Jupiter mission to explore conditions conducive to life

Jupiter is the largest planet in our solar system - a gas giant whose mass is about one thousandth that of our sun. It has more than 80 moons, making it almost like a solar system in its own right. ESA's JUICE mission will explore its three largest satellites - Ganymede, Callisto and Europa - among others. JUICE stands for Jupiter Icy Moons Explorer, and scientists believe that there may be gigantic oceans and possibly extraterrestrial life underneath the thick icy mantle of these moons. In addition to answering some fundamental questions about the formation of planetary systems, the mission also hopes to find out whether Jupiter's icy moons provide the necessary conditions for the emergence and long-term existence of life as we know it.

The space probe will take about eight years to reach the Jupiter system. After that, the four-year mission will begin, for which the probe is fitted with eleven extremely complex instruments. One of them has been provided by PSI and was developed under the direction of Wojciech Hajdas in the Laboratory for Particle Physics. The inconspicuous little box by the name of RADEM is compact and weighs three kilograms, looking more like a small car battery than a highly complex particle detector. But appearances can be deceptive: "RADEM stands for Radiation-hard Electron Monitor, an electron monitor that is resistant to radiation and that can detect high-energy particles in Jupiter's harsh environment," explains Hajdas.

In the middle of Jupiter's radiation belts

Like Earth, Jupiter has a rotating liquid metal core which generates a magnetic field. When charged particles such as electrons and protons enter this field, they are trapped inside it and accelerated along spiral paths around the planet. This acceleration is many times stronger than on Earth, resulting in high-energy synchrotron radiation, a special form of X-rays. On top of this, atoms and molecules are hurled into space by volcanic activity, which frequently occurs on Jupiter's moon Io, for example. They become ionised through collisions with the electrons - that is to say they acquire an electrical charge and thus also fall under the spell of Jupiter's gigantic magnetic field.

Such infernal radiation would be absolutely lethal, not just for human beings - they also pose a great danger to unmanned space probes and their sophisticated electronics. "Some parts of the electronics have been developed explicitly for this mission and are highly resistant to radiation. All the equipment in the probe is also specially clad to withstand the extreme conditions in Jupiter's radiation belts," explains Hajdas. "Nevertheless, a prolonged stay in certain zones can lead to damage." To prevent this, RADEM is directly connected to the probe's on-board computer. "If the radiation dose exceeds certain limits, the detector triggers an alarm signal. As it is difficult to carry out evasive manoeuvres, particularly sensitive equipment can instead be switched off in such cases, protecting it until the radiation levels are back within the permissible limits," says Hajdas.

RADEM does not simply serve as an alarm bell, however - its other task is to map Jupiter's complex radiation belts and gather information about their environment and the particles they contain. "The Jupiter system is completely unique - it is one of the most radiation-intensive environments in the entire solar system; a gigantic, natural particle accelerator," says Hajdas. "Not only does this provide an in-depth look at the fundamental laws of physics - models of the interactions occurring there could also be applied to other systems, such as solar activity and how it affects Earth's radiation belts and magnetosphere."

For its mission, RADEM is equipped with four separate detectors - one for each type of particle: electrons, protons and heavy ions. "The fourth detector registers either electrons or protons," explains Hajdas. "With an angular coverage of about 35 per cent, it allows us to determine the direction of incidence of these particles and hence the spatial distribution of the radiation environment." All this data has to be processed and stored within a very short time - and inside a very small space, so as to keep the device as light as possible.

Extraterrestrial life beneath the ice and radiation?

Jupiter's radiation belts extend several million kilometres into space - the highest particle densities and velocities around the gas giant have been recorded at a radius of around 670 000 kilometres, inside the orbit of Jupiter's icy moon Europa. At first glance, it may seem paradoxical that a moon travelling along some 780 million kilometres from the sun in such an unreal and icy death zone, should be considered a potential habitat for extraterrestrial life. However, while the radiation makes any life on Europa's surface impossible, it could lead to chemical reactions through interactions with the layer of ice, which would in turn serve as fuel for life. The lethal radiation would thus indirectly provide energy for microbial life - without photosynthesis or the presence of hydrothermal vents.

"The data collected during previous missions and from observations made from Earth have led to numerous speculations and calculations about the existence of life on Jupiter's moons," explains Hajdas. "JUICE will help us to better understand the complex Jupiter system. It's not a matter of finding life, it's about gaining a better understanding of the environment to determine whether it is a possible or impossible habitat for life."

RADEM will also monitor space weather

Many of the instruments included in JUICE will remain switched off during the eight-year voyage, but RADEM has work to do during the journey too: it is to measure the radiation within the solar system and determine how it relates to solar activity. "Between Venus and Jupiter, RADEM will determine the particle spectra and their doses in space and thus map an important parameter for the so-called space weather in this region," Hajdas explains. The sun is constantly flinging particles into space. These can not only cause radiation damage to satellites, but also disturb the Earth's magnetic field. Fluctuations in these currents can lead to excessively high voltages in electrical power grids and thus to blackouts. "The activity of our sun follows a regular cycle of about eleven years - during this time it fluctuates between a period of quiescence and a phase of more frequent solar storms. RADEM should help us to understand these activities better, and how they influence our planet and future missions, such as a potential manned mission to Mars," explains Hajdas.

This is why, unlike similar expeditions, the JUICE mission is being carried out during a so-called solar maximum - i.e. a period of high solar activity. To ensure the detector remains fully functional during this journey and especially during its stay in the Jupiter belt, it had to undergo several stress tests during its development. "With its large research facilities, PSI offers unique opportunities for simulating radiation exposure in space," explains the particle physicist. "The Proton Irradiation Facility PIF, the proton accelerator HIPA and a special vacuum chamber for low-energy electrons allowed us to create conditions similar to those that exist in space and prepare RADEM for its mission in the best possible way." Other research institutions involved in the mission, such as the University of Bern, which also developed two detectors for JUICE, brought their equipment to PSI for the beam tests too.

PSI's renowned large research facilities have been the source of unique insights with regard to the development and operation of particle detectors - knowledge that can be applied not only in laboratories, but now also in the vastness of space on this important mission.

Text: Paul Scherrer Institute/Benjamin A. Senn