While all biocrusts perform some degree of weathering, the larger grains of the grit crust are especially suited for it. The process reveals the full potential of microbes to impact their environment. A microbial skin can glue together pebbles, break them down into soil and fertilize that soil with essential nutrients. In effect, the crust can “terraform” the desert.

The power of the microbes was on full display after a disaster in 2015. Two years before Jung set foot in Pan de Azúcar, a rare flash flood ravaged the area. In just two days, the region received many years’ worth of rain. The resulting floods caused at least 31 deaths in neighboring towns.

The desert, however, burst with life. Over the following months, the dirt gave rise to a miraculous display of wildflowers — a “desierto florido.” How the plants awakened from a decades-long rest with such zest has perplexed soil biologists. But again, the key may be in the crust.

Fernando D. Alfaro, a microbial ecologist at Major University in Chile, tests that hypothesis by unleashing his own tiny floods upon the desert. He pours gallons of bottled water onto square-meter plots of desert soil. The plots that are covered in biocrust retain water for much longer, and some have managed to sprout plants in just a few weeks.

“For many years, [biocrusts] are preparing the system and the substrate to respond very fast to this input of rains,” Alfaro said. “These flower events depend on these tiny communities of microbes.”

Jung, too, has witnessed the microbes’ resilience. At 11 sites around Pan de Azúcar, he selected neighboring black and white splotches and measured their biological activity. Then he collected the top layer of grit, sterilized it in a pressure cooker, and placed it back on the ground. Within a year, the once-black areas became dark again as the microorganisms started recolonizing the sterile plots — far more quickly than usually occurs with the lichens and other microbes in biocrusts. Remote sensing data taken during the past decade has shown that 89% of the park’s surface is covered in the checkerboard pattern. Within that colonized area, about a quarter of the black-and-white design shifted over the last eight years — a surprisingly quick reaction time for the usually sluggish microbes.

Tiny Conquerors of the Land

The grit crust plays an important role in the local ecosystem, but its scientific allure doesn’t stop there. Ancient, stable and unearthly, this environment also draws the attention of astrobiologists.

For decades, scientists have used sections of the Atacama Desert as terrestrial analogues for Mars. The extreme radiation, infrequent precipitation, barren landscape and wild temperature fluctuations make the desert distinctively otherworldly. (Gutiérrez Alvarado, however, maintains that the most alien thing about Pan de Azúcar are his fellow park rangers — “definitely they are Martians,” he said, cracking a smile.)

Researchers are using Atacama biocrusts to construct a library of chemical signatures that could guide the search for microbial life on Mars. But the biocrust organisms also open a window into life on a slightly less foreign planet: the early Earth.

Fossil evidence suggests that microbes were living near deep-sea hydrothermal vents around 3.5 billion years ago. When and how life conquered the land, however, is less clear. The terrain on the continents was harder, sharper and far more forbidding than it is today.

“You wouldn’t have had nicely developed soil like you do now,” said Ariel Anbar, a geochemist at Arizona State University. “Plants that depend on there having been many generations of plants before to create an environment that’s hospitable — they would have had a tough time.”