November 26, 2018

What do we know about Mars’ interior?

The first missions to reach the surface of Mars date back 40 years, but many mysteries still lie beneath its top layer of iron oxide. To delve deeper into the red planet, the InSight lander is carrying a system able to drill down to five metres and sound Mars’ surface over a radius of thousands of kilometres. Following its arrival on Mars, we talk about what we already know and what we hope to discover there with CNES’s Francis Rocard, head of Solar System Exploration programmes, and Philippe Laudet, SEIS project leader for InSight and head of the agency’s Astronomy and Astrophysics programme.

Magnetic field and internal temperature

Mars lost its magnetic field between 3.9 and 4 billion years ago. It was generated by a dynamo effect fuelled by convection motions inside the planet’s core. But Mars’ dynamo shut down very early in its existence compared to Earth, because as a smaller planet it cooled more quickly. To measure how this energy dissipated and retrace the red planet’s geological history, InSight is going to drill down in stages of 50 centimetres.

“The HP3 sensor is going to measure temperatures over several days,” explains Francis Rocard. “That’s tough to do, because you have to drill down deep enough to where the Sun doesn’t affect readings. At a depth of 5 metres, we should find a constant temperature and that will give us the heat flow and cooling rate of Mars. It’s like going into a cave on Earth: whatever the season, it’s always 12°C, which corresponds to the heat flow of Earth.”

Nature and state of the core

Interior structure of Mars: from the centre outwards, the inner and outer core, mantle, crust and atmosphere (© IPGP/David Ducros)

From the signals it measures beneath the surface and measurements from orbit, InSight will allow scientists to learn a lot more about the core. “Its radius is estimated to be 1,700 km, give or take 300 km,” says Francis Rocard. “With this mission, we should reach a precision of 75 km, in other words four times better. We should also be able to improve significantly on the current estimate of its density of 6.4—give or take 1—to reach a precision on the order of 0.3.” In comparison, the density of water is 1 and that of iron is 7.8.

But, as Mars cooled so quickly, we don’t know if its core is still liquid or completely solid. To find out, we’ll need to measure the planet’s ‘wobble’ about its spin axis from the vantage point of orbit. To do that, the RISE instrument (Rotation and Interior Structure Experiment) will use InSight’s communication antennas and orbital relays (Mars Orbiter, Mars Odyssey, probably MAVEN and ESA’s TGO).

To illustrate, Francis Rocard uses the analogy of an egg. “When you turn a cooked egg, it doesn’t turn the same as a raw egg that’s liquid inside, which continues to turn on itself,” he says. “So we can deduce a range of parameters, depending on whether the core is liquid or solid.”

Earth’s axis of rotation is tilted 23° to the equator, which is what gives us our seasons. This tilt varies only slightly, between 22° and 24°, as a result of the stabilizing effect of the Moon’s mass. But on Mars, with its two small moons, the axis of rotation is much more unstable. It’s currently tilted 25°, but has experienced extreme shifts to more than 60° over the last billion years, and almost 90° over the last three billion years.

Geological and volcanic activity

Mars was volcanically active in the past—indeed, very active—if the volcanoes on Tharsis Rise are anything to go by. However, no subduction zones are visible at its surface, which is not being renewed. This is what has maintained the volcanoes above their hotspot and made them such towering features, Olympus Mons peaking at a height of 22.5 kilometres. The last lava flows would appear to date back only 2 million years, which is nothing on the geological timescale, so Mars may still be volcanically active today.

“Philippe Lognonné, the principal investigator for SEIS, often says that we know as much about Mars’ seismic activity and interior structure as we did about the inside of Earth a century ago,” notes Francis Rocard. “In other words, very little, as those were the early years of seismology. For Mars, we’re starting from a clean sheet, except for the models that have been developed by theorists, notably to explain the lack of plate tectonics, since there are no subduction zones visible on its surface.”

View of the surface of Mars with the Olympus Mons volcano near the equator. Credits: NASA/JPL-Caltech/University of Arizona

“Geophysics is a major goal of the mission, as 95% of Mars remains unknown and we only have seismology data for its interior,” confirms Philippe Laudet

And to understand what’s going on beneath the surface, we need to listen with a seismometer. That’s what SEIS—conceived by IPGP and built by Sodern under supervision from CNES as lead contractor responsible for its integration, testing and delivery to NASA—is designed to do. SEIS will not only detect any ‘marsquakes’ but will also sense meteorite impacts to probe the subsurface. As atmospheric pressure is much lower on Mars than on Earth, many more meteorites survive entry and hit the ground harder. “We don’t believe these meteorites cause very big quakes,” says Francis Rocard. “We therefore expect to obtain local measurements to gauge the crust’s thickness and we hope to detect impacts up to 1,000 km from InSight.”

Unearthly seismology

Before SEIS, there haven’t been many examples of extraterrestrial seismology. The Apollo 11 mission left a seismometer on the Moon in 1969, but it only operated for a month until its battery ran out. Apollo 12, 14, 15, 16 and 17 each carried a seismometer (ALSEP for Apollo Lunar Surface Experiments Package), forming a network that operated up to 1977 thanks to the instruments’ radioisotope thermoelectric generator (RTG) power supply. This experiment enabled scientists to study the Earth-Moon system and ascertain their common origin. At the same time, NASA was already preparing to send two seismometers to Mars aboard the two Viking landers. But when they arrived in 1976, Viking 1’s failed to uncage and was thus inoperable, while Viking 2’s was fixed to the lander’s deck and thus sensed all its parasitic movements, in particular those induced by winds.

The story of SEIS began in the 1990s, when Philippe Lognonné at IPGP started developing a first seismometer designed to study the interior of another planet. This instrument, called OPTIMISM, was accepted for the Russian Mars 96 mission and lifted off with it in 1996. Unfortunately, the Proton launcher experienced a mishap and after only a few orbits Mars 96 fell back to Earth and burned up in the atmosphere. Seismology on Mars would therefore have to wait for a future mission opportunity.

“After the failure of Mars 96, CNES continued funding the seismometer and sought other candidate missions for it,” recalls Philippe Laudet. “But we had to wait another 15 years: missions were planned to Mars, the Moon and asteroids, but none of them got off the ground. The last one was JAXA’s Selene 2 mission in 2010, but by then the seismometer was nothing like the one on Mars 96. And eventually in 2011, NASA selected InSight, initially called GEMS, from the 29 responses to its Discovery call for projects in 2010. Development was fast-tracked, as certain parts had been developed in advance, which isn’t often the case.”

InSight was initially planned for launch in 2016, but in late August 2015 during testing at CNES, the sphere housing the three very broad band (VBB) seismometers designed by IPGP/Sodern suffered a leak when subjected to a temperature of –50°C. To avert any possible hitch, NASA decided to push back the launch to 2018.

“You run a marathon for 20 years and then a 7-year sprint,” quips Philippe Laudet.

But unlike on the Moon, there will be nobody to deploy the seismometer on Mars. InSight’s robotic arm is scheduled to be deployed around 10 December and then set down SEIS on the surface of the planet between Christmas Day and New Year’s Day. Its shield should be positioned over it around 5-10 January 2019, and everything should be ready to start beaming back the first science data in early April.

From the outside inwards: the wind and thermal shield and the sphere housing the three pendulums/detectors. © IPGP/David Ducros

Confirmation of the landing should arrive back on Earth this Monday 26 November at 20:53:40 precisely. InSight will land in the Elysium Planitia plain, at the Martian equator, and then deploy its two circular solar panels to power its instruments for a planned two-year mission.


Head of Solar System Exploration programmes
Francis Rocard
E-mail : francis.rocard at
Tel.: +33 (0)1 44 76 75 98
Fax: +33 (0)1 44 76 78 59
Adress :
Centre National d'Etudes Spatiales, 2 place Maurice Quentin, 75039 Paris Cedex 1, France

Head of Astronomy and Astrophysics programme
Philippe Laudet
E-mail : philippe.laudet at
Tel.: +33 (0)5 61 27 31 18
Adress :
Centre National d'Études Spatiales, 18 Avenue Édouard Belin, 31401 Toulouse Cedex 9, France

InSight with its French SEIS seismometer (left) and German HP3 heat probe (right).
© NASA/JPL Caltech, 2018