X-band is a range of frequencies around 8 GHz.

The red planet possesses two caps which have the appearance of glaciers. There is a seasonal transport of CO2 vapor between the two caps forming the upper part of the caps in the winter. There are as well permanent caps of water ice. Water cannot exist in liquid form on the surface of Mars due to the too low temperature and pressure. Water does thus only exist as a gas or a solid. This transformation from solid to gas without passing through the liquid state is called sublimation and transformation of the gaseous state to solid state is called condensation. With the seasons, this sublimation and condensation arise for the CO2, increasing the caps.

The High Resolution Stereo Camera (HRSC) is one of the instruments on board Mars Express. Its role is to take pictures of the topography of Mars in three dimensions with a resolution of 10 meters or better, as well as pictures in preselected areas, with a resolution of two meters or better. The primary objectives of this instrument is to study the role of water and climate in the evolution of Mars, the temporal evolution of volcanism and tectonics, the interactions surface/atmosphere and establish a precise chronology of these events. The camera also takes pictures of the two small moons of Mars, Phobos and Deimos, to infer their volume.

Gravity is one of the four known elementary forces: electromagnetism, strong nuclear force, weak nuclear force and gravity. It is responsible for the mutual attraction exerted between two bodies of a certain mass. One prefers to say gravitational field rather than force of gravity acting between two bodies. The gravitational field extends around its source (e.g. the Earth) and any object in this field will be accelerated. It may vary over time.

The remanent magnetic field is the trace of a fossil magnetic field that existed before.

The Doppler effect is a shift in frequency of a sound or electromagnetic wave. It occurs when the signal transmitter is moving relative to the receiver of this signal, or vice versa, or a combination of both. The frequencies measured at the transmission and at the reception are thus slightly different. An illustration of the Doppler effect often suggested is the case where an ambulance passes close to someone. This person will hear sounds slightly different (more high- or low-pitched) when the ambulance comes and when it moves away. Scientists use the Doppler effect by sending signals from Earth to the spacecraft (such as Mars Express, for example through a radio-science experiment). The measurement of frequency shifts enables them to locate the spacecraft and to know how fast it moves.

Sunlight passes through the Earth's atmosphere, is absorbed by its surface and then reissues infrared radiation. The atmosphere prevents these rays from returning to space, preventing the planet from cooling. It plays the role of a greenhouse for the planet, hence the name given to this phenomenon.

A phenomenon due to an internal cause is endogenous.

A phenomenon due to an external cause is exogenous.

The moment of inertia of a system rotating around an axis p quantifies the strength of this system to a change in its speed of rotation around this axis p. It characterizes the mass distribution around an axis. The main moment of inertia depends on the mass distribution in a body around the main axis of symmetry. The more a mass is far away from this axis, the more it contributes to the main moment of inertia. The higher the moment of inertia, the larger the force required to turn in rotation. The observation of the rotation of a body allows to deduce the main moments of inertia. Knowledge of the main moments of inertia of a body allows to infer information about the mass distribution within the body.

Nutation is the periodic movement in space of the axis of rotation around its average position, nutation is added to precession. Permafrost is a frozen sub-surface/soil for a period of at least two years.

For several reasons, Mars is a very interesting planet to understand the differences in the formation of planets and their evolution. The surfaces of Mars have been acquired throughout its history, unlike the Earth which is in perpetual "renovation."

On Earth, older grounds are destroyed by tectonic movement and erosion, leaving room for new soil. The absence of tectonics on the surface of Mars, allow us to follow the cycles of transformation that the planet has undergone during its evolution. Only the volcanic past may have strongly altered the surface of Mars.

The study of the internal structure of the Red Planet brings us plenty of information on the evolution of planets. One might think that Mars has evolved much faster, because of its size and mass much smaller than the Earth.
The distance of the planet relative to the Sun is also a crucial element that determines the composition of the planet and its insulation (which is linked to the atmosphere) and therefore its thermal history. As the Earth is much more massive, it experienced a slower cooling and remains geologically active longer.
Furthermore, it is for the moment impossible to determine whether Mars has a liquid core and a solid seed as the Earth do. In spite of similar evolutions, the two planets have experienced very different fates. Can we believe that Mars reflects the future of the Earth, in the long term, when all its heat will be lost? This is, among other questions, one point that we want to lighten by studying the interior of the planet Mars. Obviously, a comparative study between the internal structure of Mars and the Earth will be very rewarding.

The precession is a change of direction of the axis of rotation planet under the action of the gravitational force of the Sun, its moons and the other planets.

This is an experiment based on the radio link between the orbiter and the Earth named MaRS (Mars Radio-Science experiment). It will, among other things, measure the gravitational field of Mars by making Doppler and ranging measurements along the line of sight. Scientists will then characterize the mass distribution within the planet. From the measurements of the global gravitational field of Mars and its temporal variations, it is also possible to characterize Mars deep interior. The experience needs no scientific instrumentation, it is based on the radio signal between the spacecraft and the Earth, sent from the Earth and observed with large antennas such as that of Perth (New Norcia, Australia) that of Madrid (Spain) or those of the U.S. network DSN (Deep Space Network located in Goldstone (California), Madrid (Spain) and Canberra (Australia). This network of antennas is used to monitor spacecrafts in orbit around planets of the solar system. The Doppler measurements are used to reconstruct the trajectory of these probes for the purpose of navigation, in order to plan the mission or for scientific purposes for the determination of geophysical parameters such as Mars' gravity field.

DSN network (credit : NASA)

When light rays from the Sun reach a celestial object, one of the moons of Mars for example, some radiation is reflected and form spectral lines which are characteristic for the chemical constituents of the object. A transponder is a device that receives a signal and transmits at different or identical frequencies.