Republished from The Conversation: Is iron rain the reason why Earth and the moon are so different?

Is iron rain the reason why Earth and the moon are so different?

Simon Redfern, University of Cambridge

New experiments show that the asteroids that slammed into Earth and the moon more than 4 billion years ago were vaporised into a mist of iron. The findings, published in Nature Geoscience, suggest that the iron mist thrown up from the high velocity impacts of these asteroids travelled fast enough to escape the moon’s gravity, but stayed gravitationally stuck on more massive Earth. And these results may help explain why the chemistry of the Earth and the moon differ.

When and how Earth’s metallic core formed is uncertain. Clues come from known differences in the preferences of certain elements incorporated in the silicate mantle or the metal core. In a mixture of silicate rock and iron metal, the atoms of certain elements, such as gold and platinum, tend to prefer to enter the metal, while others, such as hafnium, prefer the silicate.

As Earth’s iron-rich core formed it “sucked” the metal-loving elements out of the planet’s rocky mantle. However, measurements of the silicate mantle by James Day have previously shown that there are more of them left in the shallower Earth than would be expected. This has often been attributed to a late veneer of asteroids that delivered an extra dose of metal-loving elements to the rocky mantle.

The Z machine generates electric currents of up to 20 million amps, to shoot aluminium projectiles at iron targets, replicating the impacts of early asteroids.

One problem with this picture has been that the abundance of the metal-loving elements on Earth is ten to a hundred times greater than that measured on the moon, which should by this argument have the same veneer. The chemical difference between Earth and the moon has been perplexing, and casts a shadow over the prevalent idea that the moon formed from the same stuff as Earth after an impact from a Mars-sized planet early in the history of the Solar System.

Mighty Earth attracts more metal

The new paper seems to reconcile these differences. The experiment relied on Sandia National Laboratory’s “Z-machine”: a huge electromagnetic gun – twice as powerful as the world’s total generating capacity – that can launch projectiles into iron targets at ultra-high velocity.

The impact experiments by Richard Kraus and colleagues show that iron vaporises under the conditions created when an asteroid crashes into Earth or the moon. A cloud of iron mist will have wrapped around the globe after any such collision, falling to Earth as metal rain. These well-mixed droplets will have become incorporated into the mantle, delivering the excess metal-loving chemicals.

The same experiments, however, indicate that the velocity of the iron rain droplets will have been greater than the escape velocity on the moon, but below that of Earth. Earth would therefore have captured the metal cores of colliding asteroids, while the moon will have failed to. William Anderson of Los Alamos National Laboratory, US, said: “The moon may have received, but not retained, a significant portion of the late veneer.”

The results could imply that models for estimating the time scales of Earth’s core formation could be out by as much as a factor of ten, with the core forming much earlier in Earth’s history than previously recognised.

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Republished from The Conversation: Dawn breaks over distant Ceres … and perhaps reveals signs of habitability

Dawn breaks over distant Ceres … and perhaps reveals signs of habitability

Monica Grady, The Open University

NASA’s Dawn spacecraft is about to start its investigation of the largest member of the asteroid belt, 1 Ceres. It will take detailed images of the dwarf planet, and produce a geological map of its entire surface. But even before the spacecraft has reached its optimum orbit, the preliminary results just released are already surprising and delighting planetary scientists.

Up until February 2015, the best images taken of Ceres were from the Hubble space telescope, showing a near-spherical body with one area that was much brighter than the rest of the surface. As Dawn approached Ceres, its camera acquired some remarkable images, at about three times the resolution of those from Hubble. The pictures verified that there was indeed a brighter region.

Exploded map of Ceres showing ‘bright spot’.

Even better, close examination of the images showed that the area varied in brightness over the course of Ceres’ day (which is only about nine hours long), growing dimmer as the dwarf planet moved into darkness. It is interpretation of this variability that has planetary scientists buzzing.

As if that were not enough, a further series of pictures appear to show a plume emanating from the surface. Is Ceres active? Does it have a layer of water or ice below a thin crust of rock? Could it be a ball of mud, overlain by a muddy ocean, on top of which is another thin muddy crust? The exact structure of Ceres is not yet known, although it is clear that it’s not rocky all the way through – its density is too low, so there must be at least some water or ice present.
Suggestions at the 46th Lunar and Planetary Science Conference in Houston, Texas, of icy volcanism on Ceres have led to speculation that the dwarf planet could potentially be habitable. Although Ceres does not have an atmosphere, life might exist in a subsurface ocean, as has been suggested for Europa or Enceladus, moons orbiting Jupiter and Saturn respectively.

Is Ceres more slush than solid inside?

Cryovolcanism – the presence of ice volcanoes – is not the only mechanism that can produce a plume of dust and ice from a planetary surface. The Rosetta mission has delivered amazing images of plumes coming from comet P/67 Churyumov-Gerasimenko, caused by sublimation of ice that releases dust and gas trapped inside the ice. Could the bright spot be an icy plume caused by the vaporisation of Ceres’ surface as it turns towards the sun’s heat, and then dropping away as night falls? Corridor talk at the conference speculates that Ceres might be closer to a comet than the asteroid it is usually regarded as.

Fortunately, we won’t have to wait much longer before we get some more definitive answers to questions of Ceres’ physical structure and heritage. By the beginning of April, the Dawn spacecraft will be much closer and will start its imaging campaign in earnest, at which point we will start seeing craters and other surface features at better resolution.

This is not the Ceres you are looking for.
Borghese Collection, Louvre, CC BY-SA

In preparation for descriptions of such features, and bearing in mind that Ceres was the Roman goddess of the harvest, the International Astronomical Union has ruled that craters on Ceres should be named after international deities of agriculture and vegetation, while other features will be named after agricultural festivals of the world.

I’m not sure just how many of these there are, or how memorable their names will turn out to be. But as the Dawn mission’s principal investigator Chris Russell pointed out, there is one Mayan deity named Yum (Yum Kaax, god of agriculture and the jungle), who should readily be remembered. One can only hope the mission scientists find a suitably delicious feature on Ceres to give that name.

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