Archives / 2019 / October

Space travel: arrogance or vision?

Space travel: arrogance or vision?

Caroline Dorn

This week, Caroline Dorn, Ambizione Fellow at the University of Zurich, tells us her view on space travel and why we shouldn’t pack our bags just yet. 

Space exploration is experiencing a new boom, a come-back since the 60s & 70s! First discoveries of planets outside our solar system in the 90s have set the foundations of a new discipline that is exoplanet science – the field in which I am working in.

I enjoy that there is a lot of fascination from the public about the newly discovered distant worlds.

However, I observe a new fascination about space travel or space tourism among people that I find worrisome and dangerous.

Recent efforts of crewed missions to the Moon or Mars are driven by national or private egotism rather than the broadening of our understanding of other worlds. Here, I want to ask the question whether there is any value to our society in supporting space travel?

Often technological advances are put in front as a pro-argument. However, the billions of money could be directly invested in tailored research for societal benefits. It is not surprising that the vast amount of investments for space travel yields alongside innovations which find applications in other fields or our daily life. This is not unique to investments in space travel but simply a question of money and human power.

The perspective of mining other planets is often mentioned. Yet, it won’t solve the problem of limited resources that we have on Earth.  We need to find a way to deal with limited resources and how to handle our impact on the Earth’s ecosystem, which is the only one we know of. Mining other planets would only push the margin of available resources slightly further on the immense cost of energy, Earth’s natural resources, and human passion.

Yes, human passion. It takes a lot of enthusiasm to work in a specific niche such as space travel engineering or research. I think, such enthusiastic thinkers and pioneers are needed to create inclusive societies in balance with our ecosystem for present and future generations.

Travelling to other planets won’t save humankind. We are made for life on Earth. Even if humans would start living on other planets, evolution doesn’t stop. Humans wouldn’t be humans anymore. Humans are lifeforms from Earth.

In my view, the only justifiable argument for space travel is curiosity. Of course, it is fascinating to think that some people from Earth can leave the planet, travel through space and set their foot on a nearby planet that is Mars! What a fictional idea that may be possible to actually achieve!

To my disappointment, Switzerland’s only astronaut is promoting crewed space travel. For me, he is from an old generation for which the further-faster-higher principle seems still attractive. This thinking is anachronistic. Future missions must justify not only the societal and scientific benefits but also the sustainability of resource use, which is the big challenge of the 21st century.

We will stay on Earth. Enjoy this planet!

If you can think of any other argument in favour of space travel, think about it twice and if it still holds then post it in the comments below.

The Sassy Scientist – Earthquake Exoteries Nr. IV

The Sassy Scientist – Earthquake Exoteries Nr. IV

Every week, The Sassy Scientist answers a question on geodynamics, related topics, academic life, the universe or anything in between with a healthy dose of sarcasm. Do you have a question for The Sassy Scientist? Submit your question here or leave a comment below.

In a comment on a post about the key papers in geodynamics, the Curmudgeonly Commenter asked:

Could you please point out some exceptionally important papers in geodynamics and tell us something interesting about the history of the field?

Dear CC,

After discussing the earthquake cycle last week, I think we can go into friction. Are you still following me? Is this series of blog posts in depth enough? There’s more to come, don’t worry.

Rate-and-state turmoil
During the 60’s and 70’s an additional new theory was fostered in the framework of rock mechanical experiments investigating friction, which still rocks the scientific community to this very day: rate- and state-dependent friction. I cannot get into too much detail about rate-and-state friction here for brevity and clarity — and I won’t — but I’ll leave some references at your disposal for further in-depth reconnaissance. Building on earlier work, Dieterich (1978, 1979) and Ruina (1983) describe that the shear stress on a fault plane is related to the normal stress on said fault plane — as in Byerlee — but additionally the sliding velocity on the fault and a critical slip distance (i.e., a measure of the fault interface smoothness) also come into the equation. The fault interface itself is oftentimes distributed as a fault gouge — i.e., a very fine-grained, usually unconsolidated material in a fault zone between the strong blocks. Faults without such gouge are oftentimes considered immature, and may behave fundamentally different (Marone and Scholz 1988). Putting the complexity of frictional fault slip in layman’s terms: the rate-and-state formalism states that the friction on a fault interface evolves through time, between static ‘locking’ and dynamic ‘sliding’ coefficients. This includes the possibility of healing or weakening of the fault interface (Marone 1998, Scholz 1998, Sleep 2005). Rate-and-state dependent friction provides an explanation for an abundance of observations, e.g., predicting stable, conditionally stable and unstable depth ranges of seismicity, the down-dip slip distribution and loading of a fault plane during the interseismic period, and aftershocks (e.g., Marone 1998, Scholz 1998). One — relatively recent — and exciting observation is that the rate-and-state friction formalism does not only hold for earthquakes generated at interfaces between rocks, but also along the bottom of ice masses (Winberry et al. 2013; Lipovsky and Dynham 2016). That’s just a chilling coincidence.

Just one tiny, eensy-teensy, itty-bitty, wee, slightly negative comment on rate-and-state friction; it’s a kinematic description of the observations, not a mechanical explanation of what’s actually happening at depth along a ‘fault interface’. If you can even call it that.

I think that’s more than enough for one week. Or rather, the Editor-in-Chief thinks that.

Yours truly,

The Sassy Scientist

PS: This post was written in some state at some rate.

Dieterich, J. H. (1978), Time-dependent friction and the mechanics of stick-slip. Pure and Applied Geophysics, 116, 790–805
Dieterich, J. H. (1979), Modeling of rock friction: 1. Experimental results and constitutive equations. Journal of Geophysical Research, 84, 2161–2168
Lipovsky, B. P. and Dunham, E. M. (2016), Tremor during ice-stream stick slip. The Cryosphere, 10, 385–399, doi:10.5194/tc-10-385-2016
Marone, C. J. (1998), Laboratory-derived friction laws and their application to seismic faulting. Annual Reviews of Earth and Planetary Science, 26, 643–696
Marone, C. J. and Scholz, C. H. (1988), The depth of seismic faulting and the upper transition from stable to unstable slip regimes. Geophysical Research Letters, 15, 621–624
Ruina, A. (1983), Slip instability and state variable friction laws. Journal of Geophysical Research, 88, 10359–10370
Scholz, C. H. (1998), Earthquakes and friction laws. Nature, 391, 37–42
Sleep, N. (2005), Physical basis of evolution laws for rate and state friction. Geochemistry, Geophysics, Geosystems, 6, 11, doi:10.1029/2005GC000991
Winberry J. P., S. Anandakrishnan, D. A. Wiens, and Alley, R. B. (2013), Nucleation and seismic tremor associated with the glacial earthquakes of Whillans Ice Stream, Antarctica. Geophysical Research Letters, 40, 312–315, doi:10.1002/grl.50130

The geodynamics of planetary habitability

The geodynamics of planetary habitability

The Geodynamics 101 series serves to showcase the diversity of research topics and/or methods in the geodynamics community in an understandable manner. In this week’s Geodynamics 101 post, Brad Foley, Assistant Professor at the Department of Geosciences, Pennsylvania State University, talks about the geodynamics of planetary habitability and in particular the key role of CO2 cycling in the mantle. 

Figure 1: Assistant professor Brad Foley at the Department of Geosciences, Penn State University.

Earth’s mantle is the planet’s engine. The loss of heat from the interior to space drives Earth’s tectonic processes, mountain building and orogeny, volcanism, and the core dynamo generating Earth’s magnetic field. But perhaps less appreciated is that the mantle also plays a critical role in shaping the state of the atmosphere. This link between surface and interior evolution is not just important for studying the Earth, but also the other rocky planets in our solar system, and rocky exoplanets. Factors that make a planet, like Earth, a suitable home for life, such as the presence of liquid water oceans, weathering processes that provide critical nutrients to the oceans, and a temperate climate are all directly influenced by deep interior processes (Foley & Driscoll, 2016). Likewise, a complex interaction between life, atmospheric chemistry, weathering, volcanism, and sediment burial led to the rise of oxygen on Earth, which is both critical for some forms of life and a signature of the presence of life (Claire et al, 2006; Kump & Barley, 2007; Lyons et al, 2014). Thus, unravelling the factors that allowed Earth to develop into a planet teaming with life, whether those same factors are likely to be present on other rocky planets, and whether potential biosignatures, like atmospheric oxygen, are likely to arise on exoplanets if life is present, all require considering the role of the mantle.


Cycling of CO2: A key factor in planetary habitability

The abundance of atmospheric gases is determined by the balance between their sources and sinks, and the mantle acts as an important source and sink for many gases: volcanism releases volatiles locked in rocks to the surface, while subduction brings volatiles from the surface back into the interior. One of the most critical for habitability is CO2, which controls the climate state. On Earth, the cycling of CObetween surface, interior, and atmosphere involve a stabilizing feedback that acts to buffer climate (Kasting & Catling, 2003). COis drawn out of the atmosphere by weathering of silicate rocks and the formation of carbonate minerals on the seafloor, which are then subducted to return carbon to the mantle (Figure 2). Critically, the rate of silicate weathering increases with increasing surface temperature or atmospheric CO2. Thus when the climate warms the weathering rate increases, acting to cool the climate down, and when the climate is cool the weathering rate decreases, allowing outgassing of COby volcanism to warm the climate up (Walker et al, 1981).


Figure 2: Schematic cartoon of the carbonate-silicate cycle on Earth. Silicate weathering on land and seafloor weathering near mid-ocean ridges remove CO2 from the atmosphere and deposit it in the ocean crust. This carbon is then subducted, where some fraction is outgassed at arc volcanoes, with the rest returning the mantle. Outgassing from mid-ocean ridges and ocean islands returns mantle carbon to the atmosphere. From Foley & Driscoll, 2016.


However, this feedback can fail in two ways: first, rates of CO2outgassing must be high enough to keep the climate from plunging into a globally glaciated or snowball state (Kadoya & Tajika, 2014); second, there must be sufficiently high rates of physical erosion to remove weathered rock and bring fresh rock into the near-surface weathering zone (Foley, 2015). The mantle plays an important role in both CO2outgassing and surface erosion rates. The CO2outgassing rate is determined by the rate of volcanism, mantle carbon content, and oxidation state, while erosion rates are controlled by rates of tectonic uplift and mountain building over geologic timescales.


Role of the mantle in CO2 cycling: Future directions

However, there are many aspects of how the mantle influences COoutgassing and weathering rates that are still poorly understood, and exciting avenues of future research. First-order constraints on rates of volcanism and outgassing, and how they change over time, are straightforward to calculate from both simple box models of planetary thermal evolution or 2- and 3-D mantle convection calculations (Noack et al, 2017; Tosi et al, 2017). As planets cool over time, volcanic outgassing rates decline and eventually become low enough for frozen, snowball climates to develop. Factors that keep a planet’s mantle warmer for longer, such as higher rates of radiogenic heat production or tidal heating, will thus act to prolong the lifetime of habitable surface conditions (Foley & Smye, 2018; Valencia et al, 2018). Yet there are still important uncertainties, in particular on how carbon is carried into, and circulates within, the mantle that are key avenues for future research. Moreover, the connection between mantle dynamics, mountain building, erosion, and weathering rate is still poorly understood. Erosion rates are high when topographic gradients are large, as in mountainous regions. Mountain building is most likely connected to surface plate speed and the vigor of mantle convection, however just what form this connection takes is not known. How mantle convection and plate tectonics leads to the formation of topography, and hence influences weathering and erosion, is a critical area of research for understanding the controls on long-term climate evolution.


How Earth-like must a planet be to be habitable?

Ultimately one of the most important questions driving future research in planetary evolution and exoplanets, and which geodynamicists should be a central part of answering, is how Earth-like a planet needs to be in order to sustain volatile cycles that allow for the development of life and for biosignatures, such as oxygen, to accumulate in the atmosphere once life has developed (Tasker et al, 2017). Exoplanets come in a wide range of sizes (see Figure 3): planets up to about 4 Earth masses are found to be rocky, while beyond this limit planets are volatile-rich like Neptune (Rogers, 2015), and likely compositions as well (Hinkel & Unterborn, 2018). These planets could display a range of different surface tectonic modes, including plate tectonics, stagnant lids, or some intermediary style of tectonics. Oxidation states could be different, influencing the type of gases outgassed by volcanism. Instead of outgassing predominantly CO2, a planet with a more reduced mantle could outgas mostly CO or CH4. Likewise, different crustal compositions could alter weathering processes and the stability of volatiles as they are recycled into the interior at subduction zones or by crustal foundering. Exploring these issues will require interdisciplinary research including geochemists, mineral physicists, and geodynamicists, as well as biogeochemists, climate scientists, and astronomers. With future space telescopes poised to image exoplanet atmospheres, research on the role of the planetary interior in shaping the surface environment and atmosphere has never been so relevant.


Figure 3: Exoplanet population as of August 2017. Image credit: NASA/Ames Research Center/Natalie Batalha/Wendy Stenzel



Claire, M. W., Catling, D. C., & Zahnle, K. J. (2006). Biogeochemical modelling of the rise in atmospheric oxygen. Geobiology4(4), 239-269.

Foley, B. J., & Driscoll, P. E. (2016). Whole planet coupling between climate, mantle, and core: Implications for rocky planet evolution. Geochemistry, Geophysics, Geosystems17(5), 1885-1914.

Foley, B. J., & Smye, A. J. (2018). Carbon Cycling and Habitability of Earth-Sized Stagnant Lid Planets. Astrobiology18(7), 873-896.

Hinkel, N. R., & Unterborn, C. T. (2018). The Star–Planet Connection. I. Using Stellar Composition to Observationally Constrain Planetary Mineralogy for the 10 Closest Stars. The Astrophysical Journal853(1), 83.

Kadoya, S., & Tajika, E. (2014). Conditions for oceans on Earth-like planets orbiting within the habitable zone: importance of volcanic CO2degassing. The Astrophysical Journal790(2), 107.

Kasting, J. F., & Catling, D. (2003). Evolution of a habitable planet. Annual Review of Astronomy and Astrophysics41(1), 429-463.

Kump, L. R., & Barley, M. E. (2007). Increased subaerial volcanism and the rise of atmospheric oxygen 2.5 billion years ago. Nature448(7157), 1033.

Lyons, T. W., Reinhard, C. T., & Planavsky, N. J. (2014). The rise of oxygen in Earth’s early ocean and atmosphere. Nature506(7488), 307.

Noack, L., Rivoldini, A., & Van Hoolst, T. (2017). Volcanism and outgassing of stagnant-lid planets: Implications for the habitable zone. Physics of the Earth and Planetary Interiors269, 40-57.

Rogers, L. A. (2015). Most 1.6 Earth-radius planets are not rocky. The Astrophysical Journal801(1), 41.

Tasker, E., Tan, J., Heng, K., Kane, S., Spiegel, D., Brasser, R., ... & Houser, C. (2017). The language of exoplanet ranking metrics needs to change. Nature astronomy1, 0042.

Tosi, Nicola, Mareike Godolt, Barbara Stracke, Thomas Ruedas, John Lee Grenfell, Dennis Höning, Athanasia Nikolaou, A-C. Plesa, Doris Breuer, and Tilman Spohn. "The habitability of a stagnant-lid Earth." Astronomy & Astrophysics 605 (2017): A71.

Valencia, D., Tan, V. Y. Y., & Zajac, Z. (2018). Habitability from Tidally Induced Tectonics. The Astrophysical Journal857(2), 106.

Walker, J. C., Hays, P. B., & Kasting, J. F. (1981). A negative feedback mechanism for the long‐term stabilization of Earth's surface temperature. Journal of Geophysical Research: Oceans86(C10), 9776-9782.



The Sassy Scientist – Earthquake Exoteries Nr. III

The Sassy Scientist – Earthquake Exoteries Nr. III

Every week, The Sassy Scientist answers a question on geodynamics, related topics, academic life, the universe or anything in between with a healthy dose of sarcasm. Do you have a question for The Sassy Scientist? Submit your question here or leave a comment below.

In a comment on a post about the key papers in geodynamics, the Curmudgeonly Commenter asked:

Could you please point out some exceptionally important papers in geodynamics and tell us something interesting about the history of the field?

Dear CC,

We can finally get into the interesting stuff. Let’s forget about the descriptions of earthquake kinematics last week and look at actual earthquake cycles now.

Beyond an elastic half-space… towards an earthquake cycle
Slip along fault planes is not restricted to earthquakes (i.e., co-seismic displacements) only; Smith and Wyss (1968) describe additional differential surface motion in the years following the 1966 Parkfield earthquake. So elastic half-spaces are all fun and games, but something else is happening. What about the underlying asthenosphere (Barrell 1914)? Doesn’t this — low-magnitude (Haskell 1935) — viscous material affect surface motions? Indeed. Nur and Mavko (1974) elaborated that the co-seismic phase is followed by a phase of transient, time-dependent deformation, dubbed the post-seismic phase. Thatcher and Rundle (1979) provide elaborate evidence for surface-breaking events that the earthquake cycle should include significant post-seismic surface motion, without the need for exotic fault or rock properties. What could these post-seismic processes be? Is it just viscous flow of rock layers at depth (e.g., Freed and Lin 2001), or do we need to look for other mechanisms, such as a combination of the presence of water inside rock pores at depth (Nur and Booker 1972; Peltzer et al. 1998), and a-seismic or micro-seismic slip on the same fault plane after a major event (Perfettini and Avouac 2004)?

Stick-slip and earthquake statistics
In terms of understanding earthquake nucleation rather than describing the after-the-fact surface deformations, Brace and Byerlee (1966) provided the next step to better explain the earthquake source: stick-slip. Stick-slip states that the differential motion on a fault plane between two blocks does not occur smoothly, but rather ‘jerky’, i.e., the roughness of the surface ensures that the two block stick together and when the fault ‘strength’ is overcome the fault fails and an earthquake occurs. Burridge and Knopoff (1967) subsequently provided a wonderfully simple analogue that simulates the statistics of earthquakes with both small-magnitude and major earthquakes, noting similar statistics as the empirical Gutenberg-Richter (1956) and Omori (1894) laws (Utsu 1961). Burridge and Knopoff (1967) conclude that a viscous component is necessary to produce aftershocks. Byerlee (1978) — yes, the guy from the law — relates the normal stress to the shear stress on the fault plane — quite an important inference, and a step beyond the Mohr-Coulomb failure law. In this, the smoothness of the surface and therefore the presence of asperities — i.e. friction — is paramount. Asperities — domains on fault interfaces that are locked (Lay et al. 1982) — and, especially, their spatial distribution control the seismogenic nature of a particular fault interface.

So, we’re actually getting somewhere. I think I’ve whetted your apatite, but you will have to wait another week before I start explaining how friction works.

Yours truly,

The Sassy Scientist

PS: This post was written after being stuck for a while, then released quickly through a sudden burst of energy, and then some transient editing of the text. I’ll put this on repeat for the forthcoming posts in this series.

Barrell, J. (1914), The strength of the crust, Part VI. Relations of isostatic movements to a sphere of weakness — the asthenosphere. Journal of Geology, 22, 655–683
Brace, W.F. and Byerlee, J.D. (19660, Stick slip as a mechanism for earthquakes. Science 153, 990–992
Burridge, R., and Knopoff, L. (1967), Model and theoretical seismicity. Bulletin of the Seismological Society of America, 57, 3411
Byerlee, J.D. (1978), Friction of rock. Pure and Applied Geophysics, 116, 615–626
Freed, A.M., and Lin, J. (2001), Delayed triggering of the 1999 Hector Mine earthquake by viscoelastic stress transfer. Nature, 411, 180–183
Gutenberg. B. and Richter, C.F. (1956), Magnitude and energy of earthquakes. Annals of Geophysics, 9, 1-15
Haskell, N. A. (1935), The motion of a fluid under a surface load, 1. Physics, 6, 265-269
Lay, T., Kanamori, H. and Ruff, L. (1982). The asperity model and the nature of large subduction zone earthquakes, Earthquake Prediction Research, 1, 3-71
Nur, A. and Booker, J.R. (1972), Aftershocks caused by pore fluid flow? Science 175, 885–887
Nur, A. and Mavko, G. (1974), Postseismic viscoelastic rebound. Science, 183(4121), 204–206.
Omori, F. (1894), On after-shocks of earthquakes. Journal of the College of Science, Imperial University of Tokyo, 7, 111-200
Peltzer, G., Rosen, P., Rogez, F. and Hudnut, K. (1998), Poro-elastic rebound along the Landers 1992 earthquake surface rupture. Journal of Geophysical Research, 103, 30131–30145
Perfettini, H. and Avouac, J.P. (2004), Stress transfer and strain rate variations during the seismic cycle. Journal of Geophysical Research, 109, B06402.
Smith, S.W. and Wyss, M. (1968), Displacement on the San Andreas fault initiated by the 1966 Parkfield earthquake. Bulletin of the Seismological Society of America, 58, 1955-1974
Thatcher, W. and Rundle, J.B. (1979), A model for the earthquake cycle in underthrust zones. Journal of Geophysical Research, 84(B10), 5540–5556.
Utsu, T. A. (1961), Statistical study on the occurrence of aftershocks. Geophysical Magazine, 30, 521-605