Since its landing on Mars in 2018, the SEIS seismometer, carried out under the direction of the IPGP, as part of NASA’s Insight mission, has been providing unprecedented information on the internal activity that drives the red planet. But in order to be complete, it was essential to know how to determine the orientation of the tremors.
During the preparation of this mission, the problem of determining the Martian geographic north was quickly raised.
Two solutions were retained :
- The first one before landing : it consisted in using the gyroscope of InSight which worked during the descent phase of the lander in the atmosphere of Mars, in order to land the probe in the right direction. But once on the ground, the instrument stopped working.
- The second, after the landing : it was a question of using a projected shadow and from there, to deduce, with a precision if possible lower than 5°, where the geographic North was located, i.e. the direction of the rotation axis of the planet.
In this second solution, we rely on the principle of using the shadow of a gnomon, in other words a vertical stick planted on a horizontal ground.
A thousand-year-old method
This method goes back to the Babylonian astronomers who, more than 4000 years ago, started to determine the first constants of astronomy. They must have noticed very early that the shadow of a gnomon, infinite at sunrise, shortened in the morning to be the shortest possible at solar noon when the Sun peaked : the shadow then indicates either North or South depending on the place where we are. Then the shadow lengthens in the afternoon and becomes infinite again at sunset.
On Mars, the same principle was used but without being limited to the moment of culmination of the Sun :
Knowing the Martian geographical coordinates of the seismometer and having developed a theory which gives the position of the Sun on the planet according to the hour, we took a succession of images of the shadow during the day thanks to the camera located at the end of the articulated arm.
As it was not possible to equip SEIS with a real gnomon, for reasons of space, we used the hook of the seismometer whose circular shadow was projected on a graduated sight, which was far from ideal because of the ovoid shape of the shadow.
Denis Savoie, a great French specialist in gnomonics, had the heavy task of interpreting the pictures. In the end, about ten images were necessary to determine where the geographic North was located.
The comparison with the orientation obtained by the gyroscope data showed a difference of about ± 3°.
This result can be considered as excellent given the difficulties of estimating the exact position of the shadow.
Bibliography
"Finding SEIS North on Mars : Comparisons Between SEIS Sundial, Inertial and Imaging Measurements and Consequences for Seismic Analysis" in Advancing Earth and Space Science