20 September 2017
Stuart Gary
Astronomers have determined the most accurate estimate yet of the Earth’s core, finding it makes up 32.5% of the planet’s total mass.
The findings, reported in the journal Icarus and on the prepress physics platform ArXiv.org, are part of a new understanding of the Earth’s composition, which will help scientists develop new methods to determine if exoplanets orbiting distant stars are likely to be capable of supporting life.
By relating Earth’s composition to that of the Sun, researchers from the Australian National University, led by Charles Lineweaver, hope to use the results to determine the make-up of exoplanets by looking at the composition of their host stars.
Astronomers can work out the composition of stars by studying their spectra – the chemical signatures detectable in the light coming from those stars. The technique designed by Lineweaver and his colleagues will allow researchers to focus on stars likely to have planets with an Earth-like chemical composition.
As a starting point, the researchers needed to understand the composition of the elements that condensed out of the proto-planetary disk of gas and dust around the early Sun 4.6 billion years ago, from which the Earth formed.
The four most abundant elements – iron, oxygen, silicon and magnesium – make up more than 90% of the Earth’s total mass. However, determining exactly what the Earth’s core is made of proved to be far more tricky.
Seismological studies based on earthquakes provide a detailed picture of the planet’s internal structure – including information on the densities of the core, mantle and crust – but it’s difficult to convert this into a detailed elemental analysis.
The deepest humans have actually drilled into the Earth is the Kola Superdeep borehole in Russia, which reaches depths of 12,262 metres. However, that only scratches the surface of the planet’s 6400 kilometre radius.
Lineweaver and his colleagues developed new estimates of the Earth’s composition based on a meta-analysis of previous studies of the mantle and core, and a new estimate of the core’s mass. They found that the core makes up 32.5% of the planet’s total mass with an error of plus or minus 0.3%.
And it’s that tiny fraction – known as an “error bar” – which is the most useful predictive tool to emerge from the analysis.
Lineweaver says, “We’re very proud of all the work that went into determining that plus or minus 0.3 error bar, because we’re interested in exoplanets, and to do that we needed the error bars.
“When we looked in the literature to get the best understanding of Earth composition available, we found that a lot of effort has been put into determining the composition of the mantle, and a little bit of effort has been put into determining the composition of the core – but almost no effort has gone into trying to put those two together.”
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The problem was compounded because researchers use different models and haven’t been quantifying the error bars associated with each of them.
“So, in other words, here’s what the mantle’s made of, and here’s what the core is made of – but no one’s taken the trouble to say what that error bar on the mass fraction of the core is,” explains Lineweaver.
“And without that error bar you can’t put them together – and if you can’t put them together you don’t know what the bulk composition of the Earth is.”
The authors found abundances of magnesium, tin, bromine, boron, cadmium, and beryllium were significantly lower than previous estimates of the bulk Earth, while abundances of sodium, potassium, chlorine, zinc, strontium, fluorine, gallium, rubidium, niobium, gadolinium, tantalum, helium, argon, and krypton, were all significantly higher.
Lineweaver says this new analysis reveals what the real structure of the Earth.
“The next step is determining exactly what the Sun is composed of, and then comparing the two – and that’s in our next paper which is now under review,” he adds.
“We can then figure out how the Earth became devolatilised to become what it is. And it’s that devolatilisation pattern that we’re after – in other words you take something like the Sun, get rid of hydrogen, helium and noble gasses – and then you’re left with a devolatilised chunk of stuff which became the Earth, and that’s the pattern we’re looking for.”
The goal for the scientists is to use their analyses to compare the Sun and Earth to determine the chemical composition of rocky planets in the Alpha Centauri system.
“We know virtually nothing about the planets in this system, and this technique will allow us to make lots of predictions about what those planets will be like,” says Lineweaver.
He predicted that the work would show different magnesium and calcium ratios. The analysis would also throw up an estimate of the amount of orthopyroxene on the planets – a mineral that plays a crucial role in determining the amount of water that might be present.
Lineweaver says the ability to compare the composition of the Earth and the Sun will provide a semi-universal method that can help estimate the chemical compositions of rocky exoplanets around other stars.
“Even though we can’t see these rocky planets, we’ll still be able to make predictions about what their made of,” says Lineweaver.
“The statistical predictions tell us that every star has some kind of planet around it. The problem then becomes determining what kind of rocky planets are there in the habitable zone around each, and whether any of these are Earth-like and capable of supporting life.”