Scientific objectives

The ExoMars seismometer will study the seismic activity of the planet and frequency of meteorites impacts. These seismic events will be characterized by their approximate distance and azimuth, as well by their magnitude. This will allow characterization of the shallow and deep interior of the planet; target science objectives are to determine with SEIS:

• The water environment as a function of depth in the deep subsurface
• Low-viscosity zones such as shallow partial melt zones in the mantle
• The crustal thickness of the landing site
• The core size
• The mantle structure (possibly, if the seismic activity is between the middle and upper bound of present estimates)

Pressing issues that seismic exploration of Mars will address are the existence of geodynamically induced seismicity, the size of the core, and the depths of major phase transitions in the mantle. Determination of the present state of the outer core and characterization of the presence or absence of a solid inner core from seismic data would strongly constrain the evolution models and would have implications for the evolution models of the cores of the other terrestrial planets.

Marsquakes and impacts

Presently, geophysical research efforts on Mars are, by lack of suitable data, limited to the surface and shallow subsurface only. SEIS will provide a for the first time, unique and necessary data to extend research to the deep interior of Mars.

The SEIS seismometer instrument aims to register two types of seismic events: those induced by geophysical processes in the Martian interior and signals induced by meteoritic impacts. These Mars quakes give rise to the full range of seismic waves like on Earth: S-waves, P-waves, surface waves, etc., with associated wave phenomena like wave refraction and reflection at chemical boundaries and phase transitions. For large magnitudes seismic events, even the eigenvibrations of Mars may be excited (planetary oscillations), which the seismometer can record as a gravimetric signal at very low frequencies. Similarly, the seismometer will register the planet’s tidally induced deformation in response to the orbiting moon Phobos. This response is expected to be sensitive to the structure and P,T conditions of the transition zone halfway the depth of the mantle. Especially the existence of shallow low-viscosity zones might be inferred from these responses. The information in seismic signals can be mapped into a detailed map of the planet internal structure in principle only by deploying a network of seismic stations. A network is being considered as a follow-up of ExoMars, and the single station can be seen as the first necessary step towards a network. But even a single seismometer can provide unique information. By registration of the various types of signals and events, it will be possible to address the science objectives as specified above: the core size (and state), the crustal thickness of the landing site, the existence of shallow partial melt zones and the mantle structure.  Achieving these science objectives will allow to make great advances in the field of planetary research in many ways.

Comparative Planetology

If the seismometer will detect a genuine Marsquake, then this will be in itself a major front-page event. Less spectacular perhaps, but of equal significance will be the absence of such an event. In either case there are direct implications for planetary evolution models and theories. A precise measure of the core size and the presence or absence of a solid inner core bears a direct relation to the thermal evolution and overall density profile of the planet. Information of this kind is key to understanding the disappearance of the magnetic field in early history of Mars. The question why Mars differs so much tectonically from our planet Earth, a prime focus of comparative planetology in the Netherlands, can be answered only by more detailed structural data which SEIS plus its successors may provide. It is generally understood that study of the solar planetary system is required for a better understanding of the planet Earth in terms of its structure, state and evolution and even climate. Thermo-chemical evolution models for Mars are relevant in the context of comparative planetology and as such they can be related to early earth models. By advancing the knowledge of the internal structure of Mars, we will, by comparative planetology, obtain a better understanding of our Solar system and our home planet Earth.