Having been the object of neglect from space agencies on one hand, and hilarity from the general public on the other, Uranus still remains one of the most mysterious places in the Solar System.
There are currently 22 planetary spacecraft scattered throughout the Solar System, actively exploring almost every part of the Sun’s planetary family. Yet, one glaring omission from this long list of space exploration targets has been the planet Uranus, ever since NASA’s Voyager 2 spacecraft paid a brief visit there, 28 years ago this month, in January 1986.
Although it shares many similarities with neighboring Neptune, Uranus is an interesting peculiarity on its own. And even though Voyager 2’s fly-by has provided us with the bulk of our current knowledge of the planet, a greater series of even more intriguing questions about this enigmatic cyan-tinted ringed world remain unanswered to this day.
For starters, Uranus is famous for being the only planet in the Solar System with a rotational axis that is almost parallel to the plane of the ecliptic. Where all of the other major planets rotate around an axis that is somewhat perpendicular or tilted no more than 30 degrees to the ecliptic plane, Uranus’ axial tilt of 97.7 degrees means that the planet is essentially “rolling” on its side, on its 84-year-old orbit around the Sun. This axial tilt gives Uranus the unique orientation of having each pole respectively facing the Sun for 42 years consecutively during summer, with the equatorial regions being lit by a Sun that is almost always low on the horizon, except from the time of the Uranian equinoxes, during spring and autumn.
Despite this axial tilt, the equatorial regions nevertheless exhibit warmer temperatures than the brightly lit poles. The reasons behind this phenomenon remain a mystery. Could it be that an atmospheric convection mechanism for heat transfer exists in Uranus’ atmosphere?
Another perplexing mystery is Uranus’ relative lack of internal heating, compared to the other giant planets and particularly Neptune. Although Uranus’ orbit lies approximately 18 Astronomical Units from the Sun, significantly closer than that of Neptune (30 AU), the temperatures that have been recorded on their respective tropospheres were almost the same, averaging 50 K (-223 °C) for both. Neptune has been found to radiate back into space as much as 2.6 times more energy than it receives from the Sun, which could help explain the more dynamic and readily visible changes in its atmosphere. Uranus’ internal heat in comparison is less than half that, something that could possibly account for the featureless disc that Uranus displayed to Voyager 2, when the spacecraft flew past the planet’s south pole in 1986. It was this absence of any visible atmospheric features that unjustly earned Uranus the definition of the “boring” planet.
Yet in the years following the Voyager 2 encounter, more recent observations have shown that Uranus is anything but a boring world. Throughout the 1990s and 2000s, observations made with the Hubble Space Telescope and the 10-m Keck telescopes on the Keck Observatory on Mauna Key in Hawaii revealed a much more dynamic atmosphere than previously thought. While the planet was approaching its vernal equinox in December 2007, its atmosphere started to undergo significant seasonal changes. The superior observing capabilities of the Hubble and Keck telescopes in infrared and microwave wavelengths helped to reveal a planet with an ever-changing atmosphere, not unlike that of Neptune. Astronomers discovered changing bright clouds and banded structures that were invisible to Voyager 2’s instruments that observed the planet in visible and ultraviolet light. A year before the 2007 equinox, Hubble also detected a Dark Spot on Uranus for the first time, similar to that seen on Neptune by Voyager 2, albeit much smaller in size. It became clear that as the planet was approaching equinox, the mid and equatorial regions that were previously covered in darkness were responding to the increased warmth of the Sun, showcasing an interesting and volatile atmosphere, worthy of a more detailed, close-up study.
Besides these similar atmospheric characteristics, the relation between Uranus and Neptune runs deeper than that. Where the interiors of Jupiter and Saturn are believed to consist of multiple layers of different states of hydrogen on top of a rocky core, Uranus and Neptune, on the other hand, are believed to harbor vast oceans of hot and dense liquid water, methane, and ammonia on top of their rocky cores. It is because of this different layered internal structure that they are considered to be a distinct class of planets, with astronomers classifying them as “ice giants,” contrary to the “gas giant” definition of Jupiter and Saturn.
This difference in internal structure might also account for another of Uranus’ strange characteristics: the orientation of its magnetic field axis relative to the rotational axis. A similar orientation is observed in Neptune’s magnetic field, further strengthening the case for “ice giants” being a distinct sub-category of planets. Although the magnetic poles of Earth, Jupiter, and Saturn lie close to those planets’ geographic poles, the magnetic field of Uranus is severely displaced, with its axis being offset by 59 degrees to the planet’s axis of rotation. The most plausible explanation for this discrepancy is the different way in which the magnetic field is generated on Uranus. The magnetic fields of Earth, Jupiter, and Saturn are created by the motions of conducting fluids on their molten cores. The Earth’s core is composed of molten iron, and those of Jupiter and Saturn are made of metallic hydrogen. The magnetic fields of Uranus and Neptune, on the other hand, are believed to be generated by the motions of electric currents through the oceans of ionised liquid water and ammonia that probably lie at the planets’ mantles.
As intriguing as Uranus itself may be, it also comes replete with a system of 27 moons, just as interesting and inviting as the moons of Jupiter and Saturn. The five bigger of these—Miranda, Ariel, Umbriel, Titania, and Oberon—are primarily composed of rock and ice, with Titania and Oberon in particular suspected to harbor underground oceans of liquid water, as in the case of Jupiter’s moon Europa and Saturn’s moon Enceladus. The evidence for the existence of underground water oceans on Solar System objects that were previously thought of as frozen and dead has revolutionised our concept and understanding of habitability beyond Earth.
By better understanding Uranus, scientists could gain a better understanding of the internal structures and evolutions not only of the planets in our Solar System, but of those beyond as well. During the recent 223rd meeting of the American Astronomical Society in Washington this January, scientists reported on four years’ worth of data from NASA’s Kepler space telescope, showing that the vast majority of exoplanets discovered so far are planets whose sizes range from that of Earth to that of Neptune. If those extrasolar, Neptune-sized worlds are anything like the ice giants in our own Solar System, then what better way to study those distant faraway planets than to study our neighboring ice giants, Uranus and Neptune.
The importance of a dedicated Uranus mission isn’t lost on the planetary science community. A Uranus orbiter is listed as the third highest priority Flagship mission after Mars and Europa, in the 2013-2022 U.S. Planetary Science Decadal Survey. Dr. Mark Hofstadter, a planetary scientist at NASA’s Jet Propulsion Laboratory, stressed that point during a presentation of a Uranus Flagship mission concept at the January 2013 meeting of the Outer Planets Assessment Group in Atlanta, Ga. “The Group is concerned that no action was taken on its findings last year regarding a Uranus mission study,” notes Hofstadter in the presentation, “and again urges that NASA initiate such a study responsive to Decadal Survey science goals for the ice giants.” Hofstadter also presented alternate Uranus orbiter concepts that fall under NASA’s New Horizons and Discovery-class cheaper mission profiles, but none of them have been selected by the U.S. space agency to date, due to severe budget cuts to its planetary science programs in recent years. A similar orbital mission concept proposed to ESA, called “Uranus Pathfinder,” had received a very positive assessment from the European space agency, but eventually wasn’t selected for implementation either. “Being farther away makes it more difficult (read more expensive) to get there than to, say, Jupiter or Saturn,” says Hofstadter. “It’s for this reason that missions to the inner gas giants have been preferred. But, in light of technological advancements, the cost of sending a robotic mission to Uranus now is more manageable.”
A dedicated mission to the enigmatic and forgotten Uranus could be key as well for the understanding of the Solar System as a whole. “To have a complete understanding of our solar system we must study all of its components,” says Chris Arridge, a research fellow at University College London’s Mullard Space Science Laboratory. “It’s like having a huge jigsaw and only having half the pieces – we need to get all those pieces to have a chance of being able to see the big picture.”
Probably the most important reason for such a mission isn’t to answer the long list of questions that we’ve already made. As is always the case with space exploration, the most important justification might be the list of things that we didn’t expect to discover in the process, and the answers to the questions that we haven’t thought to ask yet.