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Scientists using NASA's Hubble Space Telescope went looking for evidence of one phenomenon and found quite another.

The research team studied the four largest moons of the ice giant Uranus, the seventh planet from our sun, searching for signs of interactions between Uranus's magnetosphere and the surfaces of the moons. (A magnetosphere is a region surrounding a celestial body where particles with an electrical charge are affected by the astronomical object's magnetic field.)

In particular, the team predicted that based on interactions with Uranus's magnetosphere, the "leading" sides of these tidally locked moons, which always face in the same direction in which they are orbiting the planet, would be brighter than the "trailing" sides, always facing away. This would be due to radiation darkening of their trailing sides by charged particles such as electrons trapped in Uranus's magnetosphere.

Instead, they found no evidence for darkening on the moons' trailing sides, and clear evidence for darkening of the leading sides of the outer moons. This surprised the team and indicates that Uranus's magnetosphere might not interact much with its large moons, contradicting existing data collected over near-infrared wavelengths.

Hubble's sharp ultraviolet vision and spectroscopic capabilities were critical to allowing the team to investigate the surface conditions on these moons and uncover the surprising finding.

The complicated magnetic environment of 'weird' Uranus

The four moons in this study—Ariel, Umbriel, Titania, and Oberon—are tidally locked to Uranus, so that they always show the same side to the planet. The side of the moon facing the direction of travel is called the leading hemisphere, while the side that faces backward is called the trailing hemisphere. The thinking was that charged particles trapped along the magnetic field lines primarily hit each moon's trailing side, which would darken that hemisphere.

"Uranus is weird, so it's always been uncertain how much the magnetic field actually interacts with its satellites," explained principal investigator Richard Cartwright of the Johns Hopkins University's Applied Â鶹ÒùÔºics Laboratory. "For starters, it is tilted by 98 degrees relative to the ecliptic."

This means that Uranus is dramatically tipped relative to the orbital plane of the planets. It rolls very slowly around the sun on its side as it completes its 84-Earth-year orbit.

"At the time of the Voyager 2 flyby, the magnetosphere of Uranus was tilted by about 59 degrees from the orbital plane of the satellites. So, there's an additional tilt to the magnetic field," explained Cartwright.

Because Uranus and its magnetic field lines rotate faster than its moons orbit the planet, the magnetic field lines constantly sweep past the moons. If the magnetosphere of Uranus interacts with its moons, charged particles should preferentially hit the surface of the trailing sides.

These charged particles, as well as our galaxy's cosmic rays, should darken the trailing hemispheres of Ariel, Umbriel, Titania, and Oberon and possibly generate the carbon dioxide detected on these moons. The team expected that especially for the inner moons Ariel and Umbriel, the trailing hemispheres would be darker than the leading sides in ultraviolet and visible wavelengths.

But that's not what they found. Instead, the leading and trailing hemispheres of Ariel and Umbriel are actually very similar in brightness. However, the researchers did see a difference between the hemispheres of the two outer moons, Titania and Oberon—not the moons they expected.

Like bugs on a windshield

Even stranger, the difference in brightness was the opposite of what they expected. The two outer moons have darker and redder leading hemispheres compared with their trailing hemispheres. The team thinks that dust from some of Uranus's irregular satellites is coating the leading sides of Titania and Oberon.

Irregular satellites are natural bodies that have large, eccentric, and inclined orbits relative to their parent planet's equatorial plane. Micrometeorites are constantly hitting the surfaces of Uranus's irregular satellites, ejecting small bits of material into orbit around the planet.

Over millions of years, this dusty material moves inward toward Uranus and eventually crosses the orbits of Titania and Oberon. These outer moons sweep through the dust and pick it up primarily on their leading hemispheres, which face forward. It's much like bugs hitting the windshield of your car as you drive down a highway.

This material causes Titania and Oberon to have darker and redder leading hemispheres. These outer moons effectively shield the inner moons Ariel and Umbriel from the dust, which is why the inner moons' hemispheres do not show a difference in brightness.

"We see the same thing happening in the Saturn system and probably the Jupiter system as well," said co-investigator Bryan Holler of the Space Telescope Science Institute. "This is some of the first evidence we're seeing of a similar material exchange among the Uranian satellites."

"So that supports a different explanation," said Cartwright. "That's dust collection. I didn't even expect to get into that hypothesis, but you know, data always surprises you."

Based on these findings, Cartwright and his team suspect that Uranus's magnetosphere may be fairly quiescent, or it may be more complicated than previously thought. Perhaps interactions between Uranus's moons and magnetosphere are happening, but for some reason, they're not causing asymmetry in the leading and trailing hemispheres as researchers suspected. The answer will require further investigation into enigmatic Uranus, its magnetosphere, and its moons.

Hubble's unique ultraviolet vision

To observe the brightnesses of the four largest Uranian moons, the researchers required Hubble's unique ultraviolet capabilities. Observing targets in ultraviolet light is not possible from the ground because of the filtering effects of Earth's protective atmosphere. No other present-day space telescopes have comparable ultraviolet vision and sharpness.

"Hubble, with its ultraviolet capabilities, is the only facility that could test our hypothesis," said the Space Telescope Science Institute's Christian Soto, who conducted much of the data extraction and analysis. Soto presented results from this study on June 10 at the in Anchorage, Alaska.

Complementary data from NASA's James Webb Space Telescope will help to provide a more comprehensive understanding of the Uranian satellite system and its interactions with the planet's magnetosphere.