The Mar­tian in­te­ri­or

Interior structure of Mars
In­te­ri­or struc­ture of Mars
Credit: adaptiert nach IPGP/David Ducros.

Interior structure of Mars

In­te­ri­or struc­ture of Mars – a thin at­mo­sphere cov­ers the crust that blan­kets the man­tle and core. Whether the core of Mars con­tains an in­ner sol­id and an out­er liq­uid lay­er (left) or it is en­tire­ly liq­uid (right) it is not known. Lat­est in­di­rect ob­ser­va­tions seem to favour the lat­ter ver­sion of the in­te­ri­or struc­ture.

The interior structure of a terrestrial planet is determined during the earliest stages of its evolution. Following the planet-forming process, referred to accretion, extensive melting caused by the large amount of heat present in the interior of the planet triggers the separation of metals and silicates. Metals, which are heavy elements, sink and accumulate near the centre of the planet – creating the core – while the silicates build up the rocky mantle. Subsequent melting further separates the mantle into a mantle and a crust, the latter being composed of lighter material that is enriched with heat producing, radioactive elements. This process is called differentiation. Also, during mantle melting, volatiles that were incorporated into the mantle during accretion are released through volcanic eruptions into the atmosphere and can later escape to space or, depending on the pressure and temperature conditions, condense as liquids on the surface.

At the end of the differentiation stage, a terrestrial planet is composed of an iron-rich core, a silicate mantle covered by a thin silicate crust, possibly a water layer on the surface and an atmosphere that builds up from the gases released by the magmatic processes that are a consequence of the heat production in the mantle. While the iron core and silicate mantle separate early during the planet’s history, the crust and atmosphere as we see them today are the result of billions of years of planetary evolution. As the planet cools, the initially liquid iron-rich core can start to solidify and further differentiate, building an inner solid and outer liquid layer.

Like the Earth, Mars is composed of an iron-rich core and a silicate mantle, a thin silicate crust and an atmosphere. In contrast to the Earth, however, Mars' atmosphere is too thin and the pressure therefore too low to allow for the presence of liquid water at the surface today; water would boil and evaporate immediately when poured on the surface. But the situation was different in the past. The thickness of the crust can be estimated using gravity and topography data. The slight variations in the gravitational field experienced by orbiters flying around Mars combined with topographical data obtained by the Mars Orbiter Laser Altimeter instrument on board Mars Global Surveyor can be used to derive crustal thickness maps. However, the results are not unique and depend on the density contrast between the crust and the mantle. Further layering of the crust and mantle may exist, but in the absence of seismic measurements such details cannot be resolved.

The pressure-temperature conditions in the Martian mantle may cause changes and reorientation of the crystalline structure of minerals (referred to as phase transitions), which affects the physical properties of minerals. On Earth, such phase transitions are indicated by sudden increases in density and seismic wave velocities observed at 410, 520 and 660 kilometres depth in the planet’s mantle. The difference in density between mineral phases provides buoyancy forces additional to the ones generated by temperature variations and may affect material and heat transport within the mantle. Geophysical models predict two or three phase transitions in the interior of Mars, but the number is dependent on the pressure range available in the Martian mantle and hence on the size of the Martian core. The size of the core is not well known, but changes in the orbital dynamics of satellites around Mars induced by periodic deformations of the planet, which are caused by the gravitational forcing of the Sun and the other planets of the Solar System, seem to indicate a large, possibly entirely liquid core today.

The upcoming in-situ seismic measurements planned with the InSight mission will provide an important baseline for understanding the interior structure of Mars and this will, in turn, help constrain the formation scenarios of Mars and other terrestrial planets.

Contact
  • Elke Heinemann
    Ger­man Aerospace Cen­ter (DLR)
    Ger­man Space Agen­cy at DLR
    Strat­e­gy and Com­mu­ni­ca­tions
    Telephone: +49 2203 601-2867
    Königswinterer Straße 522-524
    53227 Bonn
    Contact
  • Prof.Dr. Tilman Spohn
    HP³ Prin­ci­pal In­ves­ti­ga­tor
    Ger­man Aerospace Cen­ter (DLR)

    DLR In­sti­tute of Plan­e­tary Re­search
    Telephone: +49 30 67055-300
    Fax: +49 30 67055-303
    Linder Höhe
    51147 Köln
    Contact
  • Ana-Catalina Plesa
    HP³ project sci­en­tist of the In­Sight sci­ence team, plan­e­tary geo­physics and plan­e­tary mod­elling
    Ger­man Aerospace Cen­ter (DLR)
    In­sti­tute of Plan­e­tary Re­search
    Plan­e­tary Physics
    Rutherfordstraße 2
    12489 Berlin
    Contact
  • Ulrich Köhler
    Ger­man Aerospace Cen­ter (DLR)
    In­sti­tute of Plan­e­tary Re­search
    Rutherfordstraße 2
    12489 Berlin
    Contact

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