Where there’s water there may be life
Next stage of space exploration focuses on outer planets’ ice-covered moons
Water is life. We humans are about 60% water. Water is the necessary solvent in which biochemical reactions occur, and we know of no life that can survive without water.
Earth is a “waterworld”; 71% of its surface is covered with water. This is because, being less dense than rock, it floats.
Water makes up a mere 0.02% of the total mass of the Earth, but it is abundant on and just under the surface of the planet, where life can use it.
We are beginning our search for life elsewhere in the universe by looking at other planets and moons in our solar system that have water. This includes Mars and some of the icy moons of the planets of the outer solar system.
When the Sun and the planets formed 4.6-billion years ago, heat and the wind from the Sun blew much of the water out of the inner solar system where the rocky planets, Mercury, Venus, Earth, and Mars, formed.
In the outer solar system, where it is much colder, there is much more water. Many of the moons of Jupiter, Saturn, Uranus and Neptune are completely covered in ice.
And for some of them, under their icy crusts there are deep oceans which may possibly have life.
Saturn has a small moon named Mimas, only 400km in diameter (our moon is 3,500km across). Mimas has a giant impact crater named Herschel
that stretches 139km (1/3 the diameter of Mimas) with walls 5km high. You can see from the picture that this moon looks like the “Death Star” spaceship in the original Stars Wars movie.
Excitingly, in February 2024, astronomers at the Observatoire de Paris discovered that Mimas has a big ocean. This is a surprise. It was thought that a moon this small would be completely frozen. The newly discovered ocean lies about 2030km beneath the icy surface and the excitement is that it may harbour life.
Here on Earth, there are ecosystems communities of life that live around “black smokers” 4km beneath the surface of the ocean.
These form around hot springs that gush out water at 300°C into the surrounding 2°C deep ocean water.
Minerals in the hot water precipitate out as the “black smoke” and there are bacteria that live on the minerals.
Then sea plants, clams, shrimp, crabs, fish and other life can live on the food chain created by the bacteria.
All of these life forms survive ultimately on heat from inside the Earth. There is no sunlight down there. Some biologists think life may have first formed around black smokers more than 3.7-billion years ago.
So the hunt is on now for moons with deep oceans under ice where the heat from the inside of the moon may also produce black smokers where there may be life.
Mimas is now one such place to look someday.
Other moons covered with ice are known. Enceladus (Incell-uh-dus) is a 500km diameter ice-covered moon of Saturn with surface cracks in the ice from which the Cassini Space Mission saw sea water erupting out into space. Europa, a moon of Jupiter that is nearly
as big as our moon, is covered in an ice crust 20 to 25km thick with a huge ocean underneath.
Europa is a prime target now in the search for life in our solar system.
Juice the European Space Agency’s Jupiter Icy Moons Explorer was launched in April 2023 and will arrive at Jupiter in July 2031, where it will study Europa and two other moons, Ganymede and Callisto, both the size of the planet Mercury and covered in ice.
Then, in October this year, NASA will launch its Europa Clipper, which will arrive at
Jupiter in 2030 (ahead of Juice there is some ESA-NASA competition here).
The Clipper will pass by Europa 50 times to determine whether there are places underneath the thick ice that might be right for life.
If so, another mission is being planned to land on Europa, melt through the 25km of ice, plunge into the ocean and swim around with cameras and other equipment looking for life around black smokers.
How exciting it will be if we find we are not alone in the universe.