Phil Ritchie
Phil Ritchie is a Melbourne-based journalist.
Most life on Earth depends on the sun’s rays to survive, but an astrobiologist from the US thinks alien life may take a different approach.
Dimitra Atri from the Blue Marble Space Institute of Science in Seattle, Washington says instead of the sun’s energy, galactic cosmic rays flying between solar systems could do the trick. He published his work in the Journal of the Royal Society Interface.
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And the muse for his paper doesn’t come from above: it’s a tiny organism tucked away deep inside a South African goldmine.
In 2008, when the bacterium Desulforudis audaxviator was discovered 2.8 kilometres below the Earth’s surface, it came as a surprise. It never sees sunlight so to live, it absorbs nutrients from nearby matter that uranium and other radioactive rocks have broken into edible pieces.
“For this mechanism to work you need an energy source, radioactivity in this case, and a medium,” such as “traces of water, compounds of sulfur and iron”, Atri says.
“Radiation coming out of radioactive rocks breaks the molecules in the surrounding medium, which otherwise were useless for life, which the bug uses for its metabolism.”
And because galactic cosmic rays are a type of radiation, Atri believes they could, theoretically, provide a way for organisms lacking a star’s light to survive.
In computer simulations of Mars, Pluto, our moon, Europa, Enceladus and even comets, his calculations show they are hit with enough cosmic radiation to break apart matter.
“My calculations show that at about three to four feet [90 to 120 centimetres] below the surface of Mars, galactic cosmic radiation delivers about the same amount of energy as radioactive rocks in the South African mine,” he says.
“The radiation has the capability to break any types of bonds so all chemicals can be useful.”
Earth is a terrible place for cosmic radiation, for two reasons: we have a thick atmosphere and a functioning magnetosphere (generated by our iron core). These protect us from cosmic rays, deflecting most around us.
But on a planet with no atmosphere or magnetosphere, those particles could smash the surface unimpeded, penetrate the rocks below and bust up molecules into bite-size pieces.
Life would likely be very basic if it relied on this method of nourishment. An organism would need to partition a lot of its energy to repair the stream of damage caused by radiation – and too much radiation could kill it.
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Charley Lineweaver, an astrobiologist from Australian National University in Canberra and was not involved in the study, found the theory interesting – particularly the range of places cosmic ray-based life could exist. But he’s sceptical.
“An important question that needs answering is if there is any galactic cosmic ray-based life on Earth,” he says, adding that the existence of microbes like D. audaxviator might hinge on radioactive rocks.
He also says the amount of energy useful to this process is crucial. Not all cosmic radiation falling on a planet would break up compounds.
“If we lived inside hot plasma, there would be lots of energy but none of it would be useful. No photosynthesis would be possible,” Lineweaver says.
Atri’s next step is an experiment to see how D. audaxviator responds to changes in radiation. If it can adapt, then a dose of galactic cosmic rays similar in strength to those found on Mars would be the “ultimate test of the hypothesis”.
“Some space agencies are planning to drill on Mars and Europa, so that is very exciting too,” says Atri.
