Few topics have been as contentious among both scientists and laypeople as the age of the Earth. Many scientists are people of faith, but from the moment the ancients found seashell fossils high in the mountains, the scientific community's changing understanding of the planet's geological record has introduced tension between religious faith and physical evidence. Starchy academics had been sending angry letters about the fossil record and flipping tables at geology conferences for hundreds of years before Darwin ever sailed to the Galápagos Islands.
Scientists have calculated the Earth to be 4.54 billion years old, with an uncertainty of 50 million years on either side. But how did they arrive at that number?
Philosophers have debated the origin and age of the planet since time immemorial. As humans industrialized, mining took off, and geology and the study of Earth's age exploded in popularity.
Current estimates for the age of the Earth owe much to Lord Kelvin, the same natural philosopher after whom scientists named the Kelvin temperature scale. En homage to his work, scientists often use degrees Kelvin to discuss temperatures near absolute zero, a topic Lord Kelvin discussed at length. But Lord Kelvin started out as an electrician, and his own calculated age of the Earth was wrong.
Core Temperature
In 1864, William Thomson, 1st Baron Kelvin, published a wildly controversial age for the Earth. Kelvin did a lot of thinking on thermal energy, temperature, and cooling. A mathematician and engineer, he calculated the Earth's age to fall between 20 million and 400 million years based on how long it would have taken a homogenous planet to cool from a fully molten state.
Biologists and geologists objected that Kelvin's number was too small based on the available stratigraphic evidence. Biblical literalists, meanwhile, criticized it as too large. Kelvin, himself a devout creationist, considered his own work to conflict with Darwin's "vaguely vast age" for the Earth but was not a "flood geologist." Still, Kelvin asked, if the Earth's embodied energy wasn't enough to keep it warm so long, what could justify an estimate of billions of years for the Earth's age? Where did all that heat come from? What squared the circle for Kelvin's math was the discovery of nuclear energy.
Kelvin had based his age calculations on the melting temperature of rock, which he equated to the core temperature of the Earth. However, Marie Curie was born just a few years after Kelvin published his cooling hypothesis, and their working lives overlapped. Building on Curie's work, Ernest Rutherford famously argued at a 1904 academic lecture (attended by Kelvin!) that radioactivity was the unknown energy source Kelvin had suggested.
By the time the Second World War broke out, chemists had discovered nuclear fission. Fissile elements buried deep within the Earth still break down, and when they do, they release energy sufficient to induce fusion under sufficient pressure. That energy heats the planet hotter than simply melting the rock and keeps its core temperature propped up for much longer than it would have taken to cool down by purely thermal means.
Radiocarbon Dating
Radioactive elements decay into other isotopes over time. So, it follows that there should be a way to determine the age of a sample by analyzing its mixture of daughter isotopes. For example, carbon dating looks at subtle differences in the blend of carbon isotopes to help us date certain historical artifacts. But carbon dating can only reach back so far with any accuracy: 50,000 years or so.
Happily, there's another way. Uranium, a radioactive element, decays into many sequential daughter isotopes over before finally arriving at lead. This means that the proportions of uranium and lead isotopes can give highly accurate age readings for a given sample of up to 4.5 billion years of age and even older.
It is possible to skew age estimates from uranium-lead dating (abbreviated U-Pb dating) toward the past by lead contamination that makes its way into a sample being tested. Midcentury geochemist Clair Patterson discovered that too much lead can make a sample look older than it is, which muddies up the data. Lead contamination can come from many sources, including lead pipes, lead solder, leaded gasoline, aviation fuel, and even tobacco smoke. After widespread lead bans, babies' lead levels dropped by 75%. Under cleanroom conditions, U-Pb dating now has an expected uncertainty of 1% or less.
Stromatolites: You Can't Spell Mold Without Old
One of the big problems with Kelvin's estimates for the age of the Earth was that they disagreed with estimates from anywhere else. Science works on the principle of reproducibility: that is, if a thing is true, everyone's work should bear it out, and all roads will lead to Rome. For example, U-Pb dates work for all kinds of rocks. So, it's beloved by both biologists and geologists that U-Pb dates work on rocks that used to be alive, including fossilized bones and petrified wood.
Biomass and biodiversity are greatest in places like forests and shallow water. But what about before there were forests? What about before there were shellfish to leave their fossils in stone that would become mountaintops?
Before there were land plants, before there was multicellular life, even before the Great Oxygenation Event, there were single-celled aquatic life forms. When they took up residence in shallow pools, they created a kind of runaway biofilm known as a microbial mat. If there's anything biofilms are good at doing, it's being sticky—and over the eons, these colonies of ancient aquatic goop collected enough minerals that they mineralized themselves, forming mushroom-shaped growths of biological stone known as stromatolites. U-Pb dating of stromatolite samples has shown that these formations are truly ancient, originating some 3.4 billion years ago.
Peridot, Garnets, and Zircons
Stromatolites are sedimentary formations, fossil evidence of the earliest colony organisms. Before they learned to make colonies, even their loner ancestors left a subtle mark: a thin rime of fossilized bacteria inside crevices in rocks where a lucky cyanobacterium had taken shelter and multiplied. But Earth is made up of rigid crustal plates that jostle about, roughly floating on a viscous layer of more or less molten rock. The oldest piece of crust—or at least, the crust that has gone the longest since being formed—is in Western Australia. There, the rock that underlies the Outback is about 4.4 billion years old.
Over time, Earth's surface renews itself in a process known as the rock cycle. Old or new, sedimentary or igneous, some of the Earth's crust drifts below the surface at subduction zones, where one continental plate overlaps another. When it does, some of it melts, eventually becoming magma that turns into new igneous rock. Most of Earth's surface has renewed itself in this way. However, ancient crystals called zircons have escaped the rock cycle. Zircons found in isolated chunks of rock from an Australian sheep farm are the oldest pieces of Earth ever discovered. Better still, they contain scarcely any lead when formed because lead doesn't fit into the crystal lattice. But uranium does. This makes zircons precise clocks on the time scale of the planet.
Moon rocks, collected during the Apollo Moon landing, and meteorites that never encountered Earth's rock cycle have confirmed the dates from zircon crystals, informing our ideas on how the Moon came to be. But space exploration has still raised more questions than it answers. Mars' crust is stiffer than Earth's, its mantle less viscous. There's so much iron that its mantle is dominated by garnet, a crystal that gets its rich, deep red color from iron. Meanwhile, Earth's mantle is loaded with olivine, a green mineral known by another name in its pure crystalline form: peridot. The spacecraft we've sent to Mars have found that the Red Planet is almost entirely devoid of tectonic activity. Why did life arise here and not there? What made Mars so different from Earth? The answer may lie in the crystals.
Not only can the composition of a planet tell us how long it has been around, but it's thought that tectonic activity is an essential condition for life on Earth. Volcanoes and ocean ridges recycle elements between the mantle and crust, which influences and regulates the makeup of our atmosphere. A planet must fall within a star's habitable zone and also be made of the right stuff because not all rocks are created equal.
Earth's tectonic history is what it is because of our planet's material composition. It's not like this on all planets, nor even all the rocky planets in our own solar system. However, the garnet/peridot divide is real. Researchers in the Sloan Digital Sky Survey (SDSS) reported in 2017 that a study of 90 solar systems with rocky exoplanets showed that "garnet planets" are less likely to have plate tectonics than olivine planets such as Earth. Garnet has more silicon than olivine, so garnet is stiffer and flows more slowly. SDSS astronomer Johanna Teske said, "As we've learned more about the Earth, we have learned about how many pieces come together to make it habitable. How often will exoplanets get that lucky?"
