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Down to Earth

Phosphorous discovery stokes speculation of life beginning on Mars

Home is where the phosphorus is. NASA

The Observer has published what it regards as the top 20 questions in Science. Number one on the list is the question of what the universe is made of. Specifically the 95% of the universe that we cannot see - the nature of dark matter and dark energy. But close on the heels of this fundamental gap in our knowledge are two further questions on The Observer list - “How did life begin?” and “Are we alone in the universe?”.

A paper presented recently at the Goldschmidt conference, an international meeting of 4,000 or more geochemists in Florence, addressed both of these questions simultaneously. Unsurprisingly, it attracted wide attention.

Steven Benner of the University of Florida suggested, in his talk at the conference, that life first kicked off with the organisation of sugars into ribose, and then ribonucleic acid (RNA), one of the fundamental molecules key to life. His thesis claims that minerals containing the elements boron and molybdenum are essential templates and catalysts, needed to coax the simplest sugars into the types of biopolymers that characterise life. “There is hope through mineralogy,” said Benner.

Arid Mars vs water world

Benner posits that these minerals would have been more stable on Mars in the youthful Solar System. In his model, dry environments are needed for sugar-templating boron minerals like borax to form. Such oxidising conditions are needed for the correct form of the molybdenum catalyst to exist.

Early in its history, Earth is thought by many to have been completely covered in oceans formed by volatile elements escaping from its interior. Dry oxidised Mars is suggested to have been more hospitable than Earth for the formation of the required mineral catalysts as well as the survival of water-soluble RNA. NASA’s Curiosity Rover recently reported back a Martian landscape of water-scoured (now dry) ocean basins but in combination with dry higher mountains.

Benner, then, suggests that the development of molecules such as RNA may have first taken place on Mars, before transferring to Earth as part of the constant rain of Martian meteorites that still arrive on Earth today. “Transpermia” was the term coined to describe this process in the mid 1990s. As a concept, it touches both on the origins of life and the existence of life elsewhere in the universe.

Show me the phosphorus

Hot on the heels of Benner’s suggestions that Mars may have been a better world to kickstart life than the early Earth, is a paper by Christopher Adcock, of the University of Nevada Las Vegas, in the journal Nature Geoscience. He suggests that early Mars would also have been a better place for organisms to sequester phosphorus from their environment.

Phosphorus is a key element for life on Earth today. Adenosine triphosphate (ATP) is the compound that transports energy in all living cells, and phosphorus is a limiting nutrient in many Earth environments. Small amounts can be obtained from dissolved phosphate minerals (apatite) in rocks, but most bio-available phosphate today is derived from biological recycling. When life started, with no significant gas-phase source of the element, the phosphorus would presumably have to have come from rocks or fluids present on the planet surface.

Adcock’s work has focused on differences in the solubility of different types of apatite thought to have been present on early Earth and Mars. He finds that the chlorine-bearing apatites, characteristic of Martian meteorites and anticipated to be present in its early history, are considerably more water-soluble that the hydroxyl- and fluorine-apatites dominant on Earth. Overall, Mars also contains higher concentrations of volatile phosphorus than Earth, sitting further from the sun. This suggests that early Martian oceans would have been a better source of vital phosphorus than Earth’s seas.

Wet and dry

So, while Benner appeals to Martian equivalents of dry Death Valley for the formation of borax and molybdates, Adcock claims that the onset of life on Mars would have benefited from its phosphorus-rich oceans. In each case, the scales tip in favour of Mars over Earth, but for different reasons. While Benner prefers a dry Martian landscape to form RNA, Adcock focuses on sources of phosphorus in Martian seas. But more questions remain.

Commenting in a Nature Geoscience “News and Views” article, Matthew Pasek of University of South Florida highlights one problem with Adcock’s model. He points out that the higher phosphorus concentrations that Adcock’s chlorapatite experiments suggest may still be too dilute to promote the formation of primordial phosphate-bearing biopolymers.

Indeed, Pasek has himself recently proposed a completely different source of phosphorus for building an “RNA world”. He suggested, in a paper in the Proceedings of the National Academy of Sciences published earlier this summer, that the iron–nickel phosphide mineral, schreibersite was delivered in the rain of meteorites that dominated early planet formation. This could have led to an ocean here on Earth rich in reactive reduced phosphorus, as phosphite. Schreibersite, rather than apatite, is the key mineral in his model for RNA assembly.

While the debate goes on, it is clear is that the key questions of the origins of life and whether we are alone in the universe will need minds focused on investigating the environments and chemical processes on early Earth and, as some evidence now requires, on early Mars too.

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