The Last Time It Rained

Today began with a 2-hour drive from the coastal city of Antofagasta up to Yungay. When I arrived in Antofagasta by plane last night, the city was shrouded in fog. This coastal fog is a frequent visitor to Antofagasta, and yet the climate there is extremely dry. Aside from ornamental trees and bushes that the residents have planted, and must water conscientiously, there is no vegetation there. The town literally sprouts up out of the dirt and sand of the desert.

It was still foggy in the morning, as I began the drive from Antofagasta up into the coastal mountains, to Yungay. For about the first hour of the trip, fog covered the road, at times so thick that I could barely see the way ahead. Then suddenly, the fog cleared and a brilliant blue, sky broke through. I haven’t seen even a hint of a cloud since.

UC Berkeley graduate student Alison Skelley sampling at the Rock Garden site in the Atacama Desert in 2005.
Image Credit: Richard Mathies lab/UC Berkeley

Yungay, and the Atacama in general, is the very definition of dry. And yet, as I flew to Antofagasta from Santiago yesterday, over vast barren stretches of desert landscape, I could see out the window of the plane what appeared to be runoff channels on many of the mountains and hills I was flying over. Confusing, I thought. I had read that in the driest part of this desert it almost never rains. That’s why scientists come here. I kept waiting for the plane to cross the arid heart of the Atacama, which I knew must be somewhere long my path. I kept waiting for the runoff channels to disappear. They never did. I made a note to myself to ask one of the scientists about this.

I didn’t have to wait long. Barely a half hour after I arrived at the Yungay Desert Research Station, the base of operations for the scientists I’m shadowing, people piled into their trucks to take off to the "Rock Garden," a hillside about a five-kilometer drive from home base. There’s nothing particularly intriguing about the Rock Garden; it’s merely a site that was selected some years ago as a foucs of experimentation. Returning to the same site year after year allows scientists to correlate data from different experiments, and to observe any changes that occur over time.

Even in the Rock Garden, there were clear signs of water: runoff channels on the nearby hills, cracked mud in a few low-lying places. I asked around, "How recently did it rain here?" No one could say. "Within the last year," one scientist suggested. "It could have been a few years, 10 years, 40 years. No-one knows," said another. "Could it have been a hundred years, a thousand?" I ask. "We just don’t know. "You’d have to ask the locals." Rain is a big event here. Even so, no one may have been around to witness it.

But it was clear that, at some point in the past, the entire hillside where people were working had been soaked. Not soaked the way a long drenching rain soaks the forest. More like a flash flood. It is the desert, after all. The last time there was a rainstorm in the Rock Garden, it was a quick event, over within hours, perhaps only minutes, and water streamed down the hill in a sheet, barely penetrating the surface.

Obvious signs were everywhere. The runoff channels were easy to see. They exhibited all the classic signs: telltale shapes that water carves in the land as it erodes soil and flows around obstacles it can’t push out of the way.

Penny Boston collects rock and soil samples for laboratory analysis.
Credit: Henry Bortman

But there were more subtle signs as well. Dotted here and there across the hillside were small circular depressions, quickly dubbed "dimples" by the science team. Four to six inches across, they were filled with small dark pebbles, the size of coarse grains of beach sand. The surrounding terrain was lighter in color, covered with finer material. Both were cemented into a thin crust, known as "desert varnish."

The researchers were there to investigate these different types of terrain, and also to look underneath rocks, to see whether their instruments could detect any differences in the organic content. The dimples appeared to be places where water had pooled, so in theory they would be more likely than the surrounding terrain to contain organic material. If there were live microbes to be found here, they would make their home in the dimples, where water, however briefly, had most readily been available for conducting the business of life.

Upon our arrival – the runoff channels, tire tracks, some footprints, were obvious exceptions – the hillside had looked pretty uniform. On a grand scale, there’s not a lot of variation evident in Rock Garden terrain. A darker shade of gray here, a reddish tint there – but even most of the color has been bleached out, burned away: everything appears to have been painted with pastels.

But after a few hours, a pattern suddenly jumped into focus. The hillside yielded up a new chapter in its story. The dimples weren’t everywhere, and they weren’t distributed randomly. They occurred in chains that traced the land’s contour down the hillside, like a series of miniature pools along the course of a river. They revealed the path the water had taken as it rushed downhill. The dimples were small local patches of low-lying ground where water had, however briefly, collected.

So here, in this desiccated land, where life, if it exists at all, hangs on with great difficulty, because life needs water, and in the Atacama it is in extremely short supply – even here, the texture of the land is shaped by water. A massive precipitation event had left an unmistakable signature, etched into the landscape for all to see – if one knew how to look.

And then another discovery: in one of the small boulders that dot the hillside, a circular depression, three inches across and an inch deep, a couple of tablespoonsful of fine sand crusted at the bottom, the entire depression discolored, black, the discoloration spreading out beyond the rim of the depression into the surrounding rock. Penny Boston, known for her exploration of microbial slime that inhabits subterranean caves, lets out a shout. Other scientists rush over.

Boston is excited for two reasons. The first is that water has clearly pooled in this depression. Again, no one can say when. But it definitely happened. Water was here. And something else has happened here as well.

The circular depression in this rock may indicate that life is present even in the driest region of the Atacama Desert.
Credit: Henry Bortman

"It’s desert varnish," Boston says. Desert varnish is a thin mineral coating that is common on desert rocks. There are different types. In this case, the black color comes from manganese in the rock, manganese that has been highly oxidized, by bacteria, and excreted. It takes a lot of energy to oxidize manganese, so it’s not something that occurs readily in nature. Unless life is present – specifically microbial life that gets its energy from manganese and that oxidizes the metal in the process. There’s a debate among scientists: some think desert varnish can occur through solely chemical, as well as biological processes; others – Boston is one – believe that microbes are always involved. Has she proven her point, and found clear evidence of microbial life in the Atacama as well?

Boston collects a sample of the sand from inside the depression. She will bring it back to her New Mexico laboratory, where she will try to culture the sample for several specific microorganisms, including those known to oxidize manganese. She’ll also do a DNA analysis on the material, to see whether she can identify specific organisms through their genetic material. Later today, at sundown, she will drive back out to the site, where she will wet the rock. Then she’ll come back a couple of days later, and collect more samples. She’s hoping that these latter samples will be easier to work with.

Organisms like these can be notoriously difficult to culture in the lab. They are adapted to growing and reproducing very slowly. Sometimes it takes years before they show up in a lab culture. She’s hoping that by giving them some fake rain, she can coax them to spring into metabolic action. Here, in the Atacama, the precise mix of conditions that they need to survive is present. If the microbes are also present, and she can give them a head start, they should be much easier to culture later. But for now, all she can do is collect her samples, mark them carefully, take good notes, and speculate.

Related Web Pages

Journey to Yungay
Life is Left Handed
Microbes, Microbes Everywhere
The Martian Mortal Coil
Searching for Scarce Life
Life in Atacama
Dry Limit of Life
Trooper Zoë
Follow the Sun
The Edge of Life
Life in the Atacama, 2003