Sci­en­tif­ic ob­jec­tives

How are Earth-like planets structured, how were they formed, and what happened during their early development? The NASA InSight lander used 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 helps 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 also allow a better understanding of the formation, development and structure of extrasolar planets – planets orbiting other stars. InSight measureed the heat flow near the surface of Mars using the Heat Flow and Physical Properties Probe (HP³), to which Germany has contributed, while the French Seismic Experiment for Interior Structure (SEIS) recorded seismic waves arising from tectonic activity and caused by meteorite impacts.

Why Mars?

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 provided 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 allow the determination of the size, composition and state of the core.

DLR scientists developed the HP³ experiment for the InSight mission to answer many of the questions surrounding this issue. The experiment allowed the heat flow from the interior of the planet to its surface to be measured. Interpreting the data led to conclusions about the generation of heat in the interior of Mars and its rate of cooling. This, in turn, provided information about the composition of the planet and its activity.

Unfortunately, the Mars ‘Mole' probe of DLR’s HP³ did not fully deliver the expected measurements, because the heat flow probe could not successfully penetrate to the required depths beneath the Martian surface. Originally, the Mole was supposed to penetrate to a depth of five metres and drag a tether with temperature sensors behind it. The self-hammering probe, capable of penetrating through the familiar loose, sandy soil of other missions, was unable to find a foothold in the unexpectedly hard soil around InSight’s surroundings. The instrument was eventually able to bury its 40-centimetre probe just below the surface, collecting some valuable data on the mechanical and thermal properties of the Martian soil. The fact that the Mole was finally able to burrow in is thanks to a team effort by engineers from JPL and DLR. They used the InSight lander's robotic arm in a creative way to give the Mole additional support.