liquid water. The problem with Mars is that it is small, which results in a double whammy for the planet: its diminutive size means not only a smaller gravitational field but also a greater loss of heat from its core. As Mars’ interior began to cool, its molten core began to stiffen and the magnetic dynamo contained within began to stall. These two things conspired to allow the solar wind to strip Mars’ atmosphere, including its water vapour. (The European Space Agency’s Mars Express spacecraft has actually witnessed this stripping in action, watching Mars’ atmosphere lose oxygen at a rate ten times faster that Earth’s atmosphere; see Earth’s Magnetic Field Provides Vital Protection.
Move Mars closer in to the Sun and you could easily have a warmer Arrakis-type world. So desert worlds are feasible and you don’t require sandworms to create them either. But what about the other extreme, an ice planet like Hoth?
Life on the Outer Edge
Twice in Earth’s history – 2.5 billion years ago, and about 700 million years ago – our planet completely froze over [PG note: I must have had a typo here before; see the comments below re the 700 million year figure]. Even the oceans were covered with a thick layer of ice, right down to the equator. Dubbed ‘Snowball Earth’, what causes such events is uncertain, but a significant reduction in atmospheric carbon dioxide (possibly as a result of increased silicate weathering in the warm and wet tropics as continents gathered there) or methane (destroyed through oxidisation, as a result of an oxygen influx into the atmosphere from the first oxygen-exhaling life-forms) would do the trick. Both carbon dioxide and methane are potent greenhouse gases; without them the planet cooled and must have teetered close to the edge of an abyss from which it would never recover.
Of course, it did recover. The freezing of the planet brought the carbon-silicate cycle to a halt. Water vapour froze out of the atmosphere, which meant that precipitation ground to a halt. Ice covered the land so there could be no weathering and ice topped the oceans, preventing carbonates from reaching the sea floor. The way out of this predicament for the planet was that there was still an input into the carbon-silicate system, namely carbon dioxide belched out by volcanoes. Gradually the atmosphere accumulated carbon dioxide, with no rain to wash it out. Temperatures rose and the Earth thawed, but the point is that ice planets can very easily happen, particularly if a world lacks plate tectonics to provide that carbon dioxide input that acts as part of a thermal blanket for the world. If there were a ‘slushy’ belt around the equator, which doesn’t quite freeze over, then some life may be able to survive, although it’s hard to imagine what ecology could flourish on a planet like Hoth to permit a food chain with the monstrous yeti-like wampas at the top. Ironically, if methane was the primary greenhouse force in early Earth’s atmosphere, and was destroyed by oxygen, then the discovery of another snowball planet around another star could potentially be a biosignature indicating the presence of oxygen-exhaling life on that world.
Hoth was a world covered in ice. What about planets covered in water, such as Solaris in Stanislaw Lem’s novel of the same name (ignoring the fact that this fictional planet’s global ocean was actually a living entity)? According to the United States Geological Survey seventy percent of Earth’s surface is covered in water and simulations depicting planet formation suggest that planets could easily acquire much more water than Earth did; indeed, Earth is actually quite dry. Perhaps water is delivered to planets by comets and asteroids, or perhaps these water-worlds are born further out, beyond the ‘snow line’ where water-ice is prevalent, before migrating inwards to hotter climes where their ice melts. There’s even observational evidence for water-worlds – in February 2012 Hubble Space Telescope observations of the 6.5 Earth-mass world GJ 1214b, some forty light years distant, show that starlight passing through its atmosphere is being absorbed at the characteristic wavelength of water vapour, enough to contribute a large fraction of the planet’s mass.
All of these worlds – desert, ice and ocean planets – could potentially be habitable to a point; even in Earth’s own snowball periods, life persevered. However their occurrence was before the arrival of complex life and it is doubtful such life would have survived the onset of such a catastrophic change in climate. More to the point, Mars was once wet and warm with a thicker atmosphere, even if it was only for a short while, while still existing outside of the habitable zone. Now it is a barren. On the other hand Earth was once a frozen wilderness despite being in the habitable zone, but is now resplendent with life.
While habitable zones are a starting point, it is clear they are not necessarily the final word on habitability and locating planets within their limits does not guarantee that they are going to be Earth-like, nor does it automatically correspond that planets outside of the habitable zone will be inhospitable. Furthermore, astronomers also suspect that life could exist in such exotic locales as planets in ten hour-orbits around white dwarfs, on tidally locked worlds around red dwarfs, on exomoons orbiting gas giants and even on rogue planets that wander interstellar space, kept warm by their own innate radioactivity. Surely if any of these types of planet are discovered to be habitable it will prove that reality can be far stranger than fiction.
by Paul Gilster | Mar 7, 2013 | Astrobiology and SETI |
Among the many memorable things Freeman Dyson has said in a lifetime of research, one that stands out for me is relatively recent. “Look for what is detectable, not for what is probable.” This was Dyson speaking at a TED conference in Monterey, CA back in 2003, making the point that the universe continually surprises us, and by making too many assumptions about what we are looking for, we may miss unexpected things that can advance our understanding. Dyson has been thinking about this for a long time considering that it was way back in 1960 that he first suggested looking for the excess infrared radiation that might flag a distant Dyson sphere.
I would call this an unorthodox approach to SETI in its day except that when he first came up with it, Dyson didn’t have a SETI effort to consider. It was only in the same year that Cornell’s Frank Drake began SETI observations at Green Bank, and a scant year before that that Philip Morrison and Giuseppe Cocconi published the seminal paper “Searching for Interstellar Communications” in Nature. SETI in 1960 was a nascent field, but it would soon be focused on radio and, later, optical transmissions. Even so, Dyson’s thinking remains viable and unorthodox SETI efforts continue.
Image: Our Milky Way presents a field full of stars. How to search for signs of an extraterrestrial civilization among the countless targets? Looking for a Dyson sphere is unorthodox, but some scientists are suggesting this and other unusual ways of detecting the macro-engineering of distant civilizations. Image credit: NASA.
Luc Arnold (Aix Marseille Université) runs through the scholarship on what we might call ‘non-traditional’ forms of SETI in a new paper, noting that what he calls Dysonian SETI looks for signatures of macro-engineering projects in space. We’ve discussed many of these here before, delving particularly into the papers of Milan ?irkovi? and Robert Bradbury, but it’s worth recalling that others picked up on Dyson’s ideas earlier, including Carl Sagan and Russell Walker, who concluded as far back as 1966 that Dyson spheres should be detectable but would probably be hard to tell apart from natural objects having the same low temperatures.
In articles like Toward an Interstellar Archaeology, I’ve looked at Richard Carrigan’s searches for macro-engineering, and Luc Arnold reminds me in his new paper that Michael Harris has proposed methods of observing antimatter burning by advanced civilizations. Harris would go on in 2002 to use gamma ray observations in a first attempt to find such a signature. In fact, we can trace non-traditional SETI studies to authors as diverse as Ronald Bracewell, Michael Papagiannis and Robert Freitas, who along with several others Arnold references have followed Dyson’s lead in looking for what is detectable rather than what is probable.
Back to Arnold himself, who proposed in 2005 that transit studies like Kepler and CoRoT should be aware of the possibility of detecting an artificial signal. What the scientist has in mind is a planetary size object that orbits its star, constructed by a civilization as a celestial marker. The idea fits with something Jill Tarter said in 2001: “An advanced technology trying to attract the attention of an emerging technology, such as we are, might do so by producing signals that will be detected within the course of normal astronomical explorations of the cosmos.”
Transmission Methods and the Drake Equation
Arnold’s new paper compares radio wavelengths with laser transmissions and his own idea of artificial transits with respect to the factor L in the celebrated Drake Equation. L is generally considered to refer to the lifetime of a civilization, which obviously limits its ability to transmit a detectable signal. Working out the solid angle over which the transit could be ‘transmitted’ over one year, Arnold arrives at a figure of between 25,000 and 75,000 stars depending on stellar densities, and therefore uses a mean number of targets of 50,000 to set up comparisons between artificial transit ‘messaging’ and the more conventional radio and laser transmission options.
The idea is to examine efficiency as seen from the perspective of a transmitting civilization. A radio transmitter is the best choice for short-term messaging — a brief, highly targeted program of signaling — while lasers would require 102 times more energy. In terms of construction and maintenance, artificial transiting objects are more costly. They become interesting only for extremely long-term thinkers who are using the method to produce attention-getting signals where “…the transmitting time can be very long, possibly much longer than the lifetime on the civilization itself.”
Arnold notes that the shape of a transiting object shows up in the transit light curve, making the detection of an artificial planetary sized object a clear possibility. Weighing the costs and energy required to signal other stars, he concludes that if we make such a detection, it should be interpreted as the message of an old and perhaps defunct civilization. It would demonstrate at least that the lifetime of a technological civilization can be longer than several centuries.
It is also true that large artificial objects may be constructed for purposes other than communication. From the paper:
We may also argue that a civilization wanting to communicate with other beings also may want to leave a trace or an artifact in the galaxy that would survive much longer that the civilization itself. These two civilization behaviours seem not incompatible, but rather naturally linked and complementary, at least from an anthropocentric psychological point of view. But artificial planetary-sized objects may also be built for other technological purposes than communication, like energy gathering for example. Such macro-engineering achievements could be the result of natural technological evolution… making the will or desire of communication only an optional argument.
Arnold’s points are intriguing. SETI by radio and optical methods assumes we are looking for an active civilization. But lasers and radios fall silent when a civilization dies. Meanwhile, large artificial objects that transit their stars could remain indefinitely, markers of a culture that might have flourished billions of years ago and is now gone. The Dysonian approach of looking for macro-engineering thus offers the chance to do the kind of interstellar archaeology Richard Carrigan has championed through his exhaustive efforts. Turning up the signs of an artifact without any presumption of further communication still changes our view of the universe.
Who would construct a vast artificial occulter to send a signal to other stars? Several possibilities come to mind, including the idea that the occulting object might have been constructed for purposes other than being detected by another civilization. It is conceivable, though, that a dying culture would want to leave some trace of its existence. Because we are speculating on the motivations of extraterrestrials, we have no way of knowing. This is why Dyson’s idea of looking for what is detectable continues to resonate. Being surprised by the universe is part of our experience and there is no reason to expect that to change now.
The paper is Arnold, “Transmitting signals over interstellar distances: Three approaches compared in the context of the Drake equation,” accepted for publication in the International Journal of Astrobiology (preprint). Thanks to Antonio Tavani for the pointer to this paper.
by Paul Gilster | Mar 6, 2013 | Outer Solar System |
Europa continues to fascinate us with the possibility of a global ocean some 100 kilometers deep, a vast body containing two to three times the volume of all the liquid water on Earth. The big question has always been how thick the icy crust over this ocean might be, and we’ve looked closely at Richard Greenberg’s analysis, which shows surface features he believes can only be explained by interactions between the surface and the water, making for a thin crust of ice. See Unmasking Europa: Of Ice and Controversy for more, and ponder the prospects of getting some kind of future probe through a thin ice layer to explore the potentially habitable domain below.
Possible interactions between the surface and the ice are considered in a new paper by Mike Brown (Caltech) and Kevin Hand (JPL), one that makes the case that there are two ways of thinking about Europa. One is to see the Jovian moon purely as an ice shell upon which the bombardment of electrons and ions have created a chemical cycle. The other is to see it as a geologically active world with an internal ocean that affects what happens on the surface.
Just how much, in other words, does the chemistry of the internal ocean affect what we see from our spacecraft and telescopes? Brown and Hand now believe they can identify a chemical exchange between the ocean and the surface that we can analyze to learn more about both.
Using data from the Keck instrument on Mauna Kea, the researchers have used adaptive optics and spectroscopy to go far beyond what the instruments on the Galileo probe were able to tell us about Europa’s surface. Turning up in their results is a magnesium sulfate salt called epsomite. Magnesium could not be found on the surface unless it came from the ocean below, meaning that ocean water does make it through onto the surface, while surface materials get into ocean water. Says Brown:
“We now have evidence that Europa’s ocean is not isolated—that the ocean and the surface talk to each other and exchange chemicals. That means that energy might be going into the ocean, which is important in terms of the possibilities for life there. It also means that if you’d like to know what’s in the ocean, you can just go to the surface and scrape some off.”
Image: Based on new evidence from Jupiter’s moon Europa, astronomers hypothesize that chloride salts bubble up from the icy moon’s global liquid ocean and reach the frozen surface where they are bombarded with sulfur from volcanoes on Jupiter’s innermost large moon, Io. The new findings propose answers to questions that have been debated since the days of NASA’s Voyager and Galileo missions. This illustration of Europa (foreground), Jupiter (right) and Io (middle) is an artist’s concept. Credit: NASA/JPL.
Because Europa is tidally locked to Jupiter, the same hemisphere always leads in its orbit around the planet, while the other always trails. The difference between the two is striking: While the leading hemisphere has a yellow tint, the trailing hemisphere is streaked with a red material that has been under study for many years. It is believed that volcanic sulfur from Io accumulates on Europa’s trailing hemisphere, existing there along with a substance other than water ice that Galileo could not identify. Keck’s OH-Suppressing Infrared Integral Field Spectrograph (OSIRIS) turned out to be what was needed to map the distribution of water ice and home in on the other material.
It turns out that both hemispheres contain significant amounts of non-water ice, but on the trailing hemisphere Brown and Hand identified the spectral signature of magnesium sulfate. Interestingly, the magnesium sulfate does not itself appear to come from the ocean. Because it only appears on Europa’s trailing side, where Io’s sulfur is accumulating, the researchers surmise there is a magnesium-bearing mineral — probably magnesium chloride — everywhere on the moon that produces magnesium sulfate in the presence of sulfur. The same magnesium chloride might then make up the non-water ice detected on the leading hemisphere.
Europa’s ocean can be rich in sulfate or rich in chlorine, but Brown and Hand rule out a sulfate-rich ocean because magnesium sulfate only appears on the trailing hemisphere. This fits with other work Brown has done on Europa’s atmosphere, which identified atomic sodium and potassium as constituents. The researchers believe that sodium and potassium chlorides consistent with this atmosphere are the dominant salts on the surface of Europa. Their conclusion is that Europa’s is a chlorine-rich ocean with sodium and potassium present as chlorides. By this analysis it closely resembles Earth’s oceans. “If you could go swim down in the ocean of Europa and taste it, it would just taste like normal old salt,” Brown adds.
The paper is Brown and Hand, “Salts and radiation products on the surface of Europa,” in press at the Astrophysical Journal (preprint). More in this Keck Observatory news release.
by Paul Gilster | Mar 5, 2013 | Culture and Society |
Those of us who are fascinated with interstellar travel would love to see a probe to another star launched within our lifetime. But maybe we’re in the position of would be flyers in the 17th Century. They could see birds wheeling above them and speculate on how humans might create artificial wings, but powered flight was still centuries ahead. Let’s hope that’s not the case with interstellar flight, but in the absence of any way of knowing, let’s continue to attack the foundational problems one by one in hopes of building up the needed technologies.
Marc Millis, who ran NASA’s Breakthrough Propulsion Physics project at the end of the 20th Century, always points out in his talks that picking this or that propulsion technology as the ‘only’ way to get to the stars is grossly premature. In a recent interview with the Australian Broadcasting Corporation’s Antony Funnell, Millis joined physicist and science fiction writer Gregory Benford, Icarus Interstellar president Richard Obousy and astronomer and astrobiologist Ian Crawford in a discussion of the matter. Asked where we stood with nuclear fusion, Millis said this:
At this point it is really too soon to pick any favourites because…well, let me put it to you this way; in three different studies, one done by looking at the amount of energy available, one done by financing and one done by technology, all of them came in that there is still going to be about two centuries before we could do a serious interstellar flight. So even if you pulled off the technology for a fusion rocket, to develop the infrastructure to mine enough helium-3 to fuel it, it’s still going to take a very long time. In other words, you could make the technology but to have the amount of energy to put into it takes even longer than developing the technology.
We had much the same discussion at the 100 Year Starship symposium last year in Houston, where a backer of the project from the business world asked why an interstellar mission would be so expensive. The answer simply comes down to the amount of money it takes to create the energy needed to push a payload to the kind of speeds we’re talking about. Given all that, we continue to study everything from beam-driven sails to antimatter-induced fusion and the whole boatload of possibilities in between, hoping to find more efficient ways to drive the starship.
Image: High-intensity lasers produce particle/antiparticle pairs from the vacuum, in a concept introduced by Richard Obousy. Credit: Adrian Mann.
Timeframes Short and Long
People differ widely on time-frames — some people get positively passionate about them — but my own view is that working toward the interstellar goal is just as valuable for me if it happens three centuries from now than if it happens by 2100. I’m no seer and the few times I’ve tried to predict the future, I’ve been humbled by how surprisingly an ‘obvious’ outcome can change. I’m also reminded of what Richard Obousy often tells his own audiences, that humans tend to overestimate what they can do in the short run and underestimate what they can do over longer periods. So maybe interstellar flight is going to happen sooner than I think. Either way, I’ll keep on writing about the topic in hopes of encouraging research and public involvement.
Whenever interstellar flight comes, assuming it does, we’ll all do better by looking out for our planet in timeframes of centuries rather than years or even decades. Gregory Benford told Funnell that the reason we fall into short-term thinking traps is that we live in a tightly defined environment, one in which the great age of physical exploration on our planet is long past. Moving out into the Solar System and ultimately beyond it in search of resources may once again instill a longer-term view as we seek out elements like helium-3 that fusion reactors will demand.
New human societies should emerge from all that as we move toward the construction of an economy throughout the system. Let me quote Benford on this from the interview, in a section where he’s asked about the 100 Year Starship project and the choice of the time frame. A century may be a symbolic goal, but the act of choosing goals is itself part of the process.:
The expansion of the United States into the West began roughly around 1850 and within a century was accomplished, largely through railroads and use of coal for power. But it went on to new heights, we invented the aeroplane, and a century later we already had intercontinental air flight commonly available. It’s this kind of building upon a model that makes star flight a proper goal for the development of the inter-planetary economy that we believe is coming, and therefore sets a goal; 100 years from now let’s see if we can build a starship, and what does it look like, and is it manned or unmanned or robotic or does it have artificial intelligence aboard? Those are secondary issues. The main thing is let’s have a goal and let’s build toward it.
Thus the method: Create a concrete goal and discover a way to reach it. Benford told Funnell that prosperity grows out of these efforts because the structure is being built every step of the way. I’m also reminded here of the ‘horizon mission methodology’ that NASA has found useful in stimulating thinking in its conferences — John Anderson described this in a 1996 JBIS paper. The idea here is to present a problem that is at present impossible to solve. The team then sets about defining what breakthroughs will be needed if this problem is ever to be conquered.
The Dangers of Presumption
Defining the goal and setting the target is the beginning of the process. This is one reason for the original Project Daedalus, which set out to examine the question of whether an interstellar vessel was even possible. The reasoning was that if a culture at our own level of technological development could identify a conceivable way to reach another star, then future breakthroughs should make the job that much more realistic. No one seriously planned to send a 50,000 ton vehicle to Barnard’s Star, but the project proved, way back in the 1970s, that designs that did not violate known physics could be contemplated. Project Icarus now refines the Daedalus model.
Former astronaut Mae Jemison, who heads up the 100 Year Starship initiative, put the matter this way in her conversation with Funnell:
One of the biggest challenges is, again, to keep people from trying to design every step of the way right now, because we don’t know. And as soon as you start saying ‘I know the answer right now’, then you’re probably going to cut off other avenues. There is something that I would say when you talk about how daunting this is and whether or not people say that it’s not possible. It’s a term that I first heard associated with movies, and that term is ‘suspending disbelief’. At some point in time we have to do move forward by suspending disbelief.
Jemison is not talking about suspending disbelief in known physics, of course. What she’s saying is that setting the goal and collecting the wide range of options is how the process begins, and if we succumb to assumptions — from ‘interstellar flight is impossible’ to ‘there’s only one way to do it, my way’ — then we’re not honoring the need for lengthy and challenging research that’s ahead. Personally, I find this notion invigorating. We are beginning to realize through our exoplanet research that Earth-like planets may be out there in the billions. We now engage scientists and engineers in the great work of studying the options that may one day put a human-made payload into another solar system. Humans themselves may eventually make the journey if we are wise enough to make the foundations of this enterprise deep, strong and true.
by Paul Gilster | Mar 4, 2013 | Astrobiology and SETI |
One of the first science fiction novels I ever read was The Black Cloud, by astrophysicist Fred Hoyle. I remember that one of my classmates had smuggled it into our grade school and soon we were passing it around covertly instead of reading whatever it was we had been assigned. In Hoyle’s novel, scientists discover that the cloud, which approaches the Solar System and decelerates, may be a life-form with which they can communicate. My young self was utterly absorbed by this book and I suspect it will hold up well to re-reading.
What brings The Black Cloud to mind is recent work using data from the Green Bank Telescope in West Virginia, where scientists have been studying an enormous gas cloud some 25,000 light years from Earth near the center of the Milky Way in the star forming region Sagittarius B2(N). This cloud is not, of course, behaving as entertainingly as Hoyle’s, but it’s offering up information about how interstellar molecules that are intermediate steps toward the final chemical processes that lead to biological molecules can form in space.
Although we’ve found interesting molecules in interstellar gas clouds before, the new work suggests that the chemical formation sequences for these molecules actually occurred on the surface of icy grains in interstellar space. One of the molecules is cyanomethanimine, which scientists believe is part of the process that produces adenine, one of four nucleobases forming the ‘rungs’ in the DNA lattice. The other is ethanamine, thought to play a similar role in the formation of alanine, one of twenty amino acids in the genetic code.
The biological interest is the suggestion that life’s building blocks are widely available, as noted in one of the two papers on this work:
One important goal of the field of astrobiology is the identification of chemical synthesis routes for the production of molecules important in the development of life that are consistent with the chemical inventory and physical conditions on newly formed planets. One mechanism for seeding planets with chemical precursors is delivery by outer solar system bodies, like comets or meteorites… These objects can be chemical reservoirs for the molecules produced in the interstellar medium during star and planet formation. The chemical inventory of these objects includes the molecules that are directly incorporated from the interstellar medium and molecules subsequently formed by chemical processing of the interstellar species…
What we’re after, then, is an understanding of the process by which interstellar molecules can undergo further change relevant to the formation of life. The paper continues:
This subsequent chemical processing can synthesize larger, more complex molecules that are more directly relevant to prebiotic chemistry from the simpler molecules that can be formed in the interstellar medium. The identification of molecules in the interstellar medium is a key step in understanding the chemical evolution from simple molecular species to molecules of biological relevance and radio astronomy has played the dominant role in identifying the chemical inventory of the interstellar medium…
Image (click to enlarge): The Green Bank Telescope and some of the molecules it has discovered. Credit: Bill Saxton, NRAO/AUI/NSF.
The region under study, Sgr B2(N), turns out to be incredibly rich for this kind of work. According to the scientists, about half of the 170 molecules that have been detected in space were first found in this region. The team was able to measure the characteristic radio emission signatures of the rotational states of cosmic chemicals, using radio emission studies of cyanomethanimine and ethanamine and comparing these to the data generated by the Green Bank Telescope. “Finding these molecules in an interstellar gas cloud means that important building blocks for DNA and amino acids can ‘seed’ newly-formed planets with the chemical precursors for life,” says Anthony Remijan, of the National Radio Astronomy Observatory (NRAO).
The papers are Loomis et al., “The Detection of Interstellar Ethanimine (CH3CHNH) from Observations taken during the GBT PRIMOS Survey,” accepted in Astrophysical Journal Letters (preprint) and Zaleski et al., “Detection of E-cyanomethanimine towards Sagittarius B2(N) in the Green Bank Telescope PRIMOS Survey,” also accepted at Astrophysical Journal Letters (preprint). See this NRAO news release for more.
