It’s generally thought that the Earth was built out of asteroids called ordinary chondrites. Chondrites contain some of the most ancient minerals in the Solar System, and their composition suggests that they made up the majority of the material that collapsed to form our planet.

But recent observations have shown that the composition of the Earth’s mantle doesn’t match that of ordinary chondrites, suggesting that something else is going on. If the Earth was indeed formed from chondrites, that material must have separated into two different reservoirs, one of which we haven't identified.

Specifically, the Earth’s mantle has a lower ratio of neodymium-142 to -144 than ordinary chondrites. That means that at some point during the Earth’s formation, the chondrites that formed the planet must have differentiated into higher- and lower-ratio clumps (just as mud, stirred into a cup of water, differentiates with the densest concentration of mud ending up on the bottom). Since this requires the Earth to have been very hot, it would have happened within the first 20 million to 30 million years of the planet’s initial accretion.

In a new paper, researchers argue that the missing material, rather than differentiating and sinking deeper into the Earth, could have instead been lost to space during impact events. If so, it makes predictions for the thermal history of the Earth that conflict with current estimates. That’s because the radioactive elements inside the Earth produce heat, and much of this radioactive material would have been lost to space as part of the missing reservoir. The presence, or absence, of this radioactive material would make a big difference to the Earth, the researchers argue. Among other things, the planet’s plate tectonics would be noticeably affected, as would the planet’s habitability.

Evidence

The products of radioactive decay are detectable by a device called a mobile geoneutrino detector. If the predicted amount of radioactive material is still in the Earth, a long-term study could eventually spot it. Luckily, there are other ways to look for evidence of the missing material’s presence on Earth. For one thing, it makes testable predictions for the strength and shape of the Earth’s magnetic field, as well as for the kinds of isotopes we’d find in the continents.

Other research has argued that the radioactivity deep beneath Earth’s surface would have produced intense mantle plumes that carry much more heat than the ones we've observed. But this has not been observed, which the new paper’s authors argue is evidence against its existence inside the Earth.

The predicted heat might even stabilize the Earth’s core against the magnetic field reversals that have been going on for 200 million years—and since we observe the magnetic field continuing to reverse itself (a process which is slowly taking place right now, with the planet’s north magnetic pole slowly migrating south), this seems to be further evidence against the presence of the missing material.

The researchers further argue that if the missing material wasn’t lost to space, it has conspicuously remained hidden despite the planet’s entire history of stir-ups of various sorts, such as volcanism. This is pretty unlikely, the researchers argue. If the material had escaped into space, however, it would resolve all these apparent disparities, as it could explain how that material could exist without leaving its mark on the Earth.

Lunar evidence

There’s another constraint on when this probably happened: the Moon. That body formed after a collision event with a Mars-sized body (this one is named Theia, for the Greek titan who’s the mother of the moon goddess). The impact was powerful enough to eject a lot of Earth material into space, much of which coalesced with material from Theia to form the Moon.

If the Earth still had the missing material at the time the Moon-forming planetesimal collision happened, a lot of it likely would have gone into the Moon and we would detect it there. Instead, the Moon is closer to the Earth’s composition, implying the material was lost before the Moon-forming impact. (This isn’t certain, however, as it’s possible that if the missing material is deep enough in the Moon, it might remain relatively undisturbed.)

“It is difficult to reconcile a hidden [reservoir of the missing material] with a growing number of geomagnetic, geochemical and geodynamic constraints,” the researchers argue in their paper.

Earth, Mars, and Venus

The researchers’ hypothesis could also explain some of the differences between Earth and its two nearest neighbors. If the missing radioactive material was still in Earth, the researchers argue, it would be more likely to form a single plate. Such a world would have intermittent volcanoes; less common but more intense than the Earth has. Clearly, that’s not the case, as we have multiple plates and relatively diffuse volcanism.

But this scenario does bear a striking resemblance to Venus. The extra heat from the radioactive material never left the planet, according to the researchers’ model, because Venus never sustained an impact large enough to eject it. As such, it could have contributed to Venus' current state by fostering a runaway greenhouse effect, the researchers argue. The trapped heat continued to build up until it burned away the oceans and left a thick atmosphere, blanketed by sulfuric acid.

On Mars, meanwhile, the opposite may have happened. Mars has been pummeled quite a bit and might have lost even more radioactive material than the Earth did. That could explain the cold deadness of it. If Mars lost enough internal heating material, it could have stopped the planet’s plate tectonics before they even started.

Significance

In addition to all the significant consequences discussed above (the implications to our understandings of the Earth and the Solar System, the relationship between the planets), the work—if borne out by experiment and observation—could be important in the search for life on other planets.

If this process did indeed play a role in distinguishing Earth from its siblings Mars and Venus, it helped create conditions favorable to life. Researchers might then use this information to determine which exoplanets to look for life on. The better our understandings of the conditions necessary for life, the more likely we’ll find it.

The researchers emphasize that further work is needed to show whether their model is correct and, if so, to build on it.

Nature Geoscience, 2015. DOI: 10.1038/NGEO2488 (About DOIs)