Terra Incognita

There is a long list of things that we don’t know about these local ice giants and what they have to tell us about our solar system. Among the big questions, we seek to understand how the planets formed and why they have their current orbits. For example, some dynamicists suspect that Uranus and Neptune formed much closer to the Sun and then migrated outward due to countless gravitational interactions with small bodies in the primordial solar nebula.

Likewise, we seek to understand the configurations of extrasolar planet systems, many of which look very different from ours, with planets tightly packed close to their stars.

Hubble and ground-based telescopes can help us monitor the changing clouds on Uranus and Neptune, but to figure out which models of solar system formation are closest to the truth, we need to follow up the reconnaissance of Voyager 2 with visits by modern spacecraft.

For example, the nebula from which the Sun and planets formed had compositional gradients that changed as material was swept up or blown off, such that each planet’s unique composition pinpoints when and where it formed. While some constituents can be detected remotely, only an atmospheric probe can measure the noble gases (helium, neon, argon, xenon, and krypton) in each planet’s atmosphere, as these do not chemically react or change over time.

The addition of a satellite system that formed in place (Uranus) and one that was captured (Neptune) further constrains the gravitational interactions that occurred early on among the outer planets. Orbiting spacecraft could scrutinize more regions of these satellites, recording crucial details of geologic features as well as surface composition.

As we strive to understand the hundreds of like-sized extrasolar planets, we need detailed understanding of atmospheric processes on our own planets. Uranus and Neptune provide especially important cases for understanding atmospheric dynamics in cold atmospheres and over a range of seasonal extremes. Do the heat balances of Uranus and Neptune vary over time? Are the emissions from their interiors actually more similar than indicated by Voyager 2? To get answers, we need additional measurements of the reflected (illuminated hemisphere) and emitted (unlit hemisphere) power for each of these planets.

Even better would be detailed measurements of interior structure to understand atmospheric layering and how that affects convection of heat upward from the deep interior. This can be achieved by using orbiters to map the gravity fields close to each planet, as well as by probing deep beneath the cloud tops using radar, microwave sounding, and possibly even Doppler seismology. Atmospheric probes can measure how the temperature, pressure, and winds in these atmospheres change with altitude. This is crucial for knowing how the clouds we observe from Earth fit in global circulation models.

We also seek to understand how ocean worlds—large moons with vast subsurface reservoirs of water—came to exist and how common they may be in our solar system or elsewhere. The discoveries of subsurface oceans on Europa, Ganymede, and Enceladus provide new potential habitats for life. Does Triton also conceal an ocean under its frozen crust? Spacecraft passing near other large moons could map surface features, magnetic field deflections, and maybe even active geysers and plumes, any of which could indicate large liquid reservoirs hidden below the surface.

New missions to Uranus and Neptune can provide timely answers to many of our pressing scientific questions. The ideal scenario involves sending orbiters to both planets that could dispatch instrumented probes to plunge into their atmospheres and obtain critical measurements. If such missions were still active in 2050 (at Uranus) and 2046 (at Neptune), they’d see equinoxes at both planets, with the shifting of seasons and illumination of both poles of both the planets and their moons.

Other bodies could become additional targets of potential ice giant missions. For example, a Neptune orbiter with probe(s) could maximize its science return by also flying past a Centaur asteroid en route and then exploring Triton (a captured Kuiper belt object) after its arrival. A spacecraft heading outward to conduct a Kuiper belt flyby tour could first go past Uranus and deliver an atmospheric probe along the way.

For now, such missions are little more than engineering concepts, but that doesn’t diminish their scientific potential. Let’s go explore the ice giants in our solar system and see what secrets they hold!

What does "Ice" mean?