Early Earth May Have Swallowed a Mercury-Like Planet

The jostling of the giant planets Jupiter and Saturn in the solar system’s early days may have delivered a Mercury-type building block to baby Earth, providing the planet with the chemistry to heat its convecting, liquid metal core to this day

The jostling of the giant planets Jupiter and Saturn in the solar system's early days may have delivered a Mercury-type building block to baby Earth, providing the planet with the chemistry to heat its convecting, liquid metal core to this day, a new study shows.

The research, which is based on computer models, resolves two long-standing mysteries about Mother Nature's recipe for Earth. The first is why the planet has an abundance of the rare-earth metals samarium (Sm) and neodymium (Nd) compared meteorites, which are believed to be samples of Earth's building blocks.

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The second riddle is how the planet's metallic core has stayed hot enough over the eons to continue convection, a process that generates Earth's protective magnetic shield.

Oxford University researchers Anke Wohlers and Bernard Wood got the idea to incorporate a sulfur-rich body like Mercury into Earth-formation computer simulations after making connections between colleagues' previous studies relating rare earth elements, including samarium and neodymium, to sulfides; the elements' chemical mismatch between Earth and meteorites; and observations from NASA's MESSENGER spacecraft that Mercury has high levels of sulfur.

"Then we had to do the experiments to test the idea," Wood told Discovery News.

The models show the impacting body would have to have been 20 to 40 percent as big as Earth to produce the required chemical mix. The crash could have happened as the building blocks for Earth were melding together, or it could have been the hypothesized Mars-sized impactor, named Theia, that hit Earth and led to the formation of the moon.

With Jupiter on the move, the inner solar system was like a "mixing bowl," Wood said.

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"Under these circumstances, Mercury-like bodies could have been scattered both outwards and inwards. One could envision an early Mercury-like Earth or even a much later collision with a Mercury-like body, such as that which formed the moon," Wood wrote in an email.

The experiments explored possible chemical pathways the rare earth elements could take as they oxidized from iron metal to iron sulfide and silicates. It also explains how a sulfur-rich core would leave enough radioactive uranium and other elements to drive Earth's dynamo, the swirl of liquid iron that generates the planet's magnetic field.

"As with any new idea, there will be a lot of tests that it will need to pass first before it becomes convincing," geochemist Richard Carlson, with the Carnegie Institution for Science, wrote in an email to Discovery News.

"One of the strengths of modern geochemistry is that we have reasonably precise data for the abundance of almost every element in the periodic table, at least in Earth's outer layers. If core formation under the reducing conditions explored by the Wohlers and Wood experiments can reproduce the whole pattern of element abundances in the silicate Earth, that would give the model more support," he said.

Scientists also can look for naturally occurring telltale traces of radioactive uranium and thorium breaking down to measure concentrations inside Earth.

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These subatomic particle, called geoneutrinos can, and have been, detected, Carlson notes.

"Because neutrinos interact with matter so poorly, they can go straight through the Earth," he said, but that also makes them difficult to catch.

"The rate of geoneutrino detection is very small, a few tens per year. With the various neutrino detectors now operating in deep mines, the geoneutrino statistics are building slowly. Eventually, these measurements may provide a sufficiently accurate map of uranium and thorium distribution in Earth's interior to directly answer the question of whether there is any in the core," he said.

Wohlers and Wood's study is published in this week's Nature.

The planet Mercury, as mapped by the NASA MESSENGER probe, in color. The color variation signifies the rich, varying composition of rocks on the small world's surface.

NASA's MESSENGER probe is the first spacecraft to orbit the solar system's innermost planet and, since orbital insertion in 2011, the satellite has returned over 150,000 photographs of the enigmatic alien surface. However, many of the features the robotic probe has seen resemble very un-alien artifacts. But there isn't really snowmen, the Cookie Monster or a pirate graffiti artist on Mercury, they are all optical illusions stemming from a psychological quirk known as "pareidolia" -- the same phenomenon that makes us see the Virgin Mary in burnt toast and faces on Mars.

Shown here is a recently released photograph of Mercury's Caloris basin, featuring none other than Star Wars' Han Solo frozen in carbonite... but then again it's more likely a raised feature just tricking our brains.

In 2011, MESSENGER got into t' festive mood t' celebrate "Talk Like a Pirate" Day (Sept. 19). Well, it's that time o' t' year again me buckos, so we thought it was a very good time t' show ye t' pirate's Jolly Roger that was eyeballed inside a Mercurian crater. Look in t' starboard portal t' see t' outline o' a skull and crossbones. You see it now, right? Yarrrr!

This strange arrangement of craters give the uncanny resemblance of a certain cartoon mouse.

During a Mercury flyby in 2008, MESSENGER snapped some high-resolution images of the floor of the Caloris basin to find a spider. It was in fact an impact crater with some impressive radial troughs breaking up the surface, but the mission's science team still refer to the feature as "The Spider."

Another fortuitous arrangement of impact craters appear to have etched out The Cookie Monster's googly eyes.

Is this the X-Files logo? Or is it a classic case of "X marks the spot"? It's neither actually, this cratered "X" was formed by secondary crater chains created by ejecta from two primary impacts outside of the field of view of the photo.

One would think that Mercury was a little too close to the sun for snow, let alone snowmen! Although ice has been revealed inside the planets coldest craters, this snowman-like shape is, once again, just craters tricking the brain.