Ancient Mars Lake Could Have Supported Life

A NASA team reported 10 months ago that the first rock [nicknamed “John Klein”] Curiosity drilled at “Yellowknife Bay” on Mars yielded evidence that met the mission’s goal of identifying a Martian environment favorable for microbial life long ago. Yellowknife Bay’s clay-rich lakebed habitat offers the key chemical elements for life, plus water not too acidic or salty, and an energy source. The energy source is a type used by many rock-eating microbes on Earth: a mix of sulfur- and iron-containing minerals that are ready acceptors of electrons, and others that are ready electron donors, like the two poles of a battery.

Not only has Curiosity accomplished its primary goal of finding evidence for an ancient environment that could have supported life, but it also has provided evidence habitable conditions existed more recently than expected and likely persisted for millions of years.

The lake may have existed until as recently as 3.7 Billion years ago, much later than researchers expected which means that life had a longer and better chance of gaining a foothold on the Red Planet before it was transformed into its current cold, arid state.

Previous discoveries by Mars rovers had suggested that the Red Planet once had surface and groundwater with the quality of battery acid, but the water in this lake looks much more benign. It was a fresh water lake.

The chemistry of the lake would have been congenial to organisms known as chemolithoautotrophs — mineral-eaters. Whether such organisms, which thrive on Earth in exotic environments such as caves and deep-sea hydrothermal vents, actually existed on the young Mars is a question Curiosity lacks the tools to answer.

Additional new results from Curiosity are providing the first readings of radiation hazards at Mars’ surface, which will aid planning of human missions to Mars. Other findings will guide the search for evidence of life on Mars by improving insight about how erosion may expose buried clues of molecular building blocks of life.

New estimates of when habitable conditions existed at Yellowknife Bay and how long they persisted come from details of rocks’ composition and layering. It is thought that Mars had enough fresh water to generate clay minerals — and possibly support life — more than 4 billion years ago, but that the planet underwent drying that left any remaining liquid water acidic and briny. A key question was whether the clay minerals at Yellowknife Bay formed earlier, upstream on the rim of Gale Crater where the bits of rock originated, or later, downstream where the rock particles were carried by water and deposited.

Implications for Human Explorers

Today’s reports include the first measurements of the natural radiation environment on the surface of Mars. Cosmic rays from outside our solar system and energetic particles from the sun bombarded the surface at Gale Crater with an average of 0.67 millisieverts per day from August 2012 to June 2013, according to a report by Don Hassler of Southwest Research Institute in Boulder, Colo., and co-authors. For comparison, radiation exposure from a typical chest X-ray is about 0.02 millisievert. That 10-month measurement period did not include any major solar storms affecting Mars, and more than 95 percent of the total came from cosmic rays.

Results from the surface-radiation monitoring provide an additional piece of the puzzle for projecting the total round-trip radiation dose for a future human mission to Mars. Added to dose rates Curiosity measured during its flight to Mars, the Mars surface results project a total round-trip dose rate for a future human mission at the same period in the solar cycle to be on the order of 1,000 millisieverts.

Long-term population studies have shown exposure to radiation increases a person’s lifetime cancer risk. Exposure to a dose of 1,000 millisieverts is associated with a 5 percent increase in risk for developing fatal cancer.

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