Pluto: dazzling, dynamic, and so not dead

Abstract

Pluto is not a frozen wasteland. It is very cold at  ~ 40 K, but exhibits active geology and a dynamic atmosphere. Pluto is a beautiful example of active icy worlds we should expect to see elsewhere in the trans-Neptunian region.

Discovered in 1930, Pluto remained an under-studied mystery, hampered by technological limitations on observing the distant body, for many decades. During these Pluto “dark ages,” some astronomers suggested that Pluto might not be alone in the outer solar system (e.g., ref. ¹), a prediction that proved correct with the later discovery of thousands of Trans-Neptunian Objects (TNOs). The 2015 flyby by the New Horizons spacecraft, as well as subsequent Earth-based observations, computer modeling, and geologic inferences have revolutionized our understanding of Pluto, showing that a plethora of active processes are ongoing on the icy world. The time (and technology) is now here to search for similar active processes on Pluto’s neighbors as well.

Active processes abound on Pluto

Some of the most striking images returned by the New Horizons mission were those showing the polygon-marked surface of Pluto’s enormous ice sheet, Sputnik Planitia. Sitting inside what is likely an ancient impact basin², Sputnik Planitia is a kilometers-thick sheet of solid nitrogen ice (with some contribution of CO and CH₄³) that is thought to be undergoing solid-state convection⁴, erasing any impact craters that may form on its smooth surface⁵. Under this hypothesis, the dark polygon outlines demarcate convection cells, analogous the bubbles in a pot of boiling water, which sluggishly refresh the surface over  ~ 500,000 years⁴.

On the boundary of Sputnik Planitia, there appears to be evidence for nitrogen-ice glaciers flowing down valleys and pouring into the large ice sheet^6,7. Dark streaks trace flowlines from highlands east of Sputnik, down through narrow valleys, and terminate in fan-shaped deposits of ice. In other regions, Howard et al.⁶ interpreted dendritic topographic depressions as ancient glacial valleys, carved by nitrogen ice that has since been carried away, indicating that glacial activity could have been ongoing in Pluto’s past as well as its present. In addition to geologic evidence of glaciers, climate models have suggested a cycle of ice subliming from the northern edge of Sputnik Planitia and recondensing in the south, driven by solar insolation and counteracted by glacial flow which acts to keep the ice sheet’s surface level⁸. Over one Pluto day (about a week for us on Earth), approximately a millimeter of nitrogen ice sublimes from the north and recondenses at the southern edge of Sputnik Planitia; integrated over the past 2.8 million years, this results in a kilometer of ice transport⁸. Nitrogen can be considered a “hypervolatile" ice, meaning that even at Pluto’s frigid  ~ 35-40 K (−238 to −233 °C) temperature, it remains soft enough to flow over these timescales⁷ and exhibits thermally-driven sublimation which responds to diurnal, annual, and longer timescale changes to the received sunlight^8,9.

Singer et al.¹⁰ suggests some features on Pluto’s surface may be cryovolcanic in nature, meaning they are created when mobile material is extruded from the subsurface and covers existing surface materials. Pluto’s form of cryovolcanism may not be as explosive as Enceladus’ plumes, but nonetheless the hummocky terrain near Wright, Piccard, and Coleman Mons seems to be best explained as a cryovolcanic construction formed sometime in the past 1–2 billion years¹⁰. Elsewhere on Pluto, several graben (a trench-shaped depression) and even a putative caldera show spectral evidence of water ice, with or without ammoniated species (e.g., ref. ¹¹), which are also interpreted to be cryovolcanic in nature. Ammoniated species are thought to be destroyed over time on Pluto’s surface, so their detection at these features suggests an age for the eruptions of less than one billion years¹².

Pluto’s atmosphere may be relatively thin at 0.001% of Earth’s¹³, but it is still sufficient for active weather and climate phenomenon, such as: the production and fall-out of haze particles that color Pluto’s surface^14,15; winds on the order of 1 m/s¹⁶ that potentially leave behind dark wind streaks on the surface¹⁷; and a global volatile cycle in which nitrogen ice forms in topographic depressions and methane ice paints the mountain tops^18,19, shown in Fig. 1. The surface pressure is not constant over Pluto’s 250-year orbit; instead it varies from  ~ 0.01–12 microbar, depending on the current insolation pattern onto Pluto’s nitrogen ice deposits. This variation is suggested by models^8,9,20. Changes have also been seen in the history of observations (ref. ²¹, and references therein], especially a type of ground-based observation called an occultation, in which Pluto’s atmosphere is observed as it passes in front of a background star. Models suggest that Pluto’s atmosphere is currently near its peak pressure, and sometime in the next decade we can expect to see a sharp decrease in surface pressure as Pluto shifts from summer to winter in its northern, volatile-covered hemisphere. Stay tuned!

Fig. 1. Examples of active processes on TNOs.

A Bright methane frost covering the tops of water-ice Pigafetta Montes on Pluto. This bears resemblance to snow-capped peaks on terrestrial mountains, but forms through a different process. Adapted from Bertrand et al.¹⁹. B Schematic illustration of a possible surface activity mechanism on a TNO like Eris or Makemake. Reprinted from Icarus, 412, Christopher R. Glein, William M. Grundy, Jonathan I. Lunine, Ian Wong, Silvia Protopapa, Noemi Pinilla-Alonso, John A. Stansberry, Bryan J. Holler, Jason C. Cook, and Ana Carolina Souza-Feliciano, Moderate D/H ratios in methane ice on Eris and Makemake as evidence of hydrothermal or metamorphic processes in their interiors: Geochemical analysis, 115999, Copyright (2024), with permission from Elsevier.

A search for processes actively occurring during the New Horizons flyby did not find any detectable changes on Pluto’s surface over timescales of  ~ one week²². Despite this, it is clear from the examples above that Pluto is not a dead, dormant ice world and instead has active processes occurring over time periods of a few weeks to a billion years.

Open questions and how to answer them

Despite excellent scientific data from the New Horizons mission and other studies in the recent past, there remain many open questions related to Pluto. One large area of uncertainty is the geology and appearance of the non-encounter hemisphere (the half of Pluto that was facing away from the spacecraft during the New Horizons mission)²³. Low resolution images from the approach phase of the mission have been interpreted to show evidence for antipodal terrain created by the Sputnik-Planitia forming impact²⁴, and there is evidence for a continuation of the bladed terrain (characterized by methane-rich ice formations) onto the far side²³, but definitively answering the question of active processes on Pluto’s other half will have to wait for the next spacecraft mission to Pluto²⁵.

Similarly, much of Pluto’s southern hemisphere was experiencing polar darkness (analogous to the seasonal periods of 24-hour darkness experienced inside of Earth’s Arctic and Antarctic circles) at the time of the flyby and was thus unobservable, so there is no recent data regarding the appearance, composition, or topography south of  ~ 30^∘S. Our knowledge of this region is limited to what can be gleaned from lower-resolution HST²⁶ and mutual event observations²⁷ taken around the time of Pluto’s previous equinox in 1987, which show tantalizing evidence for a bright south pole, suggestive of seasonal refreshing of the surface though some process. The southern region of Pluto will remain terra incognita until the next equinox occurs in 2108.

In addition to these unknown surface terrains, the interior structure of Pluto is still unknown. In general terms, Pluto is though to have a dense rocky core with an overlying water-ice shell, which is indicated by the bulk density of 1854  ± 11 kg/m³²⁸, which lies between that of solid rock ( ~ 3000 kg/m³) and pure water ice ( ~ 1000 kg/m³). The authors of this Comment are convinced that the combination of theoretical thermal evolution models, possible cryovolcanic features, and young extensional tectonic features provide sufficient evidence for Pluto having had a subsurface ocean at some point, although the jury is still out on if that ocean persists to the present day [²⁹, and references therein]. Interior structure is best revealed through gravity science experiments, in which the gravitational field of a body is determined by measuring how an orbiting spacecraft is pushed and pulled by the underlying mass distribution. Thus, we withhold judgment on the question of Pluto’s interior structure until a follow on orbiter mission can be sent²⁵.

In regards to Pluto’s atmosphere, open areas of research include continuing to monitor the atmospheric pressure and structure from occultations over the next decades, as well as working to understand the origin and evolution of Pluto’s volatile inventory^30,31, which is the source of the atmosphere. The newly-commissioned JWST has aided this latter effort, providing observations with implications for the composition of Pluto’s atmosphere³², the thermal properties of the surface ices³³, and the D/H ratio, which holds implications for atmospheric loss and the chemical origin of ice on Pluto and other TNOs³². Critical laboratory work is also ongoing to further our understanding of Pluto’s hazes (e.g., ref. ³⁴) as well as the behavior of nitrogen, methane, and carbon monoxide ice mixtures at Pluto-relevant temperatures³⁵, both of which aid interpretation of telescopic observations.

Pluto: the gateway to the third zone

Pluto is not alone in its neighborhood; it is just one of thousands of icy worlds, known as Trans-Neptunian Objects (TNOs), in the outskirts of the solar system. TNOs can be thought of as leftover crumbs from the planet formation process. They tend to be smaller than Pluto (Eris is the largest, with a radius of 1163  ± 6 km, ref. ³⁶), volatile rich, and very cold owing to their wide orbits, which have semi-major axes larger than 30 AU. Pluto and Arrokoth are the only TNOs to have been explored up-close (both with New Horizons), but ground- and space-based observations are shedding light on more and more of these distant worlds.

Makemake, about two-thirds the size of Pluto, is also volatile rich and hosts a methane atmosphere (or possibly more precisely an exosphere) even more tenuous than Pluto’s³⁷. Recent observations from JWST also seem to show Makemake may have localized hot-spots on its surface, potentially signaling ongoing cryovolcanism like that seen at Enceladus³⁸. Other JWST measurements also show that Makemake and Eris’ methane inventories appear to originate from chemical reactions within their warm interiors³⁹. A retinue of other smaller bodies also seem to be having their hypervolatiles replenished, potentially from active geological processes⁴⁰. Figure 1 shows a schematic illustration of how such an active process may occur on TNOs.

As well as volatile activity, two large TNOs—Haumea and Quaoar—host ring systems^41,42 that appear to be dynamic, potentially evolving on decades-long timescales⁴³. Such ring systems can only be discovered and observed during stellar occultations. These ring systems are mysterious, provoking questions about if they could be actively replenished from active processes on their hosts’ surfaces (e.g., ref. ⁴⁴). In line with the example set by Pluto, TNO observations are finding dynamic, active surfaces rather than inert, frozen landscapes and we believe that active processes will be the norm, not an exception, on other large TNOs.

It took New Horizons nine years to get to Pluto from Earth, and a further 3.5 years to reach its second flyby target, the TNO Arrokoth. Future missions to TNOs (e.g., ref. ⁴⁵) or their closer-in cousins, Centaurs (e.g., ref. ⁴⁶), will also be hampered by long cruise times; however, the example set by Pluto, in addition to the recent TNO results discussed above, all but guarantees that any TNO mission will reveal these distant bodies to be active, exciting, and worthy of study, just like Pluto.

Author contributions

P.E.J. wrote the initial article draft, led the revision, and managed the article submission process. B.P. contributed text to the TNO section and assisted with editing both the initial draft and the revision.

Peer review

Peer review information

Nature Communications thanks Kelsi Singer for their contribution to the peer review of this work.

Competing interests

The authors declare no competing interests.