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, 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.

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.

“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.

Read more at Discovery News