Large Impact Crater, Saturn's Spin, and Black Hole Wind

Biggest Crater on Earth Discovered?

A paper in Tectonophysics this month has potentially identified the largest impact crater on Earth, the Warburton West Structure in central Australia.

The double crater structure is about 19 miles (30 kilometers) deep, spans about 250 miles (400 kilometers) across, and may represent a meteorite that split in two nearly equal pieces before or as it fell toward Earth. The responsible impactors are believed to have been more than six and a half miles across—each. For comparison, the Chicxulub impactor that killed off the dinosaurs about 66 million years ago is believed to have been around six miles (10 kilometers) in diameter and produced a crater about 110 miles (180 kilometers) across.

According to Andrew Glikson, a geologist at the Australian National University, the event may have occurred some 300 million years ago or more, although most of the surrounding rock is ranges from 300 to 600 million years old and does not show evidence of the sediment layer one would expect from such a devastating impact.

“I’m sure there will be a lot of people raising eyebrows,” says Glikson. “But I think we have a major body of evidence for an impact origin.”

While Earth has experienced a number of impacts throughout its history, most of the resulting craters, including these two, are no longer visible—buried and eroded away by plate tectonics and weather over time.

So how do we identify a crater that we can no longer see?

Unsurprisingly, big impactors have a profound effect on the geology where they land. The tremendous energy and pressure from the impact “shocks” the ground, turning rock into glass (tektites) and causing microscopic deformation of minerals such as fracture patterns in quartz crystals.

Rock samples from the Warburton site include such glassy rock and quartz grains. The quartz contains microscopic clusters of lines, some parallel and mostly straight, and others wavy and wiggly, which could represent fractures that were created during the colossal shock of a meteorite impact.

The biggest confirmed impact site on Earth is the two-billion-year-old Vredefort structure in South Africa, about 185 miles (300 kilometers) across. So the Warburton West Structure could be one for the record books! –Elise Ricard

Saturn’s peculiar spin

Saturn’s day is quite a bit shorter than Earth’s. Scientists have tracked the planet’s spin on its axis to around 10-11 hours compared to our 24. But the precise rotation of the ringed-planet was hard to pin down. It has no measureable solid surface and is covered by thick layers of clouds, foiling direct visual measurements by space probes. In addition, different parts of this sweltering ball of hydrogen and helium rotate at different speeds, and Saturn’s magnetic poles align with its rotation axis (which means we don’t observe daily variations that show up in planets where the magnetism and spin are misaligned), presenting quite a conundrum for astronomers here on Earth.

But a new method, devised by Ravit Helled, of Tel Aviv University and published this week in Nature, proposes a new determination of Saturn’s rotation period and offers insight into the internal structure of the planet, its weather patterns, and the way it formed. The technique is based on Saturn’s measured gravitational field and it oblate shape—with its east-west axis shorter than its north-south axis.

Helled and her colleagues determined that Saturn’s day clocks in at 10 hours, 32 minutes, and 44 seconds long. When the researchers applied their method to Jupiter, whose rotation period is already well known, the results were identical to the conventional measurement, reflecting the consistency and accuracy of the method.

Next, the team hopes to apply their method to other gaseous planets in the solar system such as Uranus and Neptune. Their new technique could also be applied in the future to study gaseous planets orbiting other stars. –Molly Michelson

Black hole winds affect galaxy formation

What influence do supermassive black holes have on galaxy formation? Quite a lot, according to a new study in Nature.

Astronomers studied the galaxy IRAS F11119+3257 (2.6 billion light-years away), which has an actively growing supermassive black hole at its center, consuming large amounts of gas. As material enters the black hole, it creates friction, which in turn gives off electromagnetic radiation—including X-rays and visible light.

Black holes that fit this description make their homes in active galactic nuclei (AGN), and their intense radiation output also generates powerful winds that force material away from the galactic center. The study found that these AGN winds are powerful enough to drive the large molecular outflows from deep inside the galaxy’s core that reach to the edges of the galaxy’s borders. These outflows remove massive quantities of star-making gas, thus influencing the size, shape, and overall fate of the host galaxy.

“These are not like normal spiral or elliptical galaxies. They’re like train wrecks,” explains Sylvain Veilleux, of the University of Maryland and a co-author of the study. “Two galaxies collided with each other, and it’s now a single object. This train wreck provided all the material to feed the supermassive black hole that is now driving the huge galactic-scale outflow.” –Molly Michelson