Violent Moon-Forming Impact Occurred 60 Million Years Earlier

Through the analysis of isotopes trapped inside ancient quartz crystals, geochemists have realized that the Earth-shattering cosmic impact that laid waste to our young Earth and formed the moon happened 60 million years earlier than thought.

Speaking today (Tuesday) at the Goldschmidt Geochemistry Conference in Sacramento, Calif., researchers from the University of Lorraine in Nancy, France, discussed their analysis of xenon gas isotopes trapped inside South African and Australian quartz. These geological "time capsules" were formed on the primordial Earth 3.4 and 2.7 billion years ago, respectively.

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The key problem with dating the early evolution of our planet is the lack of "classical geology" that we can tap into. This means that there is very little pristine material or layers of rock that has remained unchanged for billions of years. So geochemists have stepped in to use the technique of isotopic analysis to gauge the chemical conditions of early Earth.

During the formation of ancient quartz, pockets of atmospheric gases were trapped inside, freezing a chemical fingerprint of the early atmospheric conditions. By looking at the ancient ratios of xenon isotopes and comparing them with today's, geochemists Guillaume Avice and Bernard Marty were able to precisely zero-in on the cataclysmic Earth impact that eventually formed the moon.

In the solar system's early history, planetary collisions were common and approximately 4.5 billion years ago the Earth was hit by a hypothetical Mars-sized object - nicknamed "Theia" - unleashing huge quantities of energy, turning the planet into a searing globe of magma. The ejecta from this impact formed the moon, although the exact formation processes are not fully understood.

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But through the analysis of xenon, at least we can now define when the impact occurred.

"It is not possible to give an exact date for the formation of the Earth," said Avice in a press release. "What this work does is to show that the Earth is older than we thought, by around 60 million years.

"The composition of the gases we are looking at changes according the conditions they are found in, which of course depend on the major events in Earth's history. The gas sealed in these quartz samples has been handed down to us in a sort of ‘time capsule.' We are using standard methods to compute the age of the Earth, but having access to these ancient samples gives us new data, and allows us to refine the measurement."

It was thought that the Earth's atmosphere formed around 100 million years after the formation of the solar system. However, the primordial atmosphere would not have survived the massive Theia impact event. By studying the xenon isotope ratios from 3.4 and 2.7 billion years ago and comparing those ratios with modern Earth, Avice and Marty have been able to look back in time to find that the Earth's atmosphere likely started to form only 40 million years after the solar system's formation, meaning the Theia impact occurred approximately 60 million years earlier than previous estimates.

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"The xenon gas signals allow us to calculate when the atmosphere was being formed, which was probably at the time the Earth collided with a planet-sized body, leading to the formation of the moon," said Avice. "Our results mean that both the Earth and the moon are older than we had thought."

"This might seem a small difference, but it is important," added Marty. "These differences set time boundaries on how the planets evolved, especially through the major collisions in deep time which shaped the solar system".

The Martian surface is peppered with impact craters of all shapes, sizes and ages. However, many of the craters are just plain weird.

But just how 'weird' is weird?

Curious, Discovery News asked the High-Resolution Imaging Science Experiment (HiRISE) team which craters they considered to be the strangest. HiRISE is the most advanced camera to be put into Mars orbit. It is attached to NASA's Mars Reconnaissance Orbiter (MRO) and currently snapping features on the Red Planet's surface -- it has seen a ton of strange objects (sadly, it hasn't found a top secret military base yet, despite what you may have heard to the contrary). So, let's take a tour of some of the weirdest craters Mars has on offer...

Special thanks to Ari Espinoza of the HiRISE team for helping to compile this list (originally published Dec. 30, 2011. Updated May 7, 2013)

Crater, Horst and Graben: Is that a piece of modern art? Actually, it's an impact crater that has been bent and twisted by tectonic processes. Along the fault line that crosses this crater, blocks of rock are forced upward (called "horst") and downward (called "graben"). This is interesting to planetary scientists as it shows that tectonic activity was occurring after the crater was formed.

Rolling Stones Logo? If you squint and use a little imagination, you may see the Rolling Stones' logo. Well, that's what the HiRISE team told us anyway. (I'm still squinting...) In reality, it's an impact crater on a sloping surface. Presumably, the "tongue" of material is slipping down the slope.

Bulls-Eye Impact? Did a small meteorite have the incredible fortune to slam into the center of a larger impact crater? Probably not.

This is one of several examples of "terraced" craters where alternating layers of hard and soft material in the surface layers of the Martian surface have been hit by a single meteorite. The result is a concentric nesting of ridges inside the same crater. Pretty!

What the...? What's the weirdest kind of impact crater? The kind that may not be an impact crater at all (but looks like one). On the slopes of Pavonis Mons, one of Mars' shield volcanoes, this crater has a hole in the middle. The hole is a "skylight," or the collapsed roof of a subterranean lava tube. The loose material above the collapsed roof appears to have slumped into the skylight, creating a crater lookalike. But what caused the roof of the lava tube to collapse? Could a meteorite be to blame? No idea, but HiRISE will be taking some more photos of this little oddity to find out.

Two-for-One Crater Special: What could be worse than a meteorite hitting you? Two meteorites hitting you... at the same time! Yes, that's exactly what happened here. It seems highly likely that one object tumbled through the Martian atmosphere and split in two. In doing so, the two halves impacted in the same location. As can be seen from this example, both halves were likely the same size, producing a rather satisfying imprint.

Another Double-Whammy: Looks like double-impacts are becoming a trend! This time, in addition to the two co-joined impact craters, HiRISE has picked out the rays that are produced when space rocks slam into the Martian surface.

Hit Me Baby Three More Times? It may seem hard to believe, but Mars also has triple-impact craters! It stands to reason that after countless impacts, you might get the occasional meteorite that splits into three when blasting through the atmosphere. So here you have it, a triple-impact crater.

A Triple Ricochet Crater: Another three (likely simultaneous) impacts, only this time their craters are elongated. This suggests the meteorites hit the surface at an oblique angle.

A Simple Blemish: In an apparently featureless plain in the north polar region, a single, small crater appears as the only blemish. Looking closely, the crater seems to be filled with ice.

Bubbly Landscape: This cluster of impact craters in the northern plains of Utopia Planitia contain strange uplift features likely caused by ground ice upheaval.

Crater of Mud: The strange concentric rings inside this crater near the Martian volcano Elysium Mons are thought to be the ancient remnants of a mud flow. Therefore, it is believed this crater wasn't caused by an impact from space, but by material flowing away from under the surface. The crater was then formed as the material above slumped.

Cracked Cookie Crater? There's an odd pair of craters in Hrad Vallis that the HiRISE website describe as a "pair of odd craters." Why so... odd? Well, to me, the larger crater looks like a cracked cookie, probably crevasses and faults carved across its diameter.

The Crater with a Robot Visitor: What makes this crater weird? Well, it's not the crater, it's the little man-made robot that's parked on the crater's western rim that makes this scene weirdly awesome. It's even weirder to think that a robot in Mars orbit has taken a photo of another robot on the Martian surface a couple of hundred miles below. Robots looking out for robots on alien worlds...

This is of course NASA's tenacious Mars Exploration Rover Opportunity that keeps exploring the Martian surface since exceeding its primary mission duration of 3 months back in 2004. Opportunity now has company on the Martian surface -- on Aug. 5, 2012, the nuclear-powered Curiosity landed inside Gale Crater to look for clues behind the habitability of the red planet.

In March 2013, HiRISE spotted a series of non-impact craters in Acidalia Planitia. These may not be impact craters, but they are unlike any other crater discovered on Mars to date! So the process behind their formation will remain a mystery... for now.