UC Riverside Scientists Help Guide NASA's Search for Life in the Universe
Are we alone? Where did we come from? Where are we going? These questions get to the root of what it means to be human, and they are exactly the questions that motivate NASA-funded scientists with the Alternative Earths Astrobiology Center at UC Riverside.
“We as a university are allowed to ask fundamental questions,” said Tim Lyons, the center’s director and a professor in the UCR Department of Earth Sciences, in a recent video profile. “What we’ve been asked to do is to assemble our collective expertise studying very old things on earth to help NASA find life elsewhere.”
The exploration of life in the universe—a scientific discipline known as ‘astrobiology’—has been a priority at NASA for 20 years. Lyons, who has been doing related research since the program’s inception, now leads one of the 12 science teams that together make up the NASA Astrobiology Institute. In 2015 NASA awarded his team of 19 scientists from 11 institutions around the world ~$7.3 million to study how Earth has remained persistently inhabited through most of its 4.5-billion-year history.
“The idea is to look back in Earth history for direction in our search for life elsewhere—be it Mars, moons of Saturn, or worlds much farther way,” Lyons explained.
His team is particularly interested in planets orbiting stars outside our own solar system—so-called exoplanets. Of the more than 3,500 exoplanets confirmed so far, at least 700 are about the size of Earth. Some are even the right distance from their host star to potentially harbor the key ingredient for life as we know it: liquid water. Within the next two years, new NASA telescopes will make it possible to peer into the atmospheres of these planets and see what gases are there.
“That’s when things will really get exciting,” Lyons said.
Take Earth, for example. If you could view our planet’s atmosphere from a distant solar system, you would see water. You would also see large amounts of free oxygen, which derives entirely from photosynthesis. For modern Earth, the presence of oxygen is the strongest sign of life—what scientists call a biosignature.
Yet Earth was also inhabited at times when there was little or no oxygen in the atmosphere. So what did the atmosphere look like back then? That’s what the Alternative Earths Astrobiology Center at UCR is trying to figure out. It is conceivable that each of Earth’s widely varying planetary states— what Lyons’ team thinks of as ‘Alternative Earths’—translates to a particular atmospheric composition that could one day be detected on an exoplanet.
This is where the geologists come in. Earth’s rock record provides a means to understand the chemistry of the ancient atmosphere. Certain types of very old rocks contain chemical signatures that indicate the presence or absence of oxygen, for example.
Other chemical signatures track concentrations of important greenhouse gases, such as methane. And when scientists put them into models that take into account how these compounds cycle through the oceans and atmosphere, they can estimate concentrations of certain gases at particular times in earth’s past.
The team’s latest thinking is that relying on a single biosignature gas, such as oxygen, is not reliable. Better would be to identify certain combinations of gases would not co-exist without the influence of life. Take oxygen and methane, for example. These two gases react instantly and would destroy each other in a matter of decades if living organisms were not constantly replenishing the supply of each.
So, if some day scientists see an exoplanet with high levels of oxygen, methane, and liquid water, it will be an exciting possibility that alien life exists. But what about planets that look like earth did for the first three billion years of its history? Several lines of geochemical evidence from Lyons’ group strongly suggest that oxygen was far too sparse in earth’s atmosphere to be remotely detectable until about 600 million years ago, yet the fossil record shows that life got its start on our planet at least 3.5 billion years ago.
The team is zeroing in on answers to that question as well. In new research published last week in Science Advances, Olson and colleagues from the University of Washington reported that before about 2.5 billion years ago, earth’s atmosphere was probably a mix of methane, nitrogen and carbon dioxide in the atmosphere—a strong sign of the life that was thriving in the oceans below.
"Large fluxes of each gas in the absence of biology is really difficult to explain," said Lyons’ graduate Stephanie Olson in a recent interview with the LA Times.
The ultimate goal of the Alternative Earths team is to identify all the potential chemical cocktails from earth’s past that would be telltale signs of life. As Lyons points out, it will still be a few years before NASA will be peering into the atmosphere’s of exoplanets, but his team will be ready to guide the process when they do.
In celebration of the 20th Anniversary of the NASA Astrobiology program and his team’s latest discoveries, Lyons will be giving a free public lecture at 6 p.m. on February 13 at the UCR Palm Desert Center.
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