[meteorite-list] NASA's Hubble Observations Suggest Underground Ocean on Jupiter Moon Ganymede

[Ron Baalke]

March 12, 2015
     
NASA's Hubble Observations Suggest Underground Ocean on Jupiter's Largest Moon

NASA's Hubble Space Telescope has the best evidence yet for an underground 
saltwater ocean on Ganymede, Jupiter's largest moon. The subterranean ocean 
is thought to have more water than all the water on Earth's surface.

Identifying liquid water is crucial in the search for habitable worlds beyond 
Earth and for the search of life as we know it.

"This discovery marks a significant milestone, highlighting what only 
Hubble can accomplish," said John Grunsfeld, associate administrator of 
NASA's Science Mission Directorate at NASA Headquarters, Washington. "In 
its 25 years in orbit, Hubble has made many scientific discoveries in our 
own solar system. A deep ocean under the icy crust of Ganymede opens up 
further exciting possibilities for life beyond Earth."

Ganymede is the largest moon in our solar system and the only moon with its 
own magnetic field. The magnetic field causes aurorae, which are ribbons of 
glowing, hot electrified gas, in regions circling the north and south poles 
of the moon. Because Ganymede is close to Jupiter, it is also embedded in 
Jupiter's magnetic field. When Jupiter's magnetic field changes, the 
aurorae on Ganymede also change, "rocking" back and forth.

By watching the rocking motion of the two aurorae, scientists were able to 
determine that a large amount of saltwater exists beneath Ganymede's crust 
affecting its magnetic field.

[Image]
NASA Hubble Space Telescope images of Ganymede's auroral belts (colored blue 
in this illustration) are overlaid on a Galileo orbiter image of the moon. 
The amount of rocking of the moon's magnetic field suggests that the moon has 
a subsurface saltwater ocean.
Image Credit: NASA/ESA

A team of scientists led by Joachim Saur of the University of Cologne in 
Germany came up with the idea of using Hubble to learn more about the inside 
of the moon.

"I was always brainstorming how we could use a telescope in other ways," said 
Saur. "Is there a way you could use a telescope to look inside a planetary 
body? Then I thought, the aurorae! Because aurorae are controlled by the 
magnetic field, if you observe the aurorae in an appropriate way, you learn 
something about the magnetic field. If you know the magnetic field, then you 
know something about the moon's interior."

If a saltwater ocean were present, Jupiter's magnetic field would create a 
secondary magnetic field in the ocean that would counter Jupiter's field. 
This "magnetic friction' would suppress the rocking of the aurorae. This 
ocean fights Jupiter's magnetic field so strongly that it reduces the rocking 
of the aurorae to 2 degrees, instead of the 6 degrees, if the ocean was not 
present.

Scientists estimate the ocean is 60 miles (100 kilometers) thick - 10 times 
deeper than Earth's oceans - and is buried under a 95-mile (150-kilometer) 
crust of mostly ice.

Scientists first suspected an ocean in Ganymede in the 1970s, based on models 
of the large moon. NASA's Galileo mission measured Ganymede's magnetic field 
in 2002, providing the first evidence supporting those suspicions. The 
Galileo spacecraft took brief "snapshot" measurements of the magnetic field 
in 20-minute intervals, but its observations were too brief to distinctly 
catch the cyclical rocking of the ocean's secondary magnetic field.

The new observations were done in ultraviolet light and could only be 
accomplished with a space telescope high above the Earth's atmosphere, which 
blocks most ultraviolet light.

NASA's Hubble Space Telescope is celebrating 25 years of groundbreaking 
science on April 24. It has transformed our understanding of our solar system 
and beyond, and helped us find our place among the stars. To join the 
conversation about 25 years of Hubble discoveries, use the hashtag #Hubble25.

Hubble is a project of international cooperation between NASA and ESA 
(European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, 
Maryland, manages the telescope. The Space Telescope Science Institute 
(STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is 
operated for NASA by the Association of Universities for Research in 
Astronomy, Inc., in Washington.

For images and more information about Hubble, visit:

http://www.nasa.gov/hubble 

and

http://hubblesite.org/news/2015/09 

-end-

Felicia Chou
Headquarters, Washington
202-358-0257
felicia.chou at nasa.gov 

Ann Jenkins / Ray Villard
Space Telescope Science Institute, Baltimore, Md.
410-338-4488 / 410-338-4514
jenkins at stsci.edu [4] / villard at stsci.edu