Pop science
A new study of old data makes some interesting discoveries.
In the wisp-thin sky of Ganymede, the largest satellite in the Solar System, astronomers have detected evidence of water vapour for the first time. The discovery could shed light on similar watery atmospheres that may envelop other icy bodies in the Solar System and beyond.
Previous research suggested that Ganymede – which is larger than Mercury and Pluto and only slightly smaller than Mars – may contain more water than all of Earth’s oceans. However, the Jovian moon is so cold that water on its surface is frozen. Any liquid water would lurk about 100 miles below its crust.
Prior work suggested that ice on Ganymede’s surface could turn from a solid directly to a gas, skipping a liquid form entirely, so that water vapour could form part of the giant moon’s thin atmosphere. However, evidence of this water has proved elusive… until now.
In a new study, researchers analysed old and new data of Ganymede from NASA’s Hubble Space Telescope. In 1998 Hubble captured the first ultraviolet images of Ganymede, including pictures of its aurorae, the giant moon’s versions of Earth’s northern and southern lights. Colourful ribbons of electrified gas within these aurorae helped provide evidence that Ganymede has a weak magnetic field.
Ultraviolet signals detected in these auroral bands suggested the presence of oxygen molecules, each made of two oxygen atoms, which are produced when charged particles erode Ganymede’s icy surface. However, some of these ultraviolet emissions didn’t match what you would expect from an atmosphere of pure molecular oxygen. Previous research suggested these discrepancies were linked to signals from atomic oxygen – single atoms of oxygen.
As part of a large observing program to support NASA’s Juno mission, researchers sought to measure the amount of atomic oxygen in Ganymede’s atmosphere using Hubble. Unexpectedly, they discovered there is hardly any atomic oxygen there, suggesting there must be another explanation for the earlier ultraviolet signals. The scientists focused on how the surface temperature of Ganymede varies strongly throughout the day, with highs of about -123 degrees Celsius at noon at the equator and lows of about -193 degrees Celsius at night.
At the hottest spots on Ganymede, ice may become sufficiently warm enough to convert directly into vapour. Researchers noted that differences seen between a number of ultraviolet images from Ganymede closely match where you would expect water in the moon’s atmosphere based on its climate. CHARLES Q. CHOI
Ultraviolet signals detected in these auroral bands suggested the presence of oxygen molecules, each made of two oxygen atoms, which are produced when charged particles erode Ganymede’s icy surface.