Bengaluru: Images taken by the New Horizons spacecraft show that cryovolcanoes — which spew ice, water, and ices of gases like methane, instead of lava — have been active in the recent geological past, on a unique and large scale, on Pluto. The ice-volcanism likely occurred within the last 100 million years or so, and might still be happening.
Nearly seven years after the NASA craft flew past Pluto, in 2015, researchers have processed part of the massive volume of images and data pertaining to the surface. The team from the Southwest Research Institute in Boulder, Colorado, published their findings in the journal Nature Communications Tuesday.
The team found a large field of bumpy volcanoes rising up to various heights. As there are no impact craters nearby, unlike much of the rest of the dwarf planet, they concluded that this surface was formed after a geologically recent upheaval — definitely within the past 2 billion years.
A mountainous feature called Wright Mons, which is around 150 km wide and juts up nearly 5 km above the surface, has a central depression that is nearly 50 km across.
Another mound called Piccard Mons rises nearly 7 km high and has a width of around 225 km. The scientists conclude that this volcano’s total volume is similar to that of Hawaii’s Mauna Loa, one of the biggest volcanoes on Earth, despite the fact that Pluto’s diameter is one-sixth that of Earth.
Recent volcanism in geological terms would imply that Pluto’s core would still have residual heat from its formation, or would have retained more heat than previously believed.
The researchers analysed a large area to the south-west of the Sputnik Planitia ice sheet, which is the lighter region on the left in the now-famous heart-shaped feature on Pluto. The sheet covers an ancient impact basin that’s nearly 1,000-km wide, and is characterised by folds and rises, much like wrinkles, in the large accumulation of ice.
There are several volcanic domes in this large area, with many appearing to merge into a bigger dome. The researchers speculate that the creation of this terrain would have required multiple eruption sites located near each other, ejecting large quantities of material to create cryovolcanic eruptions that coated the region with layers upon layers of ice.
Analysis of the data also reveals that the material on the surface is primarily water-ice and not nitrogen- or methane-ice, as found in other, younger parts of Pluto’s surface, thus confirming cryovolcanic activity.
Ocean under icy crust
Pluto’s surface is on average nearly -240°C, with rock-hard ice that doesn’t melt easily so far from the Sun and other sources of heat.
A common source of heat in other geologically active bodies in the solar system, such as Jupiter’s volcano-covered moon Io, is the tug-and-pull of other gravitationally strong bodies, which flexes the interior, heating it and leading to volcanism.
But Pluto’s neighbours aren’t strong enough to generate such tidal forces and warm the interior of the dwarf planet, and the amount of rocky material inside its core is insufficient to generate heat from radioactivity.
The authors speculate that Pluto would have trapped the heat left over from its own formation, likely in a deep liquid water ocean under the icy crust. This is consistent with previous research that has alluded to the existence of a global subsurface ocean.
Other icy bodies in the solar system, such as Saturn’s moon Enceladus and Jupiter’s moon Europa, also contain such oceans. Enceladus also shows evidence of regular cryovolcanism, which has been imaged by the Cassini spacecraft, spewing icy material out into space and replenishing one of Saturn’s rings.
However, the phenomenon of mounds of icy volcanoes spread out in a field and merging together to recycle an icy surface has not been observed on any other planet or moon in the solar system.
Since New Horizons made only a flyby, it was able to observe the area just for a day. So, the researchers are unsure if cryovolcanism is a currently active and regular process on Pluto, if these mounds are dormant ice volcanoes that erupt over time, or if ice volcanism there is now extinct.
(Edited by Rohan Manoj)