Pluto and Charon Geophysics and Climate: Notes from the Conference

“Progress in Understanding thePluto System: Ten Years after Flyby” was a focused, informative conference in which scientists spent four-and-a-half days discussing Pluto in detail, comparing it with other, similar solar system bodies, and celebrating the milestone 10^th anniversary of the 2015 New Horizons flyby.

Alan Stern, New Horizons principal investigator, opened the conference by describing its goal as “bringing together everything we’ve learned about the Pluto system since the flyby.”

Because so much ground was covered during the weeklong, conference, it would be impossible to discuss everything in one entry, which is why I plan to write several for this site.

In the first session, “Pluto after Ten Years: A Holistic View,” discussion centered on Pluto’s climate, geological history, atmosphere, and chemistry. A second session focused on the geology and composition of Pluto’s largest moon and binary companion, Charon.

Throughout the conference, Pluto and its system of moons were compared and contrasted with Ceres, Haumea, Eris, Arrokoth, other Trans-Neptunian Objects and small planets, Neptune’s moon Triton, Saturn’s moons Titan and Enceladus, and Jupiter’s moon Europa.

While little is known about Haumea and Eris, every other one of these worlds is either an ocean world or a world suspected of having a subsurface ocean. This class of objects was relatively unknown just decades ago, but today, these worlds are front and center in the search for microbial life in the solar system.

In the lead up to the flyby, some scientists expected Pluto to be revealed as a geologically dead world, like our moon. Instead, many were surprised to find it, in the words of presenter Oliver White, “a geological wonderland at the edge of the solar system.”

Much focus was placed on Sputnik Planitia, the floating glacier that comprises the left side of Pluto’s “heart” feature, and the repository of the planet’s nitrogen ice. We know Sputnik Planitia is geologically young, has nitrogen ice flowing on its surface, and has no craters.

The flow of ice on Sputnik Planitia is similar to processes on Earth, but with nitrogen ice instead of water.

Pluto’s annual cycle is driven by the planet’s high obliquity or tilt toward the plane in which most but not all of the solar system’s planets orbit. Its north-facing slopes all have small deposits of methane ice. Over thousands of Pluto years, its equatorial regions have received less sunlight than its polar regions.

Pluto’s diverse geology, including bladed terrains, dunes, pitted regions, and even likely cryovolcanoes are the result of both endogenic, or internal, processes, and exogenic, or external ones.

The New Horizons team did not even know that Sputnik Planitia was there when the flyby was planned. Since this region controls almost everything that occurs on the planet, the opportunity to image it in high resolution was extremely fortuitous.

Not being a satellite of a giant planet, Pluto experiences no tidal heating. Its biggest source of energy is radioactive decay of rock.

The ancient impact that formed Charon melted ice, fractured terrain, and created Sputnik Planitia.

While the presence of a subsurface ocean on Pluto remains hypothetical, it is supported by a lot of evidence. Presenter James Tuttle Keane noted that a structure like Sputnik Planitia cannot have been created without an ocean.

Computer models are frequently used in studies to simulate conditions on remote objects like Pluto. Keane noted that “New Horizons triggered a wave of new theoretical models reshaping our understanding of Pluto and worlds beyond.”

Its axial tilt and eccentric orbit give Pluto extreme summers and winters. Its lower latitudes get at least some sunlight every day while its higher latitudes can go for long durations with no sunlight at all. These differing climates produce a variety of landscapes on Pluto’s surface.

But sunlight is not the only energy source on Pluto, which obtains a higher fraction of its energy from internal sources than the Earth does.

New Horizons found that Pluto’s atmosphere is escaping into space at a much lower rate than expected.

Much discussion centered on early migration of solar system planets. Neptune is believed to have formed closer to the Sun only to subsequently migrate outward. Now located at 39 AU (astronomical units, with one AU equal to 93 million miles or the average Earth-Sun distance), Pluto may have formed at 25 AU and then been pushed outward by Neptune.

Data obtained when various small planets occulted (passed in front of) a star indicate that Pluto, Charon, Haumea, Triton, and Quaoar all have similar densities.

“These objects are not iceballs,” emphasized presenter Bill McKinnon, a point that supports their classification as small planets. This is significant in light of the fact that the media often erroneously lump dwarf planets and tiny Kuiper Belt Objects (KBOs) in one broad category.

And KBOs, even those too small to be round, are not just giant comets—they are far more active than comets are.

Pluto’s atmosphere is similar to that of Saturn’s moon Titan, which itself is sometimes viewed as an analogue of early Earth. Its surface ices are methane, nitrogen, and carbon monoxide. Photochemical organic aerosols in Pluto’s atmosphere create its layered hazes, which extend to an altitude of more than 217 miles.

 Both Pluto and Triton, Neptune’s large moon that is believed to have once orbited the Sun on its own only to be captured by Neptune, look very different from comets. Rather than being frozen relics, both have interior energy sources. Because Triton orbits Neptune, a giant planet, it experiences far more tidal heating than Pluto.

Pluto and Triton have low levels of carbon monoxide in their surface ices. Data collected by the James Webb Space Telescope (JWST) show Eris and Makemake also lack surface carbon monoxide. As neither orbit a giant planet, their surfaces may be more like that of Pluto than that of Triton.

Since Triton is believed to have originated in the Kuiper Belt, all of this is evidence that supports classing these objects as a new subclass of planets, similar to one another but different from comets.

The distribution of ices on Pluto’s surface varies with its terrains. Nitrogen frost is found at the bottom of craters. Bright and dark regions on Pluto have different ice compositions.

Data collected by New Horizons’ MVIC and LEISA instruments was used in 2023 to create spectral maps of Sputnik Planitia, Cthulhu Macula, and Lowell Regio, regions on Pluto’s surface.

A global topography map of Pluto created in 2018 used data from New Horizons and stellar occultations to depict the distribution of volatiles, chemical elements and compounds that can be easily vaporized.

Three regions on Pluto—Kiladze, Viking Terra, and Virgil Fossae—could be cryovolcanoes. These resemble structures on Mars known to be cryogenic caldera, or depressions formed by cryovolcanism.

While Pluto’s topography is current, Charon’s is ancient. Charon may have experienced tectonics, processes that shape and create a planet’s crust, early in its history. While it may have once had a subsurface ocean, that has long since frozen solid.

Progress in Understanding the Pluto System: Ten Years After Flyby

 

It is almost impossible to believe, but tomorrow marks the 10^th anniversary of New Horizons’ historic Pluto flyby. To commemorate the anniversary and reflect on what has been learned about Pluto in the decade that has passed, the Universities Space Research Association’s (USRA) Lunar and Planetary Institute (LPI) is holding a weeklong conference titled “Progress in Understanding the Pluto System: Ten Years Since Flyby” from July 14-18 at the Johns Hopkins University Applied Physics Laboratory (JHUAPL), mission headquarters, in Laurel, Maryland.

The four-and-a-half-day conference offers an option to attend virtually, which I will be doing this week, and reporting here on the talks. Topics to be addressed include Pluto’s volatile ices, its geology and geophysics, its climate history, its atmosphere, Charon’s geology and craters; the comparative planetology of dwarf planets and Kuiper Belt Objects; history of the New Horizons mission; future exploration of Pluto and the Kuiper Belt; the origin of Pluto and its moons, and more.

On the afternoon of Thursday, July 17, presentations will address possible return missions to Pluto, including Persephone, a proposed Pluto orbiter and Kuiper Belt explorer, and the Gold Standard Mission, also a Pluto orbiter proposal and extended Kuiper Belt exploration mission.

While these are currently only in the concept stages, they matter because the New Horizons flyby raised more questions than answers, leading scientists to recognize the need for a follow up orbiter mission.

A Pluto day, one rotation of the planet on its axis, takes 6.4 Earth days. As a flyby mission, New Horizons did not have the time to study Pluto for a full rotation, as the spacecraft could not brake and slow down to observe the planet. Therefore, only one hemisphere of Pluto was imaged and studied in high resolution. The other hemisphere was imaged only on approach in low resolution, meaning less is known about it. An orbiter would reveal the secrets of that hemisphere as well as provide additional data about the side of Pluto that was explored in high resolution.

While the current political climate may not be conducive to spending money on new planetary missions, this could very well change with a different administration, so it is important to develop and study possible plans for a follow up mission.

Here, you can find a program of the conference talks along with links to the abstracts of those talks.