Earth is a less volatile version of the sun


Elemental comparison of the planet and its host star suggests a ‘universal process’ governing rocky planets across the universe. Andrew Masterson reports.


The sun and the Earth: same same, but different.

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A detailed comparison of the chemical make-up of the Earth and the sun provides insight into how rocky planets form across the universe, and may one day be a crucial factor in determining the habitability of exoplanets.

That’s the message contained in a paper written by three researchers from the Australian National University and soon to be published in the journal Icarus.

Haiyang Wang, Charles Lineweaver and Trevor Ireland prepared new estimates for metallic and non-metal elements present in both the star and the planet, and concluded that Earth is made of the same elements as the sun, but with significantly lower levels of volatile elements such as hydrogen, helium, oxygen and nitrogen.

“To first order, Earth is a devolatilised piece of the solar nebula,” they write.

To make the finding, the researchers first had to construct estimates of the chemical and mineralogical content of two places for which direct measurement is impossible – the interior of the sun, and the Earth’s mantle and core. Both are exercises, they note, that are “not straightforward”.

In the matter of the Earth, the trio rely on their earlier work, providing a set of measures for the planet’s primitive mantle, core and overall bulk. That study, published in 2009, was always intended to act as a robust comparison to solar measurements, with the aim of establishing a more precise devolatisation pattern.

That aim – assisted in the latest research by spectroscopic measurement of absorption lines in the solar photosphere – as now largely been achieved.

The researchers noted that in the Earth’s interior, elements with moderately low condensation temperatures – between 500 and 1400 degrees Kelvin – were particularly depleted. Most importantly, the level of depletion increased as the condensation temperature dropped. The patter, they note, can be described as a “volatility trend”.

“A full quantification of the volatility trend between bulk Earth and proto-Sun provides a framework for understanding the nature of devolatilisation processes active on the precursors to Earth,” they note.

“Eventually, an extension of this model could be used to estimate rocky exoplanet compositions based on the elemental composition of their host stars.”

Their analysis, they point out, makes no assumptions about, nor provides evidence to support, various theories about whether the devolatisation of any particular element “happened during the collapse of the solar nebula, during accretion within the proto-planetary disk or subsequently as a result of impacts”.

In the long run, they conclude, future research using the new estimates could calibrate a “potentially universal process associated with the formation of terrestrial planets”.

A full copy of the paper is available on the pre-print server, arxiv.

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  1. https://www.journals.elsevier.com/icarus/recent-articles
  2. https://www.sciencedirect.com/science/article/abs/pii/S0019103517302221
  3. https://arxiv.org/pdf/1810.12741.pdf
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