Detection of tiny variations in the rotation of Mars used to characterize its liquid core

Radio Science data from NASA's Insight Mission show a signature linked to the effect of the liquid core on the rotation of Mars. An in-depth analysis of this signature provides new information about the inner structure of Mars.
Published in Astronomy

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In November 2018, Nasa landed the first mission dedicated to probe the interior of another planet. Named InSight for 'Interior exploration using Seismic Investigations, Geodesy, and Heat Transport', it carries a seismometer, a heat probe, and a radio-science transponder. This last experiment, called RISE for 'Rotation and Interior Structure Experiment', communicated with  gigantic (up 70m dish) radio-telescopes on Earth with the aim of measuring the tiniest variations of Mars's rotation. With the data accumulated by RISE during the first two and a half years, the RISE team led by planetary scientists of the Royal Observatory of Belgium precisely measured the rotation of Mars and detected a resonance with a normal mode of rotation, which only exists if the core is liquid. The period of this normal mode and the strength of the resonance provided new information about the interior structure of Mars.

(artist’s concept)

Looking for InSight 

Before chasing after the liquid core in the rotation of the red planet, RISE had another mission: locate the lander on Mars, right after touchdown, when nobody yet knows where it is. So even before the pizzas of our landing celebration got cold, I was already on my computer processing the first 52 data points, freshly received from NASA-JPL via a private and secured channel. The goal was to pinpoint the location of the lander, reducing the landing ellipse uncertainty of hundreds of kilometers to only a few hundred meters, such that it could be pictured by the high resolution camera of the Martian orbiters. After struggling with the new format of RISE data, I ended up with a (hopefully) accurate prediction of the lander's location. A few hours later, the coordinates were uploaded to Mars Reconnaissance Orbiter (MRO), which took a photo of the place and found the lander! It was an exciting and challenging time. 

Reference landing ellipse (dark blue),  the RISE estimate (orange), and the actual location (red), from Parker et al. (2019).

Race for the Core

I often think that one of the main qualities of planetary scientists is patience. Patience when developing a mission. Patience during its journey. But for radio-scientists like us, even more patience is required since one needs to slowly accumulate data to see the targeted signal arising above the noise. Thus, while the first hint of the signature of the liquid core in the rotation appeared during the summer of 2020, we had to wait longer to secure our discovery and get enough precision to interpret it in terms of core-specific properties. In the meantime, InSight’s seismometer SEIS collected peculiar and unexpected data that allowed our colleagues to confirm the detection of the liquid core based on a couple of seismic observations only. While in pole position, we kind of lost this race to the liquid core detection with InSight, after the seismologists published their results a year ago. However, scientific discovery is not only a race, but also a cooperative process, since each discovery should be confirmed by independent experiments. In research, good competition is healthy and forces scientists to always go beyond. 

The Belgian touch

Belgium is a crossroads of Western Europe, where multiple cultures cohabit. Many foreigners like me feel at home there. Still, my situation in this adventure was quite peculiar, as a French scientist, born on a Caribbean island, working in a Belgian institute and leading the analysis of data from an American instrument. After growing up on “the island of a hundred windmills”, as it is called, I studied aeronautic and aerospace engineering in France. But it is in Belgium, at the Royal Observatory of Belgium in the team of Véronique Dehant, that I learned planetary and radio sciences. With my PhD in hand, I spent a few years at JPL under the direction of William Folkner, PI of RISE, before returning to Belgium. I ended up with a foot in both RISE and LaRa, a similar instrument developed at about the same time as RISE but in Belgium, under the direction of Veronique Dehant and myself and with the support of the Belgian Science Policy Office (BELSPO). 

The Royal Observatory of Belgium is an ideal nest to learn and evolve in the physics of planets, unique in its kind, as it brings together skills ranging from the development of space instruments to the physical interpretation of the results, through the analysis of the data collected by the space probes. In the planetary science department, we have specialists in the interior, the rotation and the atmosphere of planets, and Mars in particular, along with experts of spacecraft orbit determination and radio-science.

Data interpretation, a real puzzle!

“Theory is when you know everything but nothing works. Practice is when everything works but no one knows why.” But here, against all odds, and thanks to hard and thorough teamwork, we improved the theory and we know why it works!

There is a lot to tell about the analysis of RISE data. One of the many anecdotes is when we discovered a clear but unexplained signal in the data, which was finally explained (after many efforts) as resulting from Phobos perturbing Mars’s motion by only a few tens of centimeters. The RISE data are so precise that it was necessary to improve all the theoretical models used to interpret them and to calibrate them for effects hitherto neglected by other missions, such as the noise introduced by the propagation of the radio signal through the atmosphere of Mars. Another example that may be worth mentioning is the unexpected detection in the rotation of Mars inferred from RISE data of the dust storms that occurred just before and after InSight landing. This again led us to complexify the rotation model to account for the temporary change in the spin rate of Mars induced by the dust storms... In the end, our new rotation model also includes hitherto neglected quadratic terms in the three main rotation angles used to describe the direction of the spin axis as a function of time and the rotation around it, along with nutation terms at periods related to the orbital motion of Phobos and Deimos, and updated relativistic corrections to the spin variations.

Nutations and liquid core

The RISE experiment was specifically designed to measure Martian nutations, the periodic wobbles of the spin axis in space, which are not the same whether the core is solid or liquid. We detected for the first time the small nutational motions (≤40 cm) of the lander in inertial space due to the effect of the Martian liquid core. For a fluid core, the nutation amplitudes are resonantly amplified by the Free Core Nutation (FCN), a normal mode expressing a slight relative rotation of the core with respect to the mantle. From the period and the resonance strength of the FCN, we estimated the radius of the liquid core to be 1835 ± 55 km, a value close to what the InSight seismology team found, observing the reflected seismic waves at the core-mantle boundary. The core has a mean density of 5955-6290 kg/m3 and the density jump at the core-mantle boundary is 1690-2110 kg/m3

Our estimate of the FCN period allowed us to understand that the shape of the Martian core is close to that expected for a planet in hydrostatic equilibrium. But how could that be, since the surface and crust of Mars are not in hydrostatic equilibrium? We thought long and hard to identify plausible processes that could explain the unexpected flattening of the core. The only realistic way we found to explain what we observed was via internal mass anomalies deep in the mantle that we believe should exist.

Mars interior models in agreement with our estimates of the nutation parameters (gray areas). See the paper for more details.

RISE is not only about the deep inside, but also about the atmosphere and the rotation. In the future, by analyzing the complete RISE dataset, we hope to further constrain not only the interior, but also the atmosphere/caps dynamics, and provide an orientation and rotation model that can serve as a reference for the scientific community. InSight is now retired, but scientists are still working hard to analyze the huge volume of data sent back to Earth by this successful mission. 


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