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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 placed on the surface of Mars, 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 all the precision required, seismologists absolutely need to know its orientation on the ground, after it was dropped off by InSight's robotic arm on December 19th. But on Mars, it is impossible to use a conventional compass.

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

Publication date: 19/12/2018

General public, Observatories, Press, Research

Related observatories : InSight Observatory

InSight's SEIS seismometer as seen by the IDC robotic arm camera after deployment on Sol 22 (© 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 this turned out to be only fossilised. Numerous sectors of the planet’s oldest terrain, located in the southern hemisphere, do indeed retain traces of the presence of a global magnetic field, but this magnetisation is now no more than a shadow of its former self.

Analyses of the magnetic remanence imprinted in the Martian crust show that the Martian magnetic field effectively died out around 4 billion years ago, although no one knows why. The disappearance of the magnetic shield that protected Mars from the deleterious onslaughts of solar and cosmic bombardment is one of the great mysteries of the Red Planet, to which the InSight mission should provide an answer.

From a more practical point of view, the absence of a global magnetic field on Mars poses a major problem when it comes to orientation. Nor, of course, does Mars have a network of GPS satellites like those on Earth. So how do you find north? Paradoxical as it may seem, scientists are going to rely on an age-old technique: the use of a sundial, converted into a compass.

The SEIS sundial

In ancient Babylon, people were already using the shadow cast by a stick stuck in the ground to tell the time. Decades later, InSight will be using this technique on the Red Planet. At the top of the copper-coloured hexagonal thermal shield (RWEB) that surrounds the SEIS seismometer is a sight, at the centre of which is the capture handle that enabled the instrument to be deployed on the ground by the grapple of the robotic arm, and which also acts as a gnomon.

At the top of the RWEB thermal protection that surrounds the SEIS seismometer, the gripping hook also serves as a gnomon. The shadow cast by the rod on the sight will be used to determine 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 SEIS sundial pattern consists of three zones. The outermost zone has 72 sectors, each 5° apart, covering 360°. The middle zone has the same number of sectors, but they are offset by 2.5°. Finally, the innermost zone is also offset by 2.5°. Measuring 28.7 mm high, the SEIS gnomon has a particular shape (a small conical cylinder ending in a half-sphere), which is suited to its primary function – the capture of the seismometer by the InSight probe’s IDA robotic arm – but which is not all that ideal for reading its cast shadow. Its shape has nevertheless been skilfully modified to improve its role as a gnomon. The entire solar compass was designed by David Mimoun (ISAE/SUPAERO) and Ken Hurst (JPL), and built by Nicholas Onufer and Michele Wallace (JPL).

The position of the shadow cast by the gnomon, which can fall on the different zones of the sight as a function of time, makes it possible to determine the height and direction (azimuth) of the sun in the Martian sky, and therefore local Martian solar time. Although a very simple device, InSight’s sundial can provide other crucial information. In addition to the inclinometers on the levelling cradle, it can be used to determine the inclination of the seismometer relative to the ground and, more importantly, the orientation of the seismometer relative to true north (essential for interpreting seismic signals).

The SEIS sundial will operate for a very limited time on Mars. It will effectively become unusable once the seismometer is covered by the WTS wind and heat protection shield.

Determining Martian North

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

The first step was to load the images of the sight into specialised software, to correct them for the effects of parallax, and to determine with precision the direction of the shadow of the gnomon. At this stage, Denis Savoie needed several pieces of information to determine north: the precise latitude and longitude coordinates of the seismometer on Mars, as well as the exact date and time the images were taken (recorded by a very precise clock on board the InSight probe).

Entered into sophisticated calculation software, this data will be used to obtain a crucial parameter, the azimuth, i.e. the angle between the direction of the shadow on the gnomon and true north. A simple transfer to the sundial staff, where the direction of the shadow is already shown, will locate the north with an accuracy of between 1 and 2°. A second check, completely independent of the first, was carried out simultaneously in Paris by Marc Goutaudier and Andy Richard (Universcience/Palais de la découverte), this time using ray-tracing software. This enabled them to visually reproduce the SEIS pattern and the shadow of the gnomon as a function of a number of parameters, and to make a direct comparison with the images from InSight’s IDC camera.

Numerical simulation of the SEIS seismometer sight and gnomon. You can see the three zones, each offset by 2.5° and made up of 72 sectors. The small circles at the bottom right and top left on the outside of the sight are used to correct the IDC camera images for the effects of parallax. (© Marc Goutaudier)
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