Research on Uranus may shed light on a kind of planet now known to be one of the most common in the universe, now that it is the top priority of NASA planetary scientists.
Scientists have detected many types of alien worlds since astronomers discovered the first exoplanets more than 30 years ago, including ice giants. As their names suggest, gas giants such as Jupiter and Saturn are mostly gas, while ice giants such as Uranus and Neptune are predominantly ice.
Uranus and Neptune are still unknown to many.
There have been spacecraft on the solar system's six innermost planets for decades, Mercury, Venus, Earth, Mars, Jupiter, and Saturn. However, Uranus and Neptune have only experienced flybys from NASA's Voyager 2 probes over 30 years ago in 1986 and 1989. These brief encounters left behind many questions about the planets that had previously been unknown, according to planetary scientist Kathleen Mandt of The Johns Hopkins University Applied Physics Laboratory.
Due to this lack of knowledge about the ice giants, the planetary science community has chosen 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 of 2023-2032.
"The potential findings will be groundbreaking in the same way that the Cassini mission has revolutionised our understanding of Saturn, its moons — especially Titan — and its rings," Mandt said.
The proposal involves both an orbiter to gather data about Uranus over time and a probe dropped into Uranus' atmosphere to scan the planet from within. Currently referred to as the Uranus Orbiter and Probe (UOP), the mission aims to discover how Uranus and the rest of the solar system were formed and to solve mysteries about the planet, its moons, and its rings. At least five years should be required for the mission.
One of Uranus' strangest features is the fact that, unlike the solar system's other planets, Uranus is tilted so far that it essentially orbits the sun on its side, with the axis of its spin nearly pointing at the star.
"The tilt is crazy — it's the only planet in the solar system that is completely on its side," Mandt said.
Uranus may have formed after colliding with a planet-sized body, or several small bodies. "We can determine if this is true by studying what the planet is made of and its interior structure," Mandt said.
Because Uranus' orbit is sideways, it experiences extreme seasonal variations unlike any other planet in the solar system, and what little astronomers know about its weather patterns can't be explained by what they can see from Earth. With the probe gathering detailed wind and temperature data at one location and the orbiter collecting information across the entire planet, Mandt said the UOP can shed light on Uranus' atmosphere.
As a result of Uranus' tilt, astronomers are only able to see the southern hemispheres of its moons. What the probe did see, however, was unexpected. Scientists predicted Uranus' five largest moons would be cold, dead worlds because of their size, but all showed signs of surface activity recently. This raises the possibility that one or more of these moons, such as Ariel, Titania and Oberon, could have potentially habitable liquid water oceans underneath ice shells.
The 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 interiors 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 kept these rings from rapidly spreading out and losing their sharp edges. The UOP can help search for these extra shepherding moons, and also analyse the unexpectedly dark ring particles, whose composition is clearly different from that of the surfaces of Uranus' moons.
The mission to Uranus may also shed light not only on the origins and evolution of the solar system, but those of distant planetary systems as well.
"There are so many ways that the ice giants will help us to learn about exoplanets," Mandt said. "One of the largest groups of exoplanets that have been discovered are similar in size and mass to Uranus and Neptune. We want to know what these planets are made of and how the interior is structured. We also want to know more about the weather on the planet and how that compares to similar exoplanets."
It has been shown that giant planets play a major role in the birth and development of other planets as they form and migrate over time. The solar system's two gas giants have been closely observed by scientists, but more data is needed on Uranus and Neptune to reconstruct its history. In order to confirm which model of giant planet formation and migration is most accurate, Mandt said, the UOP probe can analyse nitrogen isotopes and noble gas levels in Uranus' atmosphere.
"We can see evidence in exoplanet systems that giant planets migrate in many different ways — the most obvious one is hot Jupiter's that must have formed far away and moved in really close to their stars," Mandt said. "Knowing how our planets formed and migrated helps us to know what did and didn't happen in exoplanet systems."
Besides being tilted 60 degrees from Uranus' spin axis, its magnetic field is also offset from its centre of mass. It is still unknown how the magnetic field works or how the 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 slingshot it toward Uranus. This would mean the mission would arrive well before Uranus' northern autumn equinox in 2050, ensuring full visibility of the moons. Trajectories 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 carrying fewer instruments or take longer to arrive.
"We will learn about how and where Uranus formed, what it's made of, and how the interior is structured," Mandt said. And the mission to Uranus may pave the way to its more-distant cousin Neptune as well, she added.
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