Origin of Pluto's huge 'icy heart' found

Scientists have unveiled the origin of the large heart-shaped nitrogen glacier on Pluto spotted by NASA's New Horizons spacecraft during its historic flyby of the icy-dwarf planet last year. Researchers from the Laboratoire de meteorologie dynamique in France show that Pluto's peculiar insolation and atmosphere favour nitrogen condensation near the equator, in the lower altitude regions, leading to an accumulation of ice at the bottom of Sputnik Planum, a vast topographic basin.

Through their simulations, they also explain the surface distribution and atmospheric abundance of other types of volatiles observed on Pluto. Among the types of ice covering Pluto's surface, nitrogen is the most volatile: when it sublimes (at minus 235 degrees Celsius), it forms a thin atmosphere in equilibrium with the ice reservoir at the surface. One of the most unexpected observations from New Horizons, which flew by Pluto in July last year, showed that this reservoir of solid nitrogen is extremely massive, and mostly contained in "Sputnik Planum", a topographic basin located within the tropics of Pluto, researchers said.

Methane frost also appears all over the northern hemisphere, except at the equator, while carbon monoxide ice in smaller amounts was only detected in Sputnik Planum. Until now, the distribution of Pluto's ice remained unexplained. To better understand the physical processes at work on Pluto, the researchers developed a numerical thermal model of the surface of the dwarf planet able to simulate the nitrogen, methane and carbon monoxide cycles over thousands of years, and compared the results with the observations made by the New Horizons spacecraft.

Their model shows that the solid-gas equilibrium of nitrogen is responsible for trapping ice in Sputnik Planum. At the bottom of the basin, the pressure of the atmosphere - and therefore of gaseous nitrogen - increases, and the corresponding frost temperature is higher than outside the basin, allowing nitrogen to preferably condense into ice. Simulations show that the nitrogen ice inevitably accumulates in the basin, thus forming a permanent nitrogen reservoir, as observed by New Horizons.

The numerical simulations also describe the methane and carbon monoxide cycles. Because of its volatility similar to that of nitrogen, carbon monoxide ice is entirely sequestered with nitrogen in the basin, in keeping with the New Horizons measurements. Regarding the methane ice, its lower volatility at the temperatures prevailing on Pluto allows it to exist elsewhere than in the Sputnik Planum glacier. The model shows that pure methane ice seasonally covers both hemispheres, in agreement with New Horizons data.

This scenario shows that there is no need for an internal reservoir of nitrogen ice to explain the formation of the Sputnik Planum glacier, as suggested by previous studies. The research was published in the journal Nature.