Where did Pluto get its `icy heart` from? Puzzle solved!

London: In July 2015, NASA reported glimpses of flowing nitrogen ice glaciers on Pluto, which were discovered by New Horizons during one of its flybys.

Pluto's distinctive 'heart' feature which is informally known as Sputnik Planum, has been a subject of interest for scientists ever since.

In June 2016, NASA released an image of the dwarf planet's icy heart likening it to a cosmic 'lava lamp'.

Now, scientists have managed to uncover the origin of the large heart-shaped nitrogen glacier on Pluto!

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.

Along with the nitrogen ice glaciers, NASA also observed an unexpectedly thick layer of haze in the atmosphere.

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.

(With PTI inputs)