A mission to Uranus is now the top priority of NASA planetary scientists in the future, and exploration of this mysterious ice giant could shed light on a kind of planet now known to be one of the most common in the universe, scientists say.
Since astronomers discovered the first exoplanets orbiting distant stars more than 30 years ago, one of the most common types of alien worlds scientists have discovered are ice giants. While gas giants such as Jupiter and Saturn is, as the name suggests, mostly gas, ice giants such as Uranus and Neptune are rich in heavier elements.
Much is still unknown about Uranus and Neptune. While the solar system’s six innermost planets – Mercury, Venus, Earth, Mars, Jupiter and Saturn – have all had spacecraft orbit them to gather insights for scientists, Uranus and Neptune to date have only experienced flybys more than 30 years ago , both from NASA’s Voyager 2 probe in 1986 and 1989. These brief encounters provided tantalizing glimpses of the planets that left more questions than previously imagined, Kathleen Mandt, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory in Maryland, told Space.com.
Related: Images of Uranus, the tilted giant planet
This lack of knowledge about the ice giants has led the planetary science community to select a mission to Uranus as the highest priority for NASA’s next large-scale “flagship” mission in the National Academies of Sciences, Engineering and Medicine’s 2023-2032 Planetary Science Decadal Survey.
“The potential findings will be groundbreaking in the same way that Cassini mission has revolutionized our understanding of Saturn, its moons — especially Titan — and its rings,” Mandt said.
The proposal involves both an orbiter to collect data on Uranus over time, and a probe dropped into Uranus’ atmosphere to scan the planet from the inside. The goal of the mission – currently given the placeholder name of Uranus Orbiter and Probe (UOP) – is to explore how Uranus and the rest of the Solar System formed and help solve mysteries about the planet, its moons and its rings. The assignment is recommended to last for at least five years.
One of Uranus’ strangest features is the fact that, unlike the other planets of the solar system, Uranus is tilted so far that it essentially orbits the sun on its side, with the spin axis almost pointing towards the star.
“The tilt is crazy—it’s the only planet in the solar system it is completely on his side, said Mandt.
This unusual orientation may be due to a collision with a planet-sized body, or several small bodies, shortly after Uranus formed. “We can find out if this is true by studying what the planet is made of and its internal structure,” Mandt said.
The sideways nature of Uranus causes the planet to experience extreme seasonal variations over its 84-year orbit unlike those of some of the Solar System’s other planets, and what little astronomers can see of the world from Earth cannot explain what they know about its weather patterns. UOP can help shed light on Uranus’ atmosphere, with the probe gathering detailed wind and temperature data in one place and the orbiter gathering information across the entire planet, Mandt said.
Related: Why scientists want NASA to send a flagship mission to Uranus
Uranus’ tilt also limits what astronomers can see of the moons. For example, Voyager 2 could only image the southern hemispheres of Uranus’ moons. What the probe saw was unexpected. Uranus’s five largest moons, which the researchers predicted were cold, dead worlds, too small to retain much of the heat from their creation, all showed evidence of recent surface activity. This raises the possibility that one or more of these moons, such as Ariel, Titania, and Oberon, may have potentially habitable oceans of liquid water beneath ice shells.
UOP will image the surfaces of the moons in their entirety to search for ongoing geological activity. It will also measure whether their magnetic fields vary in their interior due to the presence of liquid water, Mandt said.
In addition, Uranus has nine very dense, narrow rings around it that suggest the existence of “shepherd moons” whose gravitational influence prevented these rings from rapidly spreading out and losing their sharp edges. The UOP can help search for these additional shepherd moons, and also analyze the unexpectedly dark ring particles, whose composition is clearly different from the surface of Uranus’ moons.
The mission to Uranus may also shed light on the origin and development of the solar system, but also on distant planetary systems.
“There are so many ways that the ice giants will help us learn about exoplanets,” Mandt said. “One of the largest groups of exoplanets discovered is similar in size and mass to Uranus and Neptune. We want to know what these planets are made of and how the interior is built up. We also want to know more about the weather on the planet and how it compares to similar exoplanets.”
As giant planets form and migrate over time, they play major roles in the birth and development of other worlds. Although scientists have taken a closer look at the solar system’s two gas giants, they also need more data about Uranus and Neptune to reconstruct the history of the solar system. UOP’s probe can analyze nitrogen isotopes and levels of noble gases in Uranus’ atmosphere to help confirm which model of giant planet formation and migration may be the most accurate, Mandt said.
“We can see evidence in exoplanet systems that giant planets migrate in many different ways – the most obvious being hot Jupiters that must have formed far away and moved in very close to their stars,” Mandt said. “Knowing how our planets formed and migrated helps us know what did and did not happen in exoplanet systems.”
In addition, Uranus’ magnetic field is quite unusual, in that it is not only tilted 60 degrees from the planet’s spin axis, but also offset from the planet’s center. It remains a mystery how a planet can produce such a field, Mandt said.
The UOP is recommended to launch by 2032 to help the spacecraft use Jupiter’s massive gravity to hurl it toward Uranus. This would mean the mission would arrive well before Uranus’ northern autumnal equinox in 2050, ensuring full visibility of the moons. Orbits after 2032 that do not use Jupiter’s gravity but still arrive before the equinox are possible, but would deliver a smaller probe into orbit with fewer instruments or take longer to arrive.
“We will learn about how and where Uranus formed, what it is made of, and how the interior is structured,” Mandt said. And the mission to Uranus could also pave the way for its more distant cousin Neptune, she added.
Mandt detailed the proposed mission i a paper (opens in a new tab) in the February 17 issue of the journal Science.
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