Different materials are characterised by their ‘dielectric constant’, that is the specific way they interact with electromagnetic radiation, such as radio waves. When a radio wave crosses the boundary of different layers of material, an echo is generated and carries a sort of fingerprint from the specific materials. The MARSIS antenna booms here are seen receiving reflected signals. The red dotted line denotes the top of the ionosphere of Mars.

Credits: ESA (Animation by AOES Medialab)

Imagine yourself looking at Mars from space. If the whole planet were covered in darkness and your eyes could see the electrons in the ionosphere, this figure is what you would see. The brighter areas are areas of higher electron content.

Credits: ASI/NASA/ESA/Univ. of Rome 'La Sapienza'/Univ. of Iowa/JPL

The MARSIS experiment will map the Martian sub-surface structure to a depth of a few kilometres. The instrument's 40-metre long antenna booms will send low frequency radio waves towards the planet, which will be reflected from any surface they encounter.

Credits: ESA