Exploring Ice Giant Oceans

by Paul Gilster | May 19, 2021 | Exoplanetary Science | 9 comments

Laboratory work on Earth is, as we saw yesterday, leading to hypotheses about how planets form and the effect of these processes on subsequent life. Whether in our own outer Solar System or orbiting other stars, planets in the ‘ice giant’ category, like Uranus and Neptune, remain mysterious, with Voyager 2’s flybys of the latter the only missions that have gone near them. We also know that sub-Neptune planets are common, many of these doubtless sharing the characteristics of their larger namesake.

Thus recent experiments probing ice giant interiors catch my eye this morning. Involving an international team of collaborators, the work looks at the interactions between water and rock that we would expect to find in the extreme conditions inside an ice giant. Planets like Uranus and Neptune are thought to house most of their mass in a deep water layer, a dense fluid overlaying a rocky core, a sharp departure from terrestrial worlds. What happens at that interface is ripe for examination.

The experiments were performed at Arizona State University’s DanShimLab, which is dedicated to the study of planetary materials at a wide range of pressures and temperatures using diamond-anvil and shockwave techniques. To probe this environment, the scientists immersed the rock-forming minerals olivine and ferropericlase in water and then compressed them to high pressures using a diamond-anvil. Heating the sample with a laser, the team could then track the water/mineral reaction under these conditions by way of X-ray measurements.

The result: High concentrations of magnesium, with implications for the composition of oceans much different from Earth’s, as study co-author Sang-Heon Dan Shim (Arizona State University) explains:

“We found that magnesium becomes much more soluble in water at high pressures. In fact, magnesium may become as soluble in the water layers of Uranus and Neptune as salt is in Earth’s ocean. If an early dynamic process enabled a rock-water reaction in these exoplanets, the topmost water layer may be rich in magnesium, possibly affecting the thermal history of the planet.”

Image: A diamond-anvil (top right) and laser were used in the lab on a sample of olivine to reach the pressure-temperature conditions expected at the top of the water layer beneath the hydrogen atmosphere of Uranus (left). In this experiment, the magnesium in olivine dissolved in the water. Credit: Shim/ASU.

Laser-heated diamond anvil cells can create pressures in the range of 1-5,000,000 bars by compressing materials between diamond ‘anvils’ that are transparent to X-rays, infrared and visible light. The lab’s laser heating systems can take samples to 1,000-5,000 K, all by way of exploring how materials behave under conditions thought to exist in planetary interiors. Thus the range of such laboratory work extends through rocky worlds and into the realm of not just the ice giants but gas giants like Jupiter.

Given the lab’s finding of high concentrations of magnesium under conditions of high pressure and temperature, Shim argues that the mineral may become as soluble in ice giant interiors as salt is in Earth’s oceans. Conceivably, this finding could explain why the atmosphere of Uranus is considerably colder than Neptune’s, for magnesium in larger amounts could block heat from escaping the interior. “This magnesium-rich water may act like a thermal blanket for the interior of the planet,” says Shim.

Image: An electron microscopy image of the olivine sample shows a large empty dome structure where magnesium under high-pressure water precipitated as magnesium oxide. Credit: Kim et al.

Oceans rich in magnesium may thus be common in ice giants, with a thick layer of water covering a rocky interior. Moreover, the idea that the interior of water worlds is sharply differentiated between rock and water has been challenged in recent work, with implications for the thermal evolution of these planets. To probe deeper into these matters, the study calls for an examination of other icy materials like CO₂ and NH₃, but note this cautionary remark in its conclusion:

Extrapolation of our results beyond the pressure range covered in our experiments should [be] treated with caution because of possible changes in the properties of H₂O at very high pressures. Nevertheless, the models based on our experiments demonstrate that geochemical cycles and thermal history of water-rich planets could be sensitive to the size of the planet because of pressure-dependent chemical processes.

We have a useful methodology here to extend the study of the geochemical cycle on a range of planetary interiors. We have much to learn, for as the paper points out, the interactions of major rock-forming minerals at the interface between ocean and rock in ice giants have rarely been explored at the high pressures used in these experiments.

The paper is Kim et al., “Atomic-scale mixing between MgO and H2O in the deep interiors of water-rich planets,” Nature Astronomy 17 May 2021 (abstract).

Planet Formation Modes as a Key to Habitability

by Paul Gilster | May 18, 2021 | Exoplanetary Science | 15 comments

While a planet’s position in the habitable zone is thought critical for the development of life like ourselves, new work out of Rice University suggests an equally significant factor in planetary growth. Working at a high-pressure laboratory at the university, Damanveer Grewal and Rajdeep Dasgupta have explored how planets capture and retain key volatiles like nitrogen, carbon and water as they form The team used nitrogen as a proxy for volatile distribution in a range of simulated protoplanets.

Two processes are under study here, the first being the accretion of material in the circumstellar disk into a protoplanet, and the rate at which it proceeds. The second is differentiation, as the protoplanet separates into layers ranging from a metallic core to a silicate shell and, finally, an atmospheric envelope. The interplay between these processes is found to determine which volatiles the subsequent planet retains.

Most of the nitrogen is found to escape into the atmosphere during differentiation and is then lost to space as the protoplanet cools or, perhaps, collides with other protoplanets during the turbulent era of planet formation. The data, however, demonstrate the likelihood of nitrogen remaining in the metallic core. Says Grewal:

“We simulated high pressure-temperature conditions by subjecting a mixture of nitrogen-bearing metal and silicate powders to nearly 30,000 times the atmospheric pressure and heating them beyond their melting points. Small metallic blobs embedded in the silicate glasses of the recovered samples were the respective analogs of protoplanetary cores and mantles.”

Nitrogen, the researchers learned, is distributed in different ways between the core, the molten silicate shell and the atmosphere, with the extent of this fractionation being governed by the size of the body. The takeaway: If the rate of differentiation is faster than the rate of accretion for planetary embryos of Moon or Mars-size, then the planets that form from them will not have accreted enough volatiles to support later life.

Earth’s path would have been different. The scientists believe that the building blocks of Earth grew quickly into planetary embryos before they finished differentiating, forming within one to two million years at the beginning of the Solar System. The slower rate of differentiation allowed nitrogen, and other volatiles, to be accreted. Adds Dasgupta:

“Our calculations show that forming an Earth-size planet via planetary embryos that grew extremely quickly before undergoing metal-silicate differentiation sets a unique pathway to satisfy Earth’s nitrogen budget. This work shows there’s much greater affinity of nitrogen toward core-forming metallic liquid than previously thought.”

Image: Nitrogen-bearing, Earth-like planets can be formed if their feedstock material grows quickly to around moon- and Mars-sized planetary embryos before separating into core-mantle-crust-atmosphere, according to Rice University scientists. If metal-silicate differentiation is faster than the growth of planetary embryo-sized bodies, then solid reservoirs fail to retain much nitrogen and planets growing from such feedstock become extremely nitrogen-poor. Credit: Illustration by Amrita P. Vyas/Rice University.

This work takes the emphasis off the stellar nebula and places volatile depletion in the context of processes within the rocky body in formation, especially the affinity of nitrogen toward metallic cores. Here’s how the paper sums it up:

…we show that protoplanetary differentiation can explain the widespread depletion of N in the bulk silicate reservoirs of rocky bodies ranging from asteroids to planetary embryos. Parent body processes rather than nebular processes were responsible for N (and possibly C) depleted character of the bulk silicate reservoirs of rocky bodies in the inner Solar System. A competition between rates of accretion versus rates of differentiation defines the N inventory of bulk planetary embryos, and consequently, larger planets. N budget of larger planets with protracted growth history can be satisfied if they accreted planetary embryos that grew via instantaneous accretion.

And the nebular conclusion:

Because most of the N in those planetary embryos resides in their metallic portions, the cores were the predominant delivery reservoirs for N and other siderophile volatiles like C. Establishing the N budget of the BSE [bulk silicate Earth] chiefly via the cores of differentiated planetary embryos from inner and outer Solar System reservoirs obviates the need of late accretion of chondritic materials as the mode of N delivery to Earth.

Rajdeep Dasgupta, by the way, is principal investigator for the NASA-funded CLEVER Planets project (one of the teams in the Nexus of Exoplanetary Systems Science — NExSS — research network). CLEVER Planets, according to its website, is “working to unravel the conditions of planetary habitability in the Solar System and other exoplanetary systems. The overarching theme of our research is to investigate the origin and cycles of life-essential elements (carbon, oxygen, hydrogen, nitrogen, sulfur, and phosphorus – COHNSP) in young rocky planets.”

All of which reminds us that the essential elements for life must be present no matter where a given planet exists in its star’s habitable zone.

The paper is Grewal et al., “Rates of protoplanetary accretion and differentiation set nitrogen budget of rocky planets,” Nature Geoscience 10 May 2021 (abstract / preprint).

Alpha Centauri and the Search for Technosignatures

by Paul Gilster | May 14, 2021 | Exoplanetary Science | 89 comments

Is there any chance we may one day find technosignatures around the nearest stars? If we were to detect such, on a planet, say, orbiting Alpha Centauri B, that would seem to indicate that civilizations are to be found around a high percentage of G- and K-class stars. Brian Lacki (UC-Berkeley) examined the question from all angles at the recent Breakthrough Discuss, raising some interesting issues about the implications of technosignatures, and the assumptions we bring to the search for them.

We’re starting to consider a wide range of technosignatures rather than just focusing on Dysonian shells around entire stars. Other kinds of megastructure are possible, some perhaps so exotic we wouldn’t be sure how they operated or what they were for. Atmospheres could throw technosignatures by revealing industrial activity along with their potential biosignatures. We could conceivably detect power beaming directed at interstellar spacecraft or even an infrastructure within a particular stellar system. One conceivable technosignature, rarely mentioned, is a world that has been terraformed.

All this takes us well beyond conventional radio and optical SETI. But let’s take the idea, as Lacki does, to Alpha Centauri, which we can begin by noting that in the past several years, Proxima Centauri b, that promising world in the habitable zone of the nearest red dwarf, has found its share of critics as a possible home to advanced life, if not life itself.

Michael Hippke noted in a 2019 paper that rocket launch to orbit from a super-Earth would be difficult, possibly inhibiting a civilization there from building a local space infrastructure. Milan ?irkovi? and Branislav Vukoti? asked in 2020 whether the frequent flare activity of Proxima Centauri would inhibit radio technologies altogether. Whether abiogenesis could occur under these kinds of flare conditions remains unknown.

Alpha Centauri A and B, the central binary, offer a much more benign environment, both stars being exceedingly quiet at radio wavelengths. Moreover, we have known since the 1990s that habitable orbits are possible around each of them. We have no knowledge about whether the evolution of life would necessarily lead to intelligence; as Lacki pointed out, this is an empirical question — we need data to answer it.

The Drake Equation points to further unresolved issues. In what portion of a star’s lifespan would we expect technological cultures to emerge? Our own civilization has used radio for about a century — one part in 100,000,000 of the lifespan of the Sun. Hence the slide below, which is telling in several ways. Lacki refers here to temporal coincidence, meaning that we might expect societies around other stars to be separated in time, not just in space. Hence this simple graph of a very deep subject.

Deep time always takes getting used to, no matter how many times we think we’ve gotten a handle on figures like 4.6 billion years or, indeed, 13.8 billion years, the lifetimes of the Sun and cosmos respectively. I stared at this figure for some time. Lacki has arbitrarily placed a civilization at Centauri B, as shown by the vertical line, and another at Centauri A and C. Our own is shown in its known place in the Sun’s lifetime, except for the striking fact that the thickness of humanity’s timeline on the chart corresponds to a lifetime of 10 million years. If we wanted a line showing our 100 years of technological use — i.e., radio — the line would have to be 100,000 times thinner.

The odds that the lines of any two stars would coincide seem infinitely small, unless we are talking about societies that can persist over many millions of years. But here we can begin to turn the question around. We might want to rethink nearby technosignatures if we remind ourselves that what they represent is not the civilization itself, but the works it created, which might greatly outlive their builders. Objects like Dyson spheres would seem to fit this category and would exist at planetary scale.

Small artifacts can also last for vast periods — our own Voyagers will be intact for millions of years — though finding them would be an obvious challenge (here it’s worth thinking about the controversy over ‘Oumuamua. If a piece of dead technology were to pass near the Sun, would we be able to recognize its artificial nature? I simply raise the question — I remain agnostic on the question of ‘Oumuamua itself).

But can we be sure we are the first intelligent technological society on Earth? It’s worth considering whether we would know it if an advanced culture had existed hundreds of millions of years ago, perhaps on Earth or a different planet in the Solar System. Looking forward, if we go extinct, will another intelligent species evolve? We don’t know the answer to these questions, and the depth of our ignorance is shown by the fact that we can’t say for sure that intelligence might not evolve over and over again.

If interstellar flight is possible, and it seems to be, we can consider the possibility of intelligence spreading throughout the galaxy, perhaps via self-reproducing von Neumann probes. Even with very slow interstellar velocities, the Milky Way could be settled in relatively short order, astronomically speaking. Michael Hart pointed this out in the 1970s, and Frank Tipler argued in 1980 that at current spacecraft speeds alone, self-replicating probes could colonize the galaxy in less than 300 million years.

Sending physical craft to other stars may be difficult, but there are advantages. So-called Bracewell probes could be deployed that would wait for evidence of intelligence and report back to the home system, as well as carrying inscribed messages intended for the target system. This is Jim Benford’s ‘lurker’ scenario, one which he proposes to explore by searching nearby objects in our own system including the Moon. After all, if they might exist elsewhere, there may be a lurker here.

We don’t know how long a probe like this might remain active, but as a physical artifact, it remains a conceivably detectable object for millions, perhaps billions of years. Finding such a probe in our own system would imply similar probes around other stars and, indeed, the likelihood of a civilization that has spread widely in the galaxy. We might well wonder whether a kind of galactic Internet might exist in which information relays around stars are common, perhaps using gravitational lensing.

In other words, the question of technosignatures at Alpha Centauri doesn’t necessarily imply anything found there would have come from a civilization that originated in that system. The same civilization might have seeded stars widely in the Orion Arm and beyond.

All of these ‘ifs’ define the limits of our knowledge. They also point to a case for looking for technosignatures no matter what our intuitions are about their existence at Alpha Centauri. A lack of obvious technosignatures in our system would imply a similar lack around the nearest stars, but we haven’t run the kind of fine-grained search for artifacts that would find them on our own Moon, much less on nearby smaller objects.

Image: How quickly would a single civilization using self-replicating probes spread through a galaxy like this one (M 74)? Moreover, what sort of factors might govern this ‘percolation’ of intelligence through the spiral? We’ll be looking further at these questions in coming days. Image credit: NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration.

Intuition says we’re unlikely to find a technosignature at Alpha Centauri. But I return to the chart on civilizational overlap reproduced above. To me, the greatest take-away here is the placement of our own civilization within the realm of deep time. Given how short the lifetime of technology has been on Earth, I’m reminded viscerally of how precious — and perhaps rare — intelligence is as an emerging facet of a universe becoming aware of itself. That’s true no matter what we find around the nearest stars.