January 12, 2015 - And Ganymede Makes Four

And Ganymede Makes Four It was 405 years ago this week that Galileo looked through his small telescope and discovered a fourth moon of Jupiter (he had discovered the other three one week earlier). Over the intervening centuries, we have learned a great deal about Ganymede, the largest moon in the solar system. Its diverse surface features indicate that Ganymede underwent several periods of geologic activity. The darker regions are heavily cratered and date from the time of an early bombardment more than four billion years ago. Other regions show fault lines and other evidence of more recent internal dynamics. Half water ice and half rock, Ganymede shows us a frozen record of the geologic process by which part of its ancient, darker surface was replaced by the younger, brighter grooved terrain.

Image credit: NASA / JPL / color composite by Ted Stryk

Weekly Calendar

January 12-18, 2015

Holidays - Sky Events - Space History

Monday 12

1986: STS-61C Columbia launched
1997: STS-81 Atlantis launched
2005: Deep Impact spacecraft launched

Tuesday 13

Last Qtr Moon 4:47 AM ET

1610: Galileo discovers Ganymede, moon of Jupiter
1978: NASA selects first women astronauts
1993: STS-54 Endeavour launched

Wednesday 14

Mercury at greatest elongation (19° E)

1975: Earth Resources Technology Satellite is renamed Landsat
2005: Huygens probe lands on Titan
2008: MESSENGER spacecraft makes its first flyby of Mercury