Liquid nitrogen seeps onto Pluto, New Horizons data show

Liquid nitrogen appears to be seeping to the surface of Pluto through cracks in the northern edge of Sputnik Planitia, the frozen nitrogen glacier that forms the left lobe of the dwarf planet's heart, according to a study led by the Southwest Research Institute and published in the Planetary Science Journal on July 31.

It is the first evidence of liquid recently flowing anywhere on Pluto.

The lead author is Alan Stern, the principal investigator of NASA's New Horizons mission and an associate vice president at SwRI. "Pluto never stops surprising us," he said, "and this new result certainly does that. In addition to suggesting that liquids have recently expressed themselves on Pluto's surface, it also suggests a new kind of time-variable feature on Pluto."

The images that produced this finding were taken during a flyby on July 14, 2015, and finished arriving on Earth in October 2016. USA Times' count of the interval puts the gap between the last of that data landing and this result being published at nine years and nine months, and eleven years to the day of the encounter itself. The spacecraft has been past Pluto for more than a decade. The data set has not been exhausted.

Why the liquid has to come from below

The physics is the interesting part, because it rules out the obvious explanation before the argument begins.

	Temperature
Surface of Sputnik Planitia	about 40 K (about -233°C)
Nitrogen's melting point	63.15 K (-210.0°C)
Coldest temperature ever recorded on Earth	184 K (-89.2°C), Vostok Station, 1983

Sources: New Horizons surface temperature measurements; nitrogen triple point; World Meteorological Organization record for Vostok, Antarctica, July 21, 1983. Compiled by USA Times.

Pluto's surface is about 23 kelvin colder than nitrogen's melting point, and the atmospheric pressure there is far below nitrogen's triple-point pressure, which means liquid nitrogen cannot exist on the surface at any temperature. Nitrogen there goes straight between solid and gas. As the SwRI release puts it, liquid nitrogen rain on Pluto is physically impossible.

That leaves one place for the liquid to come from. Computer models led by Orkan Umurhan of the SETI Institute show that nitrogen ice at the base of the Sputnik glacier, kilometres down, can melt under the pressure and heat of the overlying ice, and that the resulting liquid can be driven upward through narrow conduits by buoyancy or pressure from below, in the manner of lava or geyser tubes.

Once it reaches the surface, the models find, the melt stays liquid long enough to run downhill across the nitrogen ice before it freezes or sublimes, wetting the surface as it goes.

The evidence is a resemblance, and the comparison is to Greenland

The features themselves were visible in 2015 and 2016: New Horizons images of northern Sputnik Planitia showed city-sized convection cells separated by thin dark lines and broader diffuse dark patches. What is new is the interpretation of what darkened them.

The SwRI team compared those images with NASA Landsat 9 imagery of Earth, including the Greenland ice sheet, where narrow dark surface features mark places that liquid water has wetted the ice and snow. The Pluto features look like the Greenland ones.

This is an argument from morphological analogy rather than from direct detection, and it should be read that way: nobody has measured liquid nitrogen on Pluto, and what the study establishes is that wetting explains the appearance of these features better than the alternatives do. That is a normal and respectable form of planetary evidence, and it is also the kind that later missions sometimes overturn.

The surface being studied is geologically new. "The surface of Sputnik Planitia is quite young, probably less than one million years based on modeling of the surface overturn, and thus these features that we are looking at must have formed since then," said Kelsi Singer, an SwRI principal scientist and co-author. A million years is a rounding error against the solar system's 4.6 billion, which is what makes "recently" a defensible word here.

What this changes about Pluto

Sputnik Planitia is a glacier of frozen nitrogen larger than Texas and Oklahoma combined, and it has been understood since 2015 as a convecting, resurfacing system rather than a static ice sheet. The addition here is a fluid cycle: melt at depth, transport upward, brief flow at the surface, refreeze.

Earlier work, some of it by Stern, had argued for ancient liquid flows on Pluto. The distinction in the new paper is tense. It proposes that there is liquid nitrogen under the glacier now, or was very lately, and that the surface features it produces are time-variable, meaning they may appear and disappear on timescales a future mission could watch.

Singer put the wider case for the work in terms of physical conditions rather than novelty. "Pluto has many unique terrains seen nowhere else in the solar system, and this area of Sputnik Planitia is one of them. Its surface provides a different set of conditions compared to what we are used to on Earth, and exploring that allows us to better understand how materials behave in environments that are difficult to produce on Earth."

The argument this strengthens

Nobody is going back to Pluto soon. No mission is funded, and the flight time is close to a decade even before an encounter is designed. Whatever is learned about the dwarf planet in the next fifteen years will almost certainly be learned from the New Horizons archive.

That is the practical case this study makes: a single 2015 encounter is still generating first-of-its-kind results in 2026, which is an argument about how planetary science budgets should treat archived data rather than an argument about Pluto. Analysis is cheap relative to launch. The instrument that produced this finding has been out of the neighbourhood for eleven years.

New Horizons itself is still operating, deep in the Kuiper Belt, and still returning data.

Cover image: pits and dark features in Sputnik Planitia, photographed by NASA's New Horizons spacecraft. Public domain, via Wikimedia Commons.