Editing Galileo project (section)

===Europa===
[[File:Europa-moon-with-margins.jpg|thumb|right|Europa photographed by ''Galileo''|alt=Europe is criss-crossed by lines]]
Although the smallest of the four Galilean moons, with a radius of {{convert|1565|km|sp=us}}, Europa is the sixth-largest moon in the solar system.<ref name="Europa - Jupiter's Icy Moon">{{cite web |title=Europa – Jupiter's Icy Moon |publisher= Teachlink @ Utah State University |url=http://teacherlink.ed.usu.edu/tlnasa/OtherPRINT/poster/Europa_Poster.pdf |access-date=December 3, 2020 |archive-date=February 4, 2021 |archive-url=https://web.archive.org/web/20210204170152/http://teacherlink.ed.usu.edu/tlnasa/OtherPRINT/poster/Europa_Poster.pdf |url-status=dead }}</ref> Observations from Earth indicated that it was covered in ice.{{sfn|Greenberg|2005|p=9}} Like Io, Europa is tidally locked with Jupiter. It is in orbital resonance with Io and Ganymede, with its 85-hour orbit being twice that of Io, but half that of Ganymede. Conjunctions with Io always occur on the opposite side of Jupiter to those with Ganymede.{{sfn|Greenberg|2005|pp=51–52}} Europa is therefore subject to tidal effects.{{sfn|Greenberg|2005|pp=49–51}} There is no evidence of volcanism like on Io, but ''Galileo'' revealed that the surface ice was covered in cracks.{{sfn|Greenberg|2005|pp=12–14}}

Some observations of Europa were made during orbits G1 and G2. On C3, ''Galileo'' conducted a {{convert|34,800|km|sp=us|adj=on}} "nontargeted" encounter of Europa—meaning a secondary flyby at a distance of up to {{convert|100,000|km|sp=us}}—on November 6, 1996. During E4 from December 15 to 22, 1996, ''Galileo'' flew within {{convert|692|km|sp=us}} of Europa, but data transmission was hindered by a Solar [[occultation]] that blocked transmission for ten days.{{sfn|Meltzer|2007|pp=254–256}}

''Galileo'' returned to Europa on E6 in January 1997, this time at a height of {{convert|586|km|sp=us}}, to analyze oval-shaped features in the infrared and ultraviolet spectra. Occultations by Europa, Io and Jupiter provided data on the atmospheric profiles of them, and measurements were made of Europa's gravitational field. On E11 from November 2 to 9, 1997, data was collected on the magnetosphere.{{sfn|Meltzer|2007|pp=254–256}} Due to the problems with the HGA, only about two percent of the anticipated number of images of Europa were obtained by the primary mission.{{sfn|Greenberg|2005|p=160}} On the GEM, the first eight orbits (E12 through E19) were all dedicated to Europa, and ''Galileo'' paid it a final visit on E26 during the GMM.{{sfn|Meltzer|2007|pp=256–259}}

[[File:PIA03002 Blocks in the Europan Crust Provide More Evidence of Subterranean Ocean.jpg|thumb|left|This false color image on the left shows a region of Europa's crust made up of blocks which are thought to have broken apart and "rafted" into new positions.|alt=refer to caption]]
Images of Europa also showed few impact craters. It seemed unlikely that it had escaped the meteor and comet impacts that scarred Ganymede and Callisto, so this indicated Europa has an active geology that renews the surface and obliterates craters.{{sfn|Greenberg|2005|pp=12–14}}<ref name="Europa - Jupiter's Icy Moon" /> Astronomer [[Clark Chapman]] argued that, assuming a {{convert|20|km|adj=on|sp=us}} crater occurs in Europa once every million years, and given only about twenty have been spotted on Europa, the implication is that the surface must only be about 10 million years old.{{sfn|Meltzer|2007|pp=260–216}} With more data on hand, in 2003 a team led by Kevin Zahle at NASA's [[Ames Research Center]] arrived at a figure of 30 to 70 million years.{{sfn|Zahnle|Schenk|Levison|Dones|2003|p=277}} [[Tidal flexing]] of up to {{convert|100|m|sp=us}} per day was the most likely culprit.<ref name="Off kilter">{{cite press release |title=Long-stressed Europa Likely Off-kilter at One Time |date=September 18, 2013 |publisher=NASA/Jet Propulsion Laboratory |first1=Jia-Rui |last1=Cook |first2=Elizabeth |last2=Zubritsky |first3=Nancy |last3=Neal-Jones |url=https://www.jpl.nasa.gov/news/long-stressed-europa-likely-off-kilter-at-one-time/ |access-date=December 4, 2020 |archive-date=February 19, 2021 |archive-url=https://web.archive.org/web/20210219074046/https://www.jpl.nasa.gov/news/long-stressed-europa-likely-off-kilter-at-one-time |url-status=live }}</ref> But not all scientists were convinced; Michael Carr, a planetologist from the [[US Geological Survey]], argued that, on the contrary, Europa's surface age was closer to a billion years. He compared the craters on Ganymede with those on Earth's moon, and concluded that the satellites of Jupiter were not subject to the same amount of cratering.{{sfn|Meltzer|2007|pp=260–261}}<ref name="New Images Hint">{{cite press release |id=97-66 |date=April 9, 1997 |first1=Donald |last1=Savage |first2=Jane |last2=Platt |title=New Images Hint at Wet and Wild History For Europa |publisher=NASA/Jet Propulsion Laboratory |url=https://www.jpl.nasa.gov/news/new-images-hint-at-wet-and-wild-history-for-europa |access-date=April 2, 2024 |archive-date=April 2, 2024 |archive-url=https://web.archive.org/web/20240402205836/https://www.jpl.nasa.gov/news/new-images-hint-at-wet-and-wild-history-for-europa |url-status=live }}</ref>

Evidence of surface renewal hinted at the possibility of a viscous layer below the surface of warm ice or liquid water. NIMS observations by ''Galileo'' indicated that the surface of Europa appeared to contain magnesium- and sodium-based salts. A likely source was [[brine]] below the ice crust. Further evidence was provided by the magnetometer, which reported that the magnetic field was induced by Jupiter. This could be explained by the existence of a spherical shell of conductive material like salt water. Since the surface temperature on Europa was {{convert|-162|C}}, any water breaching the surface ice would instantly freeze over. Heat required to keep water in a liquid state could not come from the Sun, which at that distance had only 4 percent of the intensity it had on Earth, but ice is a good insulator, and the heat could be provided by the tidal flexing.<ref name="New Images Hint" />{{sfn|Meltzer|2007|pp=261–263}} ''Galileo'' also yielded evidence that the crust of Europa had slipped over time, moving south on the hemisphere facing Jupiter, and north on the far side.<ref name="Off kilter" />{{sfn|Greenberg|2005|pp=173–178}}{{sfn|Sarid et al.|2002|p=24}}

[[File:Plate Tectonics on Europa.jpg|thumb|right|Plate tectonics on Europa|alt=Illustration of model of Europa with a liquid ocaen surrounded by warmer ice and then an outer layer of a cold ice shell, with outbreaks of cryolarva.]]
There was acrimonious debate among scientists over the thickness of the ice crust, and those who presented results indicating that it might be thinner than the {{convert|20|to|30|km|sp=us}} proposed by the accredited scientists on the ''Galileo'' Imaging Team faced intimidation, scorn, and reduced career opportunities.{{sfn|Greenberg|2005|pp=313–321}} The ''Galileo'' Imaging Team was led by [[Michael J. Belton]] from the [[Kitt Peak National Observatory]]. Scientists who planned imaging sequences had the exclusive right to the initial interpretation of the ''Galileo'' data, most which was performed by their research students.{{sfn|Greenberg|2005|pp=31–32}} The scientific community did not want a repetition of the 1979 Morabito incident, when [[Linda A. Morabito]], an engineer at JPL working on ''Voyager 1'', discovered the first active extraterrestrial volcano on Io.{{sfn|Chaisson|1994|p=102}} The Imaging Team controlled the manner in which discoveries were presented to the scientific community and the public through press conferences, conference papers and publications.{{sfn|Greenberg|2005|pp=31–32}}

Observations by the Hubble Space Telescope in 1995 reported that Europa had a thin oxygen atmosphere. This was confirmed by ''Galileo'' in six experiments on orbits E4 and E6 during occultations when Europa was between ''Galileo'' and the Earth. This allowed Canberra and Goldstone to investigate the [[ionosphere]] of Europa by measuring the degree to which the radio beam was diffracted by charged particles. This indicated the presence of water ions, which were most likely water molecules that had been dislodged from the surface ice and then ionized by the Sun or the Jovian magnetosphere. The presence of an ionosphere was sufficient to deduce the existence of a thin atmosphere on Europa.<ref>{{cite press release |title=Galileo finds Europa has an Atmosphere |publisher=NASA/Jet Propulsion Laboratory |first=Jane |last=Platt |date=July 18, 1997 |url=https://www.jpl.nasa.gov/news/galileo-spacecraft-finds-europa-has-atmosphere/ |access-date=January 19, 2021 |archive-date=November 30, 2021 |archive-url=https://web.archive.org/web/20211130052248/https://www.jpl.nasa.gov/news/galileo-spacecraft-finds-europa-has-atmosphere |url-status=live }}</ref>

On December 11, 2013, NASA reported, based on results from the ''Galileo'' mission, the detection of "[[Clay mineral|clay-like minerals]]" (specifically, [[phyllosilicates]]), often associated with [[organic materials]], on the icy crust of [[Europa (moon)|Europa]]. The presence of the minerals may have been the result of a collision with an [[asteroid]] or [[comet]].<ref name="NASA-20131211">{{cite web |last=Cook |first=Jia-Rui c. |title=Clay-Like Minerals Found on Icy Crust of Europa |url=https://www.jpl.nasa.gov/news/clay-like-minerals-found-on-icy-crust-of-europa/ |date=December 11, 2013 |publisher=NASA/Jet Propulsion Laboratory |access-date=January 19, 2021 |archive-date=May 28, 2019 |archive-url=https://web.archive.org/web/20190528024022/https://www.jpl.nasa.gov/news/news.php?release=2013-362 |url-status=live }}</ref>

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