One billion comets to form Pluto

Sputnik Planitia
Sputnik Planitia

An article published in the journal “Icarus” describes a research that offers an explanation for the formation of the dwarf planet Pluto. A team of scientists from the Southwest Research Institute (SwRI) put together data collected by NASA’s New Horizons space probe and data collected by ESA’s Rosetta space probe, which studied comet 67P/Churyumov-Gerasimenko, concluding that Pluto formed by the union of about a billion comets similar to it.

Two space agencies and two probes for two extraordinary space missions. New Horizons and Rosetta traveled to areas of the solar system very far from each other with very different targets. After its Pluto flyby on July 14, 2015, New Horizons continued its journey for another mission while Rosetta ended its one on September 30, 2016. In short, those look like two separate stories but at SwRI they found surprising connections between Pluto and comet 67P/Churyumov-Gerasimenko.

Dr. Christopher Glein, the article’s lead author, defined the one developed by his team the giant comet cosmochemical model to explain the formation of the dwarf planet Pluto. It’s based on the characteristics of Sputnik Planitia, the large glacier that forms the left lobe of Tombaugh Regio, the heart-shaped region in the foreground in the top image (NASA/Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute).

The glacier is rich in nitrogen in an amount that could be expected if Pluto formed by the agglomeration of about a billion comets or other celestial bodies from the Kuiper belt with a chemical composition similar to that of 67P/Churyumov-Gerasimenko.

The researchers also investigated a solar model that predicts that Pluto formed from very cold ice that would have a chemical composition more similar to that of the Sun. To do this they examined how many of the volatile elements could have escaped from the atmosphere into space in the course of the dwarf planet’s life. The problem is in the proportion between carbon monoxide and nitrogen and they needed to find a way to reconcile them to obtain a more complete picture.

On Pluto there’s a limited amount of carbon monoxide and this suggests that it’s either buried in the ice on the surface or that its elements recombined to form water. According to the researchers, initially the chemical elements were those inherited from the comets that formed the dwarf planet but that mixture was modified by liquid water and here there’s again the hypothesis that at least in the past there was an underground ocean.

The bottom image (NASA/Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute) shows four maps obtained thanks to the New Horizons space probe’s Ralph instrument. They show regions rich in methane (in purple), nitrogen (in yellow), carbon monoxide (in green) and water (blue), all frozen compounds.

The solar model still met various constraints so to find the solution the researchers used chemical tools. In other words, they reconstructed the evolution of the presence and abundance of certain chemical compounds over time. The presence of nitrogen at the beginning of Pluto’s history explains the existence of Sputnik Planitia and points towards the giant comet cosmochemical model.

The study of Pluto started thanks to the New Horizons mission brought a number of surprises and the dwarf planet’s origin could be another one of them. According to the authors of the great comet cosmochemical model it will be possible to further test their theory by analyzing the various nitrogen isotopes and also comparing them with the amount of argon. In any case, this research confirmed the usefulness of considering data collected by missions that apparently have no connections.

Maps of compounds on Pluto
Maps of compounds on Pluto

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