The Astrobiology Universe
The opening speaker at the 2008 Astrobiology Science Conference, Lord Martin Rees of the University of Cambridge, said that our universe may just be one of many. Multiple universes could be stacked sideways like sheets of paper, separated by only a thin margin of space. We would never know they were there unless we could be awakened to the existence of that other dimension.
This could have been the theme of the conference. Every morning and afternoon, nine separate talks were given simultaneously, often just separated by thin walls through which applause could be heard. Although aware of these separate astrobiology multiverses, participants could not attend most sessions due to the constraints of space and time. Perhaps Lord Rees should have helped us transcend our limited 3-dimensional existence. After all, when he received his work visa for the United States, he was classified as “an alien of extraordinary abilities.”
It’s difficult to pick just a few galaxies of thought from the universe of ideas presented at the conference. Below is an attempt to highlight those where the stars seemed to shine the brightest.
Humans Will Explore Mars
In 2007, the Human Exploration of Mars Science Analysis Group (HEM-SAG) set out to define the science goals for NASA’s human exploration of Mars. Joel Levine of NASA’s Langley Research Center said the group’s report is now online.
The group decided the first three human missions to Mars should have three different landing sites, and the sites should represent three different geological epochs in martian history. There are currently 58 possible sites, and Levine feels that biologists and life scientists should “have the highest priority” in site selection.
It takes six months to travel to Mars, and the explorers would stay on the planet for at least 500 days. For humans to travel around the surface, a pressurized rover would be needed. This vehicle would probably need to be very massive, weighing several thousand kilograms.
Such missions would have to bring a lot of equipment to Mars for in situ analysis, because it’s too expensive to bring mass back to Earth. Levine said that “when we go to Mars, it’s going to be such an important event that it’ll be an international effort.” No details about this have been worked out yet, but a Mars sample return mission should set precedents for such international collaboration.
Four Months on Mars
What would it be like to live on Mars? The Mars Society recently sent seven people to Devon Island in the Canadian Arctic to simulate a martian colony. For four months they lived in a “tin can” habitat, wore space suits when venturing outside, and ate freeze-dried food. They could communicate with Earth, but there was the same time delay Mars colonists would experience. They also gained 39 minutes each day, since Mars has longer days than Earth. Participants conducted science experiments, but also were experiment subjects: their sleep patterns were studied, as was their water use. The overall experience must have been good because one of the participants, Kim Binsted of the University of Hawaii, enthusiastically said she would volunteer for a real Mars mission.
Do We Need to Come Back?
Given the difficulties of returning humans back from Mars, Paul Davies, a physicist at Arizona State University, said a “one-way ticket” to Mars should be considered for future explorers. Going to Mars is risky, and would shorten your life expectancy. But “this is not a suicide mission,” he stressed.
The riskiest parts of space travel are the take off and landing, and by not coming back to Earth you reduce your risk by half. You also reduce the amount of zero gravity you are exposed to during space travel, which has significant hazards for health. Mars is the second safest place in the solar system, said Davies, and lava tube caves would make a good protected habitat.
The first four-person crew would establish and maintain a base, and additional people would join them over time. The initial mission “would be the first step in establishing a permanent human presence on another world,” he said.
Establishing a Mars colony also would provide humans with a lifeboat away from Earth disasters, such as asteroid impacts, plagues, or war. “But in my view, the reason to go is not to avoid disaster, but because it is the most likely object beyond the Earth to have life,” said Davies.
Scientists would do a great deal of research on Mars, and would have awards and accolades heaped upon them. “I don’t envision four miserable people sitting around on the surface waiting to die, but doing useful work,” he said.
However, Davies thinks NASA will not fund such a mission, so money would have to come from private enterprise and philanthropy. For example, “The TV rights [to] a spectacular like this, a real-life soap opera from another planet, I would think would be worth a lot of money.” The stars of the show might be all elderly people, since their life span would not be too greatly shortened by living on Mars. Davies asked for a show of hands for volunteers, and about one-third of the audience were up to the task.
Davies said he’d rather head straight to Mars and skip the moon bases planned by NASA. “Been there, done that,” he said.
The Moon Before Mars
Paul Davies might not be excited by a moon colony, but Chris McKay of NASA’s Ames Research Center thinks a human base on the moon is a necessary first step toward the human exploration of Mars.
If we wanted to send humans to Mars just to “collect some rocks, get back in the space ship and come back to Earth, we could probably do that without going to the moon,” he said. But the most effective way to search for martian life would be to establish a long-term human base there, and “we don’t know how to operate [an off-world] base for years on end.” The best place to learn what will be required, he argued, is on the moon.
Although NASA is formulating plans to send humans back to the moon, and hints at the eventual human exploration of Mars. “Mars is really the interesting world” for astrobiologists, McKay said. It’s likely that evidence of Martian life is buried below the surface, or deep within southern polar ice, accessible only by deep drilling, “maybe as deep as a kilometer.”
Have Drill, Will Explore
Since there is much less risk involved in having robots explore the other planets, one of the main goals of HEM-SAG was to define experiments humans can do that robots cannot. Peter Doran of the University of Illinois at Chicago made the case that to drill on Mars, humans will have to do it. Recent discoveries of microbes living below the surface of Earth have shown that microbial life can grow deep within a planet, protected by layers of rock and soil. Astrobiologists hope that life on Mars might live in a similarly protected niche. But deep-drilling projects are "almost an impossibility without humans" according to Doran. Because of this, human explorers could be essential in answering the question of whether Mars can support life as we know it.
A Mars-like drilling project was reported by Henry Sun, a microbiologist with the Desert Research Institute in Las Vegas, Nevada. His team wants to drill into a “natural experiment” in the Mojave Desert, a series of lava flows that occurred in the Cima Volcanic Field about a million years ago. The lava flows entombed layers of desert soil beneath them, perhaps isolating the soil layers from access to water. He’s interested in drilling into the ancient soil layers to see, after so much time has passed, what biosignatures have been preserved. The difficulty will be to perform the drilling without contaminating the ancient soils with present-day organisms. The Cima field, Sun said, is a good “training ground” for future missions to Mars that will drill into the subsurface in search of signs of life.
Robots are being used to explore places humans can’t get to. The Cliff-bot rover was tested during the 2007 field season of the AMASE (Arctic Mars Analog Svalbard Expedition) project.Developed at NASA’s Jet Propulsion Laboratory (JPL), as a prototype for a possible future mission to Mars, Cliff-bot is one of a trio of rovers that work together to access interesting rock and soil samples on steep hillsides. Two of the three rovers are anchored at the top of the cliff, and the third, which contains a set of scientific instruments, is lowered down the cliff face on tethers.
Paulo Younse, a JPL engineer working on the Cliff-bot project, said the field test included an 11-meter (35-foot) climb down an 85-degree cliff face. “Any typical rover, MER, MSL – no way they’d be able to go down” such a steep incline, Younse said, pointing out that “a lot of the interesting science” on Mars can be found along steep crater and chasm walls that have, so far, been inaccessible.
We could send robots to explore regions of our solar system that are far too cold for humans, but would the robots find any evidence of life there? We think liquid water is needed for life to exist, so the habitable zone is the region of space where water stays liquid on the surface of a rocky planet. In our solar system, the habitable zone roughly extends from Venus’ orbit out to Mars (with Earth smack in the middle). But if a planet undergoes tidal heating, it may not need to be in the habitable zone for its water to stay liquid.
According to Wade Henning of Harvard University, the gravitational influence of a star or giant planet can create a great deal of tidal heat on a rocky planet. “If you’ve got a rocky planet, I can melt it for you,” he said. That could mean there are more regions in any given solar system suitable for life as we know it. For instance, the moon Europa receives so much tidal heating from orbiting Jupiter that it has a global ocean of liquid water, even though Europa is far away from the habitable zone.
The type of body a rocky world orbits makes a big difference in how much tidal heating the planet will experience. For a G-type star like our sun and also for the slightly cooler K stars, the tidal zone and the habitable zone do not overlap (in other words, a planet in the habitable zone will not experience tidal heating as well). For M-type red dwarf stars, however, the zones do overlap. They do so for moons as well, which receive most of their tidal heating forces from the planet they orbit. Henning said that one interesting result of his analysis is that it is possible to envision a habitable planet with no star. “It would be very dark, but warm enough,” he said.
Is There Life in Alpha Centauri?
Alpha Centauri, a triple-star system only four light years from Earth, contains our closest stellar neighbors. To date, no planets have been found around these stars. Computer models suggest that gas-giant planets like Jupiter and Saturn would be unlikely to form there, and these giant planets are the easiest to detect with the radial-velocity technique that has been used to discover most of the known extrasolar planets. But models also show that Earth-like planets could form within the habitable zone around Alpha Centauri B.
Terrestrial-size planets are far more difficult to detect with radial-velocity measurements, but a group of planet-hunters thinks it may be possible if enough measurements are taken. “We’re talking about taking over 100,000 data points” over a five-year period “to detect an Earth-size planet,” said Elisa Quintana, an astronomer with the SETI Institute. Quintana and her colleagues will use a 1.5-meter telescope in Chile to make the observations, a much smaller telescope than those typically used for radial-velocity measurements. “We’re doing this from the backyard,” she said. “Hopefully, five years from now … we’re going to be able to report the detection or the non-detection” of one or more terrestrial planets around Alpha Centauri B.”
Galactic Habitable Zones
Charley Lineweaver, a cosmologist with The Australian National University, is investigating the habitable zone of galaxies. One thing that could be life-limiting on a galactic scale would be the elements available to form planets and life. Life as we know it primarily uses carbon, hydrogen, oxygen and nitrogen, but will life do so everywhere? Elements are made by stars, especially in the later years of their life. So the older a region of space, the more elements will be available. Lineweaver noted that 75 percent of stars in the galactic habitable zone are older than our sun, and this suggests that life with a far more ancient heritage than life on Earth could be out there.
|What our Milky Way Galaxy may look like from above, and the position of our star, the sun, within the galaxy. Are there certain regions within galaxies that are more habitable than others? Click image for larger view.|
Another limiting factor for life in a galaxy would be the location of the star relative to the galactic center. You want to be far enough away to avoid intense radiation and so that you’re not crowded by other stars (thereby also avoiding the supernova explosions of dying stars). How a star orbits the galactic center may be important as well. Our sun has a relatively circular orbit, while other stars do not. Lineweaver thinks perhaps a circular galactic orbit is a requirement for life to appear in a solar system, or at least survive over large time scales. Ultimately, Lineweaver said that life may be possible for less than 10 percent of all the stars ever formed in the Milky Way Galaxy.
Cycles of Diversity
Richard Muller of the University of California, Berkeley said that our place in the galaxy could influence the evolution of life on Earth. He noted that there are 62-million-year and 140-million-year cycles of fossil diversity on Earth (depending on which species you look at). Ticking down a list of possibile causes, he said that for the 140-million-year-cycle one cause could be the sun’s passage through the arms of the Milky Way. This passage would increase the possible number of comet impacts on our planet, and such impacts can lead to extinction events. As for the 62-million-year cycle, as yet no plausible causes have been found. Muller did say that strontium isotopes in the rock record also show a 62-million-year cycle. Strontium tracks weathering (the slow break-down of rocks through rain, wind, and other natural phenomena), so this could be an indication of some as-yet unknown climate cycle on Earth.
Do minerals evolve like life does? Robert Hazen of the Carnegie Institution of Washington proposed a new way to look at mineralogy on terrestrial worlds: as an evolutionary process.
Rather than the traditional static view of minerals, which focuses primarily on their crystal structures, he suggested that “every mineral is part of a narrative story, a dynamic story.” When the Earth was first coalescing, it contained only a dozen or so “ur-minerals,” leftovers from stellar explosions. Condensation, heating, pressure, water-rock interactions and plate tectonics increased that number to perhaps 1,500 different minerals. But the saturation of the Earth’s atmosphere with oxygen, the byproduct of oxygenic photosynthesis, was “the most dramatic single event in mineral diversity on Earth,” he said. “Perhaps two-thirds of all minerals on Earth, if this model is correct, are the indirect consequence of the biological changes in the atmosphere.” Hazen suggested that different planetary bodies “achieve different levels of mineral evolution, and this can be a very interesting target for astrobiology investigation.”
Are Brains Necessary?
Some forms of primitive life can teach us about the evolution of complex life. David Gold of the University of California, Los Angeles studies cnidarians. These creatures predated the Cambrian explosion and include the sea anemones, hydras, and jellyfish. Some are scyphozoan, with primitive slit eyes that can detect light orientation, while others are cubozoan, with sophisticated lens eyes extremely similar to our own. But they have no brain. “One would expect lens eyes to be useless without advanced neural processing,” said Gold. It’s not a case of form preceding function, because they use their eyes to hunt and avoid objects.
Gold has studied the more primitive Aurelia to see how gene networks that code for eyes develop. The Aurelia nervous system consists of a Giant nerve net – which may be independent brains that are connected – and a Diffused nerve net. Head-patterning genes found in bilateral animals (like humans) are also found in the development of individual nerve nets in Aurelia. Perhaps intelligence is limited by body shape. Or perhaps intelligence is not necessarily a result of evolution — many organisms have survived eons without developing “intelligent” hardware, and evolutionary history has many cases of a species losing intelligence over time.
The Human Niche
Charley Lineweaver said we are being vain when we assume all forms of intelligence evolve toward human-like intelligence, such as in the movie “Planet of the Apes” where primates evolved to be human-like. We assume that humans occupy the peak of an intelligence hierarchy, and other species want to evolve to occupy that niche to become our functional equivalent. Lineweaver said this view is not supported by the evidence. If all animals eventually evolved to develop human-like intelligence, this trait would have appeared over geologic time, especially on isolated islands that were devoid of humans. Animals had millions of years to develop, and yet no other human-like intelligences ever emerged. Lineweaver also noted that large brains are not an inevitable outcome for animals. People look to the dolphins and say that is an example of convergent evolution, since their brains are comparable to ours. However, on the tree of life humans and dolphins diverged recently, so both species had the underlying genetic tendency already built in to develop those brains.
There were broad grins all around as Denise Herzing played a video of intelligence tests on wild dolphins. The Wild Dolphin Project has been working with mostly juvenile females in the Bahamas, and they devised a simple keyboard composed of three large symbols. Each key had a musical tone that was not a normal sound in the dolphin repertoire. The key and tone represented a desired behavior, such as retrieving a brightly colored scarf that the dolphins loved to drag around.
The scientists discovered that the dolphins learned more when the divers participated in synchronized swims and other forms of mimicry, and maintained eye contact. When asked why most of the dolphins were juvenile females, Herzing said they were the only ones who had free play time. Older females were busy rearing their calves, and the young males already were fighting for territory. Herzing said the divers tried to always respect the dolphin’s social rules, but the dolphins themselves also would ensure the cultural norms were maintained. For instance, when swimming together, if a diver tried to change her place in the pack the dolphins would adjust their own positions to maintain the group order.
The Story of the Stars
The cultural aspect of astrobiology was the focus of a talk by Daniella Scalice, Education and Public Outreach Coordinator for the NASA Astrobiology Institute (NAI). In 2005, the NAI began working with the Navajo people to develop hands-on educational programs for Navajo students — resulting in what is called "So ba’ hane’" or the Story of the Stars. These programs weave together astrobiology and Navajo cultural knowledge, drawing on the experience of Navajo leaders, teachers and medicine people. Now there are plans for a more focused program on lunar science. The moon plays an important role in Navajo history and legend, and so NASA’s plan to return humans to the moon provides an opportunity to explore the cross-cultural implications of lunar research.
The Story of a Meteorite
In 1996, a team of scientists identified potential traces of microorganisms in the martian meteorite ALH 84001. Their report made headlines, and since then ALH 84001 has been the subject of heated discussions concerning life on Mars. New research presented by Andrew Steele of the Carnegie Institution of Washington identified ways the controversial ‘biosignatures’ within ALH 84001 could have been produced through non-biological chemical reactions.
The new research may appear to be a blow against the idea that Mars once supported life, but Steele was quick to point out that his work "doesn’t disprove life" — it simply shows that organic chemistry on Mars could produce structures that look similar to those produced by microbes. "If you want to find life," he stated, "you must know what signs of abiotic chemistry look like." You must first "assume that abiotic chemistry is ubiquitous," and then search for biosignatures that could not be formed by any processes other than biological activity. Ultimately, Steele feels that "if there is not life on Mars, it isn’t a negative result." Understanding how the structures in ALH 84001 could’ve formed without biological activity is essential in determining how to accurately discover biosignatures on planets beyond Earth.
Life Needs Liquid
William Baines of the Institute of Biotechnology said that although we can describe life in exquisite detail, we don’t really know what it is because we can’t make any predictions about it.
Life implies a code (genetic info) and requires catalysis for all metabolic steps. This does not require evolution or carbon chemistry, but it does imply a solvent because for things to react, they need to move around. That solvent does not need to be water, however. “Water is a dreadful solvent,” said Baines. “Chemists try to keep water out of their experiments because it reacts with everything and rips molecules apart.” Possible bio-solvents other than water include methane, ammonia, neon, argon, carbon dioxide, ethane, hydrogen, nitrogen, and sodium.
The temperature, pressure, and density of a planet will determine whether a liquid stays fluid. Solubility declines with temperature, but biochemistry still can occur in very cold places, like Jupiter’s moon Ganymede or Saturn’s moon Titan, although life there would not be as productive as life on Earth.
What We Choose to Believe
Steven Benner, a chemist with the Foundation for Applied Molecular Evolution, said exobiology is a science without a subject matter. But that’s nothing new — Galileo wanted to know whether the Earth circled the sun, but he couldn’t study that directly so he rolled balls down inclined planes to answer the question. Benner said that we often can’t rely on established science to be a guide. Lord Kelvin said the Earth and sun could NOT be billions of years old, when Darwin was arguing they were. “Who are you going to believe?” asked Benner. “The gentleman who had a temperature scale named after him, or this guy who makes a living studying bird beaks?” So science is often what we choose to believe.
To make new discoveries, we can try to prove false some of our chosen beliefs. So is there anything we can prove false in the possibility of life elsewhere? "Astrobiologists everywhere have daily evidence of the persistence of life," said Benner, "but we have little constructive knowledge of how frequently life emerges." We may never have direct evidence for life elsewhere, said Benner, but models are accepted by communities when they interconnect sufficient threads of evidence. For astrobiology, the threads include paleogenetics (tracing the tree of life backwards through time), searching the cosmos, prebiotic chemistry (trying to trace life’s origin forward in time), and synthetic biology (creating alternative life forms).
According to Benner, chemists don’t believe that life can emerge from a prebiotic soup. The Miller-Urey experiments showed that energy plus organics equals tar without evolution. “We put energy in complex chemical systems, we get pavement, not life,” said Benner. “Do an experiment. Use some glucose to make a soufflé, and leave it in the oven a little too long. [You get] asphalt.”
The RNA world preceded DNA life, but it was not necessarily the first living system. The big problem is with ribose, the “R” in RNA, which falls apart when heated and forms tar. So life may have formed with a sugar other than ribose, but in lab tests nothing else works. However, ribose-borate is a stable mineral, and Benner believes that boron makes an RNA prebiotic world more possible. Boron is associated with deserts on Earth. Benner suggested that because Mars had deserts long before Earth did, perhaps life originated there and was somehow transported to Earth (making us, in effect, Martians).
In-depth stories about some of these and other presentations will be appearing in Astrobiology Magazine in the future. Abstracts of the conference can be accessed on the website for the academic journal Astrobiology.
Note: Henry Bortman and Aaron Gronstal also contributed to this report.