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.

Advertisements

Not all waterworlds can host life

During its formation, Venus was in the Solar System’s habitable zone – much like Earth is now. Scientists think its surface contained liquid water, and its atmosphere was somewhat like Earth’s. Maybe there was life, too. However, as the levels of carbon dioxide kept increasing, its atmosphere became opaque, trapping most of the heat reflected by its surface, and Venus heated up to the point where its oceans boiled away. Today, life on the planet’s waterless surface is considered unlikely, except perhaps by those who’ve read a November 2014 study involving supercritical carbon dioxide, and those who believe in Hell.

Why can’t this be the case on alien worlds possessing water as well? Discoveries made since the mid-1990s – especially by the Kepler space telescope and probes in the Jovian and Saturnian systems – have unearthed a variety of worlds that could, or do, have liquid water on or below the surface. On Earth, life has been found wherever liquid water has been found, so liquid water on other planets and moons gets scientists excited about the possibility of alien life. Recent discoveries of a subsurface ocean on Europa and possibly on some other moons of Jupiter and Saturn have even prompted NASA to plan for a probe to Europa in the mid-2020s.

A study published online (paywall) in the Monthly Notices of the Royal Astronomical Society applies the brakes on that excitement to some extent. A kind of exoplanet which scientists think could host lots of liquid water—some 100-times the amount of water on Earth, in fact— are the so-called ‘waterworlds‘. They would have oceans so deep and wide that, according to the study, their effects on themselves and the planet’s climate would be incomparable to that on Earth – and altogether might not be hospitable to life the way we know liquid water can usually be.

The study’s authors write, “One important consequence is, for example, the formation of high-pressure water ice at the bottom of the ocean, which prevents the immediate contact of the planetary crust with the liquid ocean.” This in turn mutes the carbon-silicate cycle, a recycling of carbon and silicon compounds on the ocean floor that determines how much carbon dioxide is released from the oceans into the atmosphere.

The authors calculate that on an (at least) Earth-sized waterworld in the habitable zone of its star, there can be 25-100 Earth oceans for temperatures ranging from the freezing point of water to just beyond the boiling point. So a colder planet, say at 0° C, would have a smaller ocean and lesser liquid water to be able to absorb the carbon dioxide (and its absorptive capabilities can’t ‘power up’ without the carbon-silicate cycle). Yet, at lower temperatures the oceans are able to dissolve more gases, even as the pressure exerted by the gas on the ocean’s surface is higher. So a colder planet with a smaller ocean will dissolve more carbon dioxide from the atmosphere – turning the planet even cooler.

Similarly, a warmer waterworld will be able to absorb less carbon dioxide, letting the greenhouse gas accumulate in the atmosphere, heat the surface up and eventually boil the oceans away (like on Venus). In short, a waterworld whose temperatures are outside a specific range will become hotter if it’s warm and even colder if it’s cold. These runaway effects can occur pretty quickly, too. 

Based on the chemical properties of water and carbon dioxide, the scientists estimate that the life-friendly temperature range is from 273 K to 400 K (0° to 127° C). And even in this range, there could be threats to life in the form of ocean acidity. On Earth, limestone that’s in contact with water dissolves and keeps the water’s acidity in check, but this may not be happening on waterworlds where large landmasses could be a rarity or relatively smaller in size.

At the same time, these pessimistic speculations are offset by some assumptions the scientists have made in their study. For example, they assume that the waterworld doesn’t have tectonic activity. Such activity on Earth involves the jigsaw of landmasses grindings against each other, sometimes subducting one below the other to push down some minerals while volcanoes in other areas spew out others—in all making for a giant geological cycle that ensures the substances needed to sustain life are constantly replenished. If a waterworld were to have tectonic activity, it would also influence the carbon-silicate cycle and keep a runaway greenhouse effect from happening.

On Earth, the warming of the oceans presents a big problem to climatologists partly because its mechanisms and consequences are not fully understood – and more so to marine creatures. And as the oceans are able to dissolve more anthropogenic carbon dioxide, they also become more acidic. Yet, the effects are relatively smaller (ignoring the presence of life for a moment) compared to that on waterworlds – comprising no above-sea-level landmasses and infinite seas 100 km deep.

Featured image credit: Lucianomendez/Wikimedia Commons, CC BY-SA 4.0.

The Wire
August 23, 2015

Toll of Jure landslide mounts

On August 2, a landslide in Nepal’s Sindhupalchowk district resulted in 33 deaths, with more than 120 missing and 400 displaced. A preliminary survey by the local District Disaster Relief Committee (DDRC) revealed 115 houses had been completely destroyed. The Nepali government has since declared all missing persons dead, bringing the total death toll to 150+. Other damages include a two-km section of a highway, the reconstruction of which, according to the International Centre for Integrated Mountain Development (ICIMOD), is “nearly impossible”, and two gates of the Sun Koshi Hydropower Project.

People and households affected by the Jure landslide, by village. Source: DDRC

People and households affected by the Jure landslide, by village (link to chart and data). Source: DDRC

Satellite images of the area have shown that it’s been landslide-prone since at least 2011. Given the damage to the hydroelectric power project, the incident is another reminder that the Himalayan region presents significant geological threats to hydroelectric projects being planned in the region. GPS investigations in the past have revealed that the Lesser Himalayas, which the country of Nepal straddles, are rising at ≤ 3 mm/year due to movements on active faults, landslides being one symptom of this. As this article in Current Science argues, “If the idea is to have environment-friendly power projects, then the planners and dam builders must not ignore the geological reality of the geodynamically sensitive region”.