Physics 101 - Astronomy - Spring 2019
Class notes for day 15, March 7, 2019
Ch. 12 - Galilean moons of Jupiter, medium size moons, and Titan
The moons of the solar system can be put into three categories: 7 large (each
is distinctive), 12 medium-size, and many small (most are captured asteroids).
The Earth's Moon is the first example of a large moon. We studied it in Ch. 6.
The four Galilean moons of Jupiter have interiors that are quite distinct from
each other. Io has an iron core and a mantle that has
a magma ocean underneath a crust. Io has extensive volcanic activity due to
tidal effects, because of its interaction with both Jupiter and Europa. The
tidal forces between the moons is so strong that it has caused them to have
orbital periods related to each other by simple fractions.
Europa is further from Jupiter, with less tidal heating, and has a thick ice
layer on top of a liquid water ocean. Ganymede has a differentiated core
and mantle, and thick layer of ice. Callisto is probably undifferentiated,
without separate core and mantle.
Io and Europa are about the size of the Earth's Moon, and Ganymede and Callisto
are about the size of Mercury.
Io is the most volcanic object in the solar system, due to tidal forces between
Io, Europa, and Jupiter.
Europa has an icy crust like an arctic ice sheet, and probably has a deep ocean
underneath it. The depths of this ocean are a possible site for the existence of
some exotic forms of life because there should be some warm spots like the
hydrothermal vents in the Earth's oceans.
Ganymede has an icy surface with cracks similar to those seen on Europa and
other moons. Recent work shows that it probably has a thick layer of liquid water
underneath the icy crust.
Ganymede also has a string of craters that are probably due to a comet that
broke up at it passed Jupiter, then impacted on Ganymede. (This event would be
similar to the breakup of the Shoemaker-Levi comet that was mentioned in the
previous lecture on Jupiter.)
Callisto is similar to Ganymede, but may have a mixed up interior rather than
differentiated layers.
NASA has produced a new map of Ganymede, shown on this web page:
http://www.astronomy.com/news/2014/02/largest-solar-system-moon-detailed-in-geologic-map
Medium-sized moons (mostly around Saturn and Uranus)
The medium-sized moons of Saturn seem to be icy objects but are large enough to be round and have lots of craters. In addition, some of them show cracking like Europa, a moon of Jupiter.
Enceladus is interesting because it is relatively close to Saturn, and it erupting with geysers of water vapor and ice crystals, which contribute to a faint ring around Saturn. There is a short video from NASA about these geysers, which can be seen if you click on the link "Taking the Plunge" which is about halfway down on this web page: http://www.nasa.gov/mission_pages/cassini/multimedia/video-collection_archive_1.html
Many more news items about discoveries from the Cassini spacecraft are listed in an archive at http://www.nasa.gov/mission_pages/cassini/whycassini/cassini-latest-news-archive.html
I only showed a couple examples of the small moons of the solar system, which are probably just captured asteroids, or objects from outside the orbit of Neptune, called KBOs (Kuiper Belt Objects), which we will study next week.
Titan, the largest moon of Saturn, is the only moon in the solar system which has an atmosphere. The atmosphere is mostly nitrogen and is thicker than Earth’s atmosphere. Titan’s surface features are difficult to see in visible light from the Cassini spacecraft, so it also took infrared pictures and used radar to study the surface. Titan has lakes of liquid methane (with other unknown compounds dissolved in the liquid) and some are as large as the Great Lakes along the US-Canada border. Titan’s atmosphere has a high altitude haze, and very thick haze underneath which makes it impossible to see the surface with visible light. The surface of Titan was imaged by the probe “Huygens” when it landed. It looks like a sandy slush with some rocks strewn around (probably not really rocky stuff but chunks of solid water-ice). Rivers were seen on the surface of Titan as the probe “Huygens” descended to the surface. Lakes of methane (and ethane) near the north pole of Titan were seen by the Cassini spacecraft, using a radar imager. One of the lakes is somewhat bigger than Lake Superior. A possible volcano (a so-called cryovolcano which emits liquid water instead of lava) on Titan was seen by the Cassini spacecraft, and this may be a major source of methane on Titan.
NASA has some web pages devoted to Titan's lakes. See
http://www.nasa.gov/mission_pages/cassini/whycassini/cassini20131023.html
They have produced a new map of Titan's north pole
http://www.jpl.nasa.gov/spaceimages/details.php?id=PIA17655
This web page describes the formation of Titan's haze, with a graphic about the
composition of the atmosphere:
http://www.jpl.nasa.gov/spaceimages/details.php?id=PIA17240
Ligeia Mare, shown here in a false-color image from NASA's Cassini mission, is
the second largest known body of liquid on Saturn's moon Titan.
http://www.jpl.nasa.gov/spaceimages/details.php?id=PIA17031
Here is a view of a rotating Titan - a sequence of images.
http://photojournal.jpl.nasa.gov/catalog/PIA02146
Here is a video about the descent of Huygens probe (which was carried there on
the Cassini spacecraft, then released to begin the descent)
http://apod.nasa.gov/apod/ap130121.html
This is a video simulating what a game console might display during the descent
of the Huygens probe to the surface of Titan back in 2005.
http://photojournal.jpl.nasa.gov/catalog/PIA08117
Cassini has now flown past Titan over 100 times!
http://www.nasa.gov/jpl/cassini/titan-flyby-20140305/
Here is a selection of videos that I show in class if there is time:
A new version of imagery during the Huygens descent:
https://www.youtube.com/watch?v=9L471ct7YDo 4:41
Another video shows a zoom to the landing site: Approaching Titan a Billion
Times Closer
https://www.youtube.com/watch?v=TMxL3ZhO8A8 3:23
Soaring Over Titan: Extraterrestrial Land of Lakes 1:27
https://www.youtube.com/watch?v=RrGPtCdItBw&feature=player_embedded
The Canyon Country (fly-thru karst topography) 1:17
https://www.youtube.com/watch?v=YX7qbHOXxOA&list=PLTiv_XWHnOZpKPaDTVy36z0U8GxoiIkZa&index=23
Cassini: Four Years of Discovery (Aug 2008 tribute video) 2:18
https://www.youtube.com/watch?v=6X0cCnOZej8&index=18&list=PLTiv_XWHnOZpKPaDTVy36z0U8GxoiIkZa
Cassini: 15 Years of Exploration (Nov. 2012 tribute video) 2:29
https://www.youtube.com/watch?v=6JHv4FX0RP4&index=16&list=PLTiv_XWHnOZpKPaDTVy36z0U8GxoiIkZa
Titan Touchdown (overview of Huygens landing with big titles) 2:27
https://www.youtube.com/watch?v=msiLWxDayuA&list=PLTiv_XWHnOZpKPaDTVy36z0U8GxoiIkZa&index=2