MARSIS is a Mars orbiter mission by the European Space Agency (ESA) with participation from NASA. The acronym stands for “Mars Advanced Radar for Subsurface and Ionosphere Sounding”. The payload consists of a low-frequency radar sounder to probe the Martian subsurface up to a depth of several kilometers, and an ionospheric sounder to study the upper atmosphere.
The spacecraft was launched on 2 June 2003 aboard a Soyuz-Fregat rocket from Baikonur Cosmodrome in Kazakhstan, and arrived at Mars on 12 December 2003. After entering orbit, it deployed its 20 m long antenna boom and began science operations on 19 March 2004. The primary mission ended on 30 September 2005, but data collection continued during the extended mission until 28 May 2007. A second extended mission ran from 6 April 2008 until 30 September 2009. Data from MARSIS is helping scientists understand the structure of Mars’ deep interior, how water has interacted with rocks over time, and the processes that shape Mars’ ionosphere today.
MARSIS was designed to operate in two modes: Active mode emits pulses of radio waves towards Mars and measures the reflection coefficient as a function of frequency; Passive mode listens for natural radio emissions coming from space or Mars itself. In order to image the subsurface, active mode uses synthetic aperture radar techniques. To image the ionosphere in passive mode, MARSIS listens for spectral line emissions at altitudes where ions are present (typically 80–120 km). By measuring how these emissions vary with altitude, MARSIS can construct vertical profiles of electron density as a function of altitude. This technique is known as sounding.
The MARSIS instrument suite includes two dual-frequency antennas (one for each polarization), a low-frequency radiometer, an HF receiver/exciter, digital electronics units, and power supplies. The low-frequency antenna is used for both active and passive imaging at frequencies between 1 MHz and 3 MHz. The high-frequency antenna is used primarily for passive sounding at frequencies between 5 MHz and 10 MHz; however, it can also be used for active imaging at X band (8 GHz). Both antennas are steerable over ±60 degrees in elevation with respect to nadir (the direction perpendicular to Mars’ surface), allowing them to focus their beams onto specific regions of interest on Mars.
The radiometer measures thermal noise emitted by Earth’s atmosphere and planets like Jupiter which might interfere with measurements made by MARSIS; this information is used by algorithms that filter out Noise sources other than those coming from Mar’s surface or ionosphere . The HF receiver/exciter downconverts received signals so they can be processed by digital electronics units; it also generates high-power RF signals that are transmitted through one of the antennas during active operations . Power supplies provide electrical power to all subsystems; solar cells mounted on the spacecraft body provide most of this power , but batteries are also used during eclipses when solar cell output drops sharply