Scientist studying Jupiter have discovered the cause of the planet having water in it's stratosphere, linking it to a comet impacting the planet.
Astronomers used data from the Herschel space observatory that revealed that Jupiter's southern hemisphere contained more water in the atmosphere than the north. Using this data, scientists believe that the source of the water in the upper levels of the atmosphere must be from external origins. This origin is believed to be from the Shoemaker-Levy 9 comet that famously impacted the planet in 1994. Comets are giant orbs of ice and rock, so a comet colliding with a planet would have the power to blast huge amounts of water into the upper atmosphere of a planet, even one with such massive gravity as Jupiter.
Thibault Cavalie, leader of the study from the Laboratoire d'Astrophysique de Bordeaux in France had this to say about the results of the study:
"The asymmetry between the two hemispheres suggests that water was delivered during a single event and rules out icy rings or moons as candidate sources...Local sources would provide a steady supply of water, which over time would lead to a hemispherically symmetric distribution in the stratosphere. Depending on whether the chemical species are transported in neutral or ionized form, local sources of water would result in higher concentrations either at the poles or along the equator, but not in a north-south asymmetry."
He further clarifies in a ESA (European Space Agency) press release how much the Shoemaker-Levy 9 impact had on the planet's atmosphere. "According to our models, as much as 95% of the water in the stratosphere is due to the comet impact."
While it is not uncommon for planets, even gas giants like Jupiter and the other planets in the farther reaches of the solar system, to have water, Jupiter was unique in that the water in it's atmosphere was unevenly distributed, and not in a polar alignment like most planets. The results of this study conclude that mystery, and clarify two decades of scientific debate.