Papers by Adrian Lenardic
Geoscience frontiers, 2018
The Earth is the only body in the solar system for which significant observational constraints ar... more The Earth is the only body in the solar system for which significant observational constraints are accessible to such a degree that they can be used to discriminate between competing models of Earth's tectonic evolution. It is a natural tendency to use observations of the Earth to inform more general models of planetary evolution. However, our understating of Earth's evolution is far from complete. In recent years, there has been growing geodynamic and geochemical evidence that suggests that plate tectonics may not have operated on the early Earth, with both the timing of its onset and the length of its activity far from certain. Recently, the potential of tectonic bi-stability (multiple stable, energetically allowed solutions) has been shown to be dynamically viable, both from analytical analysis and through numeric experiments in two and three dimensions. This indicates that multiple tectonic modes may operate on a single planetary body at different times within its temporal evolution. It also allows for the potential that feedback mechanisms between the internal dynamics and surface processes (e.g., surface temperature changes driven by long term climate evolution), acting at different thermal evolution times, can cause terrestrial worlds to alternate between multiple tectonic states over giga-year timescales. The implication within this framework is that terrestrial planets have the potential to migrate through tectonic regimes at similar 'thermal evolution times' (e.g., points were they have a similar bulk mantle temperature and energies), but at very different 'temporal times' (time since planetary formation). It can be further shown that identical planets at similar stages of their evolution may exhibit different tectonic regimes due to random variations. Here, we will discuss constraints on the tectonic evolution of the Earth and present a novel framework of planetary evolution that moves toward probabilistic arguments based on general physical principals, as opposed to particular rheologies, and incorporates the potential of tectonic regime transitions and multiple tectonics states being viable at equivalent physical and chemical conditions.
Zenodo (CERN European Organization for Nuclear Research), Apr 16, 2023
Many articles and books have been written about how universities have shifted to business models ... more Many articles and books have been written about how universities have shifted to business models of operation, with associated moves toward privatization, and how that is having detrimental effects on science. It's easy to blame administrators for these shifts. It goes beyond that, however. Blame placed on one house often belongs to the whole village. The business model, with its use of metrics to quantify productivity, has been quietly adopted by working scientists. Vitae, from scientists spanning a range of academic ages, now include not only publication metrics but also the amount of monetary funding a scientist has generated and media contact metrics to quantify the amount of attention a scientist has received (attention is a cash equivalent for universities). No push back on bean counting makes life easier for bean counters. Individual push back occurs now and then but never gets far. Some of us, myself included, tell ourselves comforting stories that change will come as more scientists see the problems with turning science into an open market, competitive business. Once that group advances to positions of power, all will change.
Zenodo (CERN European Organization for Nuclear Research), Mar 3, 2023
As scientists, the terminology we choose influences our thinking as it carries our messages to co... more As scientists, the terminology we choose influences our thinking as it carries our messages to colleagues and the public. In the face of pressure to turn science into clickbait, maintaining precision in the language we use is critical to dispel misinformation and, more broadly, to enable scientific progress. T he field of exoplanet research is currently experiencing a period of integration among researchers from the planetary exploration, astrobiology, and astronomical observation communities (among others). In addition, exoplanet discoveries generate strong interest from the public and a need for frequent communication of scientific discoveries to non-scientists. Together, these factors have put significant pressure on the field to develop an easily digested scientific shorthand for frequently used concepts. While useful, such terminology is often imprecise and can ultimately mislead the very audience it was designed to reach.
<p>We investigate the past evolution of the climate of Earth and Earth-like planets... more <p>We investigate the past evolution of the climate of Earth and Earth-like planets as a coupled interior/atmosphere system. We compare climatic states obtained through parameterized modelling versus a physics-based 3D General Circulation Model (GCM). Finally, we identify characteristics in the 3D simulations that most affect the climate, and how that impacts the reliability of parameterized modeling.</p> <p>In long-term planetary evolution studies, surface conditions are often characterized using global average temperatures, and calculated using simple models (i.e., Eddington approximation, 1D radiative convective gray atmosphere). For instance, these models treat albedo and cloud cover in a parameterized way and are not always able to assess local variations (i.e., latitudinal). A more self-consistent approach uses a 3D GCM, which requires extensive computing resources and time. This makes GCMs unpractical for long-term evolution modelling. Instead, here, successive windows into the past states of the atmosphere/surface are modeled.</p> <p>The past thermal history of Earth’s interior is used as a representative case for a range of possible past states and evolution of the mantles of Earth-like exoplanets. This feeds a parameterized model for mantle thermal and dynamic evolution. From the computation of melt generation and volcanism, the volatile delivery from the mantle into the atmosphere is estimated. This produces a variety of atmospheric composition evolutionary pathways, which, in turn, govern planetary climate evolution.</p> <p>We use the ROCKE3D GCM during significant windows of the long-term evolution to understand the differences between the parameterized (coupled evolution) and more complete (GCM) approaches. We compare average surface temperatures and albedos obtained in both simulations. We then evaluate the ice coverage obtained in GCM simulations and compare it to the usual criteria for habitability (such as average temperatures above 273-258 K). Finally, we assess the reasons for discrepancies between the models.</p> <p>In particular, we study the influence of the total atmosphere pressure, and its composition (N<sub>2</sub>, CO<sub>2</sub>, O<sub>2</sub>, CH<sub>4</sub>), consistently with Earth observation, as well as solar insolation and length of day variation, depending on the different eras we consider. We further study the impact of continental distribution (i.e., present-day-like or supercontinent distributions) and topography. We use the mantle dynamics simulation output based on the thermal history to assess the characteristics of the surface features. The trend of the variations of average temperature through time (and CO<sub>2</sub> abundances) is consistent in parameterized vs. GCM models. Perturbation around the reference model result in stronger temperature variations in the GCM due to albedo feedback. Indeed the albedo variations can be significant in 3D simulations and are not considered in the parameterized approach. Supercontinent setups result in markedly dryer land than present-day Earth. Even models with average temperatures below 273-268 K have significant ice-free ground in all continental setups.</p>
AGUFM, Dec 1, 2017
Recent seismic observations, focused on mantle flow below the Pacific plate, indicate the presenc... more Recent seismic observations, focused on mantle flow below the Pacific plate, indicate the presence of two shear layers in the Earth's asthenosphere. This is difficult to explain under the classic assumption of asthenosphere flow driven by plate shear from above. We present numerical mantle convection experiments that show how a power law rheology, together with dynamic pressure gradients, can generate an asthenosphere flow profile with a near constant velocity central region bounded above and below by concentrated shear layers (a configuration referred to as plug flow). The experiments show that as the power law dependence of asthenosphere viscosity is increased from 1 to 3, maximum asthenosphere velocities can surpass lithosphere velocity. The wavelength of mantle convection increases and asthenosphere flow transitions from a linear profile (Couette flow) to a plug flow configuration. Experiments in a 3D spherical domain also show a rotation of velocity vectors from the lithosphere to the asthenosphere, consistent with seismic observations. Global mantle flow remains of whole mantle convection type with plate and asthenosphere flow away from a mid-ocean ridge balanced by broader return flow in the lower mantle. Our results are in line with theoretical scalings that mapped the conditions under which asthenosphere flow can provide an added plate driving force as opposed to the more classic assumption that asthenosphere flow is associated with a plate resisting force.
arXiv (Cornell University), Jan 27, 2018
Changes that occur on our planet can be tracked back to one of two energy sources: the sun and th... more Changes that occur on our planet can be tracked back to one of two energy sources: the sun and the Earth's internal energy. The motion of tectonic plates, volcanism, mountain building and the reshaping of our planet's surface over geologic time depend on the Earth's internal energy. Tectonic activity is driven by internal energy and affects the rate at which energy is tapped, i.e., the cooling rate of our planet. Petrologic data indicate that cooling did not occur at a constant rate over geologic history. Interior cooling was mild until 2.5 billion years ago and then increased (Figure 1). As the Earth cools, it cycles water between its rocky interior (crust and mantle) and its surface. Water affects the viscosity of mantle rock, which affects the pace of tectonics and, by association, Earth cooling. We present suites of thermal-tectonic history models, coupled to deep water cycling, to show that the petrologically constrained change in the Earth's cooling rate can be accounted for by variations in deep water cycling over geologic time. The change in cooling rate does not require a change in the global tectonic mode of the Earth. It can be accounted for by a change in the balance of water cycling between the Earth's interior and its surface envelopes. The nature and timing of that water cycling change can be correlated to a change in the nature of continental crust and an associated rise of atmospheric oxygen. The prediction that the rise of oxygen should then be correlated, in time, to the change in the Earth's cooling rate is consistent with data constraints.
Zenodo (CERN European Organization for Nuclear Research), Feb 15, 2023
Zenodo (CERN European Organization for Nuclear Research), Feb 28, 2023
We address a recently posed question: 'Why Do So Many Astronomy (and Astrobiology) Discoveries Fa... more We address a recently posed question: 'Why Do So Many Astronomy (and Astrobiology) Discoveries Fail to Live Up to the Hype?' We expand it to cover hype within science in general. Our answer relies on working definitions of hype and skin in the game, as applied to research science, and a game theory model for the stability of cooperative science. Low skin in the game allows internal feedbacks, within the research science community, to initiate increased hype and a drift toward structural instability. The instability leads to the deterioration of cooperative equilibria, which further enhances hype. Along the drift, the number of results hyped as breakthroughs will increase and more claims will fail to live up to the hype. This can lead to the public perception that science is moving backwards and a shift in the perception of what scientists, and science, values. Although a hype instability can be initiated by external nudges, a bigger role is played by the internal dynamics of the system, i.e. the collective of working scientists. Corrections for a drift toward instability should, likewise, focus on internal structure. Proposed external shifts on how research is disseminated will add restrictions to a system that can do more harm than good. Contents 4 Structure and stability of collective science 6 Perturbations and nudges toward instability 9 Conclusion, discussion and potential actions 10
Journal of Geophysical Research: Planets, 2019
Key points: 1. The characterization of terrestrial exoplanets, including interior structure and a... more Key points: 1. The characterization of terrestrial exoplanets, including interior structure and atmospheres, is becoming a primary focus of exoplanetary science. 2. The boundaries of habitability are best understood through the study of the extreme environments present on Earth and Venus. 3. There are many outstanding questions regarding Venus that are critical to answer in order to better constrain models for exoplanets.
Earth and Planetary Science Letters, 2005
An important feature of continents and oceans is that they are underlain by chemically distinct m... more An important feature of continents and oceans is that they are underlain by chemically distinct mantle, made intrinsically buoyant and highly viscous by melt depletion and accompanying dehydration, respectively. Of interest here are the influences of these preexisting chemical boundary layers on small-scale convective processes (as opposed to large-scale processes, which govern the drift of continents and the eventual fate of oceanic thermal boundary layers, e.g., subduction) at the base of the oceanic and continental thermal boundary layers. This manuscript explores the endmember in which dehydrated and melt-depleted boundary layers are assumed to be strong (in the viscous sense) and chemically buoyant enough that they do not partake in any secondary convection, that is, vertical heat transfer through these lids occurs purely by conduction. This assumption implies that the only part of the thermal boundary layer that participates in secondary convection resides beneath the strong chemical boundary layer. For oceans, this leads to the condition that the onset time of convective instability is suppressed until after the thermal boundary layer has cooled through the base of the strong chemical boundary layer, whose thickness is defined at the outset by the depth at which the solid mantle adiabat crosses the anhydrous peridotite solidus. A scaling law is presented that accounts for the presence of a preexisting strong chemical boundary layer and predicts that the onset time of convective instability beneath oceans correlates with the thickness of the chemical boundary layer, which itself correlates with the potential temperature of the mantle at the time of melting. Estimated paleo-potential temperatures required to generate old oceanic crust in the Pacific and Atlantic may in fact be correlated with onset time of seafloor flattening, but more data are needed to confirm these preliminary observations. Finally, for continents, recent numerical models suggest that the thickness of the convective sublayer, hence the total thermal boundary layer thickness, is also controlled by the thickness of a preexisting strong chemical boundary layer. Xenolith data from cratons are shown to be largely consistent with the model-predicted relationship between the thicknesses of the chemical and thermal boundary layers beneath continents. The conclusion of this study is that the nature of both oceanic and continental thermal boundary layers is likely to be linked to preexisting strong chemical boundary layers.
Geophysical Research Letters, Sep 23, 2016
The discovery of large terrestrial (~1 Earth mass (M e) to < 10 M e) extrasolar planets has promp... more The discovery of large terrestrial (~1 Earth mass (M e) to < 10 M e) extrasolar planets has prompted a debate as to the likelihood of plate tectonics on these planets. Canonical models assume classic basal heating scaling relationships remain valid for mixed heating systems with an appropriate internal temperature shift. Those scalings predict a rapid increase of convective velocities (V rms) with increasing Rayleigh numbers (Ra) and non-dimensional heating rates (Q). To test this we conduct a sweep of 3-D numerical parameter space for mixed heating convection in isoviscous spherical shells. Our results show that while V rms increases with increasing thermal Ra, it does so at a slower rate than predicted by bottom heated scaling relationships. Further, the V rms decreases asymptotically with increasing Q. These results show that independent of specific rheologic assumptions (e.g., viscosity formulations, water effects, and lithosphere yielding), the differing energetics of mixed and basally heated systems can explain the discrepancy between different modeling groups. High-temperature, or young, planets with a large contribution from internal heating will operate in different scaling regimes compared to cooler-temperature, or older, planets that may have a larger relative contribution from basal heating. Thus, differences in predictions as to the likelihood of plate tectonics on exoplanets may well result from different models being more appropriate to different times in the thermal evolution of a terrestrial planet (as opposed to different rheologic assumptions as has often been assumed). This article was corrected on 06 OCT 2016. See the end of the full text for details.
Journal Of Geophysical Research: Solid Earth, Oct 1, 2016
We use a suite of 3-D numerical experiments to test and expand 2-D planar isoviscous scaling rela... more We use a suite of 3-D numerical experiments to test and expand 2-D planar isoviscous scaling relationships of Moore (2008) for mixed heating convection in spherical geometry mantles over a range of Rayleigh numbers (Ra). The internal temperature scaling of Moore (2008), when modified to account for spherical geometry, matches our experimental results to a high degree of fit. The heat flux through the boundary layers scale as a linear combination of internal (Q) and basal heating, and the modified theory predictions match our experimental results. Our results indicate that boundary layer thickness and surface heat flux are not controlled by a local boundary layer stability condition (in agreement with the results of Moore (2008)) and are instead strongly influenced by boundary layer interactions. Subadiabatic mantle temperature gradients, in spherical 3-D, are well described by a vertical velocity scaling based on discrete drips as opposed to a scaling based on coherent sinking sheets, which was found to describe 2-D planar results. Root-meansquare (RMS) velocities are asymptotic for both low Q and high Q, with a region of rapid adjustment between asymptotes for moderate Q. RMS velocities are highest in the low Q asymptote and decrease as internal heating is applied. The scaling laws derived by Moore (2008), and extended here, are robust and highlight the importance of differing boundary layer processes acting over variable Q and moderate Ra. The goal of this study is to evaluate and expand the theoretical scaling relationships of Moore [2008]. The theory was developed for a 2-D planar system, which is designed to emulate physical tank experiments. We extend it to address a 3-D spherical shell system, designed to emulate planetary interiors. We then use a large suite of numerical experiments to test the theoretical scaling predictions. We focus on isoviscous systems in order to test the scaling theory as straight forwardly, and comparably, as possible. We will extend the theory to include temperature-and depth-dependent viscosities, as well as surface yielding in future work.
AGU Fall Meeting Abstracts, Dec 1, 2012
AGU Fall Meeting Abstracts, Dec 1, 2007
Nominally anhydrous minerals (e.g., olivine, clinopyroxene, and orthopyroxene) in peridotite xeno... more Nominally anhydrous minerals (e.g., olivine, clinopyroxene, and orthopyroxene) in peridotite xenoliths collected from the Colorado Plateau and southern Basin and Range in western North America were systematically analyzed by Fourier transform infrared spectroscopy for water contents. Measured water contents range from 2 to 45 ppm for olivine, from 53 to 402 ppm for orthopyroxene, and from 171 to 957 ppm for clinopyroxene. The Colorado Plateau has the highest water contents (up to 45 ppm H 2 O in olivine, 402 ppm H 2 O in orthopyroxene, and 957 ppm H 2 O in clinopyroxene), while San Carlos in the southern Basin and Range has the lowest water contents (up to 4 ppm H 2 O in olivine, 82 ppm H 2 O in orthopyroxene, and 178 ppm H 2 O in clinopyroxene). With the exception of San Carlos, the olivine and pyroxenes from all other localities (Dish Hill, Grand Canyon, and Navajo) have water contents close to or higher than that inferred for the fertile asthenospheric mantle. We interpret the high water contents measured here to have been introduced into the base of the lithospheric mantle by rehydration associated with the subduction of the Farallon plate beneath North America during the early Cenozoic. Application of an updated flow law for dislocation creep of wet olivine to lithospheric mantle conditions beneath the Colorado Plateau predicts that for a given background shear stress, hydration alone can result in approximately 1 order of magnitude drop in the effective viscosity at the base of the lithosphere. If viscosity alone is used to distinguish the lithosphere from underlying asthenosphere, this suggests that hydration could have resulted in more than 10 km of lithospheric thinning. Viscosity reduction and lithospheric thinning of even larger extents (up to $100 km) are predicted when thicker lithosphere (such as Archean cratons) and larger water contents (up to watersaturated conditions) are considered. If our interpretations are correct, the implications of our study go beyond western North America and hint at a possible way of recycling continental mantle, including cratonic mantle, back into the convecting mantle.
Astrobiology, Jul 1, 2016
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Papers by Adrian Lenardic