Humans have been looking up to the night sky wondering and discovering what lies beyond for thousands of years.We have detected super massive black holes, discovered distant galaxies and located extra-solar planets outside of our Solar System.Despite this we have not discovered any other life forms beyond our own planet.Perhaps we ought to be looking closer to home.


Within our Solar System, there are eight planets including Earth.The search for life on these planets has been mainly limited to the red planet, Mars.It is thought to be just on the outskirts of the habitable zone (or “Goldilocks Zone”), the area just at the correct distance from the Sun for life to exist on the surface of a planet, the area where Earth is comfortably located.Mercury and Venus have extreme temperatures thought too severe for life to exist.Jupiter, Saturn, Uranus and Neptune are all stormy gas giant planets with no surface for life to exist on, just vast and cold and toxic atmospheres.As a result scientists are also considering other ‘worlds’ in our Solar System to discover if we really are alone.


Image of Galilean Moons

Magnificent Moon Montage showing (left to right) Io, Europa, Ganymede and Callisto.(Image credit:NASA/JPL)



These other worlds are the moons of our Solar System, of which there are approximately 170 in orbit around our planets.Some of these moons are very interesting, and have diverse and unique features, very different to our barren, heavily cratered Moon.Jupiter, the largest planet in our solar system has the most moons of any planet with known 64** moons in orbit.The four moons closest to this Gas Giant are most fascinating.Discovered by Italian Galileo Galilei in 1610, these moons (Io, Europa, Ganymede and Callisto) are often known as the Galilean satellites.The European Space Agency (ESA) has recently announced plans to explore three of these Galilean moons in search for liquid water.This mission is known as JUICE (JUpiter ICy moons Explorer).


Image of jupiter-moons

Artist’s Impression of the JUICE spacecraft in orbit around Jovian moons.(Image credit ESA/AOES)


Jupiter’s moons, Ganymede, Callisto and Europa are all thought to have liquid water beneath their frozen surfaces.One of the main components for life to exist is water, making these moons a very interesting destination for ESA.The JUICE spacecraft is planned to be launched onboard an Ariane 5  just ten years from now, in  2022.After eight years in transit, it is due to arrive at Jupiter in 2030.The whole mission is due to be complete in just a few years after which it is hoped that we will know significantly more about gas giants and how their satellites have developed and evolved over time.Also by studying our nearest gas giant planet and its moons it is hoped that this will provide insight into our understanding of exo-planets, many of which have been identified as “Hot Jupiters”.


Image of Ganymede

Ganymede, the largest moon in the Solar System thought to have oceans located underneath its icy surface.(Image credit:Galileo project, JPL and NASA.)




So far we know some information about these fascinating worlds, Ganymede is the largest of all the Solar System moons.This moon is also larger than the planet Mercury and almost three quarters of the size of Mars.Under an icy surface thought to be 800km thick , this moon is rocky but has an iron core which is thought to generate a magnetic field .Ganymede has two distinctive types of features, older, darker much cratered regions and younger, lighter grooved patterns thought to be caused by movement of sub-surface water.JUICE intends to conduct investigations mainly on this moon to determine whether this world could have ever supported life.


Image of callisto

Jupiter’s Moon Callisto captured by Voyager 1 in 1979.(Image credit:NASA)



Callisto is the third largest satellite in the solar system and also thought to be the most cratered.Callisto is the most distant from Jupiter lying outside Jupiter’s main radiation belt.This moon is thought to be a dead world with few terrain changes.Its surface age is thought to be 4 billion years old.Callisto is also thought to have ice located beneath the surface and oceans of liquid water 100km below the surface.Investigation of this moon along with Europa will allow scientists to compare data with that of Ganymede.



Image of europa

Europe plans to explore Europa.(Image Credit:NASA/JPL/ Ted Stryk)



Europa is the second closest to Jupiter and is only slightly smaller than our own Moon.It is thought to have an ocean of salty water covering the entire planet but one that has frozen over, giving the planet a smooth appearance.Europa closeness to Jupiter causes tidal affects on the moon causing constant movement of water beneath the ice.Due to tidal movements this moon is not as cold as perhaps it should be being so far from the Sun.This heat created by the tides, combined with liquid sub-surface water could prove to be supportive if life were to exist on this moon.


The moons in our Solar System seem to be the only possible location left unexplored for life to exist.Three of Jupiter’s closest moons are all known to have water located beneath a frozen crust.We know here on Earth that water is critical for life to exist.So perhaps with help from the JUICE spacecraft we may discover some of these worlds will have the correct combination of criteria just right for life.

So with the exploration of these Jovian moons planned in the year 2030 at the earliest we still have some time to wait before we discover if we are alone.

(Article by Martina Redpath, Education Support Officer)


**Admin note: Please note that this article was written and published in 2012. The total number of moons orbiting Jupiter has now gone up to 79.


Anees Shah · July 18, 2018 at 10:36

Twelve new Jupiter moons are discovered. One of them is very dangerous, because eleven of the moons behave relatively normally, but one beats out of line. The researchers call him a “nerd”.
want to know more about Jupiter moons? please read our article:

    admin · July 18, 2018 at 16:17

    A nerd? Not sure I’ve seen that in the articles, but yes it is a strange little moon! It’s so cool that it moves prograde, but is in the retrograde area! The moon has been given the nickname “Valetudo” after the Roman God Jupiter’s great-granddaughter. I wouldn’t say that the moon is “very dangerous” but certainly the astronomers have said that because it is moving the way it is, it is likely that at some point in the future it will crash head on into something else 🙂

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