Martian minerals dissolved in groundwater are much more likely to yield a key building block for life – phosphate – than dissolved minerals on Earth. At least, that’s the finding of a lab-based physical simulation designed to work out the habitability of ancient environments on the Red Planet. The news comes just a few days after a prominent chemist aired his theory that only on Mars were the right chemical elements specifically boron, molybdenum and oxygen – present at the right time to produce RNA molecules. RNA is widely thought to be the precursor to DNA and therefore to life as we know it.
Both studies have brought renewed attention to the idea that life on Earth was seeded from space, a theory known as panspermia. However, they can’t both be right. One idea requires Mars to be covered in liquid water, while the other needs it to be as dry as a desert.
The latest work focuses on phosphate, a molecule made up of one phosphorus atom and four oxygen atoms. Phosphates make up the structural backbones of DNA and RNA, and many complex organisms use a version called adenosine triphosphate (ATP) to store energy from food.
“From what’s known, you can’t really have life without it,” says Christopher Adcock of the University of Nevada in Las Vegas. “It’s required for biological functions on a number of levels.”
The problem lies in how early Earth could have enough phosphate available to help spark life. The material is most stable as a solid, so the phosphate that was incorporated into early life was probably dissolved out of minerals byEarth’s oceans. But most phosphate minerals on Earth, such as fluorapatite and whitlockite, dissolve slowly and are not readily incorporated into a water-based solution, so their concentrations are relatively low.
Meanwhile, studies of Martian meteorites show that Mars’s phosphate-bearing minerals are slightly different from Earth’s. Instead of fluorapatite, Mars has chlorapatite, in which chlorine takes the place of the fluorine component. The Red Planet also has a phosphate-bearing mineral called merrillite that is not found on Earth. In addition, data from the Mars rovers Spirit and Opportunity suggest that the planet has five to 10 times the amount of phosphate as Earth. The question, then, is whether Martian phosphate minerals can dissolve more readily in water.
“Fluorapatite is very interesting, and people have done a lot of work on it because it’s so important on Earth,” says Adcock. “No one had gotten to these [other] minerals, because they’re not as relevant to Earth. But they are very relevant to Mars.”
Because of the rarity of chlorapatite and merrillite on Earth, Adcock and colleagues had to cook up the minerals from scratch. They synthesised chunks of the compounds in a kiln, then subjected them to solutions designed to mimic groundwater. The team used several batches of solution with a variety of acidities to see how fast the minerals dissolve and how much ends up in the solution.
They found that the rate of phosphate release for Mars rocks would be as much as 45 times higher than it is on Earth, and so the phosphate concentrations of a wet early Mars would have been more than twice those of Earth.
“That phosphate hurdle that terrestrial life faced during its origins may not have been such a hurdle for a potential Martian origin of life,” says Adcock.
That conclusion echoes a talk that chemist Steve Benner gave recently at the Goldschmidt conference of geochemists in Florence, Italy. Benner, of the Westheimer Institute of Science and Technology in Gainesville, Florida, argued that early Mars, not Earth, was a better chemical cradle for producing RNA molecules.
Wet vs. dry
According to Benner, young Mars would have had an abundance of oxygen in its atmosphere, which would have reacted with boron and molybdenum to make oxidised versions of these elements. The compounds could then act as catalysts that guided the formation of RNA. The molecules of life, or even hardy simple organisms, could then have been transferred to Earth via meteors. Benner’s scenario requires Mars to be a desert, because RNA falls apart if you try to build it in water. But his scenario does not address where the phosphate in Martian RNA came from. Adcock’s proposal, meanwhile, requires an ocean to dissolve the phosphate minerals, but doesn’t say how you then stop RNA from destabilising in water.
“There’s a tension between the two – one requires water, one is hindered by water,” says Matthew Pasek at the University of South Florida in Tampa, who was not involved in either project. If Mars had cycles of wetness and dryness, or if it had intermittent oceans broken up by desert continents, then both could be resolved, he says.
“In general, I think it was pretty interesting and useful work that was done,” Pasek says. “But there’s still some much larger questions as far as origins of life are concerned.”
Benner is more sanguine about the situation. He says that Adcock’s team is looking at Martian chemistry for the same reason he is: to solve a chemical problem that geologists tell us could not have been solved on early Earth. “Just like RNA requires ribose, it also requires phosphate. Just as geologists tell us that we cannot get conditions on early Earth where ribose could have accumulated, they tell us that we cannot get conditions on early Earth where phosphate is soluble. So our two papers go in the same direction,” he says.
“The logic tree from my talk allows us to escape the conclusion ‘we are all descendants from Martians’ if the geological models are wrong. The more cases we have like Adcock’s, the less likely this is to happen. Geologists cannot be wrong about everything.” n
Journal reference: Nature
Geosciences, DOI: 10.1038/NGEO1923