apr 262013
 

 

ESA’s Herschel space observatory has solved a 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.

Hubble view of the impact sites, a few days after the impacts. Image Credit: Hubble Space Telescope Comet Team and NASA

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.

ESA’s Infrared Space Observatory was launched in 1995 and was the first to detect and study water in Jupiter’s upper atmosphere. It was widely speculated that comet Shoemaker-Levy 9 may have been the origin of this water, but direct proof was missing.

This composite is assembled from separate images of Jupiter and comet Shoemaker-Levy 9, as imaged by Hubble in 1994. Comet Shoemaker-Levy 9 was discovered by astronomers Carolyn and Eugene M. Shoemaker and David Levy on 24 March 1993. It was the first comet observed to be orbiting a planet – in this case, Jupiter – rather than the Sun. The effect of Jupiter’s tidal forces tore the comet apart on its approach and, eventually, the fragments collided with Jupiter between 16 and 22 July 1994. The image of the comet, showing 21 fragments, was taken on 17 May 1994. The image of Jupiter was taken on 18 May 1994; the dark spot on the planet’s disc is the shadow of the inner moon lo. The apparent angular size of Jupiter relative to the comet and its angular separation from the comet when the images were taken have been modified for illustration purposes. Image Credit: NASA, ESA, H. Weaver and E. Smith (STScI) and J. Trauger and R. Evans (Jet Propulsion Laboratory).

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.

Distribution of water in the stratosphere of Jupiter as measured with Herschel’s space observatory. The data have been superimposed over an image of Jupiter taken at visible wavelengths with the Hubble Space Telescope. A clear asymmetry is shown in the distribution of water, with more abundant in the southern hemisphere (white/cyan) and far less detected in the northern hemisphere (darker blue shades). A new study using Herschel finally links Jupiter’s upper atmosphere water to the comet impact of Shoemaker-Levy 9 in 1994, during which 21 comet fragments struck the planet at intermediate southern latitudes. Image Credit: Water map: ESA/Herschel/T. Cavalié et al.; Jupiter image: NASA/ESA/Reta Beebe (New Mexico State University)

“Only Herschel was able to provide the sensitive spectral imaging needed to find the missing link between Jupiter’s water and the 1994 impact of comet Shoemaker-Levy 9,” says Thibault Cavalié of the Laboratoire d’Astrophysique de Bordeaux, lead author of the paper published in Astronomy and Astrophysics.

“According to our models, as much as 95% of the water in the stratosphere is due to the comet impact.”

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.

Abundance of water in Jupiter’s stratosphere.The north-south asymmetry is clearly observed. The green and red areas correspond to the highest abundances.

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.

Finally, the scientists were able to rule out any significant contributions from recent small impacts spotted by amateur astronomers in 2009 and 2010, along with local variations in the temperature of Jupiter’s atmosphere.

Shoemaker-Levy 9 is the only likely culprit.

“All four giant planets in the outer Solar System have water in their atmospheres, but there may be four different scenarios for how they got it,” says Dr Cavalié. “For Jupiter, it is clear that Shoemaker-Levy 9 is by far the dominant source, even if other external sources may contribute also.”

 

This mosaic of WFPC-2 images shows the evolution of the G impact site on Jupiter (the 21 comet fragments of Shoemaker-Levy 9 were each assigned a corresponding letter to identify the impact site; G represents the 7th fragment to strike the planet. It was also the largest impact.). The images from lower right to upper left show: the impact plume at 07/18/94 07:38 UT (about 5 minutes after the impact); the fresh impact site at 07/18/94 at 09:19 UT (1.5 hours after impact); the impact site after evolution by the winds of Jupiter (left), along with the L impact (right), taken on 07/21/94 at 06:22 UT (3 days after the G impact and 1.3 days after the L impact); and further evolution of the G and L sites due to winds and an additional impact (S) in the G vicinity, taken on 07/23/94 at 08:08 UT (5 days after the G impact). Image Credit: R. Evans, J. Trauger, H. Hammel and the HST Comet Science Team

“Thanks to Herschel’s observations, we have now linked a unique comet impact – one that was followed in real time and which captured the public’s imagination – to Jupiter’s water, finally solving a mystery that has been open for nearly two decades,” adds Göran Pilbratt, ESA’s Herschel project scientist.

The observations made in this study foreshadow those planned for ESA’s future Jupiter Icy moons Explorer mission launching towards the Jovian system in 2022, where it will map the distribution of Jupiter’s atmospheric ingredients in even greater detail.

The paper: “The spatial distribution of water in the stratosphere of Jupiter from Herschel-HIFI and –PACS observations,” by T. Cavalié et al. is published in Astronomy & Astrophysics, 553, A21, May 2013.

Source: European Space Agency (ESA) 

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