Astrobiology: "Could Life Be 12 Billion Years Old?"
"Much of the search for life outside of Earth's biological oasis has focused on examining the conditions on the other planets in our solar system and probing the cosmos for other Earth-like planets in distant planetary systems. But one team of astronomers is approaching the question of life elsewhere in the universe by looking for life's potential beginning.
Aparna Venkatesan, of the University of San Francisco, and Lynn Rothschild, of NASA's Ames Research Center in Moffett Field, Calif., are using models of star formation and destruction to determine when in the roughly 13.7 billion-year history of the universe the biogenic elements – those essential to life as we know it – might have been pervasive enough to allow life to form.We can pin down the emergence of life on Earth to somewhere around 3.5 billion years ago. Venkatesan and Rothschild want to find out what happens when you broaden the question to life throughout the universe. "Can you blast that open? Could you really start really talking about life in the universe at 12 billion years? And that's the question that we're talking about," Rothschild said. With basic estimates of the elements produced by the first several generations of stars, the pair has so far found that "most of [the essential elements] can be created fairly quickly in the early universe," Venkatesan said. Venkatesan presented their first findings last week at the 214th meeting of the American Astronomical Society in Pasadena, Calif.
For life as we know it to form and thrive, four conditions must be met: sufficient amounts of the so-called biogenic elements, a solvent (on Earth, that solvent is liquid water), a source of energy, and time "for the elements to build up and create a home and conditions for life to thrive," Venkatesan explained. The biogenic elements include carbon, nitrogen, oxygen, phosphorous, sulfur, iron, and magnesium. "Carbon in particular is very interesting," Venkatesan said. Carbon is "ubiquitous in the solar system and beyond" and "is extremely versatile chemically."
These elements, like all elements present in the universe today, are forged in the furnaces of stars. But not all stars make each element, and some produce elements much faster than others. Low-mass stars create all the elements on the periodic table through carbon, but because these stars live long lives, they produce the elements slowly. Intermediate mass stars tack on nitrogen through oxygen. Finally, the most massive stars, with their intense ovens, make all the elements up to iron and some other heavy metals. And because these stellar beasts lead such short, violent lives, they can churn out elements faster than smaller stars. The explosions that end these stars' lives can vary though, and their different signatures indicate the amounts of metals, such as iron and nickel, involved, Venkatesan said. It is thought that the first stars to form in the early universe were very massive. These stars would have characteristic compositions that in turn imply that they would have specific elemental abundances "that they create in their death throes."
The two scientists came up with the idea for applying the study of the first stars to astrobiology when Rothschild came to Venkatesan's department for a talk. While talking at dinner that night, "we began to realize it might be really fun to look at just when the first building blocks for life could be out there," Venkatesan said. "To the best of our knowledge, we didn't know anyone else out there who was at the time talking about it or thinking about it." Rothschild drew up what she calls her "wish list" of elements that she considers absolutely essential to life as we know it. Venkatesan then used current theories of star formation, from the first very massive stars to the stars that formed later from the seeds sown by the first stars, to model the build up of each of the biogenic elements.
"The number one element is carbon," Rothschild said. "And you come up with that because they're really only two elements that have any real versatility in terms of being able to create a bunch of compounds that could then form a life, and one is silicon and one is carbon." But silicon gets ruled out because it isn't as prevalent in the universe, nor as chemically versatile. "The reality check is that we're sitting on a big silicate rock, and we're not made of silicon," Rothschild said. Rounding out the list of must-haves are hydrogen, oxygen and nitrogen. "Nitrogen seems to be critical. It's found in so many compounds, and that really adds huge versatility then to the suite," Rothschild said. Nitrogen, for example, is the backbone of amino acids, which in turn are the building blocks of proteins and have been detected in interstellar space.
Secondary and tertiary lists include phosphorus, sulfur, iron and magnesium, "and all sorts of funky things which are used a lot, but I could more easily conceive of a system without it," Rothschild said. They found that "nitrogen can actually build up very quickly," Venkatesan said. But not right at the beginning, because those first massive stars "woefully under-produce nitrogen." It takes later-generation stars to boost levels high enough to what scientists think might be needed to make the element pervasive enough. Carbon also "takes a little while to build up," because it needs low- and intermediate- mass stars, Venkatesan said.
While those early massive stars would have had trouble producing nitrogen, they "are fairly efficient at producing iron early on. That is because they completely blow apart," Venkatesan said. Overall, the modeling effort found that iron and magnesium levels would have surged early on, with carbon taking at least 100 million years to build up. Though the critical masses of biogenic elements needed to allow life to form aren't known, "these amounts will be more than enough," Venkatesan said.
So by perhaps around 100 million after the universe began, many of these elements would be found in substantial enough numbers, though the timescale may be more around 500 million years for carbon and the jury is still out with nitrogen. Better models and improved knowledge of the physics at work in early stars could change the picture somewhat, changing the timescales for the buildups of the elements and the interstellar environment they are born into.
Of course, knowing which elements need to be present and whether or not they are won't answer the question of when life might have been able to spring forth. The elements must also collect in pools in significant enough amounts. "That final question is not only which elements, but what concentration do you build up locally?" Rothschild said. Once Rothschild comes up with estimates of the amounts of different elements likely required, she and Venkatesan can use models that estimate concentrations in galaxies and solar systems over time and see if they find any likely-looking spots for life to form. "All we need is one place in the universe that has the conditions, the prerequisites," Rothschild said.
Solvents, such as liquid water or methane, will also have to be factored in. Venkatesan said that in the long term, they hope to use the same methods to figure out when water might have existed in sufficient quantities. There is also the question of whether life could have thrived in the harsh, ultraviolet-dominated environments of the early stars. Ultraviolet light is thought to have both beneficial and detrimental effects on life, but which might have won out in the early universe isn't known.
Ultimately the question will become, "can we build up the building blocks" early on, Venkatesan said. Though answering that question will take some time, it could have a substantial impact on studies of the early universe, exoplanet research, and the expectations of how far along alien life might have evolved, not to mention our view of our place in the universe. "It's not going to cure cancer," Rothschild said. "But I think in a way, it's a very profound question: when can you start talking about life in our universe?"
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