November 25, 2016

Looking for life on Mars — in Chile

Two geoscientists at Arizona State University have made a discovery among hot springs in Chile that may spur scientists to revisit a location on Mars explored several years ago by NASA's Spirit rover. The discovery involves fingerlike structures that form in the hot spring deposits by processes that combine biological and non-biological activity

The Chilean hot springs are at a place called El Tatio and lie at the edge of the extremely dry Atacama Desert, one of the best "Mars analog" sites on Earth.

Co-authors Steve Ruff and Jack Farmer, of ASU's School of Earth and Space Exploration, report that El Tatio produces silica deposits with structures influenced by living organisms that appear nearly identical to those found eight years ago by Spirit in Gusev Crater on Mars. Their report was recently published by Nature Communications.

The question naturally arises whether what Spirit found on Mars might also have been influenced by life.

"Mars exploration has reached a stage where we can start looking for 'biosignatures'," said lead author Ruff. Biosignatures are naturally occurring traces that indicate the presence of life, either today or in the past.

On Earth fossils are an everyday example of a biosignature of past life. But biosignatures can take more subtle forms such as organic molecules trapped in rocks. Biosignatures can also include physical structures such as compacted mats of microorganisms called stromatolites, found in various environments on Earth.

No lander or rover on Mars has yet detected any fossils. So scientists assume that any Martian biosignature would be small — think microscopic — and difficult to identify, let alone even find, on a planet with as much surface area as all of Earth's land areas.

Home Plate is a ancient and eroded volcanic ash deposit about 100 yards across that lies in Gusev Crater's Columbia Hills. At some point in the past a hot spring was active here, producing silica outcrops that appear nearly identical to features found at the El Tatio hot springs in Chile. The silica outcrops lie next to Home Plate's right edge, near the Spirit rover (circled). Photo by NASA/JPL-Caltech/University of Arizona

Nature Communications - Silica deposits on Mars with features resembling hot spring biosignatures at El Tatio in Chile



Roaming the hills

In 2007, NASA's rover Spirit was exploring next to an eroded deposit of volcanic ash dubbed Home Plate in the Columbia Hills of Gusev Crater on Mars.

The rover's right front-wheel motor had failed, and as the rover dragged the stuck wheel like a plow across the ground, it uncovered a rich deposit of pure silica surrounded by outcrops rich in silica. This is a mineral commonly found in hot springs and geysers like those that Yellowstone National Park is famous for.

Ruff was one of the scientists who identified the silica mineral and along with Farmer, published observations supporting a hot spring origin. But the unusual nodular and fingerlike structures of the silica outcrops next to Home Plate were poorly understood.

Looking for similarities

Several years later, Ruff learned about the El Tatio hot springs from a scientific journal. The hot springs are among the highest known active thermal springs on Earth (over 14,000 feet). At night, even in summer, temperatures at El Tatio often drop below freezing, and by day lots of ultraviolet light from the sun comes through the thin, dry air. This makes El Tatio probably the best terrestrial analog for ancient Martian hot springs.

Abstract

The Mars rover Spirit encountered outcrops and regolith composed of opaline silica (amorphous SiO2·nH2O) in an ancient volcanic hydrothermal setting in Gusev crater. An origin via either fumarole-related acid-sulfate leaching or precipitation from hot spring fluids was suggested previously. However, the potential significance of the characteristic nodular and mm-scale digitate opaline silica structures was not recognized. Here we report remarkably similar features within active hot spring/geyser discharge channels at El Tatio in northern Chile, where halite-encrusted silica yields infrared spectra that are the best match yet to spectra from Spirit. Furthermore, we show that the nodular and digitate silica structures at El Tatio that most closely resemble those on Mars include complex sedimentary structures produced by a combination of biotic and abiotic processes. Although fully abiotic processes are not ruled out for the Martian silica structures, they satisfy an a priori definition of potential biosignatures.

SOURCES - Nature Communications, Arizona State University

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