Archives / 2018 / July

Thirteen planets and counting

Thirteen planets and counting

Apart from our own planet Earth, there are a lot of Peculiar Planets out there! In this series we take a look at a planetary body or system worthy of our geodynamic attention, and this week we move back to our own solar system. Many of us will clearly remember the downgrading of Pluto as a planet nearly 12 years ago to the month. In this informative and witty post, Laurent Montesi from the University of Maryland makes a case for reinstating Pluto of planetary status, plus a handful of others, or at least a review of definitions. Bring on Club Planet! 

Laurent Montesi

A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit. When Resolution 5A was passed by the International Astronomical Union (IAU) during the closing ceremony of its 2006 General Assembly, Pluto was “demoted” from the rank of the true planets to a dwarf planet. Children’s eyes filled with tears over the injustice made to “poor little Pluto”, textbooks were rewritten, and the Nine Pizzas that My Very Excellent Mother Just Served Us turned to Noodles.

I don’t really care.

I see where the IAU came from when crafting this definition, and to some extent, I agree with it. But it is not relevant to me. The thing is, I am not an astronomer. I recognize the authority of the IAU to name geological features on planets and other worlds, but I’m a geologist. Pluto, like many other solar system objects, has too much exciting geology to be ignored!

Figure 1: The five Dwarf Planets currently recognized by the IAU, proud members of Club Planet.


To me, a dwarf planet is first and foremost a planet, and what interests me in planets is their geological activity. If, as stated in part b of the IAU definition, an object is able to “overcome rigid body forces” (whatever that means), that should leave a geological trace. I don’t care if the planet cleared its neighbourhood or not.

So, I take the IAU definition as an invitation for Ceres, Pluto, Eris, Haumea, and Makemake to join the exclusive Club Planet (Figure 1). They all bring interesting geology to the Club. Look at the results of the Dawn and New Horizons missions! Ceres has mountains, fractures, oddly hexagonal craters, and a remarkable bright spot beckoning explorers to study its water-rich interior. Pluto has become a superstar of planetary exploration, with oceans of frozen nitrogen, diverse terrains, large rifts, perhaps a giant ice volcano, and the cutest heart tattoo in the solar system.

I’d like to go even further and open the door of the Club to many satellites (Figure 2). Our Moon is the doorway towards understanding the early evolution of terrestrial planets like the Earth. It taught us about giant impacts and magma oceans. If you want to find liquid water (and possibly life) today, go to Europa or Enceladus. If you are looking for alien plate tectonics, check out Ganymede and Europa. Are you searching for a thick atmosphere, rivers, and lakes? Welcome to Titan. Who has the most volcanic activity today? Please stand up, Io. What incredible rifts you have, Miranda and Charon! From a geological standpoint, satellites are as rich as any planet.

Figure 2: Knocking at the door of Club Planet are several of the satellites of the solar system: Earth’s Moon, the four Galilean Satellites Io, Europa, Ganymede, and Callisto, the large moons Titan and Triton, as well as numerous smaller, but geologically interesting satellites. They are led by Pluto’s moon Charon.


So, what actually is a planet? To the ancient Greeks, they were dots of light wandering against the rigid background of the night sky. These dots then turned out to be balls. Galileo saw four satellites around Jupiter, and in the redesigned solar system, planets could only orbit the Sun. Eventually, so many objects were found that it was decided that it mattered whether a planet “cleared their planetary neighbourhood” or not. Some objects were not enough of a bully to be regarded as a full planet, so they were called dwarfs. All along, astronomy guided our thinking about what is a planet and what is not.

Interestingly, the 2006 IAU definition merges astronomy and geophysics: what does it matter to an astronomer that the object has reached hydrostatic equilibrium? That is a geophysical criterion. Perhaps it matters in the sense that the interior is fluid enough that one should consider how dissipation influences orbital evolution. If that is the case, though, can tidal interaction with satellites be regarded separately?

I don’t know why the IAU was interested in hydrostatic equilibrium, or even if that is a valid question to consider, because, once again, I am not an astronomer. I’m a geologist. I study the geological activity and the interior evolution of… well… planets… and dwarf planets… and satellites… perhaps exoplanets one day… although not the ice giants and gas giants because, as far as I am concerned, they are different beasts altogether.

The fact is, the IAU definition does not help me. Perhaps there could be a geological definition of a planet, or whatever you want to call the various objects I am interested in. Perhaps the International Union of Geodesy and Geophysics (IUGG) — which, like the IAU, is a member of the International Science Council — could propose a definition more in line with my research interests, but as far as I know, there is no discussion of that.

In the meantime, resistance to the IAU definition is growing in our community. David Grinspoon and Alan Stern recently published a Perspective in The Washington Post1. Around twenty scientists got together to discuss a “Geophysical Planet Definition” at the start of the 2018 Lunar and Planetary Conference. One major point of agreement was that no one should feel obligated to follow the IAU’s definition (we are all rebels now), or any other definition.

At the 2017 Lunar and Planetary Conference, Kirby Runyon and coworkers proposed the following “Geophysical Planet Definition”2: A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters. I find there is a lot to like with this proposal. For example, it would allow me to consider satellites as planets. If I focus on internal evolution, it doesn’t really matter what object my planet is orbiting. Of course, this influences the possibility of tidal heating, but I can regard that as an external energy flux, like the energy of accretion for impacts.

Interestingly, the draft “Geophysical Planet Definition” does not explicitly mention hydrostatic equilibrium. In the IAU definition, the hydrostatic equilibrium criterion implies that planets have a minimum size. It also assumes that the planet behaves as a fluid. In that case, what are we to do with the solid planets, like the Earth? We have evidence of frozen hydrostatic bulges, especially for the Moon. In other words, geological bodies can be strong enough to support a significant deviation from hydrostatic equilibrium. Hydrostatic equilibrium is not the best way to define a planet from a geological standpoint.

Figure 3: Ratio of relief scaled by planetary radius against mean radius based on best fitting triaxial ellipsoid for a variety of solar system objects, drawn following Melosh (2011). The maximum relief is controlled by friction for objects smaller than ~100 km in diameter and by strength for larger objects. Note that some objects like Mercury and Venus do not appear on this graph as they have no measurable flattening, due to their small rotation rate. Gas and ice giants appear to deviate from the trend of solid planets.

Where the IAU definition focuses on the driving force, it may instead be useful to focus on the strength of the planet. In his Planetary Surface Processes book, Jay Melosh discusses the relation between strength and gravity3. He concludes that for small bodies, relief (quantified as the difference between the maximum and minimum radius of an object, divided by the average radius) is independent of size, whereas it decreases inversely with the square of the average radius for larger solar system objects. In these larger objects, relief is limited by the strength of the body. The transition between these two trends is a planetary diameter of 200 to 400 km (Figure 3). This division leaves all of the objects for which we have evidence of geological activity driven by internal processes safely within the category of planets. Ancient planetesimals were probably big enough to be regarded as planets, and indeed, evidence for internal differentiation suggests that their interior was quite active.

So, in my view, a planet is simply a body large enough to have small relief as compared to its radius. This is evidence of relatively low internal strength, which allows geological activity to take place. I don’t need to consider where it orbits, and if it cleared its “planetary neighbourhood” or not, as that doesn’t affect geology. The pitfall of my very inclusive view of what is a planet is the consequentially large number of objects to consider, but variety is the spice of life. Why limit the diversity of geological activity to consider?

There can be subcategories, as Alan Stern actually advocated: gas giants, ice giants, terrestrial planets, dwarf planets, satellite planets, even exoplanets. From a geological standpoint, the ones I am least likely to study are actually the giant planets, whose activity is dominated by atmospheric processes. But feel free to consider them.

Perhaps I should leave the term “planets” to the astronomers, and advocate instead for a new term, “geological worlds”. What remains is, whichever classification you choose to adopt should be adapted to the research you do. For me, I want to embrace the geological diversity of our solar system.



Further reading: 

David Grinspoon and Alan Stern (2018), Yes, Pluto is planet, Speaking of Science – Perspective, Washington Post, May 7

Runyon, K.D., S.A. Stern, T.R. Lauer, W. Grundy, M.E. Summers, and K.N. Singer (2017), A Geophysical Planet Definition, Lunar and Planetary Science XLVIII, Abstract 1448

Jay Melosh (2011) Planetary Surface Processes, Chapter 3, ISBN 9780511977848 

The rock whisperers…

The rock whisperers…

The Geodynamics 101 series serves to showcase the diversity of research topics and methods in the geodynamics community in an understandable manner. We welcome all researchers – PhD students to professors – to introduce their area of expertise in a lighthearted, entertaining manner and touch upon some of the outstanding questions and problems related to their fields. This month, Manar Alsaif, PhD student at Université Montpellier, discusses actual rocks and field work!

In a discipline increasingly shaped by models, what can the rocks still tell us?

Flicking through your typical geodynamics bodies of work, most of the papers are on some kind of modelling – be it analogue, numerical, or something using seismic data. This is hardly surprising considering that geodynamics is all about the depths of the Earth, where we cannot make direct observations. But at some point, we need to check the results of these models, and checking them means looking for the observables. This is ok for the global scale – we can measure gravity and magnetism and the like, but what about smaller scales? And more detail? And the kind of complexity we cannot yet model? Our observations are restricted to the Earth’s surface – but this is actually not a bad place start. There is still a lot that we can learn from good old fashioned field work. And in fact, a lot of the motivation for models comes from an observation of something not understood, or not previously thought of.
So if you need a little inspiration for your next research project, I implore you to literally take a hike!

Apart from a source of inspiration, where does field work fit into a geodynamics workflow? I’d say it fits on top (no pun intended), since all geodynamic processes have a surface expression (at least to some degree).
Take plate motion, for example. Whether a plate moves laterally or vertically, that motion is recorded in the rocks. Palaeomagnetism will trace lateral motion, while thermochronometry will give you a vertical history of the rocks. More often than not, it will reveal a complex history of the rocks and the plate in which they lie. This complexity includes a myriad of processes e.g. fluid action, metamorphism, deformation, diagnesis, etc. These are all processes that are still not fully understood but which we can address by picking up a rock and looking at its mineralogy, its texture, its veins, its contact with its surrounding rocks, its P-T history, its fractures, their strain patterns, etc.

This is by no means an exhaustive article on field methods, I merely mention some examples of how field methods can be useful. So if field geology can be so useful, why are there fewer and fewer scientists doing it? Well, there’s the popular misconception that field geology is only geological mapping, and that the world’s geological surveys have more or less taken care of that already. In reality, some geological surveys have done a marvellous job at mapping out the rock units, but half of a geological map is actually interpretation. This interpretation will constantly change with new understanding of processes and with new data, especially where rock exposures are few and/or flighty.

Apart from the misconception that all field geology has ‘been done’, there are some practical reasons why geodynamicists veer away from field studies. Firstly, there is a mismatch of scales. Generally, the smallest scale a geodynamicist will deal with is a plate – that is already a scale which is too large for field work in practice. But as we argued above, field studies can tell you so much, so what do you do? Go strategic! Pick a few practical locations on your plate, where you might find the products of the processes you’re looking at. For example, if you are looking at obduction, go look around the high pressure rocks, which have probably already been mapped – thank you, local geo survey. If you’re looking at active faulting, use topography and satellite data to help guide you, and then a little thermochronometry can go a long way. If you’re looking at processes behind magmatism, look around your magmatic rocks, and then let the powers of geochemistry come to your aid. There are so many other examples that field geologists do and new tricks that we could start to do with a little creative thinking.

Drone field geology, bridging geo-scales. Tectonic study of Northern Scandinavia by CEED U. Oslo researchers Hans Jørgen Kjøll and Torgeir Andersen. Picture provided by Hans Jørgen Kjøll.

This is all made much easier by using satellite data as a first line of attack. Never before have we had such fine satellite data to simply strategising as we do now.
So maybe it’s also time to move on from old fashioned geological mapping – especially where pretty good maps already exist- and move on to more comprehensive, strategic field campaigns. And remember, technology can be our friend, we need not shy away from it. The photo here is not merely a gorgeous landscape, it is a drone picture by Hans Jørgen Kjøll and Torgeir Andersen of CEED, U. Oslo (seen as the people-looking lines in the middle of the photo). They are seen here flying a drone to get high resolution field data in rugged, inaccessible northern Scandinavia, while simultaneously bridging the scale of typical field work to large scale tectonics.
Similar advantages can be had by using LIDAR, various GPS methods, shallow logging techniques, etc. Perhaps it’s time to stop thinking of geologists as the hammer-hand lens people, and of geophysicists as the gadget people, and of geodynamicists as the code people. Perhaps it’s time to blur the lines, work together and learn from each other.

All of this might eventually give us more real data to plug into our models, perhaps refine some of the parameterisation, or at the very least, give us something against which to compare our model predictions.

After all, George Michael said it best: “Let’s go outside”!

50 years of plate tectonics: then, now, and beyond

50 years of plate tectonics: then, now, and beyond

Even if we cannot attend all conferences ourselves, your EGU GD Blog Team has reporters that make sure all significant geodynamics events are covered. Today, Marie Bocher, postdoc at the Seismology and Wave Physics group of ETH Zürich, touches upon a recent symposium in Paris that covered one of the most important milestones of geodynamics.

On the 25th and 26th of June, the Parisian Collège de France was celebrating the anniversary of the plate tectonics revolution with a symposium entitled 50 years of plate tectonics: then, now and beyond. For this occasion, the organizers Eric Calais, Anny Cazenave, Claude Jaupart, Serge Lallemand, and Barbara Romanowicz had put together a very impressive list of presenters, starting with Xavier Le Pichon, Jason Morgan, and Dan McKenzie during the first morning!

The very impressive program of the 50 years plate tectonics symposium

Needless to say, it was a blast, and a great occasion to focus on the big picture and reflect on the evolution of Earth sciences within the last 50 years.

Watch it online!

But don’t panic if you missed it: all the presentations are available online now on the Collège de France website. So relax, brew yourself a cup of coffee, and enjoy the symposium from the comfort of your own home 🙂

Xavier Le Pichon
Image courtesy of Martina Ulvrova

Important panel
Image courtesy of Martina Ulvrova

Dietmar Müller
Image courtesy of Marie Bocher

Let’s talk about disability in geosciences

Let’s talk about disability in geosciences

Climbing towards outcrops during fieldwork for your undergraduate studies simply isn’t doable for everyone. However, this doesn’t mean that there are adequate alternative solutions available. This week, Katy Willis, PhD student on strain-localisation in the continental lithosphere at the University of Leeds, UK, discusses disability in the geosciences, because regardless of who you are a career in geosciences should be available to you.

On June 4th, 2018 at The Geological Society in London, the UK branch of the International Association for Geodiversity (IAGD) was launched under the name DiG-UK (Diversity in Geosciences, UK), and I was one of those in attendance. The IAGD focuses on the inclusion of people with disabilities within the geosciences, and DiG-UK has incorporated this aim along with championing a better representation of black, Asian, and other ethnic minority groups. Here, I am going to focus on the disability inclusion.

Before I go on – and it’s a shame that I have to type this in the 21st century – let me point out that just because a person has a mental or physical handicap it in no way detracts from their ability to study geosciences or advance in a career in either academia or industry. What hinders them is the barriers that are erected by others before their careers even start. There are a range of disabilities. Anyone may experience a temporary one, like a broken leg restricting your mobility. Some people may experience longer term issues, for example depression triggered by the death of a parent. It may be a health issue that has intermittent effects, such as Crohn’s disease. Or it could be a lifelong issue, such as partial blindness or complex mental health issues.

Geosciences is a broad term for anything from geology to paleo-climate, right up to geodynamics. The one thing that unites such studies, especially at an undergraduate level, is fieldwork. In the UK, an accredited geology degree requires a component of fieldwork, and graduation above a certain level may demand an extended independent fieldwork experience lasting weeks. This is all well and good if you are physically and mentally capable of doing such work, but each year a small proportion of students find themselves unable to go on fieldwork. The old “solution” was to give them a desktop study while the rest of the year went off to Cyprus, Scotland, or Spain. The field group would form close friendships while away, so that those left searching dusty library shelves felt partially excluded from their year group. Hardly an acceptable solution.

The inaugural DiG-UK meeting brought together academia and industry to discuss disability inclusion. An open session looking at how different organisations have got people thinking about disability inclusion and how to practically implement it got us all chatting and sharing ideas. I was delighted to see the approach the Open University had taken regarding fieldwork. It acknowledges that not all field locations are accessible to those with physical disabilities, but there is no need to prevent such students attending field trips because there will remain a number of locations that can be readily visited. For the inaccessible locations they set up a local WiFi network and use iPads so an able bodied person can stream a live image to those who can’t reach that particular outcrop. Genius!

Manchester Metropolitan University has developed a “Diversity Dash” game to help groups understand the many barriers that all students and staff can face. Each team is allocated a character, and a range of scenarios are presented (lab work, unexpected meeting on the top floor, taking notes in lectures). The teams then have to find realistic ways in which their character can take part in the scenario. The characters cover a range of people, from someone who has a pregnant partner through to the rich student that seems to have everything but certain issues are causing increased complications.

If universities automatically allow for the provision of disabled people in their fieldwork plans, then it allows students to continue their studies should they suffer a mild injury such as a sprained ankle. During my undergraduate time, I saw two people fail to finish their course because the university was unable to accommodate their disability needs. They were intelligent people, who knew that beyond graduation lay geoscience careers that did not rely on fieldwork, but they could not pass that barrier of obtaining the appropriate degree. In both cases a few simple adjustments would have allowed them to finish.

It behoves any institute to set up a framework that encourages practical and workable disability inclusion in the geosciences. Organisations such as DiG-UK and the IAGD can provide valuable information on how to do this. Our area of study – geodynamics – sees many of us sat in front of a computer, but a lot of us were required to carry out field studies at some point in our education. It added to our knowledge and experience and in my case it inspired me into the direction of geodynamics and the desire to understand the broader picture.

In September I am taking part in a field trip to Anglesey UK. There will be a range of people going and we aim to discover which methods assist inclusion and accessibility in the field (and which don’t!). Then the findings will be shared so real and practical ways of being inclusive can be implemented for geoscience fieldwork across the UK (and hopefully internationally).

So, go on. What are you going to do this month to help people with disabilities become more involved in geosciences?

Follow DiG-UK on twitter: @DiG_UK_IAGD