Earth's core captures noble gases from ancient solar wind explosions

4.5 billion years ago, when the solar system was still forming, particles in the solar wind were likely attracted to Earth's core, as it gathered from space rubble.

That's the conclusion scientists came to after analyzing an iron meteorite and found an excess of noble gas with isotopic ratios consistent with the solar wind. Since iron meteorites are thought to be similar to the formation of planetary cores, this suggests that similar abundances should be included in Earth's core.

The meteorite, named "Washington County" for the location where it was discovered in 1927, is a rare meteorite. Of all the space rocks that fall to Earth, only about 5 percent are made of iron.

Based on our understanding of planet formation, these iron meteorites are interpreted as the cores of failed planets.

Planets are thought to have formed when their stars were very young -- possibly even while the stars were still forming -- and surrounded by a thick cloud of dust and gas. The dust and pebbles in the cloud start to collide and stick together: first electrostatically, then gravitationally, as objects get bigger and can attract more matter. These objects are basically "seeds," or planetesimals, of planets.

As the planetesimals grow, they get hot and kind of melt, moving the material around. Core differentiation is the process in which the denser matter sinks inward toward the center of the object, and the less dense matter rises outward.

Not everything that starts out as a planet becomes a planet all the way. Asteroids are thought to be the remnants of planetesimals, which are destroyed and fragmented before they can reach full planetary growth; iron meteorites are thought to be fragments of differentiated planetesimal cores.

So planetary scientists study iron meteorites to better understand the formation of our own planet, which has a dense iron core.

"Washington County" appears to contain unusual isotopes of the noble gases helium and neon, which have been of interest to researchers ever since scientists first discovered it back in the 1960s.

Originally, these gases were thought to be of cosmic origin, that is, produced from interactions with galactic cosmic rays, and ferrometeids were exposed in space for billions of years.

Then, in the 1980s, astronomers found that this ratio was more consistent with the isotopic ratio of the solar wind. Now, a research team led by cosmochemist Manfred Vogt of the University of Heidelberg in Germany has confirmed this.

Using noble gas mass spectrometry, they have determined that some isotopic ratios of neon and helium found in the "Washington County" meteorite are more consistent with the solar wind than with a cosmic origin.

"These measurements have to be very accurate and precise to distinguish the Sun's signature from the dominant cosmogenic noble gases and atmospheric pollution," Vogt explained.

By extrapolating the meteorite into the planet's core, the team concluded that it is possible that similar solar wind particles could be captured by the Earth-forming core and dissolved into the liquid metal. Interestingly, observational evidence supports this conclusion.

Solar isotopes of helium and neon can also be found in igneous rocks of oceanic islands. At least some of these oceanic basalts come from deep mantle plumes, thought to extend to the core-mantle boundary 2,900 kilometers (1,800 miles) deep.

Because solar isotopes are not found in volcanic rocks from shallower material, this suggests that these isotopes came from deep inside the Earth, the researchers said.

"We've been wondering why such different gas signatures exist in the slow but constantly convective mantle," explains cosmochemist Mario Triloff of Heidelberg University.

According to the team's calculations, the observed abundance of solar neon and helium isotopes in the mantle does not require large amounts of material similar to the "Washington County" meteorite. If only 1 to 2 percent of the core components were similar, that would explain what Triloff and his team observed.

Given the turbulent environment in which the solar system formed, and the wild nature of the sun, it's perhaps not surprising that solar particles get mixed in with everything.

But the fact that these particles may be oozing from the core and into the mantle is surprising and suggests we may need to factor in core leakage in future studies and modeling, the researchers said.

"For our planet, this may provide a new solution to the problems associated with maintaining distinct mantle states with unique noble gas signatures by maintaining individual reservoir fluxes in the underlying formation," they wrote in their paper. "

"At the same time, this implies that the Earth's core plays a rather important role in mantle geochemistry and volatile geodynamics, which were previously overlooked and should therefore be included in future studies."