Interactive Europa graphic provided by NASA
We may all be focused on finding liquid water on Mars, but some planetary scientists are looking for liquid water in some very – VERY! – remote places. A new study by Noah Hammond, a professor at Wheaton College, proposed that liquid water may be found near the surface of one of the moons of Jupiter.
Some background info
Europa is a tiny moon of Jupiter. It is about one-fourth the size of Earth or about 90-percent as big as the moon. Europa orbits Jupiter about every three-and-a-half days. Jupiter is about five-times further away from the Sun than Earth, which means the sun is about 1/25th as bright on Europa as it is on Earth.
So, in order for liquid water to exist, there needs to be another source of heat. Turns out that the tidal pull of gravity does the work on Europa.
“Europa orbits Jupiter every three-and-a-half days,” Hammond said. “The orbital period does influence the rate of heating. If the orbit took less time, it would generate the same total heating but over a shorter period, so the heat flux would be higher.”
Europa consists of an ice shell that encapsulates a deep liquid ocean. And according to NASA, Europa’s ice shell is 10 to 15 miles thick with an ocean that may be up to 40 to 100 miles deep. So, if you run some back-of-the-napkin math, despite its small size, Europa’s ocean may contain twice as much water as all of Earth’s oceans combined.
So, why are there moons made entirely of ice and water out there? When I need answers about the formation of the Solar System, I turn to one of my professors from Oregon State, Adam Kent.
“Jupiter and Saturn and other giant planets are icy is because they formed far enough out in the early solar system,” Dr. Adam Kent, a Geophysics Professor and Associate Dean for Research and Faculty Advancement at Oregon State University said. “Ice could condense (it was too hot in the inner solar system and only silicate minerals and Fe-Ni alloys etc. could condense.”
And get this: Europa has an oxygen atmosphere.
But don’t get ready to take off that spacesuit! The atmosphere is very, very thin. And NASA learned that midday temperatures at Europa’s equator only reach about -225 degrees Fahrenheit.
So, you can imagine their surprise when in 2019, there was the discovery of water vapor above the moon’s surface.
“We see evidence of plumes of water vapor on Europa from Hubble space telescope,” Hammond said. “One possibility is that these water plumes are generated by frictional heating along faults.”
Yeah. Faults. Like on Earth.
But, you may be wondering, that is great Nick, but just how is there water vapor at 200-degrees below zero?!?
Scientists are working on that answer.
“We will only be able to truly test our hypothesis by going back to Europa with a spacecraft,” Hammond said. “The upcoming Europa Clipper mission will carry a radar instrument on board that can look for signs on liquid water beneath the surface. We can use the radar instrument to look for evidence of liquid water close to the surface along faults.”
New research
Let’s back up a bit first. Knowing that there is liquid water beneath the surface of Europa, and that some of it is making it to the surface, that got scientists thinking that maybe they could figure out what was going on to make that possible. Then figure out why there is water vapor.
Hammond offered this hypothesis in his latest Abstract, a scientific summary, from a recent study:
“Strike‐slip faults on Europa may slide back and forth in response to diurnal tidal stresses, which could generate significant frictional heating near the surface. Previous shear heating models assumed fault sliding rates a priori, without showing how the sliding rate is connected to the resolved stresses acting on the fault. Here I calculate the cyclic displacement along tidally driven faults. I use a Mohr‐Coulomb failure criterion to determine the frictional failure depth, which varies throughout the tidal cycle. The displacement on the fault is calculated assuming an elastic broken plate model. The magnitude of cyclic displacements along a fault depends upon the coefficient of friction and the shear modulus of the ice shell. If Europa’s ice shell is weak, diurnal tidal stress can cause faults on Europa slide back and forth by ~0.1 – 2 m each cycle. Such large amounts of cyclic slip may be enough to frictionally heat the ice and potentially produce near‐surface melting. If Europa’s ice shell has the strength of intact ice, faults become less responsive to cyclic tidal stresses and would only slide 0.01 to 0.2 m per cycle.”
In other words, using the plate tectonics on Earth as a guide for what may be happening, Hammond has devised a hypothesis that Europa’s ice shell, which has its own version of plate tectonics, may feature enough frictional heating that it causes areas of the ice to actually melt.
“We think the ice at the surface behaves like a rigid solid,” Hammond said. “However, once frictional heating warms up the ice, we do think the ice below the fault would start to flow and stretch. Once the ice below the fault can flow, we think this allows strike-slip faults to slide faster. We see a lot of strike-slip faults on Europa and we think this might be how they start. Initially only sliding back and forth, then when the ice warms up they start to slide faster and permanent fault motion occurs.”
It turns out this is a rare phenomenon.
“Europa is the only place other than Earth where we see evidence of plate tectonics,” Hammond said. “Strike-slip faults are a super important aspect of plate tectonics. This research shows how Jupiter’s tides (gravity) could help strike-slip faults develop on Europa, potentially leading to plate-tectonic-like behavior.”
Since Earth is the only other place that plate tectonics occurs, it felt prudent to check back with the terrestrial geophysicist.
“The thin outer shell of solid ice can deform brittley,” Kent said. “And tidal stresses are enough to make strike slip faults – the type of fault where rocks (or plates of ice) slide sideways past each other – like the San Andreas fault for example.”
And that ice melts then a lot closer to the surface than it would if there was no friction at all. So instead of 10 miles down, it may be only a few hundred meters.
And if that ice melts into liquid, it may ooze onto the surface.
“If liquid water made it all the way to the very surface, it would freeze extremely quickly,” Hammond said. “Since Europa has no atmosphere, the water would also sublimate (rapidly turn into a gas and drift away). Depending on how much water made it to the surface, this freezing and sublimation processes would probably happen in a matter of minutes or seconds.”
There may be times when the pressure between the ice sheets (that make up the ice shell) is high enough to spray some of that water into space. This isn’t apart of his hypothesis, but there is precedent for such an occurrence. It happens on Enceladus, a moon of Saturn.
From NASA:
If the plumes do exist, and if their source is linked to Europa’s ocean, then a spacecraft could travel through the plume to sample and analyze it from orbit, and it would essentially be analyzing the moon’s ocean (the Cassini spacecraft performed this feat at Saturn’s moon Enceladus, which is known to have an ocean spraying into space). Even if Europa isn’t ejecting free samples into space, a 2018 study concluded that samples of Europa’s ocean could get frozen into the base of the moon’s ice shell, where the ice makes contact with the ocean.
Why do we care?
Water seems to be one of the necessary ingredients for life – as far as we know. So, when scientists find water in other places int eh solar system there is a chance that life could exist.
“If faults do generate liquid water from frictional heating, these pockets of melt could be potentially habitable,” Hammond said. “Future missions could try to sample water coming out of fractures and faults and test it for chemical signatures of life.”
While you may ask, what life could live there?!
The water would be pretty cold under the surface of the ice shell, but it isn’t likely to be much colder than 32 degrees Fahrenheit. And there are plenty of life forms on Earth that survive in water that is around the freezing mark.
And if this water is closer to the surface, it may be possible to eventually sample that water to figure out if life exists.
What is next?
Like all good science, this paper will continue to be peer-reviewed and the research will be critiqued in the coming months and years. Other scientists will poke at it and try to disprove it. Some will test the math behind the findings.
Not because the people doing this are mean scientists, and it isn’t that they think Hammond is wrong. But scientists do this to everyone who proposes a new result to make sure it is accurate.
That is how science works! We all double-check and triple-check everyone’s work to make sure it is correct.
