density – Close Read

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.

Neutron stars

When the hype for the announcement of the previous GW detection was ramping up, I had a feeling LIGO was about to announce the detection of a neutron-star collision. It wasn’t to be – but in my excitement, I’d written a small part of the article. I’m sharing it below. I’d also recommend reading this post: The Secrets of How Planets Form.

Stars die. Sometimes, when that happens, their outer layers explode into space in a supernova. Their inner layers collapse inwards under the gravity of their own weight in a violent rush. If the starstuff can be packed dense enough, the collapse produces a blackhole – a volume of space where the laws of quantum mechanics and relativity break down and the particles of matter are plunged into a monumental identity crisis. However, if the dying star wasn’t heavy enough when it blew up, then the inward rush will create a very, very, very dense object – but not a blackhole: a neutron star.

Neutron stars are the densest objects in the universe that astronomers can observe. The only things we know are denser than them are blackholes.

You’d think observed means ‘saw’, but what is ‘seeing’ but the light – a form of electromagnetic energy – from an event reaching our eyes? We can’t directly ‘see’ blackholes collide because the collision doesn’t release any electromagnetic energy. So astronomers have built a special kind of eyes – called gravitational wave detectors – that can observe ripples of gravitational energy that the collision lets loose.

The Laser Interferometer Gravitational-wave Detector (LIGO) we already know about. Its twin eyes, located in Washington and Louisiana, US, have detected three blackhole-blackhole collisions thus far. Two of the scientists who helped build it are hot favourites to win the Nobel Prize for physics next week. The other set of eyes involved in the last find is Virgo, a detector in Italy.

You’ve been told that blackholes are freaks of nature. Heavy objects bend spacetime around themselves. Blackholes are freaks because they step it up: they fold it. They’re so heavy that when spacetime bends around them, it goes all the way around and becomes a three-dimensional loop. Thus, a blackhole traps one patch of the cosmos around a vanishingly small heart of darkness. Even light, if it comes close enough, becomes trapped in this loop and can never escape. This is why astronomers can’t observe blackholes directly, and use gravitational-wave detectors instead.

But neutron stars they can observe. They’re exactly what their names suggest: a ball of neutrons. And neutrons experience a force of nature called the strong nuclear force, and it can be 100,000 billion billion billion times stronger than gravity. This makes neutron stars extremely dense and altogether incredibly heavy as well. On their surface, a classic can of Coke will weigh 355,000 billion tonnes, a thousand-times heavier than all the humans on Earth combined.

Sometimes, a neutron star is ravaged by a powerful magnetic field. This field focuses charged particles on the neutron star’s surface into a tight beam of radiation shooting off into space. If the orb is also spinning, then this beam of radiation sweeps through space like the light from a lighthouse sweeps over the sea near it. Such neutron stars are called pulsars.

Rocky exoplanets only get so big before they get gassy

By the time the NASA Kepler mission failed in 2013, it had gathered evidence that there were at least 962 exoplanets in 76 stellar systems, not to mention the final word is awaited on 2,900 more. In the four years it had operated it far surpassed its envisioned science goals. The 12 gigabytes of data it had transmitted home contained a wealth of information on different kinds of planets, big and small, hot and cold, orbiting a similar variety of stars.

Sifting through it, scientists have found many insightful patterns, many of which evade a scientific explanation and keep the cosmos as wonderful as it has been. In the most recent instance of this, astronomers from Harvard, Berkeley and Honolulu have unearthed a connection between some exoplanets’ size, density and prevalence.

They have found that most exoplanets with radii 1.5 times more than Earth’s are not rocky. Around or below this cut-off, they were rocky and could hypothetically support human life. Larger exoplanets – analogous to Neptune and heavier – have rocky cores surrounded by thick gaseous envelopes with atmospheric pressures too high for human survival.

“We do not know why rocky planetary cores begin to support thick gaseous layers at about 1.5 Earth radii as opposed to 1.2 or 1.8 Earth radii, and as the community answers this question, we will learn something about planet formation,” said Lauren Weiss, a third year graduate student at UC Berkeley.

She is the second author on the group’s paper published in Proceedings of the National Academy of Sciences on May 26. The first author is Geoff Marcy the “planet hunter”, who holds the Watson and Marilyn Alberts Chair for SETI at UC Berkeley.

Not necessarily the bigger the heavier

The planets of the Solar System. Image: Lsmpascal

The group analyzed the masses and radii of more than 60 exoplanets, 33 of which were discussed in the paper. “Many of the planets our study straddle the transition between rocky planets and planets with gaseous envelopes,” Weiss explained. The analysis was narrowed down to planets with orbital periods of five to 100 days, which correspond to orbital distances of 0.05 to 0.42 astronomical units. One astronomical unit (AU) is the distance between Earth and the Sun.

Fully 26.2% of such planets, which orbit Sun-like stars, have radii 1 to 1.41 times that of Earth, denoted as R_⊕, and have an orbital distance of around 0.4 AU. Accounting for planets with radii up to 4R_⊕, their prevalence jumps to more than half. In other words, one in every two planets orbiting a Sun-like star was bound to be just as wide to eight times as wide as Earth.

And in this set, the connection between exoplanet density and radius showed itself. The astronomers found that the masses of Earth-sized exoplanets steadily increased until their radii touched 1.5R_⊕, and then dropped off after. In fact, this relationship was so consistent with their data that Weiss & co. were able to tease out a relation between density and radius for 0-1.5R_⊕ exoplanets – one they found held with Mercury Venus and Earth, too.

Density = 2.32 + 3.19R/R_⊕

So, the astronomers were able to calculate an Earth-like planet’s density from its radius, and vice versa, using this equation. Beyond 1.5R_⊕, however, the density dropped off as the planet accrued more hydrogen, helium and water vapor. At 1.5R_⊕, they found the maximum density to be around 7.6 g/cm³, against Earth’s 5.5 g/cm³.

The question of density plays a role in understanding where life could arise in the universe. While it could form on any planet orbiting any kind of star, we can’t also forget that Earth is the only planet on which life has been found to date. It forms an exemplary case.

There’s nothing inbetween

Figuring out how many Earth-like planets, possibly around Sun-like stars, there could be in the galaxy could therefore help us understand what the chances are like to find life outside the Solar System.

And because Earth leads the way, we think “humans would best be able to explore planets with rocky surfaces.” In the same way, Weiss added, “we would better be able to explore, or colonize, the rocky planets smaller than 1.5 Earth radii.”

This is where the astronomers hit another stumbling block. While data from Kepler showed that most exoplanets were small and in fact topped off at 4R_⊕, the Solar System doesn’t have any such planets. That is, there is no planet orbiting the Sun which is heavier than Earth but lighter than Neptune.

“It beats all of us,” Weiss said. “We don’t know why our Solar System didn’t make sub-Neptunes.” The Kepler mission is also responsible for not providing information on this front. “At four years, it lasted less time than a single orbit of Jupiter, 11 years, and so it can’t answer questions about the frequency of Jupiter, Saturn, Uranus, or Neptune analogs,” Weiss explained.

It seems the cosmos has lived up to its millennia-old promise, then, as more discoveries trickle in on the back of yet more questions. We will have to keep looking skyward for answers.