The planet Jupiter shows evidence of having being hit by a large object, either a comet or asteroid.
A dark mark has appeared in its atmosphere towards the southern pole.
It was first seen by Australian amateur astronomer Anthony Wesley on 19 July, and was then quickly followed up by others including the US space agency.
Nasa used its Infrared Telescope Facility on top of the Mauna Kea volcano in Hawaii to obtain detailed pictures of the disturbance.
The agency's scientists say the observations reveal a bright upwelling of particles in the upper atmosphere, and a warming of the upper troposphere with possible extra emission from ammonia gas detected at mid-infrared wavelengths.
It is 15 years since Jupiter was famously hit by Comet Shoemaker-Levy 9. It broke up into several pieces as it plunged on to the gas giant.
Scientists had plenty of warning of that impact but this latest event came out of the blue.
Researchers say that if the mark really was caused by a comet or asteroid, it should spread out in the coming days in a predictable way with jet streams .