One of the most intriguing worlds in the solar system is Europa.
It's the smallest of Jupiter's big moons and a tad smaller than our own Moon. In 1979 they sent back high-resolution images of it, revealing several surprises. The surface is very smooth and there are very few craters. The surface is young, and any old craters must have been smoothed over.
The surface is mostly water ice and covered with dark brown material that is probably complex organic molecules called tholins. Over the years, it became more and more clear that there is a thick ice shell surrounding a global ocean of liquid. Earth has less water on its surface.
The moon is squeezed and compressed by Jupiter's gravity as it circles the planet on a slightly elliptical path, keeping the ocean warm. The dark tholins might be the result of the molecules being brought up to the surface through cracks that are bombarded by Jupiter's powerful magnetic field.
The possibility of life is brought up. If those molecules can be brought back into the ocean, they could be the beginning of biology. There could be aliens swimming in the permanently sunless depths.
A lot of study is done on this question. The water below has to interact with the surface to work. Can that happen?
The connection may have been found in a new study. A team of scientists looked at a ridge in Greenland that is similar to structures on Europa, and think they have come up with a formation mechanism that has implications for life under the shell.
There are ridges running parallel along a trough between them. The most common surface feature on the moon is the ridges, which can be hundreds of meters high and hundreds of kilometers long. Water near the surface interacting with the shell and shell through compression, or heating as parts of it are sheared, are some of the different mechanisms proposed for their formation.
The lack of such structures here on Earth limits such ideas. They are ubiquitous, but have never been seen.
That was until recently. A survey used to create digital elevation models showed a double ridge on the ice in northwest Greenland with two parallel hills on either side of a trough. The hills are low and run for 800 meters. The ratio of the height of a ridge to the distance across the trough is a key characteristic. The difference between Earth and Europa's gravity is accounted for, since our stronger gravity makes hills slump if they get too big. If the formation mechanism is the same, that's important.
Maybe they form the same way. How did the one on Earth form?
Scientists think it is a multi-step process because of the ice and seasons in that region. A layer 10 or so meters thick is formed by the porous surface ice. A flattish intrusion of water is seen below that using ground-penetrating radar. The ice is very hard.
The liquid water is cold when it gets cold outside. When the water expands, it puts a lot of pressure on the ice around it. The ice under it is very hard, but the ice above it is porous and weak, so the water tends to expand upwards, freezing in place and forming a vertical slab of harder ice.
The processes can repeat if the sill thaws or more water runs into it. There is a plug of harder ice above the sill, like a wall running down the middle of a river. While the porous ice on either side is pushed up by the cold, it stays put. Lather, rinse again. You can see the ridges on either side of the sill. The height of the ridges and the distance between them are dependent on a lot of factors.
So if that works for Greenland, what about Europa? It does not have thaw and freeze seasons and no surface water. If there are cracks under the ice shell where the ocean meets it, the tides from Jupiter can force the water up to the surface. In that case, the same physics takes over and the double parallel ridges form.
It has implications for life under the moon if that is the case.
The ocean is briny because of the rocky mantle. If it gets up to the surface, it could interact with Jupiter's radiation, creating more complex molecules that are then returned to the ocean. You have all the ingredients for life, and there is also a chance that there are hydrothermal vents at the bottom of the ocean.
We have no idea if this works or not, but it is a very compelling narrative. It may be teeming with weird marine life or completely sterile. The structure and evolution of the ice shell, the size, depth, and composition of the water, and what the ocean floor looks like are not known. NASA is currently building a mission to the moon. It will map the surface in detail, including mineralogical and chemical surveys, and will do ground-penetrating radar observations to see if there are favorable conditions to life.
This is a reality and will happen. In the year of 2024 and in the year of 2030. Who knows, that's not that far in the future. It may not find life, but it may provide enough evidence to look further. We will get a better look at this weird little moon, and we will learn more about it.
Riley Culberg gave me the nifty 3D plots of the two ridges and I would like to thank him.
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