The Greatest Mysteries of Uranus

Uranus glows gorgeously in x-rays. NASA

More than a billion and a half away from Earth looms a huge cyan world with a dangerous name: Uranus.

Along with Neptune, Uranus is considered an “ice giant”, a class of planets other than the gas giants Jupiter and Saturn, which are much larger. While hydrogen and helium gas make up much of Uranus, significant amounts of water, methane, and ammonia give the planet a different color and chemistry. In terms of size, the radius of Uranus is four times that of Earth, and approximately 16 Earths could fit within the sphere of the ice giant.

Humanity hasn’t looked closely at Uranus since Voyager 2 surveyed it in 1986, and for now, a return mission is not in sight. Until we return, some of the more important mysteries will continue to be puzzling, including:

Why do the sides rotate?

In terms of their rotation, the planets and the Sun can be thought of as rotating points placed on a table that rotate on an axis more or less in the same plane.

Except for Uranus. It has an axial tilt of about 98 degrees, which means that its “north” and “south” poles are in place where Earth’s equator runs. The planet simply appears as if it has been struck from the side.

What could have done this? Unless there are more likely alternatives, scientists are betting that a body the size of Earth collided with Uranus early in the history of the solar system and brought the world down.

An impact is the only mechanism we can think of to do that.

The fact that the 13 rings of Uranus and a couple of more than a dozen moons have also flipped over, circling the planet like circles on a bullseye from our perspective, lends credibility to this theory. Maybe before the satellites formed or finished forming, everything was tilted.

Learning more about the interior of Uranus, which unlike other planets does not fit any simple model, and comparing it to its sister world, Neptune would help. There may be some compositional evidence or just an internal structure that tells us ‘okay, this took a giant hit.

Uranus stays cool

Surprisingly, Uranus radiates little or no heat into space, another thing that makes it unique among the planets in our solar system. The planets are expected to have heat within them from their formation process; The interior of the Earth, for example, remains hot and molten.

That same planetary punch that sent Uranus sideways could also explain its apparent lack of internal heat. If something giant hits Uranus, that impact could have shaken its insides. That helped bring in hot material that was very close to the surface, and thus helped Uranus cool more quickly.

A second idea is that the normal heat flow from a warm interior to a cooler surface, called convection, does not work properly. We hope that if we learn more about the interior structure of Uranus, we will see a region where convection is inhibited. Or, if we can say that the interior is really hot, we will know that the energy is trapped there and we don’t.

Where was Uranus born?

Recent models of how the outer planets of the solar system formed and have since evolved suggest that Saturn and the two ice giants were once closer to Jupiter. [What if the solar system formed closer to the edge of the Milky Way? ]

Not long after the solar system formed, the cumulative gravitational interactions of small planetesimals moving around Saturn, Uranus, and Neptune began to drift away dramatically, so in the case of ice giants. They could have doubled or tripled their distance from the Sun.

In turn, this shift in the mass of the solar system removed most of the remaining debris from the genesis of the solar system. A good number of icy bodies were probably thrown towards Earth and the inner planets during this “Final Heavy Bombardment”, which began 4.1 billion years ago. Water and organic material were deposited on our planet, perhaps critical in setting the stage for the development of life.

The best computer simulations with more data should help define this “Nice Model”, which is named after the city in France. The discovery of the history of Uranus and how it has influenced our planet speaks to the possibility of life in other solar systems: According to initial data from NASA’s planet-hunting Kepler spacecraft, ice giants could be the most common type of planet in the galaxy.

Compared to the variety of moons that circle Jupiter and Saturn, the complement of 27 satellites of Uranus is less exotic. But a moon called Miranda stands out for having one of the brightest surfaces of any known astronomical body. This tiny moon has deep canyons, scrapes, terraced layers, and a cliff about 20 kilometers deep, the deepest known in the solar system.

One theory behind Miranda’s geological clutter suggests that flowing ice inside the moon, perhaps heated by the gravitational pressure of Uranus and other moons, is pushing the surface. Another maintains that the moon broke several times and reattached itself, creating its jagged, mottled features.