What do Pluto's heart and a lava lamp have in common?

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The northern border of Sputnik Planum(Credit: NASANASA)

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Using data captured by NASA's New Horizons spacecraft, a team of researchers from Purdue University (PU) has estimated the depth of the vast nitrogen ice pool believed to exist beneath the Sputnik Planum region on the dwarf planet Pluto. Located in the dwarf planet's heart-shaped feature, the region is thought to continuously refresh its surface through a process known as convection.

Images of Pluto's "heart" grabbed the attention of astronomers and the general public alike as the New Horizons spacecraft prepared for its high-velocity flyby. In the months following the pass as the probe transmitted vast quantities of data and images, the enigmatic nature of the region now informally named Sputnik Planum moved into focus.

It was discovered that a broad swathe of the vast nitrogen plain was broken up into cellular divides. The distinctive patterns are believed to have formed via a form of convection which essentially causes this region to behave much like a lava lamp. As nitrogen ice on the surface of the plain cools, it recedes, only to be replaced by a rising blob of nitrogen that has been warmed by Pluto's meagre internal heat source.

As the centre of the cells fall away, the points at which three of more cells converge are often pinched off, leaving "x" and "y" shapes in the terrain. According to data returned by New Horizons, the cells boast a diameter between 20 – 30 km (12 -19 miles) , and are at their thickest in the centre, proceeding to taper off at the edges.

The convection process is able to occur thanks to the relative structural weakness of the nitrogen ice that composes the plain. The majority of Pluto's surface is made up of water ice, which is rendered solid in the dwarf planet's fridgid environment. An average surface temperature of 300 ºF (149 ºC) below zero allows water to build and form impressive, rugged mountain ranges, while nitrogen ice remains relatively malleable.

Data from new horizons can only tell us about the uppermost layer of Pluto's surface. Therefore, to determine the depth of the Sputnik Planum basin, the team from PU looked to the floating hills of water ice that are drawn along in the plain's powerful convection currents.

The cellular divides are believed  to be the result of a process known as convection(Credit: NASA)

The team reasoned that the reservoir must be deep enough to allow the icebergs to float along with the current without scraping the bottom. Therefore, by estimating the size of the icebergs, the team could also estimate the minimum size of the basin. As is the case with icebergs on Earth, only the tip of the structure crests above the surface.

To create a conservative estimate of the basin's depth, the team approached the water icebergs as if they were spherical in shape, as such a body would only require a shallow body of liquid in which to float. According to their calculations, the researchers estimate based on the visible mass of the icebergs that the reservoir must have a minimum depth of 5 km (3 miles)

A separate estimate of the basin depth predicated on the known ratios of the width to depth of individual convection cells suggest that the nitrogen pool boasts a minimum depth of 10 km (6 miles). Either way, the research hints at a basin depth far larger than the 500 m (1,640 ft) minimum ice depth needed for the convection process to occur.

The convection cells are thought to "turn over" at a rate of only 2 cm a year, with each cell recycling its surface once every 500,000 years. Whilst this may seem an eternity from our perspective, it explains the relative youth of Sputnik Planum, which according to the PU model should be no more than a million years old. This explains the near complete lack of impact craters and other blemishing features.

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