There’s been a lot of press coverage this week about a presentation by Steve Benner at this year’s Goldschmidt conference in Florence supporting the idea that life on Earth originated on Mars, and then spread to Earth. This would mean we’re ALL Martians. I’m not at the meeting (new baby + sequestration), so I didn’t see the presentation. From what I can piece together, it provides nice support in the form organic chemistry for a very exciting hypothesis… but the idea still needs a LOT of work in the form of geochemistry/planetary chemistry before the majority of scientists are going to accept it.
To give a little background, this idea has been pitched before. The thinking goes like this: compared to Earth, the conditions for the origins of life either (1) were more abundant on Mars; or (2) appeared earlier on in the history of Mars. One of these two things allowed life to originate on the red planet very early on. Then, a relatively large impact event would have led to the ejection of some rocks from Mars, which would have contained some of this life. (To think of how an impact would cause *ejection* of material, think about what happens when a bullet hits sand — you get sand particles spraying out from the divot you leave behind. Same thing here, but the “bullet” is the impactor and the “sand” are the rocks on Mars.) A very small amount of the ejected material would have been sent on an orbital trajectory to Earth, where it would have landed, seeding our planet with life. There are obviously lots of questions here. Could life have survived the impact event? How about the trip through interplanetary space, where radiation might be an issue? And what about entry into the Earth’s atmosphere – and impact with the surface here? So a lot of the “how” of this story has been covered.
What Benner’s work provides is a potential “why” for the story. Why would life have originated on Mars, and not on Earth? According to Benner, it appears the answer is that the dry environments and relatively high abundance of the elements Boron and Molybdenum would have enabled some of the chemistry needed for life to evolve from non-life. More specifically, these properties of the early Martian surface would have solved various paradoxes that are currently stumping chemists (such as Benner) trying to figure out how life got a foothold on Earth. Neat stuff.
As I see it, there are two outstanding issues with the idea.
First, there’s a whole different group of scientists that thinks life originated on Mars because the red planet had LOTS of liquid water early on, whereas Benner seems to argue the the LACK of liquid water was the key. In fact, just this week we saw new data published that support the idea that Mars once had oceans. So we need to figure out which of these were true. Did Mars have a lot of water, or a little? Or did it have both wet and dry areas, in different regions of the surface? (My intuition tells me that may be the best case scenario for this hypothesis – you make life in the dry areas, and then let it get carried on dust to the oceans, where it can proliferate.)
Second, we need better geochemistry in support of Benner’s proposed solutions. Specifically, we need to know if the Boron and Molydenum that Benner claims were critical to this chemistry were present in the most ancient parts of the history of Mars. Additionally, it would be good to know what other stuff was sitting in the soils of Mars at the time. Were there other things that may have made “the chemistry of making life” more difficult? We just don’t know yet, and ideally we’d see Benner or a colleague do this within a complete chemical/mineral context of an ancient Martian soil (or a mixture approximating this).
The nice thing here is that the Curiosity rover – and potentially the Mars 2020 mission yet to come – could provide us with some of these answers. Curiosity is currently looking at the history of wet environments, organic chemicals, and mineral/elemental composition for ancient Mars. Sounds just about perfect… BUT Curiosity is looking at a period of history that probably occurred much later in history than the time period before the origins of life on Earth. However, the Mars 2020 mission may look at even more ancient places on Mars. We’re not even close to knowing where that mission will land – and LOTS of other things will play larger roles in that decision – but there’s a chance we’ll get some answers then.
Either way, what I think we need to see next is some laboratory experiments where Benner’s proposed chemistry is repeated… but this time is done with soils that have the measured compositions of ancient Martian soils. Those measurements would ideally come from “scoops in the sand” measurements from Curiosity, with the admitted caveat of it being potentially the wrong point of Martian history. You’d also like to see the chemistry repeated – and shown to be impossible – with soils that have the composition of ancient Earth rocks. But again, we run into a problem… we don’t have Earth rocks old enough for this, either, because the plate tectonics has chewed them all up.
So how might we figure out what early Earth’s rocks were like, so we can compare that environment to the one our rovers may eventually uncover for ancient Mars? Here’s the cool part… the same process that may have sent life-bearing rocks from Mars to Earth long ago may have also sent stuff the other way (albeit less efficiently). So one of our rovers may one day happen across a piece of ancient Earth sitting on the surface of Mars. And if that happened, we’d have a nice little set of experiments to run back here.