X-ray emissions have been detected from most planets in the solar system and can originate from a variety of processes including the scattering of X-ray photons from the Sun; collisions between plasmas and planetary rings; and aurorae generated as solar winds interact with polar atmospheres. Until recently, however, evidence for X-ray emissions were notably absent from the solar system’s two ice giants: Uranus and Neptune.

Through new analysis of data gathered by the Chandra X-ray Observatory, Dunn’s team have identified three clear X-ray signals originating from Uranus: first in 2002, and then on two consecutive days in 2017. These observations are particularly interesting because of the planet’s unique orientation. Unlike other planets in the solar system, Uranus’s rotational axis lies parallel to its orbital plane and the planet’s magnetic field has a significant tilt relative to its axis of rotation. Indeed, the magnetic field misses the planet’s centre by roughly a third of its radius.

Complex relationship

This unusual configuration creates a complex relationship between Uranus’ magnetosphere and the solar wind. The resulting effects have already been probed at other wavelengths: during its 1986 flyby, Voyager 2 picked up patchy clusters of auroral emissions around both magnetic poles. Three decades later, the Hubble Space Telescope detected far more complex and time-variable emissions in the Uranian aurora. These results, combined with the known mechanisms for X-ray emissions on other planets, enabled Dunn and colleagues to present several theories for their X-ray observations.

The strengths of all three signals detected by Chandra were stronger than would be expected, had they originated from solar X-ray scattering. According to Dunn’s team, this could mean that Uranus is more reflective to incident X-rays than Jupiter and Saturn – but may also hint at additional mechanisms on the planet itself. These could include particle collisions in the aurora; or a glow in Uranus’ two icy rings, triggered by collisions with surrounding protons and electrons.

Further observations will be required to constrain these potential mechanisms, and to pin down the locations of X-ray sources on the Uranian surface. Dunn’s team hope this could be achieved through deeper observations with Chandra. However, future observations will be greatly improved by upcoming missions including ESA’s ATHENA X-ray Observatory, and NASA’s Lynx X-ray Observatory – both planned for launch in the 2030s. The team’s results could one day provide valuable guidance for these future observations.

Quelle: Physics World