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We present the first observations of seismic waves propagating through the core of Mars. These observations, made using seismic data collected by the InSight geophysical mission, have allowed us to construct the first seismically constrained models for the elastic properties of Mars' core. We observe core-transiting seismic phase SKS from two farside seismic events detected on Mars and measure the travel times of SKS relative to mantle traversing body waves. SKS travels through the core as a compressional wave, providing information about bulk modulus and density. We perform probabilistic inversions using the core-sensitive relative travel times together with gross geophysical data and travel times from other, more proximal, seismic events to seek the equation of state parameters that best describe the liquid iron-alloy core. Our inversions provide constraints on the velocities in Mars' core and are used to develop the first seismically based estimates of its composition. We show that models informed by our SKS data favor a somewhat smaller (median core radius = 1,780 to 1,810 km) and denser (core density = 6.2 to 6.3 g/cm3) core compared to previous estimates, with a P-wave velocity of 4.9 to 5.0 km/s at the core-mantle boundary, with the composition and structure of the mantle as a dominant source of uncertainty. We infer from our models that Mars' core contains a median of 20 to 22 wt% light alloying elements when we consider sulfur, oxygen, carbon, and hydrogen. These data can be used to inform models of planetary accretion, composition, and evolution.

Constraining the thermal and compositional state of the mantle is crucial for deciphering the formation and evolution of Mars. Mineral physics predicts that Mars' deep mantle is demarcated by a seismic discontinuity arising from the pressure-induced phase transformation of the mineral olivine to its higher-pressure polymorphs, making the depth of this boundary sensitive to both mantle temperature and composition. Here, we report on the seismic detection of a midmantle discontinuity using the data collected by NASA's InSight Mission to Mars that matches the expected depth and sharpness of the postolivine transition. In five teleseismic events, we observed triplicated P and S waves and constrained the depth of this discontinuity to be 1,006 [Formula: see text] 40 km by modeling the triplicated waveforms. From this depth range, we infer a mantle potential temperature of 1,605 [Formula: see text] 100 K, a result consistent with a crust that is 10 to 15 times more enriched in heat-producing elements than the underlying mantle. Our waveform fits to the data indicate a broad gradient across the boundary, implying that the Martian mantle is more enriched in iron compared to Earth. Through modeling of thermochemical evolution of Mars, we observe that only two out of the five proposed composition models are compatible with the observed boundary depth. Our geodynamic simulations suggest that the Martian mantle was relatively cold 4.5 Gyr ago (1,720 to 1,860 K) and are consistent with a present-day surface heat flow of 21 to 24 mW/m2.

Analyses of seismic data from the InSight mission have provided the first in situ constraints on the thickness of the crust of Mars. These crustal thickness constraints are currently limited to beneath the lander that is located in the northern lowlands, and we use gravity and topography data to construct global crustal thickness models that satisfy the seismic data. These models consider a range of possible mantle and core density profiles, a range of crustal densities, a low-density surface layer, and the possibility that the crustal density of the northern lowlands is greater than that of the southern highlands. Using the preferred InSight three-layer seismic model of the crust, the average crustal thickness of the planet is found to lie between 30 and 72 km. Depending on the choice of the upper mantle density, the maximum permissible density of the northern lowlands and southern highlands crust is constrained to be between 2,850 and 3,100 kg m−3. These crustal densities are lower than typical Martian basaltic materials and are consistent with a crust that is on average more felsic than the materials found at the surface. We argue that a substantial portion of the crust of Mars is a primary crust that formed during the initial differentiation of the planet. Various hypotheses for the origin of the observed intracrustal seisimic layers are assessed, with our preferred interpretation including thick volcanic deposits, ejecta from the Utopia basin, porosity closure, and differentiation products of a Borealis impact melt sheet.

The crustal structure of Mercury's large impact basins provides valuable insights into the planet's geological history. For a warm crust, a post‐impact basin structure will viscously relax with inward flow of crustal materials toward the basin center. This effect drastically diminishes the crustal thickness contrasts and associated Bouguer gravity contrasts between the basin center and its surroundings. Here, we analyze Bouguer contrasts of 36 basins (diameter > ${ >} $300 km) located in the northern hemisphere as a proxy for viscoelastic relaxation. Thermal evolution models, assuming the present 3:2 spin‐orbit configuration, are used to predict crustal temperatures. Our analysis reveals that the expected correlation between zones of warm crust and low Bouguer contrast from relaxation is not observed in the available data. This suggests that crustal temperatures have changed in the past, potentially due to a change in Mercury's orbit or to a major volcanic event associated with smooth plain formation. Plain Language Summary: Mercury's impact basins and their crustal structure provide important clues about the planet's geological past and present‐day state. We study Mercury's crust at large impact basins by using information from gravity measurements and temperature estimates from thermal evolution models that consider the surface temperature pattern caused by Mercury's present‐day orbit. In regions with warm interior, the crustal structure of large basins is expected to relax more readily compared to colder regions. Here, we examine 36 large basins (diameter > ${ >} $300 km) in the northern hemisphere, where the gravity data is well resolved, to investigate this process. Our results show no correlation between gravity data at basins' location, modeled local temperature, and expected impact basin relaxation. This suggests that crustal temperatures have changed in the past, potentially due to a change in Mercury's orbit or to a major volcanic event associated with smooth plain formation. Key Points: Observed Bouguer anomaly contrasts and predicted crustal temperatures are used to investigate viscoelastic relaxation of Mercury's basinsOur findings do not indicate the anticipated pattern where basins exhibiting greater relaxation are situated in areas with hotter crustCrustal temperatures varied over time, likely due to rotation state changes or major volcanic events linked to smooth plain formation [ABSTRACT FROM AUTHOR]

Mercury's low obliquity and 3:2 spin‐orbit resonance create a surface temperature distribution with large latitudinal and longitudinal variations. These propagate via thermal conduction through the thin silicate shell influencing the interior temperature distribution. We use 3‐D thermal evolution models to investigate the effects of lateral variations of surface temperature and crustal thickness on the surface and core‐mantle boundary (CMB) heat fluxes, and elastic thickness distribution. Surface temperature variations cause a long‐wavelength perturbation of the heat fluxes, as well as of the elastic lithosphere thickness, while variations in crustal thickness affect these quantities at small spatial scales. Like the surface temperature, the present‐day CMB heat flux pattern is characterized primarily by spherical harmonics of degrees two and four, which could affect dynamo generation. Placing robust constraints on the thermal evolution based on the available estimates of the age and thickness of the elastic lithosphere remains challenging due to their large uncertainties. Plain Language Summary: The orbit of Mercury around the Sun leads to a specific surface temperature distribution, with some regions receiving more insolation than others. The thickness of Mercury's crust has been derived using gravity and topography data acquired by the MESSENGER mission. Since the rocky mantle of Mercury is rather thin (≈ ${\approx} $400 km), temperature variations driven by the insolation and by crustal properties (i.e., crustal heat sources and thermal conductivity) can affect the entire mantle, down to the core‐mantle boundary. Here we model the global thermal evolution of Mercury, and investigate the influence of surface temperature and crustal thickness variations on the history and present‐day state of the interior. We find that the surface temperature variations are reflected in the variations of Mercury's surface and core‐mantle boundary heat fluxes, as well as in the thickness of the elastic part of its lithosphere. Crustal thickness variations lead to more local temperature differences, thus resulting in smaller scale variations in the lithosphere thickness and heat fluxes. The heat flux pattern at the core mantle boundary induced by the surface temperature distribution could affect magnetic field generation and should be tested by future dynamo models. Key Points: The distribution of Mercury's mantle temperature is strongly influenced by surface temperature and crustal thickness variationsSurface heat flux and elastic thickness are controlled at large scales by surface temperature, and at small scales by crustal thicknessSurface temperature variations influence the CMB heat flux distribution, which we make readily available to test dynamo models [ABSTRACT FROM AUTHOR]

The relatively unconstrained internal structure of Venus is a missing piece in our understanding of the formation and evolution of the Solar System. Detection of seismic waves generated by venusquakes is crucial to determine the seismic structure of Venus' interior, as recently shown by the new seismic and geodetic constraints on Mars' interior obtained by the InSight mission. In the next decade multiple missions will fly to Venus to explore its tectonic and volcanic activity, but they will not be able to conclusively detect seismic waves, despite their potential to detect fault movements. Looking toward the next fleet of Venus missions after the ones already decided, various concepts to measure seismic waves have been proposed. These detection methods include typical geophysical ground sensors already deployed on Earth, the Moon, and Mars; pressure sensors on balloons; and imagers of high altitude emissions (airglow) on orbiters. The latter two methods target the detection of the infrasound signals generated by seismic waves and amplified during their upward propagation. Here, we provide a first comparison between the detection capabilities of these different measurement techniques and recent estimates of Venus' seismic activity. In addition, we discuss the performance requirements and measurement durations required to detect seismic waves with the various detection methods. Our study clearly presents the advantages and limitations of the different seismic wave detection techniques and can be used to drive the design of future mission concepts aiming to study the seismicity of Venus. Plain Language Summary: We do not really know what the interior of Venus looks like. Even the first‐order structure of the size of Venus' core is plagued with large uncertainties. For other planets, such as the Earth and Mars, the interior structure is much better constrained. This is largely thanks to the seismological investigations performed on these planets that revealed their interior structure by studying the seismic waves caused by quakes. In the next decade, new missions will fly to Venus to explore its tectonic and volcanic activity, which is interesting to estimate seismicity. But these missions will not be able to detect any seismic waves. In order to help design future mission concepts, we discuss instruments that could record seismic waves, as already used on the Earth, the Moon, and Mars; instruments on balloons that could float in the Venusian atmosphere; and instruments on spacecrafts that monitor the variations of atmospheric emissions caused by seismic waves originating at the surface. We compare all these different techniques with each other and with recent estimates of Venus' seismic activity to see which of them works best in different scenarios. Key Points: The capabilities of various measurement concepts to detect quakes on Venus are estimated and compared to recent Venus seismicity estimatesGround sensors are limited by their short measurement duration, but also by an atmosphere induced noise that may be above their self noiseAtmospheric seismology concepts are limited to large quake magnitudes, and airglow imagers are favored relative to balloon measurements [ABSTRACT FROM AUTHOR]

Icy bodies with subsurface oceans are a prime target for astrobiology investigations, with an increasing number of scientists participating in the planning, development, and realization of space missions to these worlds. Within Germany, the Ocean Worlds and Icy Moons working group of the German Astrobiology Society provides an invaluable platform for scientists and engineers from universities and other organizations with a passion for icy ocean worlds to share knowledge and start collaborations. We here present an overview about astrobiology research activities related to icy ocean worlds conducted either in Germany or in strong collaboration with scientists in Germany. With recent developments, Germany offers itself as a partner to contribute to icy ocean world missions. [ABSTRACT FROM AUTHOR]

There is a growing consensus that Venus is seismically active, although its level of seismicity could be very different from that of Earth due to the lack of plate tectonics. Here, we estimate upper and lower bounds on the expected annual seismicity of Venus by scaling the seismicity of the Earth. We consider different scaling factors for different tectonic settings and account for the lower seismogenic thickness of Venus. We find that 95–296 venusquakes equal to or bigger than moment magnitude (Mw) 4 per year are expected for an inactive Venus, where the global seismicity rate is assumed to be similar to that of continental intraplate seismicity on Earth. For the active Venus scenarios, we assume that the coronae, fold belts, and rifts of Venus are currently seismically active. This results in 1,161–3,609 venusquakes ≥Mw4 annually as a realistic lower bound and 5,715–17,773 venusquakes ≥Mw4 per year as a maximum upper bound for an active Venus. Plain Language Summary: Venus could be seismically active at the moment, but it is uncertain how many earthquakes (or to use the proper term: venusquakes) there could be in a year. Here, we calculate the minimum and maximum number of venusquakes we could expect in a given year on Venus based on different assumptions. If we assume there is not much seismic activity on Venus (comparable to the interior of tectonic plates on Earth), we find that we could expect about a few hundred venusquakes per year with a magnitude bigger than or equal to 4. For an estimate of the maximum amount of venusquakes, we assume that Venus has regions with more seismic activity: the so‐called coronae, fold belts, and rifts. Depending on our assumptions, we then find that more than 17,000 venusquakes with a magnitude bigger than or equal to 4 could occur in a year. Key Points: An inactive Venus with global background seismicity like Earth's continental intraplate seismicity has a few hundred quakes ≥Mw4 per yearA lower bound on an active Venus where fold belts, coronae, and rifts are seismically active predicts a few thousand quakes ≥Mw4 annuallyThe upper bound for an active Venus results in thousands (∼5,000–18,000) venusquakes ≥Mw4 per year [ABSTRACT FROM AUTHOR]

Most impact basins are believed to have formed during the early epochs of planetary evolution. The planet's gravity, internal structure, and thermal regime have the strongest control over their formation. Because of this, we can use the geophysical constraints on Mars' interior composition, structure, and geophysical evolution derived from the InSight mission to better understand the formation of impact basins on the planet. To achieve this, we performed numerical simulations of large impacts using the iSALE shock physics code. We investigated the effects of temperature and crustal thickness variations on impact basin size and morphology. Our scaling relationships indicate that: (a) basins formed in a warmer crust have larger final diameters in comparison to basins formed in a colder crust, a difference that is further accentuated as basin size gets bigger; and (b) the largest impact basins on Mars were created by impactors ranging from 35 to 680 km in diameter, up to ∼32% larger than estimates based on classical scaling. Our results expand the current understanding of the extent of early and large impact bombardment on Mars and provide a more comprehensive knowledge of impact basin formation on planetary surfaces. Plain Language Summary: The recent advancements in the understanding of Mars' that resulted from the InSight mission can be used to better understand the early large bombardment that took place 4.4 to 3.7 Ga ago. This bombardment formed impact basins, the largest and most complex type of craters. Their size and shape depend on the interior structure and temperature of the planet when they formed. We can better understand basins by simulating their formation and comparing the results with observations. Here, we used advanced interior structure and temperature evolution models of Mars to simulate the formation of impact basins as accurately as possible. We simulated basins of various sizes forming at different locations at multiple stages during Mars' evolution. Based on that, we derived equations referred to as scaling relationships that express the connection between basin size and impact conditions for different epochs and locations on Mars. We concluded that: (a) basins formed in a warmer crust are larger than basins formed in a colder crust, and (b) the largest reported basins on the planet were created by impactors much larger than previously thought. Our results provide a more comprehensive knowledge of impact basin formation and valuable insights into the early large bombardment. Key Points: We numerically simulated impact basin formation on Mars using recent internal structure and temperature modelsWe described how thermal evolution and crustal thickness variations could have affected impact basin formation on MarsWe derived new scaling relationships for impact basins and provided an insight into the early large bombardment on Mars [ABSTRACT FROM AUTHOR]