Apr 04 2022

Europa Clipper Mission and Life on Europa

Published by under Astronomy
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What is our best bet for finding life outside of Earth? We could detect biosignatures on exoplanets, but at best we would be inferring the probably existence of some kind of life, but not be able to examine it directly. Therefore the most intriguing questions about extraterrestrial life will go unanswered. If we detect life in our own solar system there is the possibility of getting samples of that life in the not-to-distant future. Mars, as hostile as it is, is the closest thing to Earth in the solar system so we have focused a lot of attention there.

In the last few decades, however, another possibility has gained attention – life evolving in the subsurface oceans of icy worlds, most notably the moons Europa of Jupiter and Enceladus of Saturn.  In 2024 NASA will be sending the Europa Clipper mission to Jupiter. The craft will enter into an orbit around Jupiter designed to give it multiple close approaches to Europa. It is outfitted with a host of instruments to study the icy moon – a mass spectrometer to examine the constituents of the surface ice, a magnetometer, radar to probe the structures under the surface, and many others. The probe will arrive at Jupiter around 2030 so until then scientists are busy trying to learn as much about Europa as possible to make the best of the Clipper mission.

While the mission is not designed to directly detect life on Europa, it is designed to answer very specific questions about the plausibility of life. Being a frozen world distant from the sun (the sun’s light intensity is only 4% that of Earth’s), how could life evolve and survive? Beneath the icy shell of Europa is briny water, water mixed with minerals that is partially frozen but also liquid enough to have convection. Below the brine is a large liquid ocean, with more than twice the volume of water as in the oceans of Earth. Europa is not frozen solid (which it would be, given its size and distance from the sun) because of tidal forces from its massive parent, Jupiter. These tidal forces reshape Europa causing friction that heats the planet. Beneath the subsurface oceans scientists predict there are volcanoes on the sea floor.

If you have heat, liquid water, and lots of minerals fed by active volcanoes, then you have the ingredients for life. On Earth we have chemosynthetic organisms which live near ocean floor vents, feeding off the rich minerals. These bacteria undergo a chemical reaction (chemosynthesis): carbon dioxide + hydrogen sulphide + oxygen – yielding sugar + sulphur + water. They then feed off the sugar for energy. So perhaps there is chemosynthesis going on in Europa. But perhaps you noticed the problem here – the need for oxygen. On Earth the oceans are full of oxygen because of photosynthesis, which uses sunlight to turn carbon dioxide and water into sugar and oxygen. There should be no photosynthesis going on under the ice of Europa, so no oxygen, and therefore no life. Experts feel that without sunlight or oxygen Europa may be sterile, despite the otherwise favorable conditions.

But there is hope for life on Europa yet. Earlier this year biologists discovered organisms on Earth that produce oxygen without sunlight. Nitosopumilus maritimus, which is an archaea (another type of single-celled organisms similar to bacteria) is able to make their own oxygen in the dark of the deep ocean, they then use the oxygen to oxidize ammonia for their nitrogen supply. They don’t produce much oxygen, but a little more than they need, making some available in the environment.

This discovery opens the possibility of entirely new chemical cycles on Europa – life using chemistry that is simply different from what we have on Earth. Essentially we have a tiny window on Earth into “dark life” – life without sunlight. The problem with extrapolating from life on Earth is that organisms living with sunlight can be feeding those without. On Europa we need a life cycle where there is no sunlight in the ecosystem anywhere.

Recently scientists published a simulation of Europa that introduces another intriguing possibility – oxygen from Europa’s surface may be finding its way into the deep ocean. The process starts with charged radiation particles bombarding the surface of Europa, splitting the water ice into hydrogen and oxygen. The hydrogen is very light and floats away. The oxygen, however, largely remains on the surface of Europa, with some dissolving into the thin atmosphere. This surface oxygen dissolves into the surface ice of Europa forming tiny bubbles of oxygen. The simulation involves the fate of these tiny bubbles, could they work their way downward toward the ocean?

In regions of Europa called “chaos regions” because of the complex surface features, the icy shell is thinnest. Ice on the surface of Europa is 15-25 km thick, but in the chaos regions, which cover 25% of the moon’s surface, it may be only 3 km thick. Warmer briny water may be pushing up in these regions, thinning the ice and occasionally breaking through to the surface. What the simulation shows is that in these areas there may be porosity waves (think of a human wave in a stadium, but with porous sections of the ice), and these porosity waves may move oxygen bubble down through the briny frozen ice all the way to the deep ocean. This process would take thousands of years, but once started would produce a steady train of oxygen from the surface to the ocean, in significant quantities – enough to support an ecosystem of life.

This is an exciting possibility, and would solve the “dark life” problem of Europa if true. This is where the Clipper mission comes into play. Armed with this new hypothesis, the instruments on Clipper could investigate the plausibility of this process. It could measure oxygen concentrations on the surface of Europa, and the radar can map what is happening beneath the surface. Clipper is designed with the question of life on Europa in mind, although it is only able to investigate proxy questions. The instruments are not designed to directly detect life, but to answer scientific questions that will inform how plausible life there is, and studies like this one are invaluable to the research that Clipper will perform.

Although we don’t yet know what a life-cycle on Europa can and possible does look like, there are reasonable possibilities for life in the vast ocean of Europa. As Dr. Malcom said, “Life, uh…finds a way.” If there is energy and raw materials in the oceans of Europa, life will likely find a way.

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