For centuries humankind have wondered whether life exists elsewhere in the universe. But only since the mid-20th century, with great advances in technology, have scientists been able to research extraterrestrial life.
Extraterrestrial Life Outside The Solar System
Much of the search for ET has focussed on finding exoplanets in the right place and of the right kind to support life. Exoplanets is the term given to planets outside of our own solar system, i.e. those orbiting stars other than the sun.
NASA’s Kepler and TESS missions have helped to find thousands of exoplanets in our region of the Milky Way which are in or near the habitable zone – where liquid water can form – of their home star.
It’s known as the habitable zone because we believe that liquid water is a prerequisite for all life. With the right conditions some of these planets may be home to extraterrestrial life.
However, scientists may not need to go too far to find life as there are places off Earth in our solar system which may have the correct conditions for life to evolve. The most famous hunt is for life on Mars, but our best chance might be Jupiter’s moon Europa.
Europa, The Ice Covered Moon
Europa was discovered in 1610 by the astronomer Galileo Galilei and it is one of the 79 moons of Jupiter and one of the four bright Galilean moons so easy to see with a telescope.
Europa is slightly smaller than our own moon and is covered in an icy shell which is estimated to be 15 to 25 kilometres thick; it is also thought to have a rocky mantle and an iron core.
Scientists have chosen to take a closer look at Europa as they have found evidence suggesting that below the icy crust is a salty liquid water ocean. If the evidence is accurate, Europa’s ocean would have twice the amount of water than all of earth’s oceans put together.
NASA’s Galileo mission to Jupiter showed that Jupiter’s magnetic field was disrupted around Europa. This suggests that a magnetic field is being created around Europa by a thick layer of electrically conductive fluid below the icy surface; most likely an ocean of salty water.
Europa has an elliptical orbit around Jupiter and it is locked by gravity so that the same hemisphere is always facing Jupiter, just like our own moon. The elliptical orbit means that Europas distance from Jupiter varies and the near side experiences Jupiter’s gravity more strongly than the far side.
The orbital and gravitational changes as Europa circuits Jupiter create tides that in turn stretch and relax the icy surface. The flexing of these tides is likely what causes the fractures across Europa’s surface. The tidal heating caused by flexing could also lead to hydrothermal or volcanic activity, similar to the surface of earth’s oceans.
Potential For Life On Europa
Life as we know it has three main requirements: an energy source, the right organic compounds and liquid water, and it is possible that Europa has all three of these.
Water is perfect for moving chemicals about and dissolving nutrients which is why on Earth, wherever there is water, there is usually life. This is why Europa is a good candidate in our solar system for potentially harbouring life.
Europa’s icy shell is mostly water ice but due to the bombardment of radiation from Jupiter, the chemistry of the ice could be altered and different molecules can be made; for example, oxygen, carbon dioxide and sulphur dioxide.
If these molecules were to get into the subsurface ocean as part of a continuous cycle they could be involved in reactions that living things rely on where it is deep enough so any biosignatures (scientific proof of past life) are not damaged by the radiation from Jupiter.
Researchers think that Europa’s ocean is in contact with the moon’s rocky surface. This is important because the tidal flexing means that Europa is likely to have a hydrothermal or volcanic rocky surface.
Hydrothermal activity could provide the other two requirements for life, energy and organic compounds. Tidal flexing will likely provide heat energy for chemical reactions in the rocky surface. Their products, like those on Earth’s sea bed, would be released into Europa’s ocean by hydrothermal vents or volcanoes.
The more energy that Europa has the better the availability of energy for potential life, so long as its products find their way into the ocean. If Europa’s rocky interior has lots of chemical energy but can’t exchange materials with the ocean, that would limit its habitability.
Determining how well material can exchange between the icy shell, the ocean and the rocky core, and whether hydrothermal vents or volcanoes are on Europa, will help researchers to further investigate Europa’s potential to harbor life.
Europa Clipper Mission
NASA’s Europa Clipper mission is set to launch in 2023 and will perform multiple flybys of the icy moon as the spacecraft orbits around Jupiter. Clipper will be radiation tolerant and have many instruments to further research of Europa.
Cameras will be used to take high-resolution pictures of Europa’s surface and spectrometers will be used to determine the distribution of materials and identify the composition of the distant moon’s surface and ocean.
A radar will be used to identify the thickness of the icy crust and search for subsurface lakes, while a magnetometer will be used to observe Europa’s magnetic field by measuring its direction and strength.
Understanding the magnetic field will help to confirm the ocean’s existence as well as its depth and salinity. Knowing the thickness of the icy shell and the ocean’s properties will help scientists to understand whether Europa is habitable for microbial life (or, indeed, anything larger).
Drilling Through Europa’s Icy Shell
In 2012, NASA’s Hubble Space Telescope observed water vapor above the south polar region. This provided strong evidence for the existence of water plumes. If these plumes exist they may be linked to the subsurface ocean.
This would mean that they would be able to study the chemical makeup of the ocean while minimising the need to drill through the icy shell. The Europa Thermal Emission Imaging System (E-THEMIS) will be another of the instruments used on the Europa Clipper mission.
E-THEMIS will help to determine the existence of these plumes by searching Europa’s frozen surface, understanding its thermal behaviour and finding anomalies which may be vents that may have recently erupted plumes of water.
Watch the video below for more information on NASA’s Clipper mission.
Summary
While scientists are still unsure if life exists on Europa, there is a lot of evidence that shows Europa may be capable of harbouring life, even if it may only be microbial.
We will have a much clearer picture once Clipper has done its work.
