Comet smash behind mysterious water on Jupiter

The mysterious traces of water in the upper atmosphere of Jupiter came from a comet that smashed into the largest planet of the solar system in 1994, the European Space agency (ESA) has found.
ESA's Herschel space observatory solved the long-standing mystery as to the origin of water in the upper atmosphere of Jupiter, finding conclusive evidence that it was delivered by the dramatic impact of comet Shoemaker-Levy 9 in July 1994.
During the spectacular week-long collision, a string of 21 comet fragments pounded into the southern hemisphere of Jupiter, leaving dark scars in the planet's atmosphere that persisted for several weeks.

The remarkable event was the first direct observation of an extraterrestrial collision in the Solar System. It was followed worldwide by amateur and professional astronomers with many ground-based telescopes and the NASA/ESA Hubble Space Telescope.
It was widely speculated that comet Shoemaker-Levy 9 may have been the origin of this water, but direct proof was missing.
Scientists were able to exclude an internal source, such as water rising from deeper within the planet's atmosphere, because it is not possible for water vapour to pass through the "cold trap" that separates the stratosphere from the visible cloud deck in the troposphere below.

Thus the water in Jupiter's stratosphere must have been delivered from outside. But determining its origin had to wait more than 15 years, until Herschel used its sensitive infrared eyes to map the vertical and horizontal distribution of water's chemical signature.

Herschel's observations found that there was 2-3 times more water in the southern hemisphere of Jupiter than in the northern hemisphere, with most of it concentrated around the sites of the 1994 comet impact. Additionally, it is only found at high altitudes.
"According to our models, as much as 95 per cent of the water in the stratosphere is due to the comet impact" said Thibault Cavalie of the Laboratoire d'Astrophysique de Bordeaux, lead author of the study published in Astronomy and Astrophysics.

Another possible source of water would be a steady rain of small interplanetary dust particles onto Jupiter. But, in this case, the water should be uniformly distributed across the whole planet and should have filtered down to lower altitudes.
Also, one of Jupiter's icy moons could deliver water to the planet via a giant vapour torus, as Herschel has seen from Saturn's moon Enceladus, but this too has been ruled out.
None of Jupiter's large moons is in the right place to deliver water to the locations observed.