The German Aerospace Center (DLR) is the national aeronautics and space research centre of the Federal Republic of Germany. SEIS is protected from the effects of the Martian wind and the dust transported with it by a hemispherical dome consisting of several separate layers. These covers compensate for the temperature differences so that the instrument has stable measurement conditions. Because materials expand when warm and contract when cold, SEIS is equipped with a sophisticated thermal protection system in the form of several insulated shells – comparable to a ‘Matryoshka doll’. The biggest problem for reliable measurements on Mars is the large temperature differences between day and night and between summer and winter. The heart of the SEIS experiment consists of two sets of three extremely sensitive pendulums that register even the smallest movements of the Martian surface. The instrument was developed under the leadership of the French space agency CNES.
The Seismic Experiment for Interior Structure (SEIS) instrument is a seismometer for measuring movements in the Martian soil at different frequencies and consists of a total of six sensors. Since the weather becomes noticeably calmer after sunset, the first half of the night is the best time window for recording distant marsquakes, because practically no wind interferes with the measurements being performed by the ultra-sensitive experiment. The different symbols show the various types of marsquakes, which have different signal frequencies. The innermost circle is Sol 72, when the seismometer began to record continuously. The distance from the centre indicates the number of sols since InSight's landing. The slightly curved orange line indicates the times of sunrise and sunset, which vary slightly throughout the year. Midnight is at the top, followed clockwise by morning with sunrise, noon and evening with sunset. The graphic shows a 24-hour clock with Martian hours, which are therefore slightly longer than terrestrial hours. Further down, there is a several-metre-thick regolith of finely fragmented crustal rock and finally fragmented bedrock reaching deep underground.Ī day on Mars, a 'sol', lasts 24 hours and 37 minutes, almost the same length as a day on Earth. Beneath what is referred to as the 'duricrust' (from Latin 'durus', meaning hard, and 'crusta', meaning shell or crust), there is a comparatively firm crust consisting of a kind of 'cemented' sand and roughly comparable to the firm, caramelised sugar crust of a crème brûlée. A model of the soil properties has been developed using the propagation times of marsquake waves and the signals generated by the 'Mole' when it was hammering into the ground with the Heat Flow and Physical Properties Package (HP 3) geothermal measurement system, as well as the many measurements performed with the Auxiliary Payload Sensor Suite (APSS) – consisting of a barometer, an anemometer, a magnetometer and two cameras), the HP 3 radiometer and the Rotation and Interior Structure Experiment (RISE). The ground at the InSight landing site consists of three different layers and materials with different properties. This is a modelled elliptical surface of equal gravitational attraction, which is used on Mars as a ‘zero’ level in the absence of sea level.
The topographical map is based on laser altitude measurements performed by NASA’s Mars Global Surveyor spacecraft (1999-2006) and shows height differences from approximately minus 3000 metres (blue-green) to plus 7000 metres (summit of Elysium Mons), related to a reference surface referred to as an areoid. The marsquakes occurred in the Cerberus Fossae region, a young volcanic area about 1700 kilometres east of the InSight landing site located in Elysium Planitia. With the help of models of the propagation of seismic waves in the Martian subsurface, the probable source of two larger quakes (s0235b and s0173a) could be determined quite accurately, and of that another quake (s0183a), which produced fewer clear signals, with somewhat reduced accuracy. The SEIS seismometer on NASA’s InSight lander recorded a total of 174 low-intensity marsquakes between February and the end of September 2019.
InSight locates marsquakes in the Cerberus Fossae region