T he Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) mission landed on Mars on 26 November 2018 in Elysium Planitia 1,2 , 38 years after the end of Viking 2 lander operations. At the time, Viking's seismometer 3 did not succeed in making any convincing Marsquake detections, due to its on-deck installation and high wind sensitivity. InSight therefore provides the first direct geophysical in situ investigations of Mars's interior structure by seismology 1,4. The Seismic Experiment for Interior Structure (SEIS) 5 monitors the ground acceleration with six axes: three Very Broad Band (VBB) oblique axes, sensitive to frequencies from tidal up to 10 Hz, and one vertical and two horizontal Short Period (SP) axes, covering frequencies from ~0.1 Hz to 50 Hz. SEIS is complemented by the APSS experiment 6 (InSight Auxiliary Payload Sensor Suite), which includes pressure and TWINS (Temperature and Winds for InSight) sensors and a magnetometer. These sensors monitor the atmospheric sources of seismic noise and signals 7. After seven sols (Martian days) of SP on-deck operation, with seismic noise comparable to that of Viking 3 , InSight's robotic arm 8 placed SEIS on the ground 22 sols after landing, at a location selected through analysis of InSight's imaging data 9. After levelling and noise assessment, the Wind and Thermal Shield was deployed on sol 66 (2 February 2019). A few days later, all six axes started continuous seismic recording, at 20 samples per second (sps) for VBBs and 100 sps for SPs. After onboard decimation, continuous records at rates from 2 to 20 sps and event records 5 at 100 sps are transmitted. Several layers of thermal protection and very low self-noise enable the SEIS VBB sensors to record the daily variation of the

A planet’s crust bears witness to the history of planetary formation and evolution, but for Mars, no absolute measurement of crustal thickness has been available. Here, we determine the structure of the crust beneath the InSight landing site on Mars using both marsquake recordings and the ambient wavefield. By analyzing seismic phases that are reflected and converted at subsurface interfaces, we find that the observations are consistent with models with at least two and possibly three interfaces. If the second interface is the boundary of the crust, the thickness is 20 ± 5 kilometers, whereas if the third interface is the boundary, the thickness is 39 ± 8 kilometers. Global maps of gravity and topography allow extrapolation of this point measurement to the whole planet, showing that the average thickness of the martian crust lies between 24 and 72 kilometers. Independent bulk composition and geodynamic constraints show that the thicker model is consistent with the abundances of crustal heat-producing elements observed for the shallow surface, whereas the thinner model requires greater concentration at depth.

The oceanic lithosphere is a primary component of the plate tectonic system, yet its evolution and its asthenospheric interaction have rarely been quantified by in situ imaging at slow spreading systems. We use Rayleigh wave tomography from noise and teleseismic surface waves to image the shear wave velocity structure of the oceanic lithosphere‐asthenosphere system from 0 to 80 My at the equatorial Mid‐Atlantic Ridge using data from the Passive Imaging of the Lithosphere‐Asthenosphere Boundary (PI‐LAB) experiment. We observe fast lithosphere (VSV > 4.4 km/s) that thickens from 20–30 km near the ridge axis to ~70 km at seafloor >60 My. We observe several punctuated slow velocity anomalies (VSV < 4.1 km/s) in the asthenosphere between 50 and 150 km depth, not necessarily focused beneath the ridge axis. Some of the slow velocity regions are located within 100 km of the ridge axis, but other slow velocity regions are observed at distances > 400 km from the ridge. We observe a high velocity lithospheric downwelling drip beneath 30 My seafloor that extends to 80–130 km depth. The asthenospheric slow velocities likely require partial melt. Although melt is present off axis, the lack of off‐axis volcanism suggests the lithosphere acts as a permeability boundary for deeper melts. The punctuated and off‐axis character of the asthenospheric anomalies and lithospheric drip suggests small‐scale convection is active at a range of seafloor ages. Small‐scale convection and/or more complex mantle flow may be aided by the presence of large offset fracture zones and/or the presence of melt and its associated low‐viscosities and enhanced buoyancies.

Oceanic lithosphere provides an ideal location to decipher the nature of the lithosphere‐asthenosphere system which is vital to our understanding of plate tectonics. It is well established that oceanic lithosphere cools, thickens, and subsides as it ages according to the conductive cooling models. Yet this simple realization fails to explain various observations. For example, old oceanic lithosphere does not subside as predicted. Further, precise imaging of the lower boundary of the oceanic lithosphere has proven challenging. Here we use SS precursors to image the discontinuity structure across the Pacific Ocean using 24 years of teleseismic data. We image a sharp pervasive velocity discontinuity (3–15% drop over <21 km) at 30–80 km depth that increases in depth with age from the ridge to at least 36 ± 9 Myr along the 1100°C conductive cooling isotherm. Beneath seafloor >36 Myr, there is no age‐depth dependence, and we image the discontinuity at an average depth of 60 ± 1.5 km. The amplitude and sharpness of the boundary suggests that a compositional variation and/or layered carbonatitic melt may be required to explain our observations rather than temperature alone. The strength and pervasiveness of the boundary suggest that it is likely related to the lithosphere‐asthenosphere boundary. An additional deeper discontinuity at 80–120 km depth is imaged intermittently that in most cases likely represents a continuing negative velocity gradient in depth.

Scattered waves image discontinuities beneath Cascadia that are likely caused by melt beneath the young oceanic plate.