How are Earth-like planets structured, how were they formed, and what happened during their early development? The NASA InSight lander will use a robotic penetrometer to study the structure of the crust, mantle and core of Mars, together with the planet’s thermal development. Investigating the interior of our neighbouring planet should help to answer fundamental questions about the early period of the inner Solar System and the planets Mercury, Venus, Earth and Mars, which formed over 4.5 billion years ago in the hot solar nebula, but have all developed differently. The results from InSight should also allow a better understanding of the formation, development and structure of extrasolar planets – planets orbiting other stars. InSight will measure the heat flow near the surface of Mars using the Heat Flow and Physical Properties Probe (HP3), to which Germany has contributed, using a five-metre-deep borehole, while the French Seismic Experiment for Interior Structure (SEIS) will record seismic waves arising from tectonic activity and caused by meteorite impacts.
The latest information on Mars suggests that the agglomeration of most planets in the rotating primeval solar nebula occurred a few million years after the formation of the Sun. After the Solar System formed and the eight planets began orbiting the Sun as almost ‘finished’ spherical bodies, much more energy was generated within these bodies through accretion, and the heat-generating decay of radioactive elements than is the case today. The Earth-like planets were largely molten, and the heat generated in their centre was transported to the surface via the circulation of a mixture of iron and rock, and partially emitted into space from there. Over the course of a few million years, the constituents separated into an iron-rich core – a thick, possibly partially solidified silicate mantle and a rocky crust, formed by the cooling of the planetary sphere. This process is called differentiation.
Even today, the material heated by Earth's molten outer iron core and radioactive decay in the deep mantle is circulated, forming bubbles of molten rock in the places where it comes to the surface. This melting penetrates through the crust to the surface, feeding active volcanoes with magma. In the interior of Mars – a smaller planet – the scale of such upheavals is less dramatic, and the process slowed down significantly in the first billion years. This means that the 'fingerprints' of the processes that formed the core, mantle and crust of Earth-like planets may have been preserved on Mars to this day. By studying the current thermal and seismic conditions on Mars through thermal flow measurements (in other words, the rate at which the planet transfers its heat to the surface, and from there into the atmosphere and space), it is possible to determine the current structure of the planet and understand the processes that formed Mars in its first hundred million years. Putting this together with what is known about the average chemical and mineralogical composition of the mantle and core gives an insight into the structure and development of Earth-like planets.
Molten or frozen core
InSight is intended to provide indications of what happened in the first few million years after the formation of the planets and their cores, mantles and crusts, and how the Earth-like planets developed and became different from one another. Such measurements should allow the determination of the size, composition and state of the core. It is believed that Mars' core consists predominantly of iron, like that of Earth, but that it contains significantly more sulphur and other light elements. It is not clear whether and how much of the core is molten. Sulphur and other light elements lower the melting point, which may explain why the core of Mars, a relatively small planet, may still be at least partially molten. If it is already completely 'frozen' and solid, this may simply explain why Mars no longer has a magnetic field, which would protect life on the surface from cosmic radiation and the solar wind. There are a number of other possible explanations for this, including a completely liquid core, as only an entirely liquid core would fully explain why Mars no longer has a magnetic field today.
DLR scientists developed the HP3 experiment for the InSight mission to answer many of the questions surrounding this issue. The experiment allows the heat flow from the interior of the planet to its surface to be measured at a depth of up to five metres. From a depth of around three metres, this heat signal is unaffected by the seasonal fluctuations that can have an impact from the surface down. Interpreting the data will lead to conclusions about the generation of heat in the interior of Mars and its rate of cooling. This, in turn, will provide information about the composition of the planet and its activity. In addition, HP3 is intended to capture the geological stratification in the first five metres beneath the surface of Mars by measuring the thermomechanical properties of the soil. DLR scientists will also be involved in evaluating data from the seismometer experiment. The DLR Institute of Planetary Research developed an often-cited model of the strength and geographical distribution of marsquakes in 2006. This has been expanded and updated in 2018, and will be compared with the results from the SEIS experiment from 2019 onwards.