![\"""\"](\""https:\/\/images.immediate.co.uk\/production\/volatile\/sites\/25\/2019\/10\/Jupiter_Io_Shadow-f75890f-e1571310606800.jpg?quality=90&resize=620%2C210"\" "\\"\"\"\\"\/")
<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>Europa is an icy moon with a subsurface ocean lurking beneath its frozen crust, much like Saturn\u2019s moon Enceladus. As a result, it\u2019s one of the best places in the Solar System to search for conditions that could support life.<\/p>\n
Callisto is the third largest moon in the Solar System, after Saturn\u2019s moon Titan, and may be considered by some to be the lifeless, characterless member of the four Galilean moons.<\/p>\n
However, there is evidence that Callisto too hosts a subsurface ocean. Could it be another contender for life-supporting conditions beyond Earth?<\/p>\n
![\"""\"](\""https:\/\/images.immediate.co.uk\/production\/volatile\/sites\/25\/2020\/07\/Juno-Ganymede-6bb73fa.jpg?quality=90&resize=620%2C349"\" "\\"\"\"\\"\/")
<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>How did the Galilean moons form?<\/strong><\/h1>\nIt is thought that\u00a0 Jupiter\u2019s largest moons Io, Europa, Ganymede and Callisto most probably formed out of material leftover from the formation of Jupiter itself.<\/p>\n
Like the rest of the Solar System planets<\/a>, Jupiter formed out of the disc of dust and gas surrounding the young Sun. Once this gas and dust had condensed to form Jupiter, the leftover material coalesced and grew over time to form the Galilean moons.<\/p>\nThis makes Jupiter\u2019s largest moons likely as old of the rest of the Solar System: about 4.5 billion years old.<\/p>\n
![\"""\"]( "\\"\"\"\\"\/")
<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div> A Hubble Space Telescope image of Europa showing suspected plumes of water vapour erupting at the 7 o\u2019clock mark. These plumes could be evidence of a subsurface ocean below Europa\u2019s icy crust.Observing the Galilean moons<\/strong><\/h1>\nThe Galilean moons are also a favourite target of astronomers because it is possible to view them around Jupiter, provided you have a big enough telescope.<\/p>\n
If you know when and where to look, you may be able to see transits of the moons passing Jupiter, spot their shadows cast on the gas giant\u2019s surface, or even an occultation<\/a>.<\/p>\nFind out more about observing the Galilean moons<\/a>.<\/strong><\/em><\/p>\nFacts about the Galilean moons<\/strong><\/h1>\nIo<\/strong><\/h2>\n ![\"""\"]( "\\"\"\"\\"\/")
<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div> A mosaic of the Galileo probe\u2019s highest resolution images of Io, captured on 3 July 1999. Credit: NASA\/JPL\/University of Arizona<\/span><\/figcaption><\/div>\n- Diameter:<\/strong> 3,660km<\/li>\n
- Mass:<\/strong> 0.015 Earth masses<\/li>\n
- Orbital distance:<\/strong> 421,800km<\/li>\n
- Equatorial circumference:<\/strong> 11,445.5<\/span>km<\/span><\/li>\n<\/ul>\nIo is the most volcanically active body in the solar system, hosting hundreds of volcanoes that are kept active by the gravitational interactions between the moon, Jupiter, and fellow Jovian satellites Europa and Ganymede.<\/div>\n
Europa<\/strong><\/h2>\n ![\"""\"]( "\\"\"\"\\"\/")
<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div> This view of Europa shows long linear cracks crisscrossing across the surface of the icy moon. Credit: NASA\/JPL-Caltech\/SETI Institut<\/span><\/figcaption><\/div>\n- Diameter:<\/strong> 3,122km<\/li>\n
- Mass:<\/strong> 0.008 Earth masses<\/li>\n
- Orbital distance:<\/strong> 670,900km<\/li>\n
- Equatorial circumference:<\/strong> 9,807<\/span>km<\/span><\/li>\n<\/ul>\nThe smallest of the moons, Europa is an icy world known to have a liquid ocean beneath its 15-25km thick icy crust, which may contain twice as much as all of Earth\u2019s oceans combined.<\/div>\n
Ganymede<\/strong><\/h2>\n ![\"""\"]( "\\"\"\"\\"\/")
<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div> A view of Ganymede captured by NASA\u2019s Juno spacecraft during its flyby on 7 June 2021. Credit: NASA\/JPL-Caltech\/SwRI<\/span><\/figcaption><\/div>\n- Diameter:<\/strong> 5,268km<\/li>\n
- Mass:<\/strong> 0.025 Earth masses<\/li>\n
- Orbital distance:<\/strong> 1,070,400km<\/li>\n
- Equatorial circumference:<\/strong> 16,532km<\/li>\n<\/ul>
Ganymede is quite a specimen. It\u2019s the largest moon in the Solar System and the only moon orbiting the Sun with its own magnetic field. As a result it has aurorae at its north and south poles.<\/p>\n
It is thought to have a large rocky core covered with layers of ice and water, which show signs of tectonic activity.<\/p>\n
The surface of Ganymede seems to have two types of terrain: dark cratered areas and lighter terrain sporting cracks and grooves.<\/p>\n
These grooves can stretch 700 metres high and could be a result of tensional faulting, or it could be that Ganymede has a subsurface ocean that\u2019s releasing water into space.<\/p>\n
\nCallisto<\/strong><\/h2>\n ![\"""\"]( "\\"\"\"\\"\/")
<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div> An image of Jupiter\u2019s moon Callisto: the only complete global colour image of the moon obtained by the Galileo spacecraft. It was captured in May 2001. Credit: NASA\/JPL\/DLR<\/span><\/figcaption><\/div>\n- Diameter:<\/strong> 4,821km<\/li>\n
- Mass:<\/strong> 0.018 Earth masses<\/li>\n
- Orbital distance:<\/strong> 1,882,700km<\/li>\n
- Equatorial circumference:<\/strong> 15,144km<\/li>\n<\/ul>
Callisto is Jupiter\u2019s second largest moon and perhaps the dark horse of the Galilean satellites. It seems to be a geologically inactive, dark, cratered world, but planetary scientists believe it could host a liquid ocean beneath its surface.<\/p>\n
That\u2019s not to diminish what\u2019s going on at the surface, however. Callisto\u2019s surface is about 4 billion years old \u2013 a result of its geological inertness \u2013 and thought to be the oldest, most cratered surface in the Solar System.<\/p>\n
The bright spots that pepper Callisto\u2019s surface might well be water ice, frozen on the peaks of its many craters.<\/p>\n
Callisto\u2019s subsurface ocean could be interacting with a layer of rocks 250km deep, and oxygen has been detected in the moon\u2019s exosphere.<\/p>\n
Could Callisto be a key player in the search for life in the Solar System?<\/p>\n
The discovery of the Galilean moons<\/strong><\/h1>\n ![\"""\"]( "\\"\"\"\\"\/")
<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div> A fresco by Giuseppe Bertini portraying Galileo showing the Doge of Venice how to use a telescope. Credit: Giuseppe Bertini<\/span><\/figcaption><\/div>\n\u00a0<\/p>\n
On the title page of Galileo<\/a>\u2019s
This makes Jupiter\u2019s largest moons likely as old of the rest of the Solar System: about 4.5 billion years old.<\/p>\n
![\"""\"](\""https:\/\/images.immediate.co.uk\/production\/volatile\/sites\/25\/2019\/08\/Europa_water_vapour-4214f8d-e1566371545727.jpg?quality=90&resize=620%2C519"\" "\\"\"\"\\"\/")
<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>Observing the Galilean moons<\/strong><\/h1>\nThe Galilean moons are also a favourite target of astronomers because it is possible to view them around Jupiter, provided you have a big enough telescope.<\/p>\n
If you know when and where to look, you may be able to see transits of the moons passing Jupiter, spot their shadows cast on the gas giant\u2019s surface, or even an occultation<\/a>.<\/p>\nFind out more about observing the Galilean moons<\/a>.<\/strong><\/em><\/p>\nFacts about the Galilean moons<\/strong><\/h1>\nIo<\/strong><\/h2>\n ![\"""\"]( "\\"\"\"\\"\/")
<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div> A mosaic of the Galileo probe\u2019s highest resolution images of Io, captured on 3 July 1999. Credit: NASA\/JPL\/University of Arizona<\/span><\/figcaption><\/div>\n- Diameter:<\/strong> 3,660km<\/li>\n
- Mass:<\/strong> 0.015 Earth masses<\/li>\n
- Orbital distance:<\/strong> 421,800km<\/li>\n
- Equatorial circumference:<\/strong> 11,445.5<\/span>km<\/span><\/li>\n<\/ul>\nIo is the most volcanically active body in the solar system, hosting hundreds of volcanoes that are kept active by the gravitational interactions between the moon, Jupiter, and fellow Jovian satellites Europa and Ganymede.<\/div>\n
Europa<\/strong><\/h2>\n ![\"""\"]( "\\"\"\"\\"\/")
<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div> This view of Europa shows long linear cracks crisscrossing across the surface of the icy moon. Credit: NASA\/JPL-Caltech\/SETI Institut<\/span><\/figcaption><\/div>\n- Diameter:<\/strong> 3,122km<\/li>\n
- Mass:<\/strong> 0.008 Earth masses<\/li>\n
- Orbital distance:<\/strong> 670,900km<\/li>\n
- Equatorial circumference:<\/strong> 9,807<\/span>km<\/span><\/li>\n<\/ul>\nThe smallest of the moons, Europa is an icy world known to have a liquid ocean beneath its 15-25km thick icy crust, which may contain twice as much as all of Earth\u2019s oceans combined.<\/div>\n
Ganymede<\/strong><\/h2>\n ![\"""\"]( "\\"\"\"\\"\/")
<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div> A view of Ganymede captured by NASA\u2019s Juno spacecraft during its flyby on 7 June 2021. Credit: NASA\/JPL-Caltech\/SwRI<\/span><\/figcaption><\/div>\n- Diameter:<\/strong> 5,268km<\/li>\n
- Mass:<\/strong> 0.025 Earth masses<\/li>\n
- Orbital distance:<\/strong> 1,070,400km<\/li>\n
- Equatorial circumference:<\/strong> 16,532km<\/li>\n<\/ul>
Ganymede is quite a specimen. It\u2019s the largest moon in the Solar System and the only moon orbiting the Sun with its own magnetic field. As a result it has aurorae at its north and south poles.<\/p>\n
It is thought to have a large rocky core covered with layers of ice and water, which show signs of tectonic activity.<\/p>\n
The surface of Ganymede seems to have two types of terrain: dark cratered areas and lighter terrain sporting cracks and grooves.<\/p>\n
These grooves can stretch 700 metres high and could be a result of tensional faulting, or it could be that Ganymede has a subsurface ocean that\u2019s releasing water into space.<\/p>\n
\nCallisto<\/strong><\/h2>\n ![\"""\"]( "\\"\"\"\\"\/")
<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div> An image of Jupiter\u2019s moon Callisto: the only complete global colour image of the moon obtained by the Galileo spacecraft. It was captured in May 2001. Credit: NASA\/JPL\/DLR<\/span><\/figcaption><\/div>\n- Diameter:<\/strong> 4,821km<\/li>\n
- Mass:<\/strong> 0.018 Earth masses<\/li>\n
- Orbital distance:<\/strong> 1,882,700km<\/li>\n
- Equatorial circumference:<\/strong> 15,144km<\/li>\n<\/ul>
Callisto is Jupiter\u2019s second largest moon and perhaps the dark horse of the Galilean satellites. It seems to be a geologically inactive, dark, cratered world, but planetary scientists believe it could host a liquid ocean beneath its surface.<\/p>\n
That\u2019s not to diminish what\u2019s going on at the surface, however. Callisto\u2019s surface is about 4 billion years old \u2013 a result of its geological inertness \u2013 and thought to be the oldest, most cratered surface in the Solar System.<\/p>\n
The bright spots that pepper Callisto\u2019s surface might well be water ice, frozen on the peaks of its many craters.<\/p>\n
Callisto\u2019s subsurface ocean could be interacting with a layer of rocks 250km deep, and oxygen has been detected in the moon\u2019s exosphere.<\/p>\n
Could Callisto be a key player in the search for life in the Solar System?<\/p>\n
The discovery of the Galilean moons<\/strong><\/h1>\n ![\"""\"]( "\\"\"\"\\"\/")
<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div> A fresco by Giuseppe Bertini portraying Galileo showing the Doge of Venice how to use a telescope. Credit: Giuseppe Bertini<\/span><\/figcaption><\/div>\n\u00a0<\/p>\n
On the title page of Galileo<\/a>\u2019s
Find out more about observing the Galilean moons<\/a>.<\/strong><\/em><\/p>\n Ganymede is quite a specimen. It\u2019s the largest moon in the Solar System and the only moon orbiting the Sun with its own magnetic field. As a result it has aurorae at its north and south poles.<\/p>\n It is thought to have a large rocky core covered with layers of ice and water, which show signs of tectonic activity.<\/p>\n The surface of Ganymede seems to have two types of terrain: dark cratered areas and lighter terrain sporting cracks and grooves.<\/p>\n These grooves can stretch 700 metres high and could be a result of tensional faulting, or it could be that Ganymede has a subsurface ocean that\u2019s releasing water into space.<\/p>\n Callisto is Jupiter\u2019s second largest moon and perhaps the dark horse of the Galilean satellites. It seems to be a geologically inactive, dark, cratered world, but planetary scientists believe it could host a liquid ocean beneath its surface.<\/p>\n That\u2019s not to diminish what\u2019s going on at the surface, however. Callisto\u2019s surface is about 4 billion years old \u2013 a result of its geological inertness \u2013 and thought to be the oldest, most cratered surface in the Solar System.<\/p>\n The bright spots that pepper Callisto\u2019s surface might well be water ice, frozen on the peaks of its many craters.<\/p>\n Callisto\u2019s subsurface ocean could be interacting with a layer of rocks 250km deep, and oxygen has been detected in the moon\u2019s exosphere.<\/p>\n Could Callisto be a key player in the search for life in the Solar System?<\/p>\n \u00a0<\/p>\n On the title page of Galileo<\/a>\u2019s Facts about the Galilean moons<\/strong><\/h1>\n
Io<\/strong><\/h2>\n
![\"""\"](\""https:\/\/images.immediate.co.uk\/production\/volatile\/sites\/25\/2020\/07\/Jupiter-Io-moon-ee037d2.jpg?quality=90&resize=620%2C469"\" "\\"\"\"\\"\/")
<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>Europa<\/strong><\/h2>\n
![\"""\"](\""https:\/\/images.immediate.co.uk\/production\/volatile\/sites\/25\/2019\/09\/Europa-Stunning-Surface-6c79a68-e1619685372578.jpg?quality=90&resize=620%2C458"\" "\\"\"\"\\"\/")
<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>Ganymede<\/strong><\/h2>\n
![\"""\"](\""https:\/\/images.immediate.co.uk\/production\/volatile\/sites\/25\/2021\/07\/Ganymede-Juno-79c8b36.jpg?quality=90&resize=620%2C465"\" "\\"\"\"\\"\/")
<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>Callisto<\/strong><\/h2>\n
![\"""\"](\""https:\/\/images.immediate.co.uk\/production\/volatile\/sites\/25\/2019\/09\/Global-Callisto-in-Color-f768502.jpg?quality=90&resize=620%2C631"\" "\\"\"\"\\"\/")
<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>The discovery of the Galilean moons<\/strong><\/h1>\n
![\"""\"](\""https:\/\/images.immediate.co.uk\/production\/volatile\/sites\/25\/2019\/04\/Galileo-Doge-venice-f24eb91.jpg?quality=90&resize=620%2C350"\" "\\"\"\"\\"\/")
<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>