 |
Welcome to Cheshire Starwatch a weekly column by Keene, NH amateur astronomer James Maynard. |
Stardate: 8:10:97
As we have spent the last few weeks exploring the Jovian system, let us continue this week with an in-depth look at Jupiter's dynamic worldlet of Europa.
Hardly deserving to be called a worldlet, Europa is 1563 km (977
miles) across, and has an average density over three times that of water.
It was discovered by Galileo in 1612, along with Io, Ganymede, and Callisto.
This moon, one of the four "Galilean moons" of Jupiter, is composed
mostly of ices and rock. Exciting new information provides evidence of a
large ocean or oceans on this strange world.
Europa, along with Titan and Mars, has long been a source of subject matter for science fiction authors, and it was Europa on which Arthur C. Clarke placed a strange alien monolith. There is reason to believe that Europa may be capable of supporting some form of life, although no evidence has yet been found. Alien monoliths are unlikely to be a concern, however.
The first hint of liquid water on Europa
came from infrared observations made by Earth based telescopes. Photographs
taken by the Voyager spacecraft over 15 years ago revealed the surface of
Europa to be bright and smooth. The logical conclusion was a large ocean
or oceans on the surface, covered by a layer of ice. Scientists worldwide
then waited in suspense for more detailed 
observations, and that would
not come until the Galileo spacecraft reached the Jovian system at the end
of 1995. One thing the scientists were sure of was that some process was
filling in the craters nearly as quickly as they formed. This meant the
world was geologically active, remarking it's surface in a constant recycling
of material.
But this would require energy! After all,
moving rock requires an input of energy. Where is that energy coming from?
If you may recall from a couple of weeks ago, Europa's sibling moon Io also
expends more energy than it should. Like Io, Europa is too small for gravity
to be melting the core, and it's also so small, that the amount of radioactive
material left over from it's formation will now be reduced to negligible
amounts. So where is the energy coming from? It turns out the answer is
that, like Io, Europa is being warmed by tidal forces caused by Jupiter.
The surface of Europa is cracked, and to me, resembles the branches of a
bare tree in winter silhouetted against a gray sky. These cracks are up
to 1000 km (625 miles) long, and 10 - 50 kilometers (6 miles to about 30
miles) across.
Similar structures are observed on Earth where polar ice Melts, and then refreezes.
The Galileo spacecraft, on July 18 of this year, reported finding an atmosphere surrounding Europa, and the Hubble space telescope reported the presence of oxygen around this Jovian moon.
If you wish to find Europa in the summer sky, it's easily visible
- just look in the south east sky, towards the south later in the evening.
Find a map of the Jovian moons for the evening you will be observing. These
are published in most astronomy magazines whenever Jupiter is in the sky,
or many software programs will plot them for you. Any small telescope or
binoculars will reveal Europa, along with the other Jovian moons appearing
as a dot alongside Jupiter. See if you can determine which moon is which
as you observe them.
Clear skies, and good viewing.
This site operated by 
