A problem worth its weight in salt

Pictures of Jupiter’s moon Europa taken by the Galileo space probe between 1995 and 2003 support the possibility that Europa’s surface has plate tectonics. In fact, scientists think it could be one of only two bodies in the Solar System – the other being Earth – to display this feature. But it must be noted that Europa’s tectonics is nothing like Earth’s if only because the materials undergoing this process are very different – compare the composition of Earth’s crust and Europa’s ice shell. There are also no arc volcanoes or continents on Europa.1 But this doesn’t mean there aren’t any similarities either. For example, scientists have acknowledged that shifting ice plates on the moon’s surface, with some diving over others and pushing them down, could be a way for minerals on the top to plunge further interior. Because Europa has been suspected of harbouring a subsurface ocean of liquid water, a mineral cycle could be boosting the chances of finding life there. Plate tectonics played a similar role in making Earth habitable.

The biggest giveaway is that the moon’s surface is not littered with craters the way other Jupiter moons are. This meant that cratered patches of the ice shell were disappearing into somewhere and replaced with ‘cleaner’ patches. There are also kilometre-long ridges on the shell suggesting that something had moved along that distance, and they ended abruptly in some places. In 2014, a pair of geologists from Johns Hopkins and the University of Idaho used software like Photoshop to cut up Galileo’s maps of Europa and stitch them back together such that the ridges lined up. They found that there were some areas with a “big gap”. One way to explain it was that the patch there had dived beneath a neighbouring one – a simple version of plate tectonics. But tantalising as the possibility is, more evidence is needed before we can be sure.

If we’re hoping to find the first alien life inside a Jovian moon, we’ll need good models that can help us predict how life might’ve evolved there. A new paper from researchers at Brown University tries to help by trying to figure out why the plates might be shifting (To say something could be happening, it helps to have a simple way it could be happening and with the available resources). On Earth, interactions between the crust and the mantle are motivated among other factors by differences in temperature. The crust is cooler than the magma it ‘slides’ over, which means it’s denser, which assists its subduction when it happens. Such differences aren’t mirrored on Europa, where scientists think there’s a thin, cold ice shell on top and a relatively warmer one below. When a patch of ice from the top slides down, it becomes warmer because the upper layer provides insulation, which prevents the sliding layer from sliding further down because the density has been evened out.

Instead, the Brown University fellows think the density differences could arise thanks to salt content (which, by the way, could also be useful when reading their press release. It says, “A Brown University study provides new evidence that the icy shell of Jupiter’s moon Europa may have plate tectonics similar to those on Earth.” You know it’s not similar, especially if left unqualified like that.) Salt is denser than water, so ice that has more salt is more dense. A 2003 study also suggested that warmer ice will have lesser salt because eutectic mixtures could be dissolving and draining it out. So using a computer model and making supposedly reasonable assumptions about the shell’s temperature, porosity and salinity ranges, the Brown team calculated that ice slabs made up of 5% salt and saltier than their surroundings by 2.5% would be able to subduct. However, if the distribution of salt was uniform on Europa’s surface (varying by less than 1% from slab to slab, e.g.), then a subducting slab would have to have at least 22% salt → very high.

I said “supposedly reasonable assumptions” because we don’t exactly know how salinity and porosity vary around and through Europa. In their simulations, the researchers assumed that the ice has a porosity of 10% (i.e. 10% of the material is filled with pores), which is considered to be on the higher side of things. But the study remains interesting because it’s able to establish the big role salts can play in how the ice moves around. This is also significant because Galileo found the Europan magnetic field to be stronger than it ought to, suggesting the subsurface ocean had a lot of salt. So it’s plausible that the cryomagma2 on which Europa’s upper shell moves could be derived from the waters below.

The researchers also claim that if the subducting slab doesn’t lose all its salt in about one million years, it will remain dense enough to go all the way down to the ocean, where it could be received as a courier carrying materials from the surface that help life take root.3 But of you think this might be too out there, look at it in terms of the planned ESA Jupiter Icy Moons Explorer (JUICE) and NASA Clipper missions for the mid-2020s. Both Cassini and Galileo data have shown that there’s a lot going on with the icy moons of the gas giants Jupiter and Saturn, with observations of phenomena like vapour plumes pointing to heightened chances for the formation and sustenance of alien life. If JUICE and Clipper have to teach us something useful about these moons, then they’ll have to go in prepared to study the right things, the things that matter. The Brown University paper has shown that salt is definitely one of them. It was accepted for publication in the Journal of Geophysical Research: Planets on December 4, 2017. Full text here.

Featured image: An artist’s impression of water vapour plumes erupting from Europa’s south pole, with Jupiter in the background. Credit: NASA-ESA.

1Venus has two continent-like areas , Ishtar and Aphrodite terra, and also displays tectonic activity in the form of mountains and volcanoes, e.g. But it does not have plate tectonics because its crust heals faster than it is damaged during tectonic activity.

2One of the more well known cryovolcanoes in the Solar System is Doom Mons on where else but Titan.

3 On Earth, tectonic plates that are pushed downward also take a bunch of carbon along, keeping the surface from accumulating the element in amounts that could be deleterious to life.

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Could there be life on Europa? NASA okays mission to find out

The Wire
June 19, 2015

Artist concept of NASA’s Europa mission spacecraft approaching its target for one of many flybys. Credit: NASA/JPL-Caltech

Artist concept of NASA’s Europa mission spacecraft approaching its target for one of many flybys. Credit: NASA/JPL-Caltech

On Thursday, NASA okayed the development of a probe to Jupiter’s moon Europa, currently planned for the mid-2020s, to investigate if it has conditions suitable for life. The milestone parallels the European Space Agency’s JUICE (Jupiter Icy Moons Explorer) mission, also planned for the mid-2020s, which will study the icy moons of the Solar System’s largest planet.

The NASA mission has tentatively been called Clipper, and its proposal comes on the back of tantalizing evidence from the Galileo mission that Europa could have the conditions to harbour life. Galileo conducted multiple flybys of the moon in the 1990s and revealed signs that it could be harbouring a massive subsurface ocean – with more than twice as much water as on Earth – under an ice shell a few kilometres thick. It also found that the ocean-floor could be rocky, there were tidal forces acting on the water-body, and that the thick ice shell could be host to plate tectonics like on Earth.

These characteristics make a strong case for the existence of habitable conditions on Europa because they mimic similar conditions on Earth. For example, plate tectonics on Earth moves a jigsaw of landmasses on the surface around. Their resulting interactions are responsible for moving minerals on the surface into the ground and dredging new deposits upward, creating an important replenishment cycle that feeds many lifeforms. A rocky seafloor also conducts heat efficiently toward and away from the water, and tidal forces provide warmth through friction.

With NASA’s okay, the Europa mission moves to the “formulation stage”, when mission scientists and engineers will start technology development. The agency’s fiscal year 2016 budget includes $30 million for just this, according to a May 26 statement, out of a total of $18.29 billion that Congress has awarded it. NASA has already also asked for $285 million through 2020 for the Europa mission, with the overall mission expected to cost $2 billion notwithstanding delays at the time of a launch planned for 2022.

The same statement also announced the scientific payload that would accomplish the mission. Out of 33 proposals submitted, NASA selected nine – all geared toward exploring the ice- and water-related properties of the moon. They could also be pressed into observing other moons in the Jovian neighbourhood – many of which are icy and have curious surface and atmospheric characteristics resembling Europa’s. These include another of Jupiter’s moons, Ganymede, and Saturn’s Dione, Enceladus, Hyperion, Iapetus, Phoebe and Tethys.

ESA’s JUICE mission – part of its broader Cosmic Vision strategy for a class of long-term missions in the 2020s – is planned to launch in 2022 and reach Jupiter by 2030. At one point, it will enter into orbit around Ganymede. If NASA’s Clipper is at Europa by then, what the two probes find could be complementary, and be compared to infer finer details.

Europa’s ice shell could be quaking

Even before astronomers noticed last year that Europa was spouting jets of water vapor from its icy surface, they thought there was something shifty about Jupiter’s moon. While the 66 other Jovian moons are pitted with craters, Europa sports some unusual blemishes: an abundant crisscrossing of ridges tens of kilometres long. Many are abruptly interrupted by smooth ice patches.

Two geologists think they can explain why. Backed by photos taken by the Galileo space probe, they suggest Europa’s thick shell isn’t continuous but is made up of distinct plates of ice. These plates move away from each other in some places, exposing gaps which are then filled by deeper ice rising upward. In other places they slide over each other and push surface ice downward and form ridges.

“We knew that stuff has been moving over the surface, and up from beneath and breaking through, but we weren’t able to figure where all the older stuff was going,” said study coauthor Dr. Louise Prockter, a planetary scientist at Johns Hopkins. “We’ve found for the first time evidence that material is going back into the interior.” The study was published last month in Nature Geoscience.

On Earth, this kind of tectonic activity replenishes compounds necessary for life, such as carbon dioxide, by letting them move up from the interior through fissures to the surface. Now, scientists say a similar mechanism could apply to Europa. Astronomers think the moon harbors a subsurface ocean of liquid water that feeds the vapor plumes, and could be habitable.

“It’s certainly significant to find another solid body in the solar system that undergoes some kind of surface recycling,” said Peter Driscoll, a planetary scientist at the University of Washington who was not involved in the study.

Prockter, together with Simon Kattenhorn, a geologist at the University of Idaho, Moscow, worked with photographs of a part of Europa’s surface covering 20,000 km2. The pictures were shot by Galileo when it orbited Jupiter from 1995 to 2003.

“We go in using something like Photoshop and start cutting the image up,” Dr. Prockter explained. They then pieced them back together so that the crisscrossing ridges lined up end-to-end, and compared what they had to the surface as it is today.

“Once we started doing the reconstruction, we ended up with a big gap right in the middle,” she said.

The researchers concluded the missing bit had dived beneath another plate.

Although only some of Galileo’s photographs were at a resolution high enough to be useful for the study, Dr. Prockter said it was unlikely that their finding was a one-off because signs of displacement were visible all over Europa’s surface.

Nevertheless, Dr. Driscoll cautioned against using Earth’s tectonic activity as a model for Europa’s. “There are a number of missing features” that define tectonics on Earth, he said, such as arc volcanos and continents. “And many of the properties of Earth’s features may not be expected for an icy shell like Europa, where the materials are extremely different.”

A better gauge of these disparities might be a probe to the Jovian moon that NASA has planned for the mid-2020s.

“I think the timing right now is very important,” said Candice Hansen, a member of NASA’s Planetary Science Subcommittee. She says the Europa study will help scientists working on the probe secure the requisite funding and commitment from Congress.

“I am very enthusiastic about a mission to Europa, and this exciting result is one more reason to go,” she said.

Artist's concept of the Europa Clipper mission investigating Jupiter's icy moon Europa.

Artist’s concept of the Europa Clipper mission investigating Jupiter’s icy moon Europa. Image credit: NASA/JPL-Caltech