MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) is a low frequency, pulse-limited radar sounder and altimeter used on the ESA Mars Express mission. It features ground-penetrating radar capabilities, which uses synthetic aperture techniques and a secondary receiving antenna to isolate subsurface reflections.  Its capabilities complement the SHARAD radar on the Mars Reconnaissance Orbiter.
On May 4, 2005, Mars Express deployed the first of its two 20-metre-long radar booms for the MARSIS experiment. At first the boom didn't lock fully into place; however, exposing it to sunlight for a few minutes on May 10 fixed the glitch. The second 20 m boom was successfully deployed on June 14. Both 20 m booms were needed to create a 40 m dipole antenna for MARSIS to work; a less crucial 7-meter-long monopole antenna was deployed on June 17. The radar booms were originally scheduled to be deployed in April 2004, but this was delayed out of fear that the deployment could damage the spacecraft through a whiplash effect. Due to the delay it was decided to split the four week commissioning phase in two parts, with two weeks running up to July 4 and another two weeks in December 2005.
The deployment of the booms was a critical and highly complex task requiring effective inter-agency cooperation ESA, NASA, Industry and public Universities.
Nominal science observations began during July 2005.
A 2012 paper by the MARSIS team measured a difference between the dielectric constant of the northern and southern high-latitude regions. This is evidence that the material that fills the northern basin is a lower-density material, which could be interpreted as evidence of an ancient northern ocean.
- MARSIS instrument home page
- R. Orosei et al., "Science results from the MARSIS and SHARAD subsurface sounding radars on Mars and their relevance to radar sounding of icy moons in the Jovian system", EPSC2010-726, European Planetary Science Congress 2010, Vol. 5 (accessed Nov. 17 2014)
- Glitch strikes Mars Express's radar boom - space, New Scientist, May 9, 2005
- Mars Express's kinky radar straightened out - space, New Scientist, May 12, 2005
- ESA Portal, Mars Express radar ready to work
- Jérémie Mouginot, Antoine Pommerol, Pierre Beck, Wlodek Kofman and Stephen M. Clifford, "Dielectric map of the Martian northern hemisphere and the nature of plain filling materials," Geophysical Research Letters, 39, No. 2, 19 Jan 2012 (abstract)(article at UCI) accessed Nov. 17 2014
- Emily Lakdawalla, "Has Mars Express MARSIS data proved that Mars once had a northern ocean?" Planetary Society, February 7, 2012 (accessed Nov. 17 2014)
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