Water sculpted the surface of Mars, covers 70 percent of Earth's surface, and – most likely as steam – cooked the surface of a young Venus. But questions remain about where the water came from and how early it became a fixture in the inner solar system.
New measurements indicate that water started to migrate into the inner solar system about 8 to 10 million years after the solar system began to form some 4.6 billion years ago. This arrival time for water is earlier than previously thought.
At the time, Earth would have been about half its current size. Gravity from a young Jupiter would have encouraged water-enriched asteroids known as carbonaceous chondrites to migrate into the inner solar system, where budding inner planets could accrete them.
Carbonaceous chondrites are comprised of 22 percent water, bound up in minerals, and carry organic compounds as well. Their composition is thought to be a close match to the makeup of the primordial cloud of dust and gas from which the sun and planets emerged.
“No one really knew when these asteroids brought water. Now we're putting a time stamp on it,” says Adam Sarafian, a PhD student in geophysics with the MIT/Woods Hole Oceanographic Institution Joint Program in Massachusetts, and the lead author of a paper describing the measurements in today's issue of the journal Science.
At the time, Earth would have been about half of its current size, he continues. Over time, volcanism would have released the mineral-trapped water as vapor. The vapor was available to condense and accumulate as the planet cooled and the atmosphere grew thick enough to allow liquid water to remain stable on the surface.
To unravel the mystery of water's arrival, however, Mr. Sarafian and colleagues from Woods Hole, the American Museum of Natural History in New York, the University of Bristol in Britain, and the University of New Mexico in Albuquerque had to rely on out-of-this-world rocks.
Evidence in rocks on Earth for events that old have long since been erased by a continuous rock recycling process known as plate tectonics. Old crust is driven back deep into the Earth, propelled by fresh magma rising up through ridges in the sea floor elsewhere to form new crust. Volcanoes, such as the erupting Kilauea in Hawaii, also refresh Earth's surface.
The composition of carbonaceous chondrites has been gleaned through the study of meteorites that are chips off carbonaceous-chondrite asteroids. In particular, the hydrogen molecules associated with water locked up in minerals have a distinct ratio of deuterium to hydrogen. Deuterium is a stable isotope in which a neutron has been added to the electron and proton that make up a typical hydrogen atom.
Water on Earth has a similar ratio to that of the mineral-bound water in the carbonaceous chondrites. That speaks to a cosmic origin, but says nothing about timing.
The team turned to another class of meteorite known as eucrites to provide the “when.” These contain water bound up in minerals as well – in this case in phosphate-rich apatite.
Many eucrites come from the asteroid 4 Vesta, which NASA's Dawn mission orbited between July 2011 and July 2012. At 326 miles across, Vesta orbits in the main asteroid belt between Jupiter and Mars and is one of the largest asteroids in the solar system. It is widely considered a planet-wannabe with a serious case of arrested development.
Eucrites crystallized some 4.559 billion to 4.547 billion years ago, the team notes, roughly 8 million to 20 million years after the first clumps of material rich in aluminum and calcium grew out the disk of dust and gas surrounding the young sun.
Finding and measuring the ratio of deuterium to hydrogen in apatite grains was a challenge, Sarafian says. It meant hunting across the surfaces of thin slices of meteor samples less then a quarter of an inch square for the presence of phosphate. The American Museum of Natural History, the Smithsonian Institution, and NASA provided the samples.
The apatite grains were no more than about 20 microns across. Within that area, the team would sample a spot only about 10 microns across, using a device known as an ion probe.
When they measured the ratio of deuterium to hydrogen in the apatite's mineral-bound water, it matched that of the carbonaceous chondrites as well as that of water on Earth. The timing of crystallization for eucrites serves as the basis for estimating the arrival of water into the inner solar system.
The results help answer the question: When? But they don't say anything about how much water carbonaceous chondrites contributed to the inner solar system's reservoir. The team notes that water molecules also could have been clinging to dust grains in the inner solar system, grains that budding planets also would have accreted.
The next step, Sarafian says, is to answer the question: How much water did these early asteroids contribute?