The SEIS seismometer is equipped with a solar compass to find geographic North | INSTITUT DE PHYSIQUE DU GLOBE DE PARIS


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  The SEIS seismometer is equipped with a solar compass to find geographic North

Despite the incredible complexity of the Mars probes deposited on the planet’s surface, finding the position of the geographic North Pole on the Red Planet is no easy task. To be able to use the SEIS seismometer with the required accuracy, seismologists need to know its orientation on the ground, after it was left by InSight's robotic arm on 19 December. But on Mars, it is impossible to use a conventional compass.


The SEIS seismometer sundial seen by the IDC camera of the robotic arm in sol 25, with the shadow of the gnomon on the dial. (© NASA/JPL)

Unlike Earth, the Red Planet no longer has a global magnetic field. In 1997, during its aerobraking manoeuvres, the American probe Mars Global Surveyor detected magnetic activity on Mars, but it turned out to be only fossil-related. Many areas of the oldest terrain on the planet, located in the southern hemisphere, do indeed retain traces of the presence of a global magnetic field, but this magnetisation is now only a shadow of what it was.


Analyses of the magnetic remanence printed in the Martian crust show that the Martian magnetic field effectively turned off about 4 billion years ago, without anyone knowing why. The disappearance of the magnetic shield that protected Mars from the harmful attacks of solar and cosmic bombardment is one of the great mysteries of the Red Planet, to which the InSight mission should be able to provide an answer.


On a more practical level, the absence of a global magnetic field on Mars is a major problem with regards to orientation. Of course, Mars also does not have a network of GPS satellites like those on Earth. Under these conditions, how can it be possible to find North? As paradoxical as it may seem, scientists will rely on an ancestral technique: the use of a sundial, converted into a compass.


The SEIS sundial

In ancient Babylon, humans already used the shadow cast by a stick planted in the ground to determine the time. Tens of centuries later, InSight will use this technique on the Red Planet. At the top of the hexagonal copper thermal protection box (RWEB) surrounding the SEIS seismometer is a dial, in the centre of which is the handle which allowed the instrument to be placed on the ground by the grabber of the robotic arm, and which also acts as a gnomon.


At the top of the RWEB thermal protection surrounding the SEIS seismometer, the handle also serves as a gnomon. The shadow cast by the rod on the dial will make it possible to know the direction and height of the Sun in the sky and to deduce the position of the geographic North of Mars (© IPGP/David Ducros)


The dial of the SEIS sundial consists of three zones. The outermost has 72 sectors each spaced 5° apart, covering 360°. The middle zone has the same number of sectors, but they are offset by 2.5°. Lastly, the innermost zone is also offset by 2.5° from the previous one. With a height of 28.7 mm, the SEIS gnomon has a particular shape (a small conical cylinder ending in a half-sphere), adapted to its primary function, providing the IDA robotic arm of the InSight probe with a handle for moving the seismometer, but which is less than ideal for reading a shadow. However, its shape has been carefully modified to improve its role as a gnomon. The entire solar compass was designed by David Mimoun (ISAE/SUPAERO) and Ken Hurst (JPL), then built by Nicholas Onufer and Michele Wallace (JPL).


The position of the gnomon's shadow, which can fall on the different areas of the dial depending on the time, determines the height and direction (azimuth) of the Sun in the Martian sky, and therefore the local Martian solar time. Although the InSight sundial is a very simple device, it can also provide other crucial information. Independently of the inclinometers of the levelling cradle, it can thus determine the angle of the seismometer with respect to the ground and, more importantly, its orientation with respect to geographic North (a data point that is essential for the interpretation of seismic signals).


The SEIS sundial will operate for a very limited time on Mars. It will effectively become unusable when the seismometer is covered by the wind and thermal shield (WTS).


The determination of Martian North

To accurately determine the position of the Red Planet’s geographic North, and thus the orientation of the SEIS seismometer, several images of the sundial will be obtained at specific times by the InSight robotic arm's IDC camera. One shot will be taken at noon, and another when the sun is lower on the horizon, with the shadows on the ground being longer. One of the world's leading sundial specialists, Denis Savoie of France, travelled specially to the Jet Propulsion Laboratory (JPL), NASA's centre responsible for the InSight mission in California, to analyse and interpret the data provided by the SEIS sundial.


Digital simulation of the SEIS seismometer dial and gnomon. We distinguish the three zones, each offset by 2.5° and composed of 72 sectors. The small circles at the bottom right and top left outside the dial allow the IDC camera images to be corrected for parallax effects. (© Marc Goutaudier)

The first step will consist in loading the images of the dial into specialised software, to correct them for parallax effects and to more precisely determine the direction of the gnomon’s shadow. At this stage, Denis Savoie needs several pieces of information to determine North: the precise latitudinal and longitudinal coordinates of the seismometer on Mars, as well as the exact date and time of the shots (recorded by a very precise clock on board the InSight probe).


Once entered into sophisticated calculation software, these data will make it possible to obtain a crucial parameter, the azimuth: the angle between the direction of the shadow on the gnomon and geographic North. A simple transfer to the dial of the sundial, where the direction of the shadow is already shown, will make it possible to locate North with an accuracy of between 1 and 2°. A second verification, totally independent of the first, is carried out simultaneously in Paris by Marc Goutaudier and Andy Richard (Universcience/Palais de la Découverte), who this time use ray tracing software. This allows makes it possible to very accurately visually reproduce the SEIS pattern and gnomon shadow based on numerous parameters, and to make a direct comparison with the images of the InSight IDC camera.