Belgian Instrument : LaRa


Mars

Exploring Mars

Exploring the Mars's moons, Phobos and Deimos

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ExoMars : LaRA

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mars express_LaRA at ROB
      Instrument : LaRa ( 
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mars project
The radio science experiment LaRa

LaRa (Lander Radioscience) is an X-band transponder to be put on landers. The coordinator of the Team Coordinator/Principal Scientist is Veronique Dehant, researcher at the ROB.

The two main goals of this experiment are to obtain information on the interior of Mars in order to gain a better understanding of the formation and evolution of Mars on one hand, and to improve our knowledge about process of condensation/sublimation of the CO2 ice caps in order to gain a better understanding of the movement and dynamics of the atmosphere of Mars on the other hand.

LaRa is an instrument using a radio link between the Earth and Mars. A radio signal is sent from Earth to Mars and LaRa sends it coherently back to Earth as a mirror would do. On Earth, the researchers measure the frequency shift between the transmitted signal and the received signal. This shift is due to the Doppler effect induced by the relative speeds of the Earth and Mars. It occurs when the issuer of the signal is moving relative to the receiver of these signals, or vice versa, or a combination of both. The frequencies measured at the transmission and at the reception are thus slightly different. This difference will allow us to obtain the orientation of Mars in space and its rotation (precession, nutation and variations of the length-of-day).

Current knowledge of the internal structure of Mars has been developed mainly on three parameters: the total mass, the size and the moment of inertia of the planet. The gravitational field felt by any body around Mars also depends on the mass distribution within the planet. The observation of the position of an artificial satellite in orbit around Mars gives information on the mass distribution (the moments of inertia) in the planet. The study of the rotation of Mars should thus complete the information on the inside of this planet.


L'intérieur de Mars

LaRa will allow the researchers to determine the direction and variation of the rotation of Mars over time. One will then be able to accurately determine the changes in speed (and thus the length-of-day) and the orientation of the planet in space. From those data, we will infer some very interesting parameters concerning the internal structure of Mars (physical properties, density and dimensions of the core for example) and obtain the variations of mass and pressure in the atmosphere relative to the seasonal sublimation / condensation of polar caps, as explained in the following paragraphs.



Precession and nutation of Mars
Similar to the Earth, the position of the axis of rotation of Mars varies over time due to the gravitational pull of the Sun, the natural satellites Phobos and Deimos and the surrounding planets. Because of the existence of an equatorial bulge (like the Earth, Mars is flattened at the poles), the attraction of the Sun tend to rock the equator in the plane of the orbit of Mars (ecliptic). As Mars is in rotation on itself, it reacts to this forcing like a gyroscop and its axis of rotation describes a broad cone around the perpendicular to the plane of the ecliptic: this movement is called precession.
La précession et la nutation de Mars


LaRa will allow us to calculate the precession with a precision four times higher than presently known for the nominal duration of the mission (120 days on the surface of Mars). With this, one can calculate the moment of inertia of the entire planet and the radius of the core with accuracy up to a few tens of kilometers.

The precession is disturbed by a series of periodic variations, called nutations, due to the relative positions of Mars, the Sun, Phobos, Deimos and the nearby planets periodically changing over time. The fact that Mars is deformable and that it could contain a liquid core induces additional disruptive effects, in particular a resonance in nutations forced by the Sun. According to their shape, structures and constitution, some objects present resonances at certain frequencies (like a bell); a very small excitation can give rise to very large movements. It is for a similar reason that soldiers must break their steps while passing over a bridge. To observe a mode of vibration, one must have induced an excitation at a frequency very close to the natural frequency. The resonance in nutations does only exist if the core of Mars is liquid. Therefore, if the data collected by LaRa show the existence of this resonance, we have the certainty that the kernel is at least partially liquid. Otherwise, the kernel must be completely solid. The same reasons distinguish a raw egg to an egg cooked by the way it turns on itself.


La nutation de Mars pour un noyau liquide ou solide
La nutation de Mars pour un noyau liquide ou solide
The measure of variations in the speed of rotation of Mars (length-of-day) will allow us to calculate variations in the angular momentum of the planet. It changes regularly with the mass transfer between atmosphere and ice poles (by angular momentum conservation between the solid planet an the atmosphere). Knowing the variations of angular momentum, we will determine the amount of mass transferred. This exchange of matter is responsible for changes in rotation speed and therefore also for changes in length-of-day. A quarter of the Martian atmosphere is involved in the phenomenon of condensation/sublimation of CO2 in the polar caps with the seasons. Therefore, knowledge of changes in length-of-day will improve our knowledge about this phenomenon and at the same time those on the circulation and dynamics of the Martian atmosphere. In conclusion, the precession and nutation will be studied to improve our knowledge about the inside of Mars, while the change in length-of-day will be studied to clarify our questions about the Martian atmosphere.
Precession and nutation of Mars

To visualize in 3D the motion, rotation and orientation of Mars in space, click here.


Design and construction of LaRa

The design and construction of LaRa are made by the OMP company (Orban Microwave Products) residing in Leuven in Belgium (their website). The space quality tests in Martian environment, vibration tests necessary to prove the resistance of LaRa to launching and landing on Mars, and the decontamination are done by the Space Center of Liege (SCL).



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