Water is the building block of life, and evidence of it suggests the possibility of life on other worlds. This week, one of the Great Observatories may have unlocked the key to life on a distant, strange world. The Hubble Space Telescope (HST), which will soon enter its 25th year of operation, recently added another feather to its illustrious cap: its observations have shown strong evidence that a large underground saltwater ocean may possibly exist within Jupiter’s biggest moon, the icy Ganymede. While scientists have suspected since the 1970s that this may have been the case, this week NASA announced that the amount of “rocking” by the moon’s magnetic field supports the idea of such an ocean. Joachim Saur of Germany’s University of Cologne, who led the team that made this finding, enthused, “Our new HST observations provide the best evidence to date for the existence of an ocean on Ganymede.”
Ganymede has its own magnetic field and is able to generate aurorae surrounding its poles (similar to the aurorae seen surrounding Earth in time-lapse videos from the International Space Station). The moon, also the largest in our Solar System, is also located within Jupiter’s strong magnetic field; this causes the aurorae to show a “rocking” motion.
The Galileo space probe previously attempted to prove the theory of an underground ocean within Ganymede. However, according to NASA, “[The 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.” NASA stated that a group of scientists from Germany’s University of Cologne instead used HST to better observe this phenomenon. The team was led by Saur, who described the motivation to use Hubble’s capacity to view the ultraviolet spectrum in this unique manner:
“I was always brainstorming how we could use a telescope in other ways. 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,” he related.
NASA stated it’s theorized that the large ocean “fights” the magnetic field, causing the aurorae to rock by two degrees, as opposed to six degrees if no such ocean existed. In addition, the findings from the team show that the ocean is thought to be “ … 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.” The full report on these findings was published Thursday, March 12, in the Journal of Geophysical Research: Space Physics.
John Grunsfeld, NASA’s assistant administrator of its Science Mission Directorate, echoed Saur’s enthusiasm: “This discovery marks a significant milestone, highlighting what only Hubble can accomplish. 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,” he stated. Grunsfeld, a former astronaut who will soon be inducted into the U.S. Astronaut Hall of Fame, performed eight spacewalks during his spaceflight career over three missions in efforts to service HST.
HST was deployed from Space Shuttle Discovery during STS-31 on April 25, 1990. A previous AmericaSpace article detailed some of the early challenges faced by the space telescope.
The author wrote: “However, shortly after [HST’s] deployment it was discovered that the telescope’s primary mirror contained a serious flaw that made it produce ‘nearsighted,’ distinctly out-of-focus images … But all was not lost. Two instruments were designed to help address the flaw. The Wide Field and Planetary Camera 2 (WFPC2) and the Corrective Optics Space Telescope Axial Replacement (COSTAR) system were designed to work together and jointly correct the abberation in Hubble’s primary mirror. By December 1993, these instruments were ready to do their work. In addition, other repairs were scheduled to lengthen the telescope’s operational life and replace failing components.” The space telescope was famously returned to health during the STS-61 (Endeavour) mission, which launched that month.
It is hoped HST will last through 2020, encompassing 30 years in orbit. At present time, the James Webb Space Telescope (JWST) is being manufactured, with its scientific “heart” recently undergoing a “deep freeze” test—JWST is meant to operate at one of the distant Lagrangian points, and will experience extremely cold temperatures. Like HST, JWST is also a cooperative project between NASA and the European Space Agency (ESA); it is scheduled to launch aboard an Ariane 5 launch vehicle from Europe’s Spaceport in Kourou, French Guiana, in 2018. While that telescope is being billed as Hubble’s successor, even at its ripe age HST still continues to deliver scientific evidence about our Solar System and Universe that stuns scientists and casual observers.