BOULDER, Colo. — You can find it all over the world in almost any desert that has deep canyons and soaring cliffs.
It's visible in many areas of southeastern Utah, so much so that ancient cultures used it as a sort of canvas for early rock art. And yet scientists have been puzzled by the phenomenon known as "desert varnish" ever since Charles Darwin wrote about it nearly two centuries ago.
The mystery comes from the fact that no one has been able to prove whether the vivid coating on sandstone cliffs is produced by some unknown form of life. If it is created by something that's alive, a Colorado scholar says, the organisms may be so "weird" or "alien" that we don't have scientific tools capable of detecting them.
"Nobody has an adequate explanation," professor Carol Cleland said in an interview in her office at the University of Colorado, Boulder. For several years she's been urging NASA scientists to study desert varnish and develop new scientific tools for detecting unusual forms of life. Eventually, those methods might help answer one of humanity's most profound questions.
"Those techniques then could be used to look for life on Mars," she said.
Cleland is not a working biologist; her academic specialty is the philosophy of science. She directs a think tank on the Boulder campus called the Center for the Study of Origins. Her ideas about desert varnish are in the realm of speculation — some might say "far out" speculation — rather than provable science. She believes it's possible we are surrounded on Earth by what she calls a "shadow biosphere" of organisms that we don't know how to detect.
"Looking for the shadow biosphere," she said, "even if it doesn't exist, would allow one to develop methods for looking for life that's different than ours, even modestly different than ours. Because even modestly different life, we're going to have problems detecting."
Desert varnish can be big and bold and easy to see, yet it's so unobtrusive it's often overlooked in the stunning scenery of southeastern Utah. It forms in large and small dark blotches — a film — on the exposed surface of sandstone cliffs. It's usually black, sometimes red and it's often very shiny as it reflects the light of the desert sky. Ancient cliff dwellers often made rock art by scratching their designs right into it.
So what is desert varnish and how did it get there?
Tourists in canyon country often speculate about it. Standing at the rim of a cliff overlooking the Colorado River, Texas tourist Mike Harrel said, "I had actually, just looking at it from here, had thought it was probably a form of lichen."
California tourist Bo Soe's speculation took him a different direction.
"The rock and the layer, it's exposed with the oxygen and might have like oxidation," Soe said. "So it changes color over the years. That's what I think about it."
Although some scientists have claimed they solved the mystery, Cleland said nobody has really proven what causes it since Darwin himself puzzled over those dark patches of varnish in the 1800s.
"He himself was wondering if they were biological," Cleland said. "He might be the first person who wondered if they were biological."
In other words, is some form of microscopic life involved? Something is at work on the sandstone, capturing iron and manganese from the soil and air and gluing it to the rocks. It's the sort of thing that bacteria might do, but no one has yet detected any microbes that do it. So scientists are divided on whether desert varnish is caused by something alive, or not.
"You have different sides trying to explain it," Cleland said. "Each one thinks they're right, and each can't explain certain striking features of it."
Cleland speculates that a microscopic form of life may have been producing desert varnish for eons, but scientists simply haven't figured out how to detect it. Science has good tools for finding life that's chemically like us. Everything on the so-called Tree of Life — from E. coli to elephants to Englishmen — are all related by chemistry and descended from the very first living thing billions of years ago. That's what scientists know how to look for.
"We're looking for proteins and nucleic acids — DNA and RNA — that's like ours," Cleland said. "That's the only thing we can detect."
If Cleland's idea of a "shadow biosphere" really exists, it means there may be an entire menagerie of microbes we can't detect because they're chemically different from us. Perhaps they came from outer space. More likely though, in Cleland's view, is that life might have started on Earth more than once. Perhaps one time it created a life form that evolved into the Tree of Life that includes us and all life we know; perhaps another time life originated and spawned a tree of "weird" life that we may never detect.
"If life originated on Earth, I believe," Cleland said, "this is extremely likely, that there were multiple origins."
Cleland plans to publish a book in October called "The Quest for a Universal Theory of Life; Searching for Life as We Don't Know It." In an email, she told the Deseret News it will discuss desert varnish "as an intriguing possibility for an alternative form of microbial life on Earth — descendants of an alternative origin of Earth life that never developed into a multicellular form of life."
If scientists, especially those at NASA, follow Cleland's advice and use desert varnish to develop new detection methods, perhaps they'll discover a previously unknown form of "weird" life.
"Oh, it would be enormously significant," Cleland said in the on-campus interview. "It would revolutionize biology. Biology would now have a second example of life."
Three NASA scientists who exchanged emails with the Deseret News are aware of Cleland's speculative idea and are interested in it. But they are noncommittal and seem to be scientifically skeptical.Comment on this story
Desert varnish is "probably a combination of microbial activity (by known organisms) and weathering under desert conditions," wrote Rocco Mancinelli. However, he and fellow NASA scientist Lynn Rothschild agree that more studies should be done using modern molecular microbiology tools.
That might please tourist Mike Harrel, who is willing to buy into Cleland's speculation.
"No, I don't think it's wacky at all," Harrel said. "Just because everything here is carbon-based, it doesn't mean everything is carbon-based everywhere."