Seismic constraints from a Mars impact experiment using InSight and Perseverance

NASA’s InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) mission has operated a sophisticated suite of seismology and geophysics instruments on the surface of Mars since its arrival in 2018. On 18 February 2021, we attempted to detect the seismic and acoustic waves produced by the entry, descent and landing of the Perseverance rover using the sensors onboard the InSight lander. Similar observations have been made on Earth using data from both crewed1,2 and uncrewed3,4 spacecraft, and on the Moon during the Apollo era5, but never before on Mars or another planet. This was the only seismic event to occur on Mars since InSight began operations that had an a priori known and independently constrained timing and location. It therefore had the potential to be used as a calibration for other marsquakes recorded by InSight. Here we report that no signal from Perseverance’s entry, descent and landing is identifiable in the InSight data. Nonetheless, measurements made during the landing window enable us to place constraints on the distance–amplitude relationships used to predict the amplitude of seismic waves produced by planetary impacts and place in situ constraints on Martian impact seismic efficiency (the fraction of the impactor kinetic energy converted into seismic energy).

Solar evolution and extrema: current state of understanding of long-term solar variability and its planetary impacts

The activity of stars such as the Sun varies over timescales ranging from the very short to the very long—stellar and planetary evolutionary timescales. Experience from our solar system indicates that short-term, transient events such as stellar flares and coronal mass ejections create hazardous space environmental conditions that impact Earth-orbiting satellites and planetary atmospheres. Extreme events such as stellar superflares may play a role in atmospheric mass loss and create conditions unsuitable for life. Slower, long-term evolutions of the activity of Sun-like stars over millennia to billions of years result in variations in stellar wind properties, radiation flux, cosmic ray flux, and frequency of magnetic storms. This coupled evolution of star-planet systems eventually determines planetary and exoplanetary habitability. The Solar Evolution and Extrema (SEE) initiative of the Variability of the Sun and Its Terrestrial Impact (VarSITI) program of the Scientific Committee on Solar-Terrestrial Physics (SCOSTEP) aimed to facilitate and build capacity in this interdisciplinary subject of broad interest in astronomy and astrophysics. In this review, we highlight progress in the major themes that were the focus of this interdisciplinary program, namely, reconstructing and understanding past solar activity including grand minima and maxima, facilitating physical dynamo-model-based predictions of future solar activity, understanding the evolution of solar activity over Earth’s history including the faint young Sun paradox, and exploring solar-stellar connections with the goal of illuminating the extreme range of activity that our parent star—the Sun—may have displayed in the past, or may be capable of unleashing in the future.

Hydrodynamic instability at impact interfaces and planetary implications

Impact-induced mixing between bolide and target is fundamental to the geochemical evolution of a growing planet, yet aside from local mixing due to jetting – associated with large angles of incidence between impacting surfaces – mixing during planetary impacts is poorly understood. Here we describe a dynamic instability of the surface between impacting materials, showing that a region of mixing grows between two media having even minimal initial topography. This additional cause of impact-induced mixing is related to Richtmyer-Meshkov instability (RMI), and results from pressure perturbations amplified by shock-wave refraction through the corrugated interface between impactor and target. However, unlike RMI, this new impact-induced instability appears even if the bodies are made of the same material. Hydrocode simulations illustrate the growth of this mixing zone for planetary impacts, and predict results suitable for experimental validation in the laboratory. This form of impact mixing may be relevant to the formation of stony-iron and other meteorites.

Thermodynamicsof giant planetary impacts from ab initiosimulations

<h3>Impacts are highly energetic phenomena. They abound in the early stages of formation of the solar system, when they actively participated to the formation of large bodies in the protoplanetary disk. Later on, when planetesimals and embryo planets formed, impacts merged smaller bodies into the large planets that we know today. Giant impacts dominated the last phase of the planetary accretion, with some of these impacts leaving traces observable even today (planets tilts, moon, missing mantle, etc). The Earth was not spared, and its most cataclysmic event also contributed to the formation of the Moon.</h3><h3>Here we present the theoretical tools used to explore the thermodynamics of the formation of the protolunar disk and the subsequent condensation of this disk. We show how ab initio-based molecular dynamics simulations contribute to the determination of the stability field of melts, to the equilibrium between melts and vapor and the positioning of the critical points. Together all this information helps building the liquid-vapor stability dome. Next we investigate the supercritical regime, typical of the post-impact state. We take a focused look to the transport properties, the formation of the first atmosphere, and compare the properties of the liquid state typical of magma oceans, to the super-critical state, typical of protolunar disks.</h3><h3>We apply this theoretical approach on pyrolite melts, as best approximants for the bulk silicate Earth. These simulations help us retrace the thermodynamic state of the protolunar disk and infer possible condensation paths for both the Earth and the moon.</h3><h3> </h3><p>RC acknowledges support from the European Research Council under EU Horizon 2020 research and innovation program (grant agreement 681818 – IMPACT) and access to supercomputing facilities via the eDARI gen6368 grants, the PRACE RA4947 grant, and the Uninet2 NN9697K grant. STS was supported by NASA grants NNX15AH54G and 80NSSC18K0828; DOE-NNSA grants DE-NA0003842 and DE-NA0003904.</p>

A new equation of state applied to planetary impacts

Observed FeO/MgO ratios in the Moon and Earth are inconsistent with simulations done with a single homogeneous silicate layer. In this paper we use a newly developed equation of state to perform smoothed particle hydrodynamics simulations on the lunar-forming impact, testing the effect of a primordial magma ocean on Earth. This is investigated using the impact parameters of both the canonical case, in which a Mars-sized impactor hits a non-rotating Earth at an oblate angle, and the fast-rotating case, in which a half-sized Mars impactor hits a fast-spinning Earth head-on. We find that the inclusion of a magma ocean results in a less massive Moon and leads to slightly more mixing. Additionally, we test how an icy Theia would affect the results and find that this reduces the probability of a successful Moon formation. Simulations of the fast-spinning case are found to be unable to form a massive-enough Moon.

The Ecological Cost Of Consumption

The link between global ecosystem decline, trade, and human consumption suggests that trade-based biodiversity footprints should be regarded as a critical indicator of planetary impacts. Here we integrate a global input-output economic framework that encompasses global trade between 15909 sectors, with range and impact data on 10518 terrestrial plant, 17234 terrestrial animal, 6101 freshwater and 5059 marine species, to specify the biodiversity footprints associated with global trade and consumption across domestic and international supply chains. Our framework characterises global species loss as driven by domestic trade in emerging market economies including China, Brazil, Mexico, India, and Ecuador, and exacerbated by consumption in high-income countries, especially those in the G7, that are driving species loss in emerging markets and low-income nations. We attribute the largest sector-scope footprints to construction in China, Colombia, and India, agriculture commodity trade in Madagascar, Mexico, Tanzania, and Peru, and food manufacture in Mexico, Germany and France.

Doing Things With Natures

The article expands on Lewis and Maslin’s “double two-step” historicization of the Anthropocene, with two major transitions in energy (agriculture and fossil fuels) and two in social organization (modernity and the Great Acceleration). Insofar as planetary impacts arise from “what we spend our time doing” – foraging, farming, feudal then waged labour, finally unsustainable consumption – such “doing” is understood as precisely ‘performative’ in the sense that its effects only arise from a massive social repetition that is confused with essential nature and thus concealed. Through a graphic model of such ‘plural performativity,’ four consecutive Anthropo(s)cenes are sketched: the Giving World of agriculture and state formation; the New World of colonial pillage and world trade; the Netherworld of wage labour and fossil capital; then ‘All the World’ but not with all of “us” as players. Apart from environmental changes, the paper targets performances of power and inequality: normative histories of ‘common sense’ on the one hand, concealing ‘people’s histories’ of conflict and opposition, on the other – the Anthropocene arising not simply from what the majority of people have been doing, but from what they have always beenforced to do.

A new equation of state applied to planetary impacts

We present a new analytical equation of state (EOS), which correctly models high pressure theory and fits well to the experimental data of ɛ-Fe, SiO2, Mg2SiO4, and the Earth. The cold part of the EOS is modeled after the Varpoly EOS. The thermal part is based on a new formalism of the Gruneisen parameter, which improves behavior from earlier models and bridges the gap between elasticity and thermoelasticity. The EOS includes an expanded state model, which allows for the accurate modeling of material vapor curves. The EOS is compared to both the Tillotson EOS and ANEOS model, which are both widely used in planetary impact simulations. The complexity and cost of the EOS is similar to that of the Tillotson EOS, while showing improved behavior in every aspect. The Hugoniot state of shocked silicate material is captured relatively well and our model reproduces vapor curves similar to that of the ANEOS model. To test its viability in hydrodynamical simulations, the EOS was applied to the lunar-forming impact scenario and the results are presented in Paper II and show good agreement with previous work.

Collisional formation of massive exomoons of superterrestrial exoplanets

ABSTRACT Exomoons orbiting terrestrial or superterrestrial exoplanets have not yet been discovered; their possible existence and properties are therefore still an unresolved question. Here, we explore the collisional formation of exomoons through giant planetary impacts. We make use of smooth particle hydrodynamical collision simulations and survey a large phase space of terrestrial/superterrestrial planetary collisions. We characterize the properties of such collisions, finding one rare case in which an exomoon forms through a graze and capture scenario, in addition to a few graze and merge or hit and run scenarios. Typically however, our collisions form massive circumplanetary discs, for which we use follow-up N-body simulations in order to derive lower limit mass estimates for the ensuing exomoons. We investigate the mass, long-term tidal-stability, composition and origin of material in both the discs and the exomoons. Our giant impact models often generate relatively iron-rich moons that form beyond the synchronous radius of the planet, and would thus tidally evolve outward with stable orbits, rather than be destroyed. Our results suggest that it is extremely difficult to collisionally form currently-detectable exomoons orbiting superterrestrial planets, through single giant impacts. It might be possible to form massive, detectable exomoons through several mergers of smaller exomoons, formed by multiple impacts, however more studies are required in order to reach a conclusion. Given the current observational initiatives, the search should focus primarily on more massive planet categories. However, about a quarter of the exomoons predicted by our models are approximately Mercury-mass or more, and are much more likely to be detectable given a factor 2 improvement in the detection capability of future instruments, providing further motivation for their development.