Hot enough for gemstones Planet Observer/Universal Images Group via Getty Images

IT IS the hottest temperature ever recorded on Earth’s surface.

When a space rock struck the ground nearly 40 million years ago in what is now Canada, the impact briefly heated rocks to 2370 °C. That’s halfway to the temperature at the sun’s surface. The blistering heat has been revealed by an unexpected source: gemstones.

When space debris crashes into Earth, the impact creates hellishly hot temperatures in the collision zone. Such impacts when our planet was young changed its atmosphere and crust, and affected its habitability.


But inferring the heat generated by these ancient meteorites is tricky. Not only did they hit Earth millions of years ago but both the meteorites and the surface rocks often vaporised in the resulting shock waves.

This is a pity, since such rocks hold temperature clues. We can predict maximum temperatures – estimated to be over 2000 °C. However, a lack of physical evidence in the rock record meant it was not possible to test the most extreme predictions.

“What kinds of records can survive an event capable of vaporising rocks?” asks Benjamin Black at the University of California, Berkeley.

Now clues left behind in the Mistastin Lake crater in Labrador, Canada, which is 28 kilometres across, are revealing just how hot the impact site became.

Nicholas Timms at Curtin University in Perth, Australia, and his team found that the crater was once hot enough to transform the common mineral zircon into gem-like cubic zirconia. The zircon acts as a thermometer, because the minimum temperature necessary for this transformation is 2370 °C.

Timms’s team traced the zircon’s history back to the point of impact, about 38 million years ago (Earth and Planetary Science Letters, doi.org/cczc).

“These new results underscore just how extreme conditions can be after asteroids strike”

“Nobody has even considered using zirconia as a recorder of temperatures of impact melts before,” says Timms. “This is the first time that we have an indication that real rocks can get that hot.”

“These new results underscore just how extreme conditions can be in the seconds to minutes after asteroids strike a planet,” says Black. Understanding the upper limits of temperatures during these impacts might improve our picture of the conditions on Earth’s surface over 4 billion years ago, when the newly formed planet was repeatedly bombarded from space.

What secrets does the Earth’s mantle hold? Find out more at New Scientist Live

Black says superheating from more frequent and larger bombardments could have baked Earth’s crust, keeping hydrogen, carbon and sulphur in the atmosphere. All these elements are considered essential for life: for instance, without hydrogen and oxygen there can be no water. But too much could have affected the planet’s climate and chemistry, making it less habitable.

This article appeared in print under the headline “Record heat from space rock impact”