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

Clues to a planet’s geologic history are contained in its interior structure, particularly its core. We detected reflections of seismic waves from the core-mantle boundary of Mars using InSight seismic data and inverted these together with geodetic data to constrain the radius of the liquid metal core to 1830 ± 40 kilometers. The large core implies a martian mantle mineralogically similar to the terrestrial upper mantle and transition zone but differing from Earth by not having a bridgmanite-dominated lower mantle. We inferred a mean core density of 5.7 to 6.3 grams per cubic centimeter, which requires a substantial complement of light elements dissolved in the iron-nickel core. The seismic core shadow as seen from InSight’s location covers half the surface of Mars, including the majority of potentially active regions—e.g., Tharsis—possibly limiting the number of detectable marsquakes.

ol 185 was a typical sol on Mars (a Mars sol is 24 h 39.5 min long, and we number sols starting from landing). The ground acceleration spectrogram recorded by the very broadband (VBB) instrument of SEIS 1-3 (Seismic Experiment for Interior Structure; Fig. 1a) is dominated by the noise produced by the weakly turbulent night-time winds and by the powerful, thermally driven convective turbulence during the day 4. Around 17:00 local mean solar time (lmst), the wind fluctuations die out quite suddenly and the planet remains very quiet into the early night hours. Several distinctive features can be seen every sol on Mars. Lander vibrations activated by the wind appear as horizontal thin lines with frequency varying daily as a result of temperature variations of the lander; almost invisible during quiet hours, they are not excited by seismic events (for example, the lander mode at 4 Hz in Fig. 1a). We also observe a pronounced ambient resonance at 2.4 Hz, strongest on the vertical component, with no clear link to wind strength but excited by all the seismic vibrations at that frequency. The relative excitations of the 2.4 Hz and 4 Hz modes serve as discriminants for the origin of ground vibrations recorded by SEIS, allowing us to distinguish between local vibrations induced by atmospheric or lander activity and more distant sources of ground vibrations. On Sol 185, two weak events can also be spotted in the quiet hours of the early evening, one with a broadband frequency content and a second 80 min later, centred on the 2.4 Hz resonance band (Fig. 1a).

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Human activity causes vibrations that propagate into the ground as high-frequency seismic waves. Measures to mitigate the COVID-19 pandemic caused widespread changes in human activity, leading to a months-long reduction in seismic noise of up to 50%. The 2020 seismic noise quiet period is the longest and most prominent global anthropogenic seismic noise reduction on record. While the reduction is strongest at surface seismometers in populated areas, this seismic quiescence extends for many kilometers radially and hundreds of meters in depth. This provides an opportunity to detect subtle signals from subsurface seismic sources that would have been concealed in noisier times and to benchmark sources of anthropogenic noise. A strong correlation between seismic noise and independent measurements of human mobility suggests that seismology provides an absolute, real-time estimate of population dynamics.

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On November 26, 2018, NASA's InSight lander successfully touched down on the Martian surface in Elysium Planitia (Figure 1). The scientific goals of InSight are to determine the interior structure, composition, and thermal state of Mars, as well as to document the present-day seismicity and impact rate. To achieve these goals, InSight carried the seismometer package SEIS (Seismic Experiment for Interior Structure) to Mars including a very broadband (VBB) and short period (SP) instrument that cover the seismic bandwidth 0.01-5 Hz (Lognonné et al., 2019). These two instruments are used to locate and classify Marsquakes, to

The InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) mission began collecting high quality seismic data on Mars from February 2019. This manuscript documents the seismicity observed by SEIS, InSight’s seismometer, since this time until the end of March, 2020. Within the InSight project, the Marsquake Service (MQS) is responsible for prompt review of all seismic data collected by InSight, detection of events that are likely to be of seismic origin, and curation and release of seismic catalogues. In the first year of data collection, MQS have identified 465 seismic events that we interpret to be from regional and teleseismic marsquakes. Seismic events are grouped into 2 different event families: the low frequency family are dominated by energy at long period below 1 second, and the high frequency family primarily include energy at and above 2.4~Hz. Event magnitudes, from Mars-specific scales, range from 1.3 to 3.7. A third class of events with very short duration but high frequency bursts have been observed 712 times. These are likely associated with a local source driven by thermal stresses. This paper describes the data collected so far in the mission and the procedures under which MQS operates; summarises the content of the current MQS seismic catalogue; and presents the key features of the events we have observed so far, using the largest events as examples.