The
Moons of Jupiter – Then & Now
On January 7, 1610 Galileo
first viewed Jupiter through his telescope. What caught his eye was not the planet itself, but three bright
stars that were arranged in a perfect line on either side of the planet. Galileo sketched Jupiter and the three
stars, thinking at first they were simply a chance alignment. Some of his
original sketches are below.
Above: GalileoÕs sketches of the moons of
Jupiter made on the nights of February 3 and 4, 1610. (Ori means Orient, or East, Occ Occident, or West). Image Credit: Octavo Corp./Warnock
Library)
The next night Galileo
decided to check in on Jupiter again.
There were the three stars, but now in different positions relative to
the planet itself. The next night
was cloudy, but on the 10th, he saw a fourth star in the same
line. As he continued to study
Jupiter on successive nights, Galileo came to the realization that these four points
of light were not background stars, but mini planets orbiting around
Jupiter. The existence of four new
worlds was amazing in itself, but the discovery would help bring about a
revolution in astronomy and our understanding of the cosmos.
Was the Earth or the Sun the
center of the Universe? The debate
was raging in the first decade of the 17th century. Does the Sun move around a stationary
Earth, or does the Earth, as simply another planet, move around a stationary
Sun? Neither camp had any hard
evidence to back its case, but the Earth-centered supporters had a strong
argument. If the Earth moved,
wouldnÕt it leave the Moon behind? (Remember, this is before we had an
understanding of gravity).
Galileo, a supporter of the Sun-centered universe, could now counter
that argument: If Jupiter can move
and take its moons with it, then surely the Earth can carry its Moon through
space as well. Although not proof that
the Earth moved, it was one significant piece of evidence that would help pave
the way for acceptance of a heliocentric (Sun-centered) universe.
Above: MicroObservatory image of JupiterÕs
moons. Notice that the planet
itself is over-exposed to show the fainter moons.
What did YOU See with MicroObservatory?
How
do the Moons of Jupiter appear in your images taken by MicroObservatory? To see
the images in more detail, you may want to open your image in our MicroObservatory
image processing software.
In
the archive image above, the planet itself looks smeared out because it is
overexposed to show the fainter moons, Can you see the positions of the moons
change with respect to Jupiter?
The inner moons move in their orbits faster, so they will show more of a
change. If you requested more than
one image of JupiterÕs Moons, you can combine
the images into an animation to see how the moons move.
The
Moons of Jupiter - 400 years later
The moons of Jupiter are
seen today as unique worlds in their own right. The Jovian system, comprising
the four Galilean and (at last count) 23 smaller moons, has been visited by
seven space probes since1973. The
most significant mission, fittingly named Galileo, studied Jupiter and its
moons for eight years between 1995 and 2003. What do we know about the moons of
Jupiter now?
Io the innermost Galilean moon, is perhaps the most amazing
moon in the solar system. Looking like a messy pizza, its orange-yellow surface
is covered in sulfur-belching volcanoes and lava lakes. IoÕs geology is so active because the
poor little moon is stretched and squeezed by the gravity of Jupiter and the
other outer Galilean moons, heating and melting its surface. (Image credit:
NASA)
Callisto is the second largest moon of Jupiter and is about
the same size as the planet Mercury. Whereas Io has the youngest surface in the
Solar System, Callisto has the oldest. Its crust dates back 4 billion years,
just shortly after the solar system was formed. Its ancient history is evident by having the most cratered
surface of any moon in the solar system. (Image credit: NASA)
Ganymede is the largest moon of Jupiter. If it orbited the Sun instead of
Jupiter it would be classified as a planet. Like Callisto, Ganymede is most
likely composed of a rocky core with crust of rock and ice. Ganymede has had a
complex geological history. It has mountains, valleys, craters and lava flows.
GanymedeÕs dark crust is mottled with bright spots where recent meteorite
impacts have exposed clean bright ice from below the surface. (Image credit:
NASA)
Europa is among the brightest moons of the solar system, a
consequence of sunlight reflecting off a relatively young icy crust. Its
surface is also among the smoothest, lacking the heavily cratered appearance
characteristic of Callisto and Ganymede. Lines and cracks wrap the exterior
suggest a solid layer of ice above a liquid water ocean. Liquid water is found
on only one other body in the solar system: Earth. (Image credit: NASA)
Find
Out More
2009 is International Year
of Astronomy, chosen to commemorate the 400th anniversary of
GalileoÕs discoveries with the telescope.
Find out what else is happening by visiting these web sites:
The World site for International Year of
Astronomy.
The United States national site for
International Year of Astronomy.
NASAÕs International Year of Astronomy
website.
Learn more about the Galileo
Mission, at:
http://solarsystem.nasa.gov/galileo/
For more information on the
planets of the Solar System, go to:
http://solarsystem.nasa.gov/planets/index.cfm
The space probes that have
visited Jupiter are Pioneer 10 (1973), Pioneer 11 (1974), Voyager 1 and Voyager
2 (1979); Galileo (1995) and New Horizons (2007). To read about all NASAÕs missions, go to http://www.nasa.gov/missions/index.html
Take
a look at the full list of objects in MicroObservatoryÕs Galileo activity,
and see how our understanding has evolved over the last four centuries.