One of the most amazing strides that's been made in science in the last hundred years is our ability to figure out how old stuff is.
Geologists have known for a long time that the Earth is old; how old, on the other hand, was a matter of serious debate. Scottish geologist James Hutton, who pioneered the idea of uniformitarianism -- that the same slow, steady processes we see going on today have proceeded at essentially the same rate throughout Earth's history -- guessed that our planet was at least tens of millions of years old, and a far cry from the six-thousand-odd years the creationists of both his day and ours believe. In fact, because the rocks he studied seemed to have been melted, eroded, remodeled, and remelted over and over, he thought it was entirely possible that the Earth was infinitely old; "The result of our present enquiry," he wrote, in his 1738 book Theory of the Earth, "is that we find no vestige of a beginning, no prospect of an end."
It was undeniable, though, that things had changed over time. A century later, English geologist William Smith went all over the British Isles tracking similarities in rock outcroppings, and used index fossils -- fossil organisms characteristic of only short geological timespans, and therefore useful in dating strata -- to create a map of the country by geological age. (That map has stood the test of time; in the nearly two centuries since he created it, there have been very few changes needed.)
But still, Hutton and Smith could only speculate as to how old particular rock outcrops were. There might be Jurassic fossils to be found in Lyme Regis (on the Dorset coast) and in Cleveland (in Yorkshire), suggesting they're close to the same age, but what is their actual age? It wasn't until American chemist Bertram Boltwood had a major brainstorm in 1907, realizing that the steady breakdown of radioisotopes in rock samples could act like a natural clock, that geologists had a tool to determine exactly how old various rock strata are.
Still, it's not easy. Radioisotope dating rests on the assumption that the rock in question hasn't been significantly altered since formation. If something has changed the amount of the radioisotope you're using (or its decay product), it will throw off your estimate of the age. (That's why there's still argument over the Shroud of Turin; although radiocarbon dating has pretty conclusively shown that it's from the Middle Ages, there was a fire in the church where it was housed that deposited soot in the cloth, potentially altering the amount of carbon-14 in the fibers. Almost all scientists, however, are of the opinion that this doesn't affect the calculation enough to increase its age by the twelve hundred years required to buy its divine origin.)
So radioisotope dating is a cool idea, but rests on some serious assumptions. How do you make it more accurate?
Enter the humble zircon.
Zircons -- mostly made of zirconium silicate -- are crystalline minerals found kind of everywhere. When big enough, they're decent semiprecious gemstones, but geologists love them for a different reason; they are amazingly good for geochronology. They crystallize in many kinds of igneous rocks, and once they form, they are incredibly durable, resisting both physical and chemical weathering. They contain trace amounts of radioactive elements, and when those decay, the decay products stay put, allowing zircons to act as extremely accurate radiochemical clocks. They also trap the gases that were in the atmosphere at the time of formation, and the ratio of two oxygen isotopes (oxygen-16 and oxygen-18) gives a good idea of what the environment was doing at the time of formation.
This is how zircons from the Jack Hills Formation in Australia have been found to date from over four billion years ago -- and to show that even at that time, the Earth was cool enough to have a liquid water ocean.
The reason all this comes up, though, is not because of a terrestrial rock, but a Martian one. The meteorite NWA 7034, found in Western Sahara in 2011, was blasted out of the surface of Mars by a (different) meteorite impact, ultimately landing on Earth; we know it's from Mars because of gas bubble inclusions that have a gas composition matching what we know of the Martian atmosphere. And NWA 7034 contains zircon crystals that not only date back to 4.45 billion years ago...
... they show that they were formed in the presence of hot water.
The banding pattern shows alterations in iron, aluminum, and sodium concentration indicating that it formed in contact with high-temperature water, perhaps a hydrothermal vent system.
So amazing as it sounds, considering the Red Planet's current dry, dusty, windswept surface, four billion years ago it had liquid water, maybe even oceans of it. Its lower gravitational pull meant that its atmosphere gradually leaked away to space, lowering the pressure and evaporating away the water it had. But for a time, Mars was a wet planet.
And given how ecosystems flourish around Earth's hydrothermal vents, it may even have had life.
Even fervent aficionados of extraterrestrial life like myself doubt that Mars had time to evolve life of any great complexity; so I'm afraid C. S. Lewis's vision of the intelligent Hrossa and Séroni and Pfifltriggi in Out of the Silent Planet are going to remain forever in the realm of fiction.
But it's entirely possible that it might, at some point, have had microbial life. There's a slim (but nonzero) chance it still exists somewhere underground; what's more likely is that it left microfossils that could potentially be detected with more careful study of Martian rocks. At this point, we don't know for sure, but the new study of the Western Sahara Martian meteorite certainly seems to support the possibility.
Whether or not that pans out, it's still pretty incredible that in only a little over a hundred years we've gone from "okay, this rock is probably about the same age as that rock" to being able to say "this tiny crystal formed on Mars near a hydrothermal vent 4.45 billion years ago, then got blasted into space and landed here." Science will always have a capacity to astonish us.
And if you're curious about the universe around you, the one certain thing is that you'll never be bored.
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