All About Space

WATER VAPOUR DETECTED ON HUGE JUPITER MOON GANYMEDE FOR THE FIRST TIME

This could be a trend for icy bodies throughout the Solar System and beyond

- Reported by Charles Q. Choi

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.

Research has 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 160 kilometres (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 (-190 degrees Fahrenheit) at noon at the equator and lows of about -193 degrees Celsius (-315 degrees Fahrenheit) at night.

At the hottest spots on Ganymede, ice may become sufficient­ly warm enough to convert directly into vapour. They 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.

“Water vapour in the atmosphere matches the data very well,” study lead Lorenz Roth, a planetary scientist at the KTH Royal Institute of Technology in Stockholm, said. The main reason previous research failed to detect water in Ganymede’s atmosphere is because the ultraviole­t signal from molecular oxygen is very strong. “Within this stronger oxygen signal, it’s hard to find other signals,” Roth said. “These findings suggest that water vapour actually exists in the atmosphere­s of icy bodies in the outer Solar System,” he continued. “Now we might see it in more places.”

“FINDINGS SUGGEST WATER VAPOUR ACTUALLY EXISTS IN THE ATMOSPHERE­S OF ICY BODIES IN THE SOLAR SYSTEM”

LORENZ ROTH

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 ??  ?? Left: Ganymede is Jupiter’s largest moon, but only seventhclo­sest to the giant planet
Left: Ganymede is Jupiter’s largest moon, but only seventhclo­sest to the giant planet

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