What do Pluto's heart and a lava lamp have in common?
Using data captured byNASA's New Horizons spacecraft, a team of researchers from PurdueUniversity (PU) has estimated the depth of the vast nitrogen icepool believed to exist beneath the Sputnik Planum region on the dwarfplanet Pluto. Located in the dwarf planet's heart-shaped feature, theregion is thought to continuously refresh its surface through aprocess known as convection.
Images of Pluto's"heart" grabbed the attention of astronomers and thegeneral public alike as the New Horizons spacecraft prepared for itshigh-velocity flyby. In the months following the pass as the probetransmitted vast quantities of data and images, the enigmatic natureof the region now informally named Sputnik Planum moved into focus.
It was discovered thata broad swathe of the vast nitrogen plain was broken up into cellular divides. The distinctive patterns are believed to have formed via aform of convection which essentially causes this region to behavemuch like a lava lamp. As nitrogen ice on the surface of the plaincools, it recedes, only to be replaced by a rising blob of nitrogenthat has been warmed by Pluto's meagre internal heat source.
As the centre of thecells fall away, the points at which three of more cells converge areoften pinched off, leaving "x" and "y" shapes inthe terrain. According to data returned by New Horizons, the cellsboast a diameter between 20 – 30 km (12 -19 miles), and are at their thickest in the centre, proceeding to taper off atthe edges.
The convection processis able to occur thanks to the relative structural weakness of thenitrogen ice that composes the plain. The majority of Pluto's surfaceis made up of water ice, which is rendered solid in the dwarfplanet's fridgid environment. An average surface temperature of 300 ºF (149 ºC) below zero allows waterto build and form impressive, rugged mountain ranges, while nitrogenice remains relatively malleable.
Data from new horizonscan only tell us about the uppermost layer of Pluto's surface.Therefore, to determine the depth of the Sputnik Planum basin, theteam from PU looked to the floating hills of water ice that are drawnalong in the plain's powerful convection currents.
The team reasoned thatthe reservoir must be deep enough to allow the icebergs to floatalong with the current without scraping the bottom. Therefore, byestimating the size of the icebergs, the team could also estimate theminimum size of the basin. As is the case with icebergs on Earth,only the tip of the structure crests above the surface.
To create aconservative estimate of the basin's depth, the team approached thewater icebergs as if they were spherical in shape, as such a bodywould only require a shallow body of liquid in which to float.According to their calculations, the researchers estimate based onthe visible mass of the icebergs that the reservoir must have aminimum depth of 5 km (3 miles)
A separate estimate ofthe basin depth predicated on the known ratios of the width to depthof individual convection cells suggest that the nitrogen pool boastsa minimum depth of 10 km (6 miles).Either way, the research hints at a basin depth far larger than the500 m (1,640 ft) minimum ice depthneeded for the convection process to occur.
The convection cellsare thought to "turn over" at a rate of only 2 cm a year,with each cell recycling its surface once every 500,000 years. Whilstthis may seem an eternity from our perspective, it explains therelative youth of Sputnik Planum, which according to the PU model should be no more than a million years old. Thisexplains the near complete lack of impact craters and otherblemishing features.
Source: Purdue University