Until its downgrading in August 2006 by the International Astronomical Union (IAU), it was considered the 9th planet of the solar system. Since then, Pluto has been one of the multitude of dwarf planets, the largest and the second most massive among them — and yet its diameter (2,377 kilometers) is far surpassed by the seven largest moons of the solar system.
Since NASA’s spacecraft New Horizons flew past Pluto at a distance of 12,500 kilometers on July 14, 2015, knowledge about the small icy world has multiplied. How it was formed be created, however, remains unclear. Now US scientists have presented a new spectacular theory based on data from New Horizons and ESA’s Rosetta mission: Pluto could have come from the clustering of hundreds of millions of comets according to a team led by Christopher Glein of the Southwest Research Institute in San Antonio, Texas, in an upcoming issue of the journal Icarus.
“We’ve developed something we call the ‘gigantic comet model’ of pluto ,” explains Glein. At the center of the research was the nitrogen-rich ice sheet in the equatorial region of Sputnik Planitia, which occupies the western half of Pluto. “We found a striking coincidence between the amount of nitrogen inside this ice sheet and the mass of nitrogen that would be expected if the dwarf planet were the result of an agglomeration of one billion comets or other Kuiper belt objects,” says Glein.
As the basis of this model, the researchers chose the chemical composition of the Rosetta comet 67P / Churyumov-Gerasimenko. As an alternative to the cometary thesis, Glein and his colleague Hunter Waite Jr., also from the Southwest Research Institute, investigated the possibility that Pluto was formed from very cold ice. According to this so-called “solar model”, the dwarf planet would have parallels to the chemical composition of the sun.
Important for the new comet model is not only the current nitrogen distribution on Pluto, but also how much of the volatile element has possibly escaped from the ice and the atmosphere into space. For a reasonably consistent picture of the geochemical conditions on Pluto, the relationship between carbon monoxide and nitrogen is also important.
The analysis of available data from the distant dwarf planet ultimately produced the evidence for a small amount of carbon monoxide, which suggests, according to the researchers, that liquid water once existed there as well. “Our findings provide plausible evidence that Pluto’s original chemical composition, a legacy of his comet building blocks, was caused by liquid water, possibly even an underground ocean,” says Glein, though adding that the solar model would explain certain phenomena.
“All in all, there are still many questions left,” say the scientists. “Our research into the origin and evolution of Pluto builds on the great successes of the New Horizons and Rosetta missions,” says Glein. “Using chemistry as a kind of detective tool, we can trace some of today’s structures on Pluto back to their ancient origins – these details enrich Pluto’s ‘life story’, but also show us how much we do not yet know.”