The Frenchman Philippe Lognonné is the origin of the main instrument of the exploration mission on mars: InSight from the Nasa, the seismometer SEIS. The same one that will probe the bowels of the red planet. A few hours of the landing of InSight on Mars, it explains what to expect during the next few hours, or even the next few weeks.

The Point : How will it unfold, on Earth, the landing of InSight on Mars ?

Philippe Lognonné : The landing of InSight will take place this evening around 21 hours (Paris time). A priori, about a quarter of an hour after, we should receive an image, rather rudimentary, transmitted by the intermediary of two small satellites called MarCO (March Cube One) and MarCO (B, acting as a radio relay. However, this is not completely certain since these small satellites, which are not included in the nominal scenario of the mission, the quality is a little lower. Thus, they can more easily fail, but, for the moment, they are in good health. In addition, we will have, throughout the descent a signal of type semaphore, that is to say, a kind of beep that will indicate that each critical step, from atmospheric entry to landing, happened nominally. If all goes well, towards 21: 30, we will have a good idea of what happened and maybe even an image. Then, a little later in the night, around midnight – an hour in the morning, other data, should happen, in particular after the deployment of the solar panels of the lander. Finally, the next morning, we will know definitively what happened.

Read also InSight or the motionless journey in the heart of March

Y a-t-is there a risk of rebound of the nose landing gear, as what we had been seeing for the robot Philae to the onslaught of the comet Rosetta ?

Yes, if the rétrofusées do not stop… In this case, when it’s going to happen on the ground, InSight will take off and will only stop when its fuel will be exhausted. But this is not quite as for Philae. Indeed, on the comet Rosetta, the big problem was that there was no gravity. On Mars, on the other hand, there is a gravity that is roughly one-third of the earth’s gravity. In summary, on March, one falls without too many problems whereas on Rosetta, it is not so simple to arrive by land. In contrast, on the red planet, it arrives with a speed very important and therefore it is necessary, in thirty kilometers, to achieve the curb to get to the ground at a speed near zero. A part of the braking is done with a shield, another with a parachute and, finally, the last one with rétrofusées. So you have a succession of braking systems and it is sufficient that only one of these steps will be a failure, or even just a half-failure, to ensure that the entire result is compromised. This is why it is never easy to land on Mars. After that, there are several ways of seeing things. The positive way, it is clear that Nasa has, in general, been quite successful these landings martians. There is only one time where this failed : it was the Mars Polar Lander in 1998. The way that is a little more worrying to see things, that is to say that the lander is the same as for Phoenix, which was successful, but also the same as for the Mars Polar Lander that failed. That said, there’s nothing better than a failure to find out why and to correct it…

Read also Auscultate March : the dream of a scientific French touch to his purpose

If all goes as planned tonight, what will be the next steps of the mission ?

After that, there’s going to be the deployment. The seismometer SEIS will remain on the bridge until around December 10. After which, it will be deployed on the ground. We should have first the data a little before Christmas. The week of December 17th, we will measure the direction of north with a small sundial, which is located at its front surface to cover the seismometer with its shield around the beginning of next year. Finally, SEIS will be permanently in operation, day and night, in the middle of the month of January.

how to know the interior of Mars may help us to understand its evolution ?

To make it simple, March is a big heat engine that derives its energy from two sources. The first is the accretion, the gravitational energy that it has absorbed during its formation. The second is the radioactivity. Since its birth, this machine heat cools down. We can compare this to the knowledge of a car engine. If you see the engine from the outside, the car will accelerate, slow down, but you will not understand why. In the same way, the internal structure of the planet is going to inform us about the characteristics of the heat engine in March. For example, the average thickness of the crust we will provide information on the insulation of the mantle generated by this crust, or even on the amount of radioactive elements that come from the mantle and that are found in this crust. This will give us, at the same time, constraints on the amount of radioactive materials that are still present in the mantle. About the size of a kernel, we investigate the energy of accretion. Once you have all of these characteristics, we can begin to better model the history of the thermal evolution of Mars. It should be understood that any martian mission will not be sufficient in itself. They have a sense of collectively. Up here, we had a lot of information on the structure, mineralogy, isotopic composition, on the amount of uranium in the crust, but we had no information on the deep structure to allow us to extrapolate this information from the surface to the interior. InSight and the seismometer SEIS should address.

But then how and what will you review in the seismic waves that SEIS will analyze ?

We will see basically two things. The first thing is time of arrival : the sismogramme is flat, and, all of a sudden, it starts to oscillate because you have the energy which is propagated and which arrives at the seismometer. This energy can be spread in several ways : in the form of the P waves (compression wave), S-waves (shear waves) or surface waves. From these distinct types of waves, because their time and their path of propagation are a little bit different, one is able to determine both the speed and the distance between the recording station and the source of the seismic wave. Here, for the first type of analysis. As for the second, if one has very large earthquakes, at this time, the planet will begin to vibrate, a little like a bell. Well, thanks to the sounds of the bell, one is able to deduce what type of bell is March.

On the same subject, InSight or the motionless journey in the heart of March

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