GD
Geodynamics
Diogo Lourenço & Antoine Rozel

Guest

Find out more about the blog team here.

Writing the Methods Section

Writing the Methods Section

An important part of science is to share your results in the form of papers. Perhaps, even more important is to make those results understandable and reproducible in the Methods section. This week, Adina E. Pusok, Postdoctoral Researcher at the Department of Earth Sciences, University of Oxford, shares some very helpful tips for writing the Methods in a concise, efficient, and complete way. Writing up the methods should be no trip to fantasy land!

Adina Pusok. Postdoctoral Researcher in the Department of Earth Sciences, University of Oxford, UK.

For my occasional contribution to the Geodynamics blog, I return with (what I think) another essential topic from The Starter Pack for Early Career Geodynamicists (see end of blog post): how to write the methods section in your thesis, report or publication. Or using the original title: “Writing up the methods should be no trip to fantasy land”. Don’t get me wrong, I love the fantasy genre, but out of an entire scientific manuscript that pushes the boundaries of knowledge (with additional implications and/or speculations), the methods section should be plain and simple, objective and logically described – “just as it is”.

The motivation for this post came some months ago when I was reviewing two articles within a short time interval from each other, and I felt that some of my comments repeated – incomplete methods sections, assumptions let to be inferred by the reader, and which ultimately made assessment of the results more difficult. But I also consider it is not ok to write harsh reviews back (for these reasons), since again, there is little formal training for Early Career Scientists (ECS) on how to write scientific papers. Moreover, even when there is such formal training on academic writing, it is often generalized for all scientific disciplines, ignoring some important field-specific elements. For example, a medical trial methods section will look different from an astrophysics methods section, and within Earth Sciences, the methods section for a laboratory experiment on deformation of olivine will contain different things compared to a systematic study of numerical simulations of subduction dynamics.

A common approach by most students (especially first time) is to dump everything on paper and then hope it represents a complete collection of methods. However, with increasing complexity of studies, this collection of methods has neither heads nor tails, and is prone to errors. Such pitfalls can make the manuscript cumbersome to read or even question the validity of the research. Generally, journals do have guidelines on how the methods should be formatted, how many words, but not necessarily what to contain because it varies from field to field. I believe there should be a more systematic approach to it. So in this post, I aim at describing some aspects of the Methods section, and then propose a structure that (mostly) fits the general Geodynamics studies.

1. The scientific Methods section

The Methods section is considered one of the most important parts of any scientific manuscript (Kallet, 2004). A good Methods section allows other scientists to verify results and conclusions, understand whether the design of the experiment is relevant for the scientific question (validity), and to build on the work presented (reproducibility) by assessing alternative methods that might produce differing results.

Thus, the Methods section has one major goal: to verify the experiment layout and reproduce results.

It is also the first section to be written in a manuscript because it sets the stage for the results and conclusions presented. So, what exactly do you need to include when writing your Methods section? The title by T.M. Annesley (2010) puts it perfectly into words: “Who, what, when, where, how, and why: The ingredients in the recipe for a successful methods section”.

  • Who performed the experiment?
  • What was done to answer the research question?
  • When and where was the experiment undertaken?
  • How was the experiment done, and how were the results analyzed?
  • Why were specific procedures chosen?

Across sciences, the Methods section should contain detailed information on the research design, participants, equipment, materials, variables, and actions taken by the participants. However, what that detailed information consists of, depends on each field.

2. The Methods section for numerical modeling in Geodynamics

I propose below a structure for the Methods section intended for numerical simulations studies in Geodynamics. I want to mention that this structure is meant as a suggestion, especially for ECS, and can be adapted for every individual and study. Geodynamics studies may have different aspects: a data component (collection, post-processing), a theoretical (mathematical and physical) framework, a numerical framework (computational) and an analog component (laboratory experiments). The majority of studies have 1-2 of these components, while few will have all of them. In this post, I will focus primarily on studies that use numerical simulations to address a question about the solid earth, thus having primarily a theoretical and numerical component.

Before I start, I think a great Methods section is like a cake recipe in which your baked cake looks just like the one in the photo. All the ingredients and the baking steps need to be explained precisely and clearly in order to be reproduced. We should aim at writing the Methods with this in mind: if someone were ‘to bake’ (reproduce) my study, could they succeed based on the instructions I provided? There are many ways how to write your Methods, my way is to break it into logical sections, going from theoretical elements to numerical ones.

Proposed structure:

  1. Brief outline – A general paragraph describing the study design and the main steps taken to approach the scientific question posed in the Introduction.
  2. Theoretical framework – Any numerical simulation is based on some mathematical and physical concepts, so it’s logical to start from here. And from the most important to the least important.
    • 2.1 Governing equations – Section describing the conservation of mass, momentum, energy.
    • 2.2 Constitutive equations – Section describing all the other elements entering the conservation equations above such as: rheology (deformation mechanisms), equation of state, phase transformations, etc. Each of these topics can be explained separately in subsections. For example,
      • 2.2.1 Rheology
        • 2.2.1.1 Viscous deformation
        • 2.2.1.2 Plastic deformation
        • 2.2.1.3 Elastic deformation
      • 2.2.2 Phase transformations
      • 2.2.3 Water migration in the models
    • Figures and tables:
      • Table of parameters – for quick definition of parameters used in equations.
  3. Computational framework – Section explaining how the theory (Section 2) is solved on the computer.
    • 3.1 Numerical methods – code details, discretization methods, programming language, solvers, software libraries used, etc. If you are using a community code, these details should be provided in previous publications.
    • 3.2 Model setup – Section describing the layout of the current experiment.
      • 3.2.1 Details: model geometry, resolution (numerical and physical), parameters, initial and boundary conditions, details on rheological parameters (constitutive equations), etc.
      • 3.2.2 Must motivate the choice of parameters – why is it relevant to address the scientific questions?
    • Figures and tables:
      • Table of parameter values, rheological flow laws used.
      • Table with all model details (to reduce text).
      • Figure illustrating the model geometry, initial and boundary conditions.
    • *NOTE: If you are testing/implementing a new feature in the code, you should allocate a new section for it. Also, spend more effort to explain it into details. Do not expect many people to know about it.
  4. Study design – Section describing the layout of the study.
    • 4.1 What is being tested/varied? How many simulations were performed (model and parameter space)? Why perform those simulations/vary those parameters?
    • 4.2 Code and Data availability – code availability, input files or other data necessary to reproduce the simulation results (i.e., installation guides). Many journals today only accept for publication studies in which data and code availability is declared in standard form (i.e., AGU journals). Some other questions to answer here: where were the simulations performed? how many cores? can I reproduce data on laptop/desktop or do I need access to a cluster?
    • Figures and tables:
      • Simulations table – indicating all simulations that were run and which parameters were varied. When the number of simulations is high (i.e., Monte-Carlo sampling) you should still indicate which parameters were varied and the total number of simulations.
  5. Analysis of numerical data – details on visualization/post-processing techniques, and describe how the data will be presented in the results section. This is a step generally ignored, but be open about it: “visualization was performed in paraview/matlab, and post-processing scripts were developed in python/matlab/unicorn language by the author”. If your post-processing methods are more complex, give more details on that too (i.e., statistical methods used for data analysis).

 

Before you think you’ve finished the Methods section, go over your assumptions, and make sure you’ve explained them clearly! Geodynamics is a field in which you take a complex system (Earth or other planetary body) and simplify it to a level that we can extract some understanding about it. And in doing so, we rely on a physically consistent set of assumptions. It is important to bear in mind that this set of assumptions may not always be obvious to the audience. If your reviewers have questions about your methods and interpretation of results (that you think is obvious), it means that something was not clearly explained. Be pre-emptive and state your assumptions. As long as they are explicit and consistent, the reviewers and readers will find less flaws about your study. Why that choice of parameters? Why did you do it that way?

3. A few other things…

It’s good practice to write a complete Methods section for every manuscript, such as one following the structure above. However, some journals will ask for a short version (1-2 paragraphs) to be included in the manuscript and have the complete Methods section in a separate resource (i.e, Supplementary Data, Supporting information, repository) such that it’s made available to the community. For some other journals, it will be difficult to find a balance between completeness (sufficient details to allow replication and validity verification) and conciseness (follow the guidelines by journals regarding word count limits).

To master the writing of the Methods section, it is important to look at other examples with similar scope and aims (especially the ones you understood clearly and completely). It is also a good idea to keep notes and actually start writing up your equations, model setup, and parameters as the study progresses (such as the mandatory lab notebook).

Finally, some tips on the style of writing of the Methods section:

  • be clear, direct, and precise.
  • be complete, yet concise, to make life easy for the reader.
  • write in the past tense.
  • but use the present tense to describe how the data is presented in the paper.
  • may use both active/passive voice.
  • may use jargon more liberally.
  • cite references for commonly used methods.
  • have a structure and split into smaller sections according to topic.
  • material in each section should be organized by topic from most to least important.
  • use figures, tables and flow diagrams where possible to simplify the explanation of methods.

The Starter Pack for Early Career Geodynamicists

In the interest of not letting the dust accumulate, the growing collection of useful Geodynamics ECS posts (from/for the community):

References:

Kallet R.H. (2004) How to write the methods section of a research paper, Respir Care. 49(10):1229-32. https://www.ncbi.nlm.nih.gov/pubmed/15447808

Annesley, T.M. (2010) Who, what, when, where, how, and why: the ingredients in the recipe for a successful Methods section, Clin Chem. 56(6):897-901, doi: 10.1373/clinchem.2010.146589, https://www.ncbi.nlm.nih.gov/pubmed/20378765

The geodynamics of Enceladus: exotic and familiar

Enceladus

This week, Gael Choblet, CNRS research associate in the Laboratoire de Planétologie et Géodynamique (University of Nantes and Angers), tells us everything about the interior of Enceladus, an interesting icy moon of Saturn!

Gael Choblet

Gaël Choblet

This is my first contribution in these pages. The choice of Saturn’s small moon Enceladus as a topic mostly results from my acquaintance with this planetary body. Yet, the reader probably remains to be convinced that this particular subject belongs to that specific section of EGU Geodynamics blog – I also do, although I noticed that some colleagues have proposed texts that are adjacent to the “geodynamics” central theme, albeit on far more important or at least far more urgent topics than Enceladus research. The ongoing extinction of living species is probably more pressing than the discovery of life outside the Earth, of which Enceladus could be one of the most accessible laboratories (probability not provided here – incidentally, this is not the subject of this text). Given this, the idea I propose in the following lines is to highlight the connection between progress in the knowledge on Enceladus’ interior and Geodynamics as a discipline, using a chronological approach. In the end, we (author, reader) will see whether this choice made sense: whether a dual perspective, exotic and familiar, ensues or whether the bridges built prove artificial and ephemeral.

Full disclosure: I have co-authored some of the work I mention and the following account certainly involves bias.

Enceladus, the moon, is known to humans since the middle of the french revolution although not because of it. Saturn being more distant than Jupiter, the discovery of its largest moon Titan waited 45 years after Galileo had spotted Jupiter’s four large moons of approximately the same size. In the case of Enceladus, almost precisely one order of magnitude smaller than Titan in radius, further instrumental developments (and a huge telescope, about one order of magnitude larger than Galileo’s) were necessary for german-born William Herschel, appointed Astronomer of the King of Great Britain at the time, to discover Enceladus. William’s younger sister Caroline probably helped: besides knowing how to make tea (illustration) and being also german-born, she participated in her brother’s astronomical research and conducted her own. For this reason, she won the Gold medal of the Royal Astronomical Society then became a member (first woman to achieve each of these distinctions).

Then, strangely enough, humankind lost interest in Enceladus.

Establishing a first connexion here with the Earth is hazardous since little is known by me about human’s discovery of its birth planet.

That Enceladus surface is made of water ice appeared likely in the 1970s, even before direct spectro- scopic evidence, owing to the bulk density of icy moons as well as the fact that Saturn’s rings spectra seem compatible with that of H2O ice. French engineers had already associated Enceladus to water when designing a fountain for the “Bosquet de l’Encelade” in the gardens of the Versailles castle (illus- tration). Yet, one has to admit that rather than a dazzling prescience (the moon was not known yet), their choice of water as a material was probably motivated by the inability to construct a magma foun- tain: Enceladus (the mythological figure), was one of these proto-gods, brothers and sisters of Saturn, whose access to fame mostly sums to their defeat against the (true) olympian deities. Early geodynam- icists often explained the known volcanic activity at the time by the fact these Giants/Titans were then buried beneath the ground and angry. For example, Giant Enceladus is told to lie beneath the ground of mount Etna after Athena gave him a beating. This volcanic character probably motivated the choice of a powerful water jet for the bosquet, which could also be fantasized as some form or foreknowledge, a concept not easily favored by geodynamicists.

So, water. Lots of water. Earth’s hydrosphere corresponds to a 2.5 km-thick shell at the surface. In fact, so little water that solid Earth’s topography is enough for continents to emerge and, strange thing (or not, you tell me), mountains are roughly as high as oceans are deep. Nothing of the sort for Enceladus: the hydrosphere thickness is 60 km (for a global radius of about 250 km) so that the surface of the rock component lies very deep beneath the surface. To say something slightly different in a slightly other way: water is half of Enceladus mass while the most extravagant hypotheses for Earth (if a number of water oceans are stored in the mantle and even more were present in the core) still provide a budget smaller than 2 %.

Quite early after I was born although not because of this, bold colleagues suggested that Enceladus’ location in the Saturn’s system, in the densest region of the otherwise very diffuse E-ring made of tiny ice particles, could be at the origin of this peculiar ring: as we will see below, this would prove a correct guess.

Enceladus viewed by NASA

Enceladus, an active little moon (NASA/JPL/Space Science Institute)

Because then occurred the real birth of Enceladus as a major body for planetary science (yes, knowledgeable reader, I intentionally mute the achievements of the Voyager spacecrafts for the sake of brevity): in the second half of 2004, the long planned Cassini-Huygens mission arrived in Saturn’s environment after a journey of about 7 years. Over the course of a remaining 13 years-long mission, this unique spacecraft with the mass of an elephant (launch mass: African bush elephant; dry mass: African forest elephant), carrying an impressive series of instruments developed in the 1990s fulfilled all the scientific expectations. Saturn itself and the architecture of the Saturn’s system were scrutinized. The planetary-size moon Titan (larger than Mercury) which, everything suggested, was certainly a fascinating body but whose orange glow formed by a dense atmosphere had removed the surface from the endeavors of previous space probes, turned out to be a fascinating body, arguably more so. For the reader more versed in serendipity, nevertheless, Enceladus’ activity is definitely the favorite pick in the list of Cassini’s top-ten discoveries. During the first months of Cassini’s Saturn trip, it occurred that a conductive cloud was located above Enceladus’ south pole. Further examinations by the multiple instruments revealed that this plume emerged from the moon (illustration), was composed of water (ice and vapor) and contaminants ejected from Enceladus interior through individual jets emanating from large parallel fractures in Enceladus’ icy crust (illustration) and, yes, Enceladus plume is feeding the E-ring (illustration). While Earth’s history of magmatism is known to significantly contribute to the evolution of its atmosphere, Enceladus’ gravity at the surface, a hundred times smaller, rather enables its volcanic activity to shape its orbital environment.

Still, such an activity remained puzzling given the moon’s dimension: infrared emission in the south polar region only (heat flowing through other terrains is too small to be detected) was soon estimated to be 10-15 GW. Equivalent to the geothermal power used by humans worldwide, this amount might seem modest to a non-geodynamicist as a negligible fraction of the Earth’s heat budget of internal origin (46 TW). Yet, the reader will have noticed that, given its size and rock content, Enceladus could be expected to expel only 50 times less energy if radiogenic decay were the sole provider of heat. This might not come as a surprise, though, for the planetary scientist aware of the peculiar heat budget of some icy moons: after all, Cassini’s older sibling devoted to the exploration of the Jupiter system, the Galileo spacecraft, had already estimated that 100 TW were transported through the volcanic surface of Io, the closest of the Galilean moons, and that the existence of Europa’s putative internal ocean also required another powerful heat source, identified even before the spacecraft observations to likely result from the dissipation of tidal deformation enabled by these two satellites’ orbits.

In the case of Enceladus, the localized emission at the South Pole, accompanied by complex tectonic features there confirming the ongoing activity witnessed by Cassini, led part of the research community to hypothesize the deep presence of a regional sea in the Southern hemisphere, beneath the ice, above the rocky core. Only such a liquid layer decoupling the motion of the ice shell and the rocky core would permit a sufficient deformation to dissipate large amounts of heat – viscous dissipation of tidal deformation was mostly envisioned in the ice layer as the supposedly cool rocks appeared too stiff, contrary to Io and its much hotter rocky mantle that most probably includes an asthenosphere much more developed than the Earth’s at present.

This was problematic, nevertheless. The poor knowledge of Saturn’s interior and possible dissipation mechanisms it may host implies that estimates of dissipation only rely on the observed motion of its natural satellites. At the time (second half of 2000s), this led to envision a necessarily episodic release of heat by Enceladus, in the fashion of Wilson cycles on the Earth where deviations of 25% around the long- term trend of the mean oceanic heat flux are postulated. Only, in the case of Enceladus, this episodic outburst appeared really exceptional (at best corresponding to maybe 1% of the duration of the moon’s duty cycle). A fairer comparison would thus be with the almost mythological catastrophic resurfacing event once postulated to explain the crater distribution on Venus if this one involved the recycling of a significant part of the lithosphere – a view that is probably in the process of being abandoned, anyway. At this stage of Enceladus research (early 2010s), the community was left with the difficult idea that it might be witnessing precisely an extraordinary phase of the evolution of the moon, a comforting notion possibly being that the sample of icy moons has a cardinal number larger than that of the terrestrial planets of the solar system (to the point where the use of statistics could almost begin to make sense).

Models of Enceladus

Interior models for Enceladus – left: before, right: after (NASA/JPL-Caltech)

Putting aside these uncertainties, a team composed Czech colleagues and people in our group in Nantes proposed an original way to constrain the interior of Enceladus. In the absence of seismology, a now classic approach for the study of planetary interiors relies on the use of the global shape (or topography) and gravity field, even if the results are more uncertain (more interior models satisfy a given observation). But in the case of Enceladus, a small moon, volcanic activity is so powerful that it could also reflect the global structure. Cassini has shown variations in the amplitude of this activity measured in terms of particle flux (the quantity of gas seems more or less constant) during the moon’s orbit. Never absent, it seems maximum at a precise point in the orbit (with a period of a little more than a day) which is repeated from one orbit to another. This seemed to indicate a control of the activity by tidal stresses that evolve with the position of Enceladus around Saturn and confirmed the prominent role played by tides. But this activity is delayed when compared to what would be expected of an elastic body: although other phenomena may contribute to this delay, we have bet that the viscous structure of the moon could be at the origin of it. We therefore built a method (certainly less precise than seismology, and with more uncertain foundations) to probe models of internal structure in terms of the rheological behavior that they oppose to tidal forcing. Several families of models could be suitable, some with a global ocean. But probably guided by a more or less conscious bias not unfamiliar to the geodynamicists that contributed to the debate on plate tectonics, our favorite candidates belonged to a group with a deep regional sea (illustration). Which proved wrong.

Pschiiit

Pschiitt (ina.fr)

Unlike in the case of plate tectonics, the paradigm shift was very rapid. The causes of this effectiveness would still have to be studied but rather than a change in the nature of scientific practice, I imagine (without any proof) that this shall be attributed to a much smaller research community as well as to the speed of change of opinions since the Cassini spacecraft mowed down the models as fast as new radar swaths were acquired. In fact, about a year before the end of the mission (2017), the meticulous evaluation of the motion of Enceladus’ surface from the compilation of numerous Cassini images indicated a very marked dynamic of the moon’s rotation (precisely, its physical libration, the motion that enables the visible side of the Moon from the Earth to represent a little more than half of the total surface). Unequivocally, such an amplitude made it necessary that the ice shell was free to move without being mechanically anchored to the rocky core. As a french president said to describe a completely different subject, the idea of a regional sea had made “pschitt”.

 

With this powerful estimate of the libration as well as chemical evidence that jets emerged from a salty water reservoir, Enceladus became the planetary body for which the existence of a global internal ocean is the least doubtful (illustration). It is now emblematic of an increasingly large family named since “Ocean worlds”.

In the meantime,

  • colleagues had demonstrated that dissipation in the Saturnian system could be much larger than anticipated so that the present-day activity of Enceladus might in the end correspond to a steady-state,
  • two independent analyses of the jets materials and ice particles in the E ring (tiny sand grains and endogenic hydrogen) revealed the likelihood of ongoing hydrothermal activity, pushing Enceladus in the short list of planetary bodies worth studying for the emergence of life (all the more so that the ocean delivers samples to spacecrafts flying through the plumes (as did Cassini several times) or would land on the surface, beneath the snow).

But this takes us away from geodynamics.

In the second and last episode, dear reader, I will show

  • how the principle of isostasy dear to geodynamicists since the 19th century was key to finally (?) understanding the interior of Enceladus,
  • how the thermal convection dear to geodynamicists from the beginning of the 20th century to describe the Earth’s mantle might also occur within the rocky core of Enceladus, with different modalities,
  • and perhaps how oceanic hotspots dear to geodynamicists from the second half of the 20th century might bear resemblance to Enceladus’ erupting centers (or not).

Join us next time, won’t you?

Searching for future directions in tectonic modelling

Searching for future directions in tectonic modelling

Geoscientists frequently use forward geodynamic simulations to test hypotheses derived from geophysical and geologic observations. While numerical simulations of lithospheric deformation have lead to key advances in our understanding of tectonic processes, in many cases it remains difficult to ascertain whether numerical models reproduce observations for the correct underlying regions.  This week, John Naliboff and Jolante van Wijk discuss this issue, and talk about a White Paper being prepared by the Computational Infrastructure for Geodynamics (CIG) Long-Term Tectonics Working Group on this topic.

John Naliboff. Assistant Research Scientist in the Department of Earth and Planetary Sciences, UC Davis.

In recent years, advances in numerical methodology, high-performance computing and elucidation of complex geologic observations have enabled 3-D simulations of long-term lithospheric deformation at kilometer-scale resolution and with complex non-linear material behavior. The lithosphere models generally include rheological and compositional layering that delineate a brittle upper crust, ductile (viscous) to brittle mid- to lower crust and a brittle to ductile lithospheric mantle overlying a purely viscous asthenosphere. Inherently, the rheological behavior of distinct layers varies temporally through complex feedbacks between temperature, grain-size, strain-rate, phase transitions, flexural stresses and finite deformation. Across a wide range of tectonic settings, numerical investigations incorporating this type of behavior have qualitatively and in some cases quantitatively reproduced key first- and second-order geologic observations.

Jolante van Wijk. Associate Professor in the Department of Earth and Environmental Science, New Mexico Tech.

Despite these successes, numerous significant challenges remain as the computational tectonics community looks toward investigations that account for physical processes acting across a wide range of spatial and temporal scales (Figure 1). Geodynamic model development currently evolves around modifying existing models to include surface processes, thermodynamically consistent melt and volatile transport, metamorphic reactions, and brittle failure to reproduce characteristic features of the seismic cycle.

While many of these processes or features are active areas of research and have been addressed on an individual basis, it often remains unclear at best as to how one should numerically validate even the simplest models of lithospheric deformation. In other words, one can ask whether numerical models of lithospheric processes are reproducing key observations for the correct underlying reasons. Significantly, this question of validation equally applies to observational studies: given that many geologic processes contain significant feedbacks across vast spatial and temporal scales, to what degree can a set of specific observations be interpreted to meaningfully reflect first-order processes?

Continued close collaboration between observational, experimental, and computational Earth scientists is needed to overcome these challenges. At present, the CIG Long-Term Tectonics Working Group is preparing a white paper draft that outlines a 5-10 year vision for collaboration between computational Earth scientists and experimental and observational communities. Given the vast series of topics and disciplines associated with lithospheric dynamics, the White Paper will be organized around Transitional Domains within the lithosphere. The Transitional Domains include Earth’s surface, the brittle-ductile transition, the Moho, the mid-lithosphere discontinuity, and the lithosphere-asthenosphere boundary. For each of these domains we will address the following questions:

  1. How are these domains characterized in the Earth’s lithosphere?
  2. (How) have these domains been modeled previously?
  3. What steps can we take to improve the characterization of these transitional domains in numerical models?
  4. What new methods need to be developed to implement the domains?

 

Figure 1. Spatial and temporal scales associated with distinct lithospheric processes, which was published in Cooper et al., 2015, GSA Today, v. 25(6), pp. 42-43 (Figure 1a).

 

A new generation of geodynamic models will be developed to include the transitional domains. These models will need to be validated, using datasets from the observational and experimental communities, with newly developed techniques.

The White Paper will also include sections on increasing the value of lithospheric models for other scientific communities, and on a pathway toward increasing societal relevance of our modeling efforts.

Please contact John Naliboff (jbnaliboff@ucdavis.edu) and Jolante van Wijk (jolante.vanwijk@nmt.edu) for suggestions or questions. A draft White Paper will be presented at CIG’s annual meeting at AGU 2019 in San Francisco.

Iron volcanism on metallic asteroids?

Metallic asteroid

This week, Francis Nimmo, professor in the Department of Earth and Planetary Sciences (University of California Santa Cruz), tells us about volcanism on metallic asteroids! Around and after the formation of the solar system (4.5 billion years ago onwards), volcanoes on some of the gigantic bodies of the asteroid belt might have erupted … iron. Explanations.

Francis Nimmo

Francis Nimmo

One way in which we can learn about the insides of planetary bodies is by looking for signs of volcanism. Volcanism transports molten material from the interior to the surface of a body, where it solidifies. Something similar may happen on icy bodies, where cryovolcanism is thought to occur. Together with Coby Abrahams, one of my grad students, I recently wrote a paper proposing “ferrovolcanism”, which is the eruption of metallic iron. We suggested that this may have happened in the past on metallic asteroids.

 

What are metallic asteroids? Some asteroids, such as 16 Psyche are thought to be made mainly of iron and nickel, on the basis of their spectra and radar signatures. They probably originated from a catastrophic collision. The picture is that a proto-planet, which had already formed a dense metallic core, was broken apart so violently that its core was fragmented, and the fragments then cooled to form free-floating metallic bodies. Such an event would also explain the large number of iron meteorites in our collections. So far we have never seen a metallic asteroid up close, but NASA is planning to send an orbiter which will reach 16 Psyche in 2026.

Iron meteorite

Artistic view of an iron meteorite with ferrovolcanism (Elena Hartley)

How would “ferrovolcanism” work? Immediately before the catastrophic collision, the core would have been liquid (because the silicate mantle is a good insulator). After the collision, the surface of a free-floating iron blob would have initially cooled very rapidly. Analysis of iron meteorites tells us that, in some cases, the blob developed a strong, cold iron crust. The melt (liquid iron) beneath the crust would have been less dense than the crust – just like molten magma on Earth. And so, just like magma on Earth, the liquid iron would tend to rise to the surface – hence ferrovolcanism.

There are a few factors that make ferrovolcanism less likely than silicate volcanism. One is that, as the iron crust cools, it contracts, putting the whole body into compression and making it more difficult for melt-filled fractures to open up. Another is that iron is both ductile and strong, so that fracture propagation is harder. And iron is also much more thermally conductive than rock, so that melt-filled fractures will cool and solidify rapidly. Nonetheless, we don’t think any of these factors is fatal. For instance, Mercury also experiences global compression but still shows signs of volcanism. Iron eruptions might be aided by volatiles coming out of solution, just as with terrestrial eruptions, although the catastrophic impact may have removed some volatiles.

How do we test this hypothesis? One way is to wait for images of 16 Psyche! If we see things that look like iron volcanoes, that would be interesting. But identifying signs of ancient volcanism is notoriously hard – this paper documents several proposed volcanic or cryovolcanic features which turned out to be something else on further investigation. And our calculations show that the volume of erupted material will be only about 0.1 percent of the total volume of Psyche, so these features may be hard to spot – especially after 4 billion years of erosion by impacts.

Another way is to look at the iron meteorites in our collections, because ferrovolcanism should produce anomalous-looking meteorites. For instance, one might find vesicle-rich samples, or ones happening late in the crystallization sequence but showing extremely rapid cooling (because they were erupted to the surface).

How did we write the paper? It was more or less accidental. Coby was working on how iron asteroids might lose their volatiles early when one day he turned to me and said “I think they’ll erupt!”. I thought this was a neat idea, and so we started to look at how the eruptive process might work.

The strangest part of writing our paper was that at the LPSC conference someone came up to me and said “I just invented iron volcanism!”. It turned out that another group had been working on ferrovolcanism completely independently. They suggested that pallasite meteorites might be evidence of ferrovolcanism (something that I had completely missed). This simultaneous but independent development of ideas seems to be quite common in the Earth Sciences – but that is a topic for another day.