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Hammering down to Martian depths – countdown to NASA's InSight mission

(3 May 2018 - DLR) The formation of planets and the occurrence of volcanism and earthquakes are determined by the thermally driven forces acting inside a planet. Continents and life as we know it emerged on Earth. On Mars, the internal development dynamics slowed rapidly.

To decipher the interior of Mars and its past in more detail, and to find out what makes Earth so unique, an Atlas launch vehicle will lift off from Vandenberg Air Force Base in California at 13:05 CEST (04:05 local time) on 5 May, carrying NASA's Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) lander to Mars.

Upon its arrival on 26 November 2018, InSight will touch down just north of the equator, on the Elysium Planitia plain, where it will commence its work as a geophysical observatory.

This will be the first mission to Mars to focus on exploring the planet's interior and its 4.5-billion-year history. The German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) is contributing one of the three principal experiments of the NASA InSight mission, HP3 – a small probe that will hammer five metres deep into the Martian soil to measure temperature and thermal conductivity at various depths to determine the heat flow from deep inside the planet. The resource-saving key technology developed by DLR has already been used in road construction in China, for agriculture in Poland and in avalanche surveillance in Switzerland.

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Artist’s impression of the NASA InSight lander on the Martian surface (courtesy: NASA/JPL-Caltech)

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For the InSight mission, a landing area was sought that had to meet several scientific criteria, but above all space systems engineering criteria. For a secure energy supply from solar power and to avoid extreme diurnal and seasonal temperature variations, it could not be too far north or south of the equator, it had to be flat and, as far as possible, not covered by rocks. After a long selection process, engineers and scientists selected an area in a plain southwest of the large volcanic complex surrounding Elysium Mons. The chosen site is in Elysium Planitia, north of the Martian equator and the highland border – just a few hundred kilometres north of Gale Crater (below the ‘4’ in the latitude label), in which the NASA rover Curiosity has been driving since 2012. The image is a section of a global topographic map of Mars; blue and green are low-lying areas, yellow and red are elevated terrain. The image is about 5000 kilometres wide. (courtesy: ASA/JPL/USGS (MOLA))

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DLR has contributed the HP3 experiment to the NASA InSight mission. HP3 stands for ‘Heat Flow and Physical Properties Package’ and the instrument was developed under the leadership of the DLR Institute of Planetary Research. The thermal conductivity of the material below the landing site and the heat flow from the interior of Mars to the surface will be measured using a penetrometer hammered five metres deep into the Martian regolith. The experiment is designed for an operational life of two Earth years. Essential components of HP3 are the ‘Mole’ and the ribbon cable with the temperature sensors, which the Mole will pull behind it into the ground to perform measurements. (courtesy: NASA/JPL, DLR)

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The DLR HP3 experiment also includes the RAD radiometer, which is mounted on the lander platform. This determines the temperature of the Martian surface by observing its infrared radiation. The surface can reach a maximum of just over 20 degrees Celsius during the day, but can fall below minus 70 degrees Celsius at night. Knowledge of the surface temperature is important in order to be able to calculate disturbances in the temperature distribution in the subsurface. RAD was built at the DLR Institute of Optical Sensor Systems. (courtesy: DLR)

More precise knowledge of Mars' interior and other terrastial planets

"The InSight mission fulfils the long-standing desire of planetary scientists to operate a geophysical observatory on a terrestrial planet," explains Tilman Spohn from the DLR Institute of Planetary Research and Principal Investigator of the HP3 experiment. "Mars is an ideal destination – easy to reach and comparable with Earth," Spohn continues. "The processes that played out after the formation of a metallic core at the Martian centre, its surrounding rocky mantle and the crust slowed down much earlier than on Earth." It is therefore possible that the 'fingerprints' of the events that led to the formation of an earth-like planetary core, mantle and crust have been preserved to this day. "Understanding how this occurred on Mars will enable us to acquire a far better insight into the events that unfolded on Earth up to the emergence and continued existence of life, as well as into the evolution of the Moon, Venus and Mercury. It might even allow us to learn a great deal about the formation of rocky planets around other stars – or extrasolar planets." The researchers are curious as to whether – as in Earth’s interior – a hot molten core is at the heart of Mars.

Launch from the United States West Coast

InSight was selected in August 2012 as the twelfth Discovery Mission. Like all previous missions in this NASA programme, InSight is a comparatively small mission targeted at a specific subject within planetary research. Together with the upper stage for injection into a transfer orbit, the total mission mass is just 727 kilograms, and the actual lander weighs just 360 kilograms. This makes it possible to launch the mission from the US airbase at Vandenberg, California, using an Atlas V-401 launch vehicle. It will be the first time for a planetary mission to be launched from this NASA site. The 70-metre antennas of NASA's Deep Space Network in California, Australia and Spain will be used to maintain contact with the lander on its voyage to Mars and during the mission itself.

A geophysical observatory for the Red Planet

The lander’s principal structure is a two-metre-diameter platform carrying most of the system components, namely the experiments in 'travel mode', the antennas, the thrusters, the fuel tanks and three telescopic legs. Upon arrival on Mars, a robotic arm will deploy the Seismic Experiment for Interior Structure (SEIS) onto the surface first. The seismometer will be used to record waves propagating through the planet from marsquakes and from sites impacted by meteors. DLR is also involved scientifically in the SEIS experiment. Afterwards, in early January 2019, the HP3 experiment developed by DLR will be taken from the platform and lowered onto the Martian ground. HP3 is an abbreviation for Heat Flow and Physical Properties Package. The experiment consists of a housing – referred to as the support structure – placed on the surface of Mars that holds a 40-centimetre-long hammering probe with a diameter of 27 millimetres – nicknamed 'the Mole' by the scientists. Powered by an electric hammering mechanism, it will burrow its way centimetre by centimetre into the surface of Mars over a period of several weeks. The support structure and the 'Mole' were built by the Bremen-based DLR Institute of Space Systems in cooperation with external partners. For example, the Mole's impact mechanism was developed and built with support from Astronika and the Space Research Centre of the Polish Academy of Sciences (Centrum Badań Kosmicznych; CBK), both located in Warsaw. The maximum achievable depth is five metres.

Hammering with 14,000 times gravitational acceleration

HP3 is not a 'drill' as it does not rotate. Instead, the mole advances using a special hammering mechanism in which a spring is repeatedly compressed, causing a hammer to be accelerated forward towards the inner lining of the tip of the 'Mole' each time the spring is released. These impacts generate an acceleration of up to 14,000 times that of Earth's gravity, which is why the sensitive measurement technology inside the probe requires special shock absorption techniques to withstand the stresses. For this reason, the DLR Microgravity User Support Center (MUSC) used systems for impact and vibration minimisation devised by the DLR Institute of Composite Structures and Adaptive Systems in Braunschweig. A special system isolates the sensors from the impacts and in doing so minimises their exposure to stress. For this purpose, the system is fitted with specially patented double helix springs – also known as 'galaxy springs'. In addition to the vibration dampening springs in the Mole, methods for equipping the temperature sensors and feed lines on the measuring cable are technical highlights that make use of the Mole on Earth appealing for physical measurements in remote areas with scarce resources.

Temperature sensors on the science tether measure the temperature in the subsoil

At the heart of the experiment is a tether equipped with temperature sensors – developed by the DLR Institute of Planetary Research – which the Mole pulls into the Martian soil. Once the Mole has reached its final depth below the surface, the system will record ground temperature measurements for up to two years in order to determine the temperature gradient in the subsurface. The sensors are able to measure temperature differences of just a few thousandths of a kelvin (or one degree Celsius), in order to determine the very small geothermal temperature gradient. Mounted on the lander, the experiment also includes the radiometer RAD, which is designed to measure the temperature of the Martian surface. Knowledge of the surface temperature is essential to calculate disturbances in the temperature distribution in the subsurface. RAD was developed and built by the DLR Institute of Optical Sensor Systems.

The HP3 instrument on the NASA InSight mission

The InSight mission is being conducted by the Jet Propulsion Laboratory (JPL) in Pasadena, California, on behalf of NASA’s Science Mission Directorate. InSight is a mission in the NASA Discovery Programme. DLR is contributing the HP3 experiment to the mission. Scientific leadership lies with the DLR Institute of Planetary Research, which was also in charge of developing the experiment in collaboration with the DLR institutes of Space Systems, Optical Sensor Systems, Space Operations and Astronaut Training, Composite Structures and Adaptive Systems, System Dynamics and Control, as well as the Institute of Robotics and Mechatronics. Industrial partners are Astronika and the CBK Space Research Centre, Magson GmbH and Sonaca SA. The scientific partners are the ÖAW Space Research Institute in the Austrian Academy of Sciences and the University of Kaiserslautern. The DLR Microgravity User Support Center (MUSC) in Cologne is responsible for HP3 operations.