Geologist says Curiosity's images show Earth-like soils on Mars

July 17, 2014
Geologist Gregory Retallack, based on images and data gathered by the Mars rover Curiosity, believes 3.7 billion-year-old rocks at the bottom of the Gale impact crater contain fossilized soil. Such soil, called paleosols, have been found in the oldest Earth soils and their presence suggests microbial life was likely. Credit: Tim Christie

Soil deep in a crater dating to some 3.7 billion years ago contains evidence that Mars was once much warmer and wetter, says University of Oregon geologist Gregory Retallack, based on images and data captured by the rover Curiosity.

NASA rovers have shown Martian landscapes littered with loose rocks from impacts or layered by catastrophic floods, rather than the smooth contours of soils that soften landscapes on Earth. However, recent images from Curiosity from the impact Gale Crater, Retallack said, reveal Earth-like profiles with cracked surfaces lined with sulfate, ellipsoidal hollows and concentrations of sulfate comparable with soils in Antarctic Dry Valleys and Chile's Atacama Desert.

His analyses appear in a paper placed online this week by the journal Geology in advance of print in the September issue. Retallack, the paper's lone author, studied mineral and chemical data published by researchers closely tied with the Curiosity mission. Retallack, professor of geological sciences and co-director of paleontology research at the UO Museum of Natural and Cultural History, is an internationally known expert on the recognition of paleosols—ancient fossilized soils contained in rocks.

"The pictures were the first clue, but then all the data really nailed it," Retallack said. "The key to this discovery has been the superb chemical and mineral analytical capability of the Curiosity Rover, which is an order of magnitude improvement over earlier generations of rovers. The new data show clear chemical weathering trends, and clay accumulation at the expense of the mineral olivine, as expected in soils on Earth. Phosphorus depletion within the profiles is especially tantalizing, because it attributed to microbial activity on Earth."

The ancient soils, he said, do not prove that Mars once contained , but they do add to growing evidence that an early wetter and warmer Mars was more habitable than the planet has been in the past 3 billion years.

Curiosity rover is now exploring topographically higher and geologically younger layers within the crater, where the soils appear less conducive to life. For a record of older life and soils on Mars, Retallack said, new missions will be needed to explore older and more clayey terrains.

Surface cracks in the deeply buried soils suggest typical soil clods. Vesicular hollows, or rounded holes, and sulfate concentrations, he said, are both features of desert soils on Earth.

"None of these features is seen in younger surface soils of Mars," Retallack said. "The exploration of Mars, like that of other planetary bodies, commonly turns up unexpected discoveries, but it is equally unexpected to discover such familiar ground."

The newly discovered soils provide more benign and habitable soil conditions than known before on Mars. Their dating to 3.7 billion years ago, he noted, puts them into a time of transition from "an early benign water cycle on Mars to the acidic and arid Mars of today." Life on Earth is believed to have emerged and began diversifying about 3.5 million years ago, but some scientists have theorized that potential evidence that might take life on Earth farther back was destroyed by plate tectonics, which did not occur on Mars.

In an email, Malcolm Walter of the Australian Centre for Astrobiology, who was not involved in the research, said the potential discovery of these fossilized soils in the Gale Crater dramatically increases the possibility that Mars has microbes. "There is a real possibility that there is or was life on Mars," he wrote.

Retallack noted that Steven Benner of the Westheimer Institute of Science and Technology in Florida has speculated that life is more likely to have originated on a soil planet like Mars than a water planet like Earth. In an email, Benner wrote that Retallack's paper "shows not only soils that might be direct products of an early Martian life, but also the wet-dry cycles that many models require for the emergence of life."

Explore further: In Brief: Martian soil oxidation-reduction potential not too extreme for life

More information: Geology: … 14/G35912.1.abstract

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5 / 5 (3) Jul 17, 2014
"Life on Earth is believed to have emerged and began diversifying about 3.5 million years ago, but some scientists have theorized that potential evidence that might take life on Earth farther back was destroyed by plate tectonics, which did not occur on Mars."

They might want to double check that figure.
5 / 5 (1) Jul 17, 2014
Retalllack is the astrobiological crackpot that uses his perfectly fine soil science to persistently claim that geological features is "fossil lichen", despite that the other scientists have moved on, showing it wan't a fruitful theory:

"Retallack's initial taphonomy work interpreting some Ediacaran biota as lichens[44] was questioned[45] for its applicability to all Ediacaran fossils. The recent Retallack proposal that Ediacaran fossils were preserved in paleosols and thus could not be marine fossils,[11] is a provocative challenge to prior interpretations,[2] and has been supported in some quarters,[46] but disputed in others.[47] However this hypothesis of Retallack is not generally accepted by the paleontological community. [48][49] ..."

5 / 5 (2) Jul 17, 2014

... Nature called it a "controversial claim" in a news report, in which paleontologist Guy Narbonne said "Most of us appreciated that Retallack's lichen hypothesis was innovative thinking and tested his ideas critically, but it quickly became clear that there are simpler explanations for the features Retallack had validly noted, and most of us moved on to more promising explanations."[50]"

[ http://en.wikiped...etallack ]

Benner is balanced but a chemist, geology would be outside his expertise. I'm not confident that anything of the anonymous article isn't Retallack self-advertising poor support such as throw away emails.

But if this work can be supported, it is interesting.
5 / 5 (2) Jul 17, 2014
@rockwold: Right, it is a typo. But it is also a conflation between the earliest accepted fossils at 3.5 billion years before present [Ga bp] and the date of emergence.

The earliest survivable environment appeared ~ 4.4 Ga bp as attested by zircons, and the dated phylogenies covering the archea-bacteria split or the archaea root go all the way back to 4.0 - 4.3 Ga bp. (See Timetree of Life, for example.)

Now they have found 4.0 Ga bp rocks in the Acasta Gneiss, which metamorphized rocks go back to 4.2 Ga bp, that shows a relation to the pristine Icelandic rocks and predates the early land plate TTG rocks. Iceland is a fresh new thick land plate, a result of a confluence between a hotspot that can be tracked from Canada under Greenland to Iceland and the submarine plate ridge that runs through at the moment. But early Hadean Earth was hotter and hotspot mantle upwells would have been enough to seed land plates.

5 / 5 (2) Jul 17, 2014

[ http://en.wikiped...a_Gneiss ]

Presumably Hadean Earth had only submarine plates which is why we have few older remains such as the oldest Acasta material, such plates recycle by subduction in 100 million years. And the first land cratons appeared concurrently with the first splits between Bacteria and Archaea.

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