Spirit's concluding journey around Home Plate and ending location. NASA/JPL-Caltech/Cornell - NASA

Spirit’s concluding journey around Home Plate and ending location.
NASA/JPL-Caltech/Cornell – NASA

During its wheeled treks on the Red Planet, NASA’s Spirit rover may have encountered a potential biosignature of past life on Mars, report scientists at Arizona State University.

To help make their case, the researchers have contrasted Spirit’s study of “Home Plate” — a plateau of layered rocks that the robot explored during the early part of its third year on Mars – with features found within active hot spring/geyser discharge channels at El Tatio in northern Chile.

The work has resulted in a provocative paper: Silica deposits on Mars with features resembling hot spring biosignatures at El Tatio in Chile.

A portion of the volcanic hydrothermal system at El Tatio in Chile. Credit: ASU/Ruff & Farmer

A portion of the volcanic hydrothermal system at El Tatio in Chile.
Credit: ASU/Ruff & Farmer

Field work

As reported online in Nature Communications, field work in Chile by the ASU team, Steven Ruff and Jack Farmer in the School of Earth and Space Exploration at Arizona State University in Tempe, 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,” they report.

Ancient setting

The Mars rover Spirit encountered outcrops and regolith composed of opaline silica (amorphous SiO2nH2O) in an ancient volcanic hydrothermal setting in Gusev crater.

Image shows the location of Spirit rover on September 29, 2006. Toward the top of the image is "Home Plate," a plateau of layered rocks that Spirit explored during the early part of its third year on Mars. Spirit itself is clearly seen just southeast of Home Plate. Also visible are the tracks made by the rover. Image was taken by the Mars Reconnaissance Orbiter's HiRISE instrument. Credit: NASA/JPL-Caltech/University of Arizona

Image shows the location of Spirit rover on September 29, 2006. Toward the top of the image is “Home Plate,” a plateau of layered rocks that Spirit explored during the early part of its third year on Mars. Spirit itself is clearly seen just southeast of Home Plate. Also visible are the tracks made by the rover. Image was taken by the Mars Reconnaissance Orbiter’s HiRISE instrument.
Credit: NASA/JPL-Caltech/University of Arizona

An origin via either fumarole-related acid-sulfate leaching or precipitation from hot spring fluids was considered possible. “However, the potential significance of the characteristic nodular and mm-scale digitate opaline silica structures was not recognized,” Ruff and Farmer note.

Home Plate opaline silica occurs in nodular masses with digitate structures that resemble those at El Tatio. Credit: ASU/Ruff & Farmer

Home Plate opaline silica occurs in nodular masses with digitate structures that resemble those at El Tatio.
Credit: ASU/Ruff & Farmer

El Tatio: Mars-like conditions

The physical environment of El Tatio offers a rare combination of high elevation, low precipitation rate, high mean annual evaporation rate, common diurnal freeze-thaw and extremely high ultraviolet irradiance.

“Such conditions provide a better environmental analog for Mars than those of Yellowstone National Park (USA) and other well-known geothermal sites on Earth,” suggest Ruff and Farmer. “Our results demonstrate that the more Mars-like conditions of El Tatio produce unique deposits, including biomediated silica structures, with characteristics that compare favorably with the Home Plate silica outcrops. The similarities raise the possibility that the Martian silica structures formed in a comparable manner.”

Spirit imagery shows opaline silica nodular outcrops adjacent to Home Plate showing typical stratiform expression.White outline highlights nodular silica outcrop. Rover wheel tracks are roughly one meter apart. Rolling wheels did not deform the roughly15 centimeter high outcrop (lighter tracks) compared with the inoperative dragging wheel in a later traverse (darker track). Credit: ASU/Ruff & Farmer

Spirit imagery shows opaline silica nodular outcrops adjacent to Home Plate showing typical stratiform expression.White outline highlights nodular silica outcrop. Rover wheel tracks are roughly one meter apart. Rolling wheels did not deform the roughly15 centimeter high outcrop (lighter tracks) compared with the inoperative dragging wheel in a later traverse (darker track).
Credit: ASU/Ruff & Farmer

Biosignature definition

Previously, a NASA science team defined a potential biosignature as “an object, substance and/or pattern that might have a biological origin and thus compels investigators to gather more data before reaching a conclusion as to the presence or absence of life.”

“Because we can neither prove nor disprove a biological origin for the microstromatolite-like digitate silica structures at Home Plate, they constitute a potential biosignature according to this definition,” Ruff and Farmer comment.

Spirit of Future exploration

The Spirit rover bogged down on Mars in May 2009, becoming stuck in soft soil.

In late January 2010, after months of attempts to free the rover, NASA dubbed the wheeled robot mission a stationary research platform. The lack of mobility and the harsh climes of Mars conspired to seal the fate of the robot, with attempts to regain contact with the robot ending in May 2011. Subsequently, NASA announced the end of contact efforts and the completion of Spirit’s mission.

Spirit of exploration - NASA rover. Credit: NASA/JPL

Spirit of exploration – NASA rover.
Credit: NASA/JPL

The ASU researchers suggest that a future and specially-instrumented rover mission could perhaps provide a more definitive assessment of possible biogenicity of Home Plate silica structures.

“However, because of the challenges in obtaining unambiguous evidence in situ, coordinated microscopic and compositional analyses of samples returned to laboratories on Earth may be required to reach a robust conclusion as to the presence or absence of past Martian life in these rocks,” Ruff and Farmer state.

 

 

 

 

 

 

Their work – Silica deposits on Mars with features resembling hot spring biosignatures at El Tatio in Chile – has been published online in Nature Communications, dated November 17, 2016.

It can be viewed here: http://www.nature.com/articles/ncomms13554

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