by Jupiter’s icy moon Europa has long been thought to be one of the most habitable worlds in the Solar System. Now the Juno mission to Jupiter has directly sampled its atmosphere in detail for the first time. The results, published in Nature Astronomy, show that Europa’s icy surface produces less oxygen than we thought.
There are plenty of reasons to be excited about the possibility of finding microbial life on Europa. Evidence from the Galileo mission has shown that there is an ocean beneath the moon’s icy surface containing about twice as much water as Earth’s oceans. Also, models derived from Europa data show that the ocean floor is in contact with rock, enabling chemical water-rock interactions that produce energy, making it the prime candidate for life.
Telescope observations, meanwhile, show a tenuous, oxygen-rich atmosphere. It also looks like plumes of water would occasionally erupt from the ocean. And there is some evidence that the surface contains basic chemical elements – including carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur – that life on Earth uses. Some of these may seep down into the water from the atmosphere and surface.
The warming of Europa and the oceans is partly a result of the moon’s orbit around Jupiter, which produces tidal forces to warm an otherwise frigid environment.
Although Europa has three basic ingredients for life – water, the right chemical elements and a heat source – we still don’t know if there has been enough time for life to develop.
The other main candidate in our solar system is Mars, the target of the rover Rosalind Franklin in 2028. Life could have started on Mars at the same time as it did on Earth, but then probably stopped because climate change.
The third candidate is Saturn’s moon Enceladus, where the Cassini-Huygens mission discovered plumes of water from a salty subsurface ocean, also in contact with rock at the sea floor.
Titan is the second closest in fourth place, with its thick atmosphere of organic compounds including hydrocarbons and toluene, born in the upper atmosphere. These swim to the surface then cover it with ingredients throughout their life.
Oxygen was lost
The Juno mission contains the best charged particle instruments sent to Jupiter to date. It can measure the energy, direction and composition of charged particles on the surface. Similar instruments at Saturn and Titan found toluene (a type of organic substance) there. But they also measured particles that suggested atmospheres at Saturn’s moons Rhea and Dione, as well as those at Titan and Enceladus.
These particles are called pick-up ions. Planetary atmospheres contain neutral particles, but the top of an atmosphere becomes “ionized” (meaning it loses electrons) in sunlight and through collisions with other particles, forming ions (charged atoms that have lost electrons) and free electrons.
When plasma – a charged gas that makes up the fourth state of matter beyond solid, liquid and gas – flows over an atmosphere of newly created ions, it disturbs the atmosphere with electric fields that can accelerate the new ions – the first part of ions picking process.
These pick-up ions then spiral around the planet’s magnetic field and are usually lost from the atmosphere, while some hit the surface and are absorbed. The accretion process has rid the Martian atmosphere of particles after the red planet’s magnetic field was lost 3.8 billion years ago.
Europa also has a collection process. The new measurements show the visible signs of the collection of molecular oxygen and hydrogen ions from the surface and atmosphere. Some of these escape Europa, but some hit the icy surface increasing the amount of oxygen on the surface and below the surface.
This indeed confirms that oxygen and hydrogen are the main constituents of Europa’s atmosphere – in agreement with remote observations. However, the measurements suggest that the amount of oxygen being produced – released from the surface to the atmosphere – is only around 12kg per second, at the lower end of earlier estimates of around 5kg to 1,100 kg per second.
This would indicate that there is little corrosion on the surface. The measurements suggest that this may be only 1.5cm of Europa’s surface per million years, which is less than we thought. So Europa is constantly losing oxygen through accretion processes, with only a small amount of additional oxygen being released from the surface to replenish it and ending up back on the surface.
So what does that mean for his chances of hosting life? Some of the oxygen trapped in the surface may find its way to the subsurface ocean to support any life there. But based on the study’s estimate of overall oxygen loss, this should be less than the 0.3kg-300kg per second estimated earlier.
It remains to be seen whether this rate, recorded on 29 September 2022, is normal. It may not be representative of the total oxygen on the moon. Plume eruption, orbital position and upstream conditions may increase and decrease the rate at certain times, respectively.
Nasa’s Europa Clipper mission, due to launch later this year, and the Juice mission, which will make two Europa flybys on their way to orbit Ganymede, will be able to follow up on these measurements, and provide much more information on the habitability of Europa.
This article from The Conversation is republished under a Creative Commons license. Read the original article.
Andrew Coates receives funding from STFC and UKSA (UK).
