Links of planetary energetics to moon size, orbit, and planet spin: A new mechanism for plate tectonics

ABSTRACT Lateral accelerations require lateral forces. We propose that force imbalances in the unique Earth-Moon-Sun system cause large-scale, cooperative tectonic motions. The solar gravitational pull on the Moon, being 2.2× terrestrial pull, causes lunar drift, orbital elongation, and an ~1000 km radial monthly excursion of the Earth-Moon barycenter inside Earth’s mantle. Earth’s spin superimposes an approximately longitudinal 24 h circuit of the barycenter. Because the oscillating barycenter lies 3500–5500 km from the geocenter, Earth’s tangential orbital acceleration and solar pull are imbalanced. Near-surface motions are enabled by a weak low-velocity zone underlying the cold, brittle lithosphere: The thermal states of both layers result from leakage of Earth’s internal radiogenic heat to space. Concomitantly, stress induced by spin cracks the lithosphere in a classic X-pattern, creating mid-ocean ridges and plate segments. The inertial response of our high-spin planet with its low-velocity zone is ~10 cm yr–1 westward drift of the entire lithosphere, which largely dictates plate motions. The thermal profile causes sinking plates to thin and disappear by depths of ~200–660 km, depending on angle and speed. Cyclical stresses are effective agents of failure, thereby adding asymmetry to plate motions. A comparison of rocky planets shows that the presence and longevity of volcanism and tectonism depend on the particular combination of moon size, moon orbital orientation, proximity to the Sun, and rates of body spin and cooling. Earth is the only rocky planet with all the factors needed for plate tectonics.

1. Beginnings

‘Beginnings’ discusses the general processes that form planetary systems, particularly the Solar System. Most of the Universe is made of a mysterious substance called ‘dark matter’, and an even more mysterious substance called ‘dark energy’. After the birth of the Universe in the Big Bang, the tiny bits of stardust which have accumulated contain the heavier elements (baryonic matter) that make it possible to form beings like ourselves, and the planets on which we live. We mustn't forget the importance of the formation of protostars, as well as gas and ice giant planets, the evolution of the proto-Sun, and the formation of inner rocky planets.

New Perspectives on the Exoplanet Radius Gap from a Mathematica Tool and Visualized Water Equation of State

Recent astronomical observations obtained with the Kepler and TESS missions and their related ground-based follow-ups revealed an abundance of exoplanets with a size intermediate between Earth and Neptune (1 R ⊕ ≤ R ≤ 4 R ⊕). A low occurrence rate of planets has been identified at around twice the size of Earth (2 × R ⊕), known as the exoplanet radius gap or radius valley. We explore the geometry of this gap in the mass–radius diagram, with the help of a Mathematica plotting tool developed with the capability of manipulating exoplanet data in multidimensional parameter space, and with the help of visualized water equations of state in the temperature–density (T–ρ) graph and the entropy–pressure (s–P) graph. We show that the radius valley can be explained by a compositional difference between smaller, predominantly rocky planets (<2 × R ⊕) and larger planets (>2 × R ⊕) that exhibit greater compositional diversity including cosmic ices (water, ammonia, methane, etc.) and gaseous envelopes. In particular, among the larger planets (>2 × R ⊕), when viewed from the perspective of planet equilibrium temperature (T eq), the hot ones (T eq ≳ 900 K) are consistent with ice-dominated composition without significant gaseous envelopes, while the cold ones (T eq ≲ 900 K) have more diverse compositions, including various amounts of gaseous envelopes.

Olivine-rich achondrites from Vesta and the missing mantle problem

Mantles of rocky planets are dominantly composed of olivine and its high-pressure polymorphs, according to seismic data of Earth’s interior, the mineralogy of natural samples, and modelling results. The missing mantle problem represents the paucity of olivine-rich material among meteorite samples and remote observation of asteroids, given how common differentiated planetesimals were in the early Solar System. Here we report the discovery of new olivine-rich meteorites that have asteroidal origins and are related to V-type asteroids or vestoids. Northwest Africa 12217, 12319, and 12562 are dunites and lherzolite cumulates that have siderophile element abundances consistent with origins on highly differentiated asteroidal bodies that experienced core formation, and with trace element and oxygen and chromium isotopic compositions associated with the howardite-eucrite-diogenite meteorites. These meteorites represent a step towards the end of the shortage of olivine-rich material, allowing for full examination of differentiation processes acting on planetesimals in the earliest epoch of the Solar System.

Rock traits drive complex microbial communities at the edge of life

Antarctic deserts are among the driest and coldest ecosystems of the planet; there, some microbes hang on to life under these extreme conditions inside porous rocks, forming the so-called endolithic communities. Yet, the contribution of distinct rock traits to support complex microbial assemblies remains poorly determined. Here, we combined an extensive Antarctic rock survey with rock microbiome sequencing and ecological networks, and found that contrasting combinations of microclimatic and rock traits such as thermal inertia, porosity, iron concentration and quartz cement can help explain the multiple complex and independent microbial assemblies found in Antarctic rocks. Our work highlights the pivotal role of rocky substrate heterogeneity in sustaining contrasting groups of microorganisms, which is essential to understand life at the edge on Earth, and for searching life on other rocky planets such as Mars.

How does volcanic outgassing differ on geological time scales between planets with and without plate tectonics?

&lt;div&gt;One of the main factors to assess the possible habitability of a rocky planet (either in or beyond our solar system) is its capability to maintain an atmosphere that allows for moderate temperatures at the surface and would allow water to occur in a liquid form, and that can help shield surface life from harmful radiation.&lt;/div&gt; &lt;div&gt;The existence of an atmosphere depends on several factors - possible accretion from the nebula and catastrophic degassing from the crystallizing magma ocean during planet formation, later delivery of volatiles via comets, sinks of atmosphere gases to the surface or to space, and last, but definitely not least, volcanic release of volatiles from the mantle that where stored in the planet's interior during its formation stage.&lt;/div&gt; &lt;div&gt;For planets of masses not too different from Earth, volcanic degassing plays a major role for the question if the planet could have an atmosphere. Lower-mass planets might not be able to keep an atmosphere but loose it entirely to space, and much more massive super-Earth planets will likely keep the primordial, catastrophically outgassed atmosphere during magma ocean crystallization, and may never be habitable at their surface due to a thick atmosphere rather comparable to Venus. The &quot;Goldilocks zone&quot; for potentially habitable rocky planets is therefore limited to a range from&amp;#160;above Mars' mass to a few Earth masses. However, planets of a few Earth masses may not be able to efficiently outgas volcanic gases, if they are in a stagnant-lid regime. This may be different, though, for planets experiencing plate tectonics like Earth, where hot, molten material reaches the surface at plate boundaries and may therefore build up or replenish an atmosphere. The work presented here compares the efficiency of interior volatile depletion and degassing to the surface for rocky planets of different size and composition, either in the stagnant-lid or in the plate-tectonics regime.&lt;/div&gt;

Atmospheres of lava exoplanets. Where and what to look for.

&lt;p&gt;Strongly irradiated rocky planets will have sufficiently high surface temperatures to sustain dayside magma lakes and oceans that will outgas low-pressure days-side silicate atmospheres. The surface magma will preferentially outgas its most volatile components and eventually equilibrate itself with the formed atmosphere. However, the volatile constituents may be depleted due to transport and condensation to the cooler night side or escape to space. If the atmospheric evolution is slower than the surface-interior exchange, the melt composition will remain coupled with the planet's interior. In such a case the atmospheric component will be continuously replenished. If atmospheric evolution is fast, the magma pool will lose its volatile component and the atmospheric composition will be irreversibly changed. We employ numerical models of outgassing, atmospheric chemistry, and radiative transfer to model the possible observable emission features of evolving magma atmospheres for all confirmed rocky lava worlds. . Our results highlight the best possible observable features in the atmospheres of hot rocky exoplanets that may give insight in their interior and atmospheric evolution. We also propose a set of ideal targets for JWST and ARIEL missions.&lt;br&gt;&lt;br&gt;&lt;br&gt;&lt;/p&gt;

The interior-atmosphere coupling of rocky worlds

&lt;p&gt;Earth has been the only known habitable world and thus used as a reference to understand habitability. The origin of life on Earth is not yet clearly understood, but known traces are up-to the Archean (&amp;#8764; 3.5Ga, Ga-billion years). Earth had water and continents from the Hadean Earth (&gt; 4.0Ga), which had different atmospheric conditions compared to the Archean Earth. Similarly, the current state and composition of atmosphere does not represent its future state. Climate changes are partly attributed to feedback mechanism between the internal processes and the atmosphere. And as such, each atmospheric state is depictive of an instance a long a trajectory path of a coupled evolution of Earth system. Venus was thought to be habitable until into the 1960s, when its surface was observed to be oven-hot with surface pressure a hundred times that of Earth. Why and when the evolutionary paths of Venus and Earth, which are similarly sized and should have similar internal compositions, started to diverge? Moreover, known exoplanets, planets and moons have very different geophysical characteristic from Earth. This implies exotic life might vary substantially from what we know. As a result, understanding evolution of rocky planets, that is their interior structure, atmospheres and climate regardless of their habitability is of great importance. In this work we study the relation between a rocky planet&amp;#8217;s internal properties and its observable surface and atmosphere properties over time.&amp;#160;&amp;#160; We explore the different convection regimes (stagnant lid, episodic-lid and tectonic), studying the relation between a planet&amp;#8217;s viscous state, its interior composition and structure. Focusing on the effects of mantle convection on volatile recycling processes such as CO2 outgassing that influence the atmospheric state and climatic conditions over time. The computed models are then used to compute observables, that ultimately can be tested with observations.&lt;/p&gt;

Coupled models of magma oceans and their primordial atmospheres: volatile speciation, cooling history, and impact of condensables

&lt;p&gt;The earliest atmospheres of rocky planets originate from extensive volatile release during magma ocean epochs that occur during assembly of the planet. These establish the initial distribution of the major volatile elements between different chemical reservoirs that subsequently evolve via geological cycles. Current theoretical techniques are limited in exploring the anticipated range of compositional and thermal scenarios of early planetary evolution. However, these are of prime importance to aid astronomical inferences on the environmental context and geological history of extrasolar planets. In order to advance the potential synergies between exoplanet observations and inferrences on the earliest history and climate state of the solar system terrestial planets, I will present a novel numerical framework that links an evolutionary, vertically-resolved model of the planetary silicate mantle with a radiative-convective model of the atmosphere. Numerical simulations using this framework illustrate the sensitive dependence of mantle crystallization and atmosphere build-up on volatile speciation and predict variations in atmospheric spectra with planet composition that may be detectable with future observations of exoplanets. Magma ocean thermal sequences fall into three general classes of primary atmospheric volatile with increasing cooling timescale: CO, N&lt;sub&gt;2&lt;/sub&gt;, and O&lt;sub&gt;2&lt;/sub&gt; with minimal effect on heat flux, H&lt;sub&gt;2&lt;/sub&gt;O, CO&lt;sub&gt;2&lt;/sub&gt;, and CH&lt;sub&gt;4&lt;/sub&gt; with intermediate influence, and H&lt;sub&gt;2&lt;/sub&gt; with several orders of magnitude increase in solidification time and atmosphere vertical stratification. In addition to these time-resolved results, I will present a novel formulation and application of a multi-species moist-adiabat for condensable-rich magma ocean and archean earth analog atmospheres, and outline how the cooling of such atmospheres can lead to exotic climate states that provide testable predictions for terrestrial exoplanets.&lt;/p&gt;