Galileo was equipped with a magnetometer \u2013 an instrument for measuring magnetic fields, a bit like an extremely sensitive compass needle \u2013 and every time the spacecraft performed a flyby of Galilean moons<\/a> Europa or Callisto it sensed the field lines of Jupiter\u2019s powerful magnetic field<\/a> draping around the moons.<\/p>\n The satellites seemed to possess a weak magnetic field that was interacting with that of Jupiter.<\/p>\n More from Lewis Dartnell:<\/strong><\/em><\/p>\n These tiny, frozen moons can\u2019t have been generating the magnetic field themselves by an active internal dynamo, as happens in Jupiter and Earth\u2019s molten metal cores.<\/p>\n It was being created indirectly, as the moons moved through Jupiter\u2019s own field.<\/p>\n In order for this induced magnetic field to arise, there must be a layer of something hidden beneath the frozen face of the moons that is electrically conductive.<\/p>\n By far the most likely option was salty water \u2013 Europa and Callisto both harbour subsurface oceans, and it is these that are interacting with Jupiter\u2019s magnetic field.<\/p>\n This same technique could be used to remotely sense the presence of subsurface oceans on other moons too.<\/p>\n Corey Cochrane at the Jet Propulsion Laboratory and his colleagues argue that Uranus<\/a> and Neptune also offer a great opportunity for using magnetic sounding to detect oceans within their moons.<\/p>\n Neptune has only one major moon, Triton, which is thought to be a captured Kuiper Belt Object that destroyed Neptune\u2019s original system of satellites when it arrived.<\/p>\n There\u2019s even a mission in the pipeline, called Trident<\/a>, that would send a spacecraft to explore Triton.<\/p>\n Uranus has a diverse family of moons (named after Shakespearean characters), including Puck, Miranda, Ariel, Umbriel and Oberon.<\/p>\n Voyager 2 visited both the Uranus and Neptune systems, but it didn\u2019t pass close enough to any of the moons to detect the tell-tale signature of magnetic induction.<\/p>\n Photographs taken by the probe, however, show widespread evidence in the landscapes on both Miranda and Ariel of recent tectonic and cryovolcanic<\/a> activity.<\/p>\n Perhaps these moons have subsurface liquid water oceans that persist even today.<\/p>\n And Cochrane says that available technology could readily detect such oceans with just a single flyby.<\/p>\n What\u2019s more, Uranus\u2019s quirky magnetic field would allow a probe to deduce a great deal about a subsurface ocean.<\/p>\n The ice giant\u2019s magnetic field is not only pretty intense, but crucially is tilted significantly relative to the orbits of the major moons \u2013 by around 60\u00b0 \u2013 and is also offset from the centre of the planet.<\/p>\n The planet\u2019s satellites experience a magnetic field that is constantly varying. The induction effects of this within the moon\u2019s interior mean that researchers could even start working out the depth, thickness and conductivity of different subsurface layers.<\/p>\n![\""An\"](\""https:\/\/images.immediate.co.uk\/production\/volatile\/sites\/25\/2019\/09\/Galileo_spacecraft-39f2bdf.jpg?quality=90&resize=620%2C471"\" "\\"\"An\\"")
<\/div>Detecting oceans on the moons of the Solar System<\/span><\/strong><\/h1>\n
![\""Voyager\"](\""https:\/\/images.immediate.co.uk\/production\/volatile\/sites\/25\/2020\/08\/Voyager-2-triton-6dd2225.jpg?quality=90&resize=620%2C482"\" "\\"\"Voyager\\"")
<\/div>![\""Uranus's\"](\""https:\/\/images.immediate.co.uk\/production\/volatile\/sites\/25\/2020\/05\/uranus-moon-miranda-bde072d.jpg?quality=90&resize=620%2C434"\" "\\"\"Uranus's\\"")
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