1 Making the case for life having originated on Mars on Sat Nov 30, 2013 9:17 pm
The Institute for Advanced Study, which has played host to such luminaries as Albert Einstein and Kurt Gödel, held a series of talks to celebrate the birthday of another one of its famous faculty: Freeman Dyson, who has made important contributions to a huge variety of fields (and gave us the concept of the Dyson Sphere). The talks covered many of the fields that have been influenced by Dyson; here, we'll describe the talk by Caltech's Joseph Kirschvink.
In the past few weeks, there has been a lot of talk about the prospect that life originated on Mars. I had been considering tracking down more details on the idea when the Institute for Advanced Study conveniently brought them to me via a talk by Joseph Kirschvink. Kirschvink made a number of parallel cases for life getting its start on the red planet, but they all come back to the early environment there and its contrasts with the early Earth.
Kirschvink started out by noting that the earliest geological period on Earth was called the Hadean, its reference to the underworld evoking the rather hellish conditions that prevailed in the wake of the collision that formed the Moon. In contrast, the same time on Mars is called the Noachian due to the evidence of large amounts of surface water.
An argument from chemistry
But there were also some specific features of Mars that may have made the chemistry there suitable to kickstarting biochemistry. One of the key ingredients of life on Earth is ribose, the sugar that forms most of the backbone of DNA and RNA. If you attempt to make ribose from some of the simple chemicals that were present on the early Earth, you get a thick sludge of organic molecules that doesn't dissolve in water. If you add some simple minerals (such as borate and some molybdenum oxides), you get lots of ribose.
Kirschvink noted that these minerals only form in dry, desert environments on Earth. And in his view, the early Earth was likely to be very wet. In fact, he suggests that almost its entire surface would have been under water during the Hadean, as plate tectonics couldn't get started in the thin, warm crust that would have prevailed at the time.
In contrast, he argues that Mars had the right environment to form these minerals. Meteorites that originated on Mars suggest that it once had a strong magnetic field, which Kirschvink thinks would have led to an ozone layer. Because of Mars' distinctive geology, the giant volcanoes of the Tharsis region would have extended up into the ozone, bringing minerals there to be oxidized under dry conditions. As these spread to the lower slopes, they would reach environments where they could catalyze chemical reactions that might have given life its start.
Rapid (and cool) transit
If life did start on Mars, it would still have had to made the transit to Earth. To indicate that it could, Kirschvink turned to a Martian meteorite that contains grains of magnetized material. His team heated these grains and found that they would start to lose their magnetic orientation at about 40°C, from which he concludes that they never ended up above that temperature during their entire trip from Mars' surface to Earth.
Modeling, he said, indicates how this could happen. If a large meteor strike occurs on the surface of Mars, it can punch directly through the crust before creating an explosive vaporization of materials. Given Mars' relatively weak gravity, the sub-surface explosion can loft overlying bits of crust off the planet without subjecting them to excessive heat or a shock wave. Orbital models suggest that the chunks would start arriving at Earth within nine months.
As for reentry into the Earth's atmosphere, Kirschvink says that the outer layers of these rocks would evaporate off at high temperatures in a way that shielded the inside, much like the ablative heat shields used on spacecraft. The end result: rapid, safe delivery of any microbes to Earth.
Magnets: How do they form?
Direct evidence for life making its way to Earth has a somewhat awkward history. Back in 1996, some NASA researchers announced that they had found suggestively shaped carbon deposits inside a Martian meteor, but those results were met with widespread skepticism, and researchers subsequently suggested that there were ways in which they could have formed naturally. (TheSmithsonian Magazine has a good rundown on the controversy.)
Kirschvink is basically taking the same general approach, but he is focusing on something entirely different: magnetite. Various forms of life on Earth, from bacteria to birds, create chains of magnetite structures that let them orient themselves in magnetic fields. And Kirschvink thinks he has found similar-looking structures in a meteorite that started out on Mars.
Unlike the carbon deposits, Kirschvink says that these clusters of magnetite can't form under natural conditions—among other reasons, they're too pure. Normally, magnetite incorporates material from the environment it forms in, while these structures show no signs of contamination. Thus, Kirschvink argues, Mars is still sending indications of life to the Earth.
Does it all add up?
Even Kirschvink admitted that the idea that life originated on Mars was somewhat contingent on how you viewed the chemistry needed to get life started. In a flowchart he showed during the talk, he made it clear that the argument for a Martian origin was much more persuasive if you accepted that minerals like borate were needed to form the first biochemicals. If you can bring your own chemistry, Kirschvink suggested, then Mars looks less necessary.
And in fact, other people have brought their own chemistry, finding reactions that can take hydrogen cyanide and building the full sugar-base combination that is used in DNA and RNA. And they don't need to use ribose as an intermediate to get there.
If he's right about the magnetite, it would imply that all life on Earth probably descended from a magnetic-sensing organism. That claim is a bit hard to evaluate at all. A paper published just a few months ago suggests that we're not yet sure how exactly the magnetite structures form in living things, and thus we can't be certain whether similar conditions can occur naturally.
Kirschvink did make a fairly convincing case that microbes could have hitched a ride from Mars to Earth and that the environment of early Mars is so radically different from its present that we should work on understanding the chemistry that could have occurred there. But I'd like to see a lot more before buying into the idea that life got its start there.]