How Uranus and Neptune are key to unlocking…

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Clues to the origin and evolution of giant planets lie in the specific elements that make up their atmospheres, ones other than the hydrogen they amply inherited from the Sun’s disk. For example, noble gases like helium undergo very few chemical reactions inside giant planets so measuring their abundance compared to other gases will tell scientists how and where each planet acquired its heavy elements over time.

The trouble is that other than the direct gas measurements from inside Jupiter’s atmosphere by NASA’s Galileo probe in 1995, we lack such information on Saturn, Uranus and Neptune. This is particularly pressing for the ice giants because their noble gases are the most unaltered reflections of the planet-forming materials in the early outer solar disk. And yet Neptune and Uranus are the least-explored planets of our solar system, only flown past once by NASA’s Voyager 2 spacecraft in the last century, which also flew by Jupiter and Saturn.

Sending spacecraft to Jupiter and Saturn again has already proved to be world-changing – literally. The most recent gravity data from NASA’s Juno spacecraft, which entered Jovian orbit in 2016, provided evidence that Jupiter does not have a distinct rock-metal core. Jupiter’s core is larger and fuzzier than expected, likely caused by the intense gas pressures in its mantle dissolving the core into an exotic substance called metallic hydrogen, or due to Jupiter absorbing a planet with 10 Earth masses during its formation.

Data from NASA’s Cassini mission suggests Saturn has a fuzzy core too, spanning 60% of the planet’s diameter. These discoveries have led scientists to think that Uranus and Neptune might also have large diluted cores. The only way to know is to send a mission to the ice giants.

Upcoming exploration of Uranus and Neptune

The 2023-2032 Planetary Science Decadal Survey – a report produced every 10 years by the US scientific community to guide future NASA missions – recommends sending a spacecraft to Uranus as the highest priority. If commissioned, the Uranus Orbiter and Probe (UOP) mission will measure the planet’s complex and unique magnetic field, map gravity variations and note the nature of its atmospheric wobbles to determine if the planet really sports a fuzzy core and what it is made of. A probe would enter Uranus’ atmosphere and precisely measure gases and their relative abundance.

The UOP mission would thus paint us a clear picture of where and how Uranus formed, and how it subsequently evolved. Because of the two ice giants’ similarity, these insights would also likely apply to Neptune. The mission would also provide us with missing information necessary to understand the migration of the gas giants and the connected evolution of our solar system and early Earth.

In the best-case scenario, UOP could launch on a SpaceX Falcon Heavy rocket in 2031 or 2032 and reach Uranus 12 to 13 years later. There’s strong international interest in the mission too. The 2021 report of ESA’s Voyage 2050 Senior Committee recommends that the agency contributes to an ice giant mission led by an international partner, similar to how ESA provided the Huygens landing probe to Saturn’s moon Titan as part of Cassini.

ESA is also considering a Uranus orbiter mission. China is considering a Voyager-like mission to interstellar space that would launch in 2024 and fly past Neptune in 2038. The spacecraft would include an atmospheric probe. In the meanwhile, observing Jupiter-like and Neptune-like planets in different star systems lets us witness snapshots of planets forming, migrating and evolving. The next generation of telescopes, like the Nancy Grace Roman Space Telescope, will advance such studies by letting us observe ice giant worlds in various stages of formation.

The ice giants in our backyard are essential to understanding how planets form and evolve. It’s high time we give them a proper visit.