A nebular winter could have doomed the dinosaurs. The clue is a thick layer of an extraterrestrial element on the ocean floor, now claimed to be the result of Earth colliding with a galactic cloud.
The most-heard explanation for the dinosaurs’ demise is an asteroid impact, which left a crater off the coast of Mexico and a worldwide 30-centimetre-thick layer of iridium, an element otherwise rare on Earth.
But every so often, astronomers suspect, the solar system ploughs into a giant nebula of molecular gas and dust much denser than typical interstellar space. The resulting galactic fog would have darkened skies and cooled the ground until a harsh winter set in. It would also have destroyed the ozone layer and halted photosynthesis.
Now Tokuhiro Nimura of the Japan Spaceguard Association in Ibara and colleagues claim such a collision could have ended the dinosaurs’ 150-million-year reign. At a site in the Pacific Ocean, the team found an iridium-rich sediment layer 5 metres thick, too thick for a sudden event like an asteroid strike to account for it.
They argue that a collision with a giant molecular cloud – which could take Earth at least a million years to pass through, picking up iridium steadily as it does so – fits the discovery much better. “Such an encounter and related perturbation in global climate [is] a more plausible explanation for the mass extinction than a single impact event,” they write.
Adrian Melott at the University of Kansas in Lawrence says the idea is wacky but not impossible. “We know that these clouds exist,” he says. “We know that we’re likely to run into one sooner or later. And they have some evidence from the ocean floor in favour of their idea.”
Others question that evidence. David Kring at the Lunar and Planetary Institute in Houston, Texas, argues that the deposit could have been disturbed by currents, organisms and drilling, broadening the layer locally.
“There have been several studies specifically designed to determine the thickness of the iridium anomaly,” he says. “They all indicate the anomaly is sharp, not broad,” which would favour the impact hypothesis.
Even so, a collision with a cloud could have dislodged rocks from the fringes of the solar system and sent them hurtling toward Earth – perhaps explaining the crater and leaving a thin iridium layer intact.
We could test this idea by searching the ocean floor for radioactive elements likely to be replenished from space, like uranium and plutonium, says Alexander Pavlov at NASA’s Goddard Space Flight Center in Maryland. Most uranium isotopes aren’t even found in the solar system, so finding them along with iridium would prove that something interstellar, like a vast nebula, was the true culprit.
Journal reference: Gondwana Research, accepted, arxiv.org/abs/1603.06136