When the Cassini mission ended in 2017 after more than a decade studying Saturn, its rings, and its diverse assortment of moons in tantalizing detail, the prospects for outer Solar System exploration were not looking up. The Juno mission had arrived at Jupiter the year before, but there was little else planned, with NASA shifting focus to missions to Mars and small bodies. For the first time in close to half a century the outer Solar System was emptying out of spacecraft, despite the scientific questions left unanswered by Cassini and earlier missions.

Artist’s concept of NASA's Europa Clipper spacecraft. Credit: NASA/JPL-Caltech

Just as the Solar System is divided into two halves by the asteroid belt, so too has been the field of planetary science. The science of the rocky planets is commonly thought akin to the science of our own rocky planet, often conducted by researchers with earth science backgrounds, attending geoscience conferences, and based in geoscience departments. In contrast, the science of the gas giants and their moons has been more closely affiliated with astrophysicists and astronomers. This divide is also seen at Nature, with inner Solar System research usually handled by the Earth team of editors and outer Solar System research usually by astronomy, and is to some extent reflected in the range of planetary science research published in Nature Geoscience versus Nature Astronomy. That such a division of sorts evolved is not surprising: a geologist has walked on the Moon, but much of what we know about the outer planets is from telescopes and spacecraft remote sensing.

Yet, as outer Solar System bodies have come into focus at increasing resolution, new expanses of geoscience have opened up to understand the formation, composition, evolution, and environment of these far-off worlds. Two papers in this issue exemplify the range of geoscience to be found in the outer Solar System, with familiar processes operating under alien parameter ranges.

Pluto may no longer be considered an official planet, but when the New Horizons mission flew past almost a decade ago, the images showed it to be most certainly planetary in its geology1. An Article by Denton et al. explores the formation of Pluto and its largest moon, Charon. Like the Earth–Moon system, it is thought that Pluto and Charon share an origin in a giant impact. But, instead of Charon forming from debris like in Moon-formation models, the researchers suggest a collisional scenario that could result in the capture of Charon by Pluto relatively intact — suggesting a very different geological evolution for this system than our own.

Even amidst the rings of Saturn geoscience processes are to be found. An Article by Hyodo et al. weighs in on the debate over the age of Saturn’s rings by examining how readily the icy ring particles are dirtied by collisions with non-icy micrometeoroids. The researchers find that the vapourized products of micrometeoroid impacts are mostly removed under the gravitational and electromagnetic fields of Saturn, with little material deposited on the ring particles. Thus, due to this self-cleaning surface process, the relatively bright rings aren’t necessarily young in age.

With the end of the Cassini mission and funding shifting away from the outer Solar System, it is not often that we have two outer Solar System papers in an issue. This is not due to a lack of interest: our journal seeks to bridge, not divide, the Solar System. The same goes for our sibling journal Nature Astronomy. Nature Geoscience and Nature Astronomy both publish planetary science from across the Solar System, the main difference being that the readerships target the broader geosciences and space sciences communities respectively. The journals’ scopes do not aim to propagate historic divides in the field between the inner and outer planets, but intentionally overlap.

Fortunately, the outer Solar System has not been abandoned by exploration and a new era of outer planet research is ramping up. The Juno mission has been extended and remains in orbit. ESA’s Jupiter Icy Moons Explorer (JUICE) mission launched in 2023 and will study Ganymede, Callisto, and Europa. NASA’s Europa Clipper mission launched in October and aims to determine whether habitable conditions for life exist beneath Europa’s ice. NASA’s Dragonfly mission to Saturn’s moon Titan is slated for 2028. Meanwhile the planetary community continues to push urgently for a mission to Uranus2 that would give insight into the formation and evolution of the little-studied ice giant planets.

These new missions are not arriving to their destinations until the 2030s and beyond, but they are all motivated by science objectives rooted in geoscience. Whether focused on our planet or Pluto, we welcome top geoscience research from anywhere in the Solar System.