How did earth get its water?

Sodium chloride, better known as table salt, isn’t exactly the type of mineral that captures the imagination of scientists. However, a smattering of tiny salt crystals discovered in a sample from an asteroid has researchers at the University of Arizona Lunar and Planetary Laboratory excited, because these crystals can only have formed in the presence of liquid water.

Even more intriguing, according to the research team, is the fact that the sample comes from an S-type asteroid, a category known to mostly lack hydrated, or water-bearing, minerals. The discovery strongly suggests that a large population of asteroids hurtling through the solar system may not be as dry as previously thought. The finding, published in Nature Astronomy, gives renewed push to the hypothesis that most, if not all, water on Earth may have arrived by way of asteroids during the planet’s tumultuous infancy.

Tom Zega, the study’s senior author and a professor of planetary sciences at the UArizona Lunar and Planetary Laboratory, and Shaofan Che, lead study author and a postdoctoral fellow at the Lunar and Planetary Laboratory, performed a detailed analysis of samples collected from asteroid Itokawa in 2005 by the Japanese Hayabusa mission and brought to Earth in 2010.

The study is the first to demonstrate that the salt crystals originated on the asteroid’s parent body, ruling out any possibility they might have formed as a consequence of contamination after the sample reached Earth, a question that had plagued previous studies that found sodium chloride in meteorites of a similar origin.

“The grains look exactly like what you would see if you took table salt at home and placed it under an electron microscope,” Zega said. “They’re these nice, square crystals. It was funny, too, because we had many spirited group meeting conversations about them, because it was just so unreal.”

Zega said the samples represent a type of extraterrestrial rock known as an ordinary chondrite. Derived from so-called S-type asteroids such as Itokawa, this type makes up about 87per cent of meteorites collected on Earth. Very few of them have been found to contain water-bearing minerals.

“It has long been thought that ordinary chondrites are an unlikely source of water on Earth,” said Zega who is the director of the Lunar and Planetary Laboratory’s Kuiper Materials Imaging & Characterization Facility. “Our discovery of sodium chloride tells us this asteroid population could harbour much more water than we thought.”

Today, scientists largely agree that Earth, along with other rocky planets such as Venus and Mars, formed in the inner region of the roiling, swirling cloud of gas and dust around the young sun, known as the solar nebula, where temperatures were very high -- too high for water vapour to condense from the gas, according to Che.

Science Daily