On Monday, September 26th, after some teasing beforehand, NASA provided an update on the venting of water by Jupiter’s icy moon, Europa.
As I noted in my last Space Sunday report, Europa is covered by shell of water ice, much of it discoloured by mineral deposits and by deep cracks, beneath which it is believed to have a liquid water ocean about 100 km (62.5 miles) deep. The ocean is believed to be made possible by tidal flexing enacted by the massive gravity of Jupiter as well as from the other large Galilean moons. This generates heat within Europa, and this heat stops the water from freezing solid.
In 2012, The Hubble Space Telescope (HST) captured what appeared to be a huge plume of water erupting some 200 kilometres (125 mi) above the surface of Europa, using its Space Telescope Imaging Spectrograph (STIS) instrument. The update offered on September 26th provided information on further plumes, strengthening the case of water existing under the ice crust of Europa in the process – a crust which may be far thinner than thought.
Over a 15-month period, astronomers used Hubble’s STIS to observe Jupiter and Europa in the ultra-violet spectrum. During that time, Europa occulted (passed in front) of Jupiter on 10 separate occasions. The observations were an attempt to examine a possible extended atmosphere around the moon, which is slightly smaller than our own. However, on three of the passes, astronomers witnessed what appeared to be plumes of water erupting from the surface – and in pretty much the same location as seen in 2012. Analysis of the plumes revealed they were made up of hydrogen and oxygen consistent with water vapour being broken apart by Jupiter’s radiation in a process known as radiolysis.
The plumes are not constant, but rather flare up intermittently, possibly as a result of the surface ice on Europa flexing in response to the same gravitational influences that are keeping the ocean beneath the ice from freezing out. This suggests that the icy crust is, at least around the region where the plumes are occurring, thinner than had been thought. This is important, because it could mean that any automated mission sent to Europa could have a fair chance of cutting its way through the ice to deploy a submersible vehicle which could then search for any evidence of life in Europa’s salty ocean – which contains between two and three times as much water as all of Earth’s oceans combined.
The Gentle Crunch: Rosetta Mission Ends
The European Space Agency’s Rosetta spacecraft said farewell on Friday, September 30th, bringing the 12-year mission that bears its name to a close.
Launched in 2004, Rosetta was a daring attempt to rendezvous with a short-period comet, 67P/Churyumov-Gerasimenko, then orbit it and study it as it swept through the inner solar system and around the sun on its (roughly) 6-year obit. The aim was to give us unique insight into cometary behaviour and – more directly – to study one of these tiny lumps of mineral and chemical rich rock “left over” from the solar system’s formation, and thus gain greater understanding as to how things came to be, and perhaps how life itself might have begun.
Rosetta travelled almost 8 billion km (5 billion miles), including three flybys of Earth and one of Mars, and two asteroid encounters, before finally arriving at 67P/C-G in August 2014. In November of that year, The Philae lander was deployed in the hope of studying the comet from the surface and gathering samples of its material for analysis. Unfortunately, Philae’s anchoring mechanism failed, sending the little lander bouncing across the comet, until it came to rest in a location where it was receiving insufficient sunlight to recharge its batteries. Nevertheless, in the time it did have before its batteries were almost depleted, the washing machine sized lander some 80%+ of its science goals.
Meanwhile, Rosetta studied the comet in the long fall towards the Sun, and carried out an extensive mission of study, analysis and image capture, much of which has completely altered thinking around comets like 67P/C-G. For example, the mission discovered that water within the comet has a different ‘flavour’ to that of Earth’s oceans, suggesting that the impact of such comets with primordial Earth played far less of a role in helping start Earth’s oceans than had been thought.
As the comet became more active during its approach to the Sun, Rosetta found complex organic molecules – amino acid glycine, which is commonly found in proteins, and phosphorus, a key component of DNA and cell membranes – were present in the dust vented by 67P/C-G, reinforcing the idea that the basic building blocks for life may have been delivered to Earth from an early bombardment of such rocks. The mission also confirmed that the comet’s odd shape – two potato-like lobes of different sizes joined at a narrow waist – was the result of a very slow-speed collision very early in the comet’s 4.5 billion-year age.
In all the spacecraft operated in the harsh environment of the comet for 786 days, made a number of dramatic flybys close to its surface, survived several unexpected outgassings, and made two full recoveries for potentially serious “safe mode” situations. However, all things must inevitably come to an end, and with its manoeuvring propellants almost exhausted, on September 29th, Rosetta set course for a gentle crash landing on 67P/C-G.