mission reveals surprising secrets from inside the red planet – new research

We may have walked on the moon and sent probes through the solar system, but we know very little about what is going on inside the other planets. Now, for the first time, we have been able to see the interior of one of them, thanks to NASA’s Mars InSight probe. The probe, which landed in 2018, is equipped with a solar-powered lander bristling with equipment, including a seismometer (a very sensitive vibration detector).

The results, published in three studies in Science, bring unexpected discoveries about the interior of Mars, including a very large nucleus.

Although Mars does not have tectonic plates, the first “earthquakes” were detected a few months after the probe landed. These can result from vibrations caused by meteorites hitting the surface or from processes inside the planet.

Earthquakes on Mars are difficult to detect, in part because the seismometer is subjected to extremes of Martian weather, with seasonally changing windy periods obscuring the data. The signals used to probe the Martian interior all come from relatively small earthquakes, the best of the hundreds detected so far.

Planets develop by accumulating matter (accretion) early in the life of a solar system. But their interiors aren’t a uniform blend of those original ingredients – they also undergo differentiation, where some lighter minerals “float” to the surface, while heavier components like iron flow toward the center of the planet. We expect rocky planets like Mars to have an iron-rich core, followed by a silicate layer called the mantle, and an outermost skin known as the crust. Until now, the area of ​​Mars occupied by each of these layers was unknown.

Metallic heart

It is impossible to obtain a sample of the core of Mars. Instead, to estimate its size, we used seismic waves (created by earthquakes). On Earth, the radius of the nucleus was first estimated by finding its “shadow”, an area where the nucleus disrupts the arrival of seismic waves from distant earthquakes. Our study must have relied on a special type of slow lateral displacement waves called S waves which were reflected back to the surface by the interface between the nucleus and the mantle.

Careful seismic processing by seismologists around the world revealed the signals of six earthquakes relatively close to the probe. Combined with information from mineral physics and seismic waves passing through the mantle, we were able to estimate the size and density of the Martian core. This suggests that the radius is 1,830 km (plus or minus 40 km) – just over half the radius of the planet, which is larger than we thought.

The larger-than-expected core requires a relatively large proportion of lighter elements to mix with its iron. From our work, we now know that the Martian core is expected to contain a high fraction of sulfur and other light elements. Experiments show that liquid iron compounds containing so much sulfur are unlikely to solidify at the pressures and temperatures expected in the center of Mars, so it is unlikely to have an internal solid core like Earth. It can help us understand why there is no planetary magnetic field on Mars today, unlike on Earth.

Layers and layers

The crust of a planet comprises a tiny fraction of its mass. But the chemical and thermal interactions of the Martian crust with the atmosphere, and with any presence of water or ice, help define the conditions that determine whether life can exist there.

In the second new study, another team looked at seismic waves that converted from P waves, which are fast compression waves, to S waves (or vice versa) when they encountered different rock materials, and an assessment background vibrations and gravity, to probe the Martian crust. This suggests that the average possible thickness of the Martian crust is between 24 km and 72 km. This means that we can exclude earlier estimates of up to around 100 km.

From more than 100 years of seismology on Earth, we have known that beneath the thin crust is the mantle, but the mantle itself is not uniform down to the core. The upper mantle and crust, collectively called the lithosphere, are rigid, while the lower mantle is a solid that can flow. On Earth, it is the lithospheric plates that move as part of the plate tectonics, but on Mars, it is not known what role the lithosphere plays.

To sample different depths of the mantle, we can use both direct and reflected seismic waves. Direct P or S waves dive deep into the mantle and then rise to the surface. The depth to which they descend depends on the structure of the planet and the distance between the earthquake and the seismometer. The reflected waves return to the surface and then plunge back two or three times. A third study identified eight low-frequency earthquakes that produced both direct and reflected waves, and used them to create and test different models of the Martian crust and mantle.

By comparing data and models, they found that Mars’s lithosphere is between 400 km and 600 km thick. This is considerably thicker than any rigid layer observed on Earth and implies that the Martian crust has a higher concentration of heat-producing radioactive elements than previously thought.

We now know more about the ingredients that went into the construction of Mars and that it has a very thick lithosphere, allowing our smaller sister planet to retain its internal heat. While future astronauts don’t have to worry about the small earthquakes we used to probe the red planet, the lack of a magnetic field generated by the sulfur-rich core will force them and their equipment. , to pay more attention to the harshness of the sun. wind.

Our new understanding of the Martian interior is part of a new era of planetary seismology, more than fifty years since the Apollo missions landed seismometers on the Moon. New seismometers will be deployed to the Moon as part of the Artemis mission, while the Dragonfly mission will place a seismometer on Saturn’s Titan moon in the mid-2030s. These experiments will help us better understand how planets form and evolve – seeing deep into Mars is just one piece of a puzzle the size of a solar system.