Pluto’s dark side spills its secrets – including hints of a hidden ocean

A splashy start

When Clyde Tombaugh, a young astronomer at Lowell Observatory in Flagstaff, Arizona, discovered Pluto 90 years ago, it was a speck of light barely visible to the largest ground-based telescopes. It wasn’t until 1988 that a chance alignment gave astronomers a lucky break, when Pluto happened to cross in front of a distant star.

As the starlight filtered through Pluto’s atmosphere, scientists were able to disentangle the molecules there (including nitrogen, carbon monoxide and methane). Then in 1996, the Hubble Space Telescope helped scientists to finally see surface details at a resolution of 500 kilometres. Although fuzzy, the images revealed a world — at that time still defined as a planet — that had more large-scale contrast than any other in the Solar System, except Earth.

A mosaic of images shows the parts of Pluto captured in high resolution, called the near side, and the lower resolution ‘far side’. Credit: NASA/JHUAPL/SwRI.

A mosaic of images shows the parts of Pluto captured in high resolution, called the near side, and the lower resolution ‘far side’. Credit: NASA/JHUAPL/SwRI.

It was a tantalizing hint that suggested Pluto might be a dynamic world — and was quickly verified in July 2015 when New Horizons famously spotted a heart-shaped feature just north of the near side’s equator. Within the heart’s ‘left ventricle’ is Sputnik Planitia — an icy basin, churning and flowing with massive glaciers, that scientists now know exerts an extraordinary influence over Pluto’s activity. As sunlight warms the frozen plain, a pulse of ice sublimates into vapour that wafts upwards, before dropping back down at the end of the day. The heart might have even knocked Pluto on its side.

Shortly after the first images of the near side arrived at Earth, Francis Nimmo, a planetary scientist at the University of California, Santa Cruz, and his colleagues realized that Sputnik Planitia was in a strange place: it is aligned almost exactly opposite Pluto’s largest moon, Charon. It could be an accident, but the likelihood of that is a mere 5%. Instead, models suggest that when the basin formed, an underground ocean began to well up into the chasm. Afterwards, nitrogen gas in Pluto’s atmosphere condensed and froze in the frigid basin. The weight of the new water and ice created a heavy load that tipped Pluto into its current alignment².

The idea of a subsurface ocean has existed for some time, but the far-side images have helped to bolster this idea. Some of the strongest evidence comes from a feature known as chaotic terrain — a muddled mess of ridges, cracks and plains on the exact opposite side of Pluto from Sputnik Planitia (see ‘Coming into focus’).

A close-up of Pluto’s chaotic terrain on its far side. Credit: NASA/JHUAPL/SwRI

A close-up of Pluto’s chaotic terrain on its far side. Credit: NASA/JHUAPL/SwRI

Scientists have seen such pairs before — on Mars, Mercury and Europa, one of Jupiter’s moons — where they suspect that a collision from an asteroid or a comet sent seismic waves racing through, and around, the body. Once those quakes converged on the opposite horizon, they tore up the surface in ways resembling what appears on Pluto’s far side.

To test the origins of this chaotic terrain, Adeene Denton, a graduate student in planetary geology at Purdue University in West Lafayette, Indiana, simulated how an asteroid impact would send shock waves across the dwarf planet. The work, which was presented virtually at the Lunar and Planetary Science Conference this March, verifies that such a collision would have created the terrain, but with one caveat: it would have been possible only if Pluto had a 150-kilometre-thick subsurface ocean of liquid water.

Oliver White, a planetary scientist at the SETI Institute in Mountain View, California, who built a geologic map of the far side but was not involved in the study, argues that, although the feature does seem to resemble those seen elsewhere in the Solar System, the resolution in the image is low. And other scientists agree. “We will never know until we go back, someday,” says William McKinnon, a planetary scientist at Washington University in St. Louis, Missouri, who is deputy lead of the New Horizons geology team.

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