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Geodynamics

Geodynamics

The Venus enigma: new insights into ‘Earth 2’

The Venus enigma: new insights into ‘Earth 2’

Apart from Earth, there are a lot of Peculiar Planets out there! Every 8 weeks, we look at a planetary body worthy of our geodynamic attention. This week Richard Ghail, lecturer in Engineering Geology at Imperial College London in the United Kingdom, discusses Earth’s sister: Venus.

Geologists have long held the view that they only have the results of one experiment: Earth. The growing list of Earth-like planets around other stars (exo-Earths) means that such a view is no longer valid, even if we have limited knowledge of those worlds. Surprisingly, perhaps, our own Solar System boasts two exo-Earths: and the other one is not Mars, as you might think, but Venus. Our nearest planetary neighbour is also the most similar to Earth: at about 80% of its mass and 95% of its radius, and orbiting arguably within our Sun’s habitable zone, Venus would be recognised as an Earth-like exoplanet if it were in orbit around another star. Yet the results of these two experiments could not be more different: Earth may not quite be Eden, but Venus is certainly the closest place we know to hell. Its dense global shroud of sulphuric acid clouds hides a surface on which a person would be simultaneously roasted (at 450°C), crushed (at 90 atmospheres pressure), poisoned and asphyxiated (its atmosphere is 95% CO₂), and corroded (not by sulphuric acid, which decomposes under the extreme conditions, but by HCl and even HF!). The armoured Soviet Venera landers survived only a couple of hours on the surface, but still managed to return tantalising pictures of a barren rocky landscape bathed in an orange light.

A geodynamic surprise: Catastrophe or not?
NASA’s Magellan mission (1989-1994) revealed that geodynamically too, Venus was a surprise. A wealth of volcanoes, rifts and mountains cover its surface but there is little evidence for the spreading ridges and deep trenches that characterise plate tectonics on Earth. More perplexing was the realisation that the 950 or so impact craters – implying a youthful 500 Ma average age – were distributed apparently at random. Had the whole planet been somehow catastrophically resurfaced in one go, half a billion years ago? As strange as it might sound, there appeared to be good reasons to think so: that 450°C surface temperature is enough to stop the crust subducting, effectively shutting down plate tectonics. Without that safety valve, the interior of Venus must be getting ever hotter at the same time that exterior cools and thickens the lithosphere. Such a situation is inherently unstable and calculations showed that Venus should ‘blow its top’ every 500 Ma or so – explaining both the lack of plate tectonic features and the crater distribution. The Venus enigma was solved.

Or was it? The theory divided the community into bitterly opposed sides for the next decade or more. One group could see a global sequence of events in its geological features that seemed to confirm the theory; while the other could see an array of geological complexity at odds with it. ESA’s Venus Express mission (2005-2012) focussed on the planet’s atmosphere but it revealed a remarkably dynamic and changeable system that must somehow reflect geodynamic activity below. It even found tantalising hints of recent volcanic activity. Understanding both the geological evidence and the crater distribution turns out to depend on the very thing that set Venus apart from Earth: its extreme surface conditions. The high temperature not only makes the crust buoyant, but weak, especially so at about 10 km depth, where it is able to shear relative to the mantle below. In this new geodynamic view, plate tectonics does operate on Venus much as it does on Earth, but under 10 km of crust, not 5 km of ocean (Ghail, 2015). As well as explaining the large-scale features of Venus, including its geoid, calculations show that this subcrustal rejuvenation, as it is called, is able to maintain the heat balance on Venus). No catastrophic events are required.

If the crater distribution is not the result of a global catastrophe, what caused it? Mechanically, the basaltic crust of Venus behaves much like Earth’s granitic continental crust, and is similarly broken into many small plates, or terranes, on the order of 500 to 1500 km across, characterised by low strain interiors and highly deformed margins, similar to terrestrial continental blocks. On Earth these terranes are driven by far-field plate tectonic stresses but on Venus they are driven by subcrustal rejuvenation stresses that jostle the terranes against one other but do not move them far across the surface. Impact craters are preserved in terrane interiors but rapidly destroyed at their margins, so that the average crater spacing is similar to the size of terranes. The preserved terrane-interior craters are only destroyed when the terrane is itself destroyed, most likely by interaction with a subcrustal plate boundary (rift or collision), which by inference is something that occurs on average every half billion years. This new geodynamic understanding refines our appreciation of how the geochemistry and geomechanics of the outer few kilometres of the planet profoundly influence stagnant-lid behaviour, promising new insights into early-Earth geodynamics and the nature of newly-discovered exo-Earths.

Welcome to hell Venus. I want you to explore it!
Credit: Pixabay

Exploring Venus
So our views of Venus have changed; our Solar System’s second experiment is, geodynamically, rather more similar to Earth than once imagined. Even so, these ideas have yet to be tested, and our nearest neighbour retains many secrets. Almost nothing is known about its interior, its rates of activity, or even how Venus maintains such a hostile atmosphere. A new phase in Venus exploration is called for, and within Europe the most promising is the proposed EnVision mission, which is currently undergoing evaluation by ESA. EnVision will use an advanced Earth Observation heritage interferometric radar to measure and monitor geological activity over a 5-year period and obtain images at up to 1 m resolution – sufficient to locate and track the Venera landers, providing the precise geodetic control needed to measure terrane deformation. A radar sounder will probe the near subsurface and an IR/UV emission spectrometer will map geochemistry and follow volcanic gases from their source to the upper atmosphere. NASA has proposed landers that could probe interior seismicity, and in the future balloons may directly sense cloud chemistry and dynamics. Unlike the Moon and Mars, these missions will be exploring a world that is – in a geodynamic sense, at least – very much alive.

References
Ghail, R. (2015). Rheological and petrological implications for a stagnant lid regime on Venus. Planetary and Space Science, 113, 2-9.

NetherMod Day 4 & 5 – Secret Summary

NetherMod Day 4 & 5 – Secret Summary

I hear you exclaiming: “Why merge day 4 and 5 together? What happened to the secret summary of day 4?” Well, I have to admit, apart from being your daily NetherMod reporter, I also needed to present my poster, so apologies for the delay and merge.

Day 4 started with a keynote talk from Dave Stegman with a surprise topic (He had never handed in his abstract. Apparently this is a possibility. Probably only tolerated for keynote speakers though). Dave presented a symphony in four movements about the inconvenient truth of the possibility of a molten lower mantle in the early Earth. We were also very lucky to see the one and only picture of the moon forming impact: truly impressive. Steering the discussion towards politics, Dave introduced two great slogans:

The geodynamics liberation front

and

Viva la subducción

Talking about subduction, Fanny Garel had a great keynote talk about modelling subduction zones, followed swiftly by a talk by Yury Podladchikov on resolving ductile strain localisation and porous fluid channeling due to thermo- and hydro-mechanical coupling (a “scary talk” according to chair Dan Bower) and Manuele Faccenda’s talk on coupled geodynamic and seismological modelling of subduction zone dynamics. During the final discussion with everyone, Carolina Lithgow-Bertelloni imposed on us the scary truth that

Models are only useful if they fail

I think that’s particularly encouraging for every ECS, although we might be talking about different flavours of model failing…

Day 5 (the final day! how time flies when you’re having fun!) was an excellent day for your NetherMod reporter, because of the incredible amount of bad jokes, puns, and food related analogies in the talks. Thanks! I can work with that!

Paul Tackley kicked of Thursday morning with an interesting talk on the influence of melting on the Earth’s evolution. One of his animations caused the most seasoned scientists to giggle. Unfortunately, I can’t comment more on that; in fact, I have been specifically asked by Dave Stegman to refrain from quoting him on this subject. Indeed, I am a classy girl and this is a classy blog, so I won’t go into details, but props to those who know what I am talking about!

Next, Carolina Lithgow-Bertelloni talked about the carbon compensation depth and she once again had a great comment on modelling etiquette (I see a pattern here):

The goal of a study should be defined before the tools are chosen

Clint Conrad discussed how we could infer dynamic topography from bathymetry and plate motions. He noticed that the degree 1 net motion characteristics of plate velocities all seem to point towards North Korea. Interesting point, isn’t it? I won’t make any (political) remark about that, because, you know, I like my home nuke-free. A lovely quote from Dan Bower during the subsequent question round was that

One person’s observation is another person’s model

Think about it. Keep it in mind.

Things really started to get spicy when Louise Kellogg discussed comparisons of different models and chemical geodynamics for mantle convection. She gave a very nice introduction about the concept of modelling (which was open for debate, of course) and particularly hastened to broaden René de Borst’s previous statement about the fact that the devil is in the boundary conditions: the devil is also very much present in the initial conditions and perturbations. There is no escaping him, really. Louise also added several (chemical) spices in our mantle dynamics curry by looking at the Earth as a chemical factory with several reservoirs connected by fluxes.

Now that we have had potatoes, sausages, and a dynamical curry at NetherMod, the desert was served by Louise when she compared the Earth to a marble cake. I will leave you this time with that delicious idea for a dinner party. NetherMod 2017 finished after a splendid party until the wee hours. Next time it will be in Italy (with a masked ball? Can we please make sure that happens? Like Romeo & Juliet? Pretty please?), and maybe I will report that as well.

For now though, I leave you with the suggestion that we should get together soon for a dinner party. Anyone who would like to volunteer to cook the potatoes?

NetherMod Day 5 – Putten an end to Nethermod: interviews with attendees

NetherMod Day 5 – Putten an end to Nethermod: interviews with attendees

Today is the fifth and final day of the XVth International Workshop on Modelling of Mantle and Lithosphere Dynamics, or “Nethermod”, here in Putten, The Netherlands. Despite the overcast conditions outside, the lively scientific program included keynotes by Paul Tackley and Carolina Lithgow-Bertelloni in the morning and Clint Conrad and Louise Kellogg in the afternoon. With over 120 attendees, and a program built around selected keynote presentations with plenty of time for posters and discussions, Nethermod offers a unique meeting format. Today’s post includes interviews with three attendees at different stages of their career – student, mid career and more established – and asks about their experiences of the workshop and their perspective on the future of geodynamics.


Kiran Chotalia here at Putten

— Early Career Researcher —
Kiran Chotalia (University College London).
Kiran is entering the third year of her PhD, and is a first-time attendee to the workshop.

– What is your PhD project about and what did you present here?
My project looks into the effects of water on mantle circulation, firstly using parameterized models and then 2-D models. I had a poster on day 2 which presented some parameterized models that included a time-lag to simulated delayed mixing.

– The conference is aimed at leaving extra time to develop student-keynote interactions. What have your impressions been? Do you have any suggestions for changes for future workshops?
Considering the format and length of the lectures (45 mins + 15 questions), I think that 30 mins with the keynotes presenters was sufficient. However, there are around 40-50 other student attendees so perhaps the option to write some anonymous questions to help find consensus within the broader group’s needs could be incorporated. This would also help achieve a more overview style session, which cannot be covered in the lectures.

– What are your plans for after the PhD – is the Brexit process seriously weighing into your decision making?
I am definitely now more open to considering the idea of moving outside the UK. In any case, with time I have felt more integrated with the geodynamics community, and have a broader picture of who else is also out there and what the cutting-edge ideas are.

– What are your takeaway potatoes of wisdom from the meeting?
Dave Stegman’s comment – “Don’t always believe what you read” has stuck with me. It is important to be reminded of this fact! Science is only the best description at that time, and also considering the current state of pressure to publish, it is easy loose this perspective. It is also nice to be at a smaller, manageable conference with other researchers doing similar things. It is a chance to meet and talk to those beyond the home institute and I feel more up-to-date with what others are up to.

 


Fanny Garel at the poster of her PhD student Manar Alsaif

— Mid Career Researcher —
Fanny Garel (Géosciences Montpellier)
Fanny is a lecturer at Montpellier, and was an invited keynote presenter from the “Subduction and mantle flow modelling” session. Nethermod is her third workshop of this series.

– Your presentation built upon work from your earlier paper (Garel et al., 2014) that has had quite a reception in the community. Can you please summarize the talk for us?
My presentation was on numerical models that (re)produce the seismically imaged slab morphologies by varying different parameters and understanding the physical controls (e.g., slab sinking and bending).

– From your perspective as a keynote speaker, how did the daily student question session go?
Half an hour was perhaps slightly too short. The session is a key opportunity to open the discussion to more of the limitations and assumptions of the model. Students can ask more about the basics which you cannot fit into a presentation. Teaching is not just presenting, and vice versa. It is also helpful for speakers in terms of feedback for their own presentations!

– What are some of the biggest and outstanding questions in the modelling community?
Understanding when present-day subduction zones initiated, is one. Exploring a self-consistent interaction between single subduction zone specific-scales and global scales, both spatially and temporally, is still outstanding.

– Any tips or suggestions for ECS researchers at the end of the thesis and thinking of whether to continue for a postdoc?
You are already experts of your PhD subject, so keep your options open and try to change topic or your approach for a postdoc. There are plenty of different scales in geodynamics to explore, and perhaps it is best to change tools rather than objects. Consider your longer term view too and what the hot topics are that will lead to new opportunities in the next ca. 5 years.

– What are your takeaway potatoes of wisdom from the meeting?
There is always the requirement for simple models to help understand the Earth. Models can have lots of complexity but we can lack a fundamental and first-order understanding of problems – including the physics and relevance of feedbacks e.g. the Marianas trench, and surface and deep dynamics.
The generation and modelling of melt was also discussed, including the different ways of approaching it, and the links chemical evolution and dynamic evolution in a self-consistent manner.

 


Dave and the sunset

— Established Career Researcher —
Dave Stegman (Scripps Institute of Oceanography)
Dave is an Associate Professor of Geophysics and was a keynote speaker within the “Global modelling of Early and recent Earth” theme.

– This is the 15th time it has been run – how many conferences within the series have you attended?
My first in the series was in 2001 as a grad student and I have missed two of them since then. Some of the fellow students I met there the first time are here this week, so there is a real community building aspect to this meeting series and it is really important for the fabric of the community.

– How has it evolved since your first conference?
The format similar is pretty similar and has become increasingly student focused. The attendance of a high proportion of students makes the meeting fresh and dynamic.

– Can you summarize your presentation from earlier this week?
The take home message of my presentation was to shift our mindset in order to allow for scenarios that permit a molten lower mantle and a magnetic field in places outside the core. It was provocative, sure, but ensured a healthy scientific discussion.

– Origins of the geodynamo and core formation is a shift from your earlier work. Can you comment on the interdisciplinary aspects of modelling on vastly different temporal and spatial scales? 
This work really integrates geo- and paleo-magnetism, geodynamics and mineral physics, which really inform each other. Collaboration is required to progress.

– The “Geodynamics Liberation Front” was a big success and you are rolling out a new geodynamics themed t-shirt. Can you tell us more?
It will come in different sizes. The first was wildly popular – some more community themed fabric. Email me if you are interested in knowing more, else I’ll be bringing a suitcase full to AGU.

– What are some of the biggest or outstanding questions in the modelling community generally?
Those regarding Earth’s evolution and it’s entire history. The roadmap that makes most sense to me is to firstly calibrate our models of plate tectonics to present-day or recent timescales before going back in time, or to exoplanets.

– As a non-EU based researcher, do you have many active collaborations with researchers back on this side of the Atlantic?
Science is an international and you need to follow problems wherever they take you. Our community is really open to collaborations and in-person opportunities are important; they add much more value to Skype level meetings.

– Any tips for the next generation of ECS members of the geodynamics community, or those PhD students not sure whether to transition to postdoc?
Don’t underestimate yourself. The skills required to accomplish a PhD are valued in many settings… persistence, attention to detail, the ability to think at different levels, time management. Students often don’t realize they have these abilities and they are a starting point for many paths.

– Finally, what are your takeaway potatoes of wisdom from the meeting?
The informality of the venue and meeting format enables everyone to expand from their comfort zone. This is critical for learning as you cannot be inhibited to ask questions and start discussions. The financial support to bring so many students to the meeting really tips the scales… when students and ECSs are the dominant force they do not feel intimidated to make the most of it.


Thanks Dave, Fanny, and Kiran for your time!  

NetherMod Day 4 – A typical day

NetherMod Day 4 – A typical day

Today, Ági Király, postdoc at CEED (Centre for Earth Evolution and Dynamics) at the University of Oslo, Norway, is sharing her experience of NetherMod.

After the wine tasting and karaoke party of yesterday evening, this morning’s keynote speakers had quite a difficult job to shake up the sleepy audience. Dave Stegman even dropped in a promise of a free beer for the most awake student (won by  René Gassmöller, cheers!). But what really helped to wake up our little community was the cutting edge science that Dave and later Fanny Garel presented. Dave shared some new modelling results about early earth dynamics, showing the necessity of having a liquid lower mantle to reproduce the Archaeans strong magnetic field. Fanny Garel then showed us results on subduction modelling, reminding us again that “every model is wrong but some are useful”. In the afternoon Yury Podladchikov first talked about how to solve ductile strain localization and porous fluid channelling due to thermo- and hydro-mechanical coupling using parallel computations. It was followed by the talk of Manuele Faccenda showing results of combined geodynamics and seismological modelling to calculate olivine lpo and sks anisotropy in subduction systems. Manuele drew our attention to the artifacts on seismic tomographic models, which can be present due to the mantle anisotropy and the way earthquakes sample the Earth’s interior.

And now about my favourite part in these meetings! A. K. A. why should students always come to meetings and workshops like this. First of all, it’s a relatively small/medium sized meeting, where everyone is staying, eating, drinking, laughing, and brainstorming together all week. So, young scientists have plenty of possibilities to meet fellow young and senior scientists, and do some serious networking. Furthermore, every day we have a half an hour long session where students meet with the keynote speakers during which students and early career researchers have the opportunity to ask questions and discuss with the keynote speakers of the day, without the judging eyes of the other senior participants. This is the place and time where we can ask the seniors to clarify things we didn’t understand, but also to get some more insight into where science is going; what we have to keep always in mind; and what are the biggest challenges in our community. While in the plenary discussion students usually don’t speak up, during this session they can challenge the seniors and exchange ideas.

A typical day at NetherMod. Note the discussions dedicated to students!