The planet closest to the sun seems to be doomed to a tragic fate.
Mercury has never been a giant. With its 4,879 kilometers in diametershe would be almost pale appears next to the earth, three times wider. However, this little pebble grilled by the sun intrigues. For billions of years, it has narrowed: its diameter would have lost at least 14 kilometers Over time!
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Mercury, a planet to no other in our solar system
Mercury is a telluric planet, that is to say made of rocks and metals. It is also the one whose orbit is the shortest in our solar system: 88 days are enough to take a complete tour around the sun. On the surface, temperatures play extremes: 450 ° C during the day, bordering on –170 ° C at night. This brutal variation is explained by the almost total atmosphere of atmosphere.
But what especially fascinates planetologists is the nucleus. At Mercure, it occupies more than 60 % of its volume, against only 17 % for the earth. In other words, this planet is a metal heart with a thin rock crust.
The great thrill: a geological contraction
In the 1970s, the mariner probe 10 marks of strange lobed escarpments. These giant flaws, several hundred kilometers long, cross the crust of mercury like scars. They even distort the craters, proof that they appeared After The impacts.
Between 2011 and 2015, the American Mission Messenger confirms the hypothesis: Mercury shrinks as its core cools. This cooling leads to a global contraction of the planet, comparable to an apple left in the sun which loses water and ratatines.
Under the crust, the contraction metal shoots on the rock, which fractures to absorb the shock. Result: a pleated topography, bristled with nearly 6,000 identified faults, 9 to 900 kilometers long, according to a study published in Nature Geoscience.
Mercury is not a planet like the others
- Mercury does not have an atmosphere in the classic sense. What surrounds him is a exospherea tenuous cloud of atoms torn off by the solar wind.
- This exosphere contains a little oxygen, sodium, potassium and traces of hydrogen, but in quantities less than a millionth of the earth’s atmosphere.
- This lack of sparkling envelope explains the extreme temperature differences: Up to 450 ° C during the day, and –170 ° C at night.
- Mercury is the fastest planet in the solar system: it orbit around the sun at almost 170 000 km/h.
- Its orbit is strongly ellipticalbringing it closer to the sun at 46 million kilometers and then moving it away at 70 million kilometers.
- Despite its small size, Mercury has a magnetic fieldabout 100 times lower than that of the earth, but still active.
- This field is asymmetrical and shifted northwhich could betray an internal dynamic still partially active or vestiges of a more turbulent past.
Bepicolombo: Sensor of secrets in orbit
To unravel the mysteries of Mercury, Europe (via ESA) and Japan (via Jaxa) launched the Bepicolombo mission in 2018. Objective: to better understand the composition, geology, magnetic history and mercury interactions with the solar wind.
As early as 2023, the on -board sensors detected amazing anomalies of the magnetic field at the South Pole. Unpublished measures, which suggest that the nucleus is still partially active and that the thermal history of the planet is more complex than expected.
The mission is underway, with an orbit planned around Mercury in December 2025. It should provide valuable data on the internal contraction mechanisms and on the dynamics of a nucleus that does not turn round.
Mercury, a moon that survived its giant?
Some researchers are considering an even more spectacular hypothesis: Mercury could have one day was much larger, before losing its rocky coat in a cataclysmic collision with another celestial body. This scenario would explain its oversized nucleus, a vestige of a flapped planet.
This idea, although debated, finds a certain echo in digital simulations. A super Mercurereduced to the state of metallic core, would have become the densest planet in the solar system, with 5.43 g/cm³. It is more than the earth, however much more massive.
Some figures on Mercury:
| Characteristic | Value |
| Diameter | 4,879 kilometers |
| Masse | 3,3 × 10²³ kg |
| Max temperature (day) | 450 °C |
| Minimum temperature (night) | –170 °C |
| Duration of one year | 88 land days |
| Composition of the nucleus | 60 to 70 % of the volume |
| Estimated diameter reduction | At least 14 kilometers |
| Number of fabrics measured | 5 934 |
| Length of the faults | 9 to 900 kilometers |
Source :
Man, B., Rothery, D.A., Balme, M.R. et al. Widespread small grabens consistent with recent tectonism on Mercury. Nat. Geosci. 16, 856–862 (2023). https://doi.org/10.1038/s41561-023-01281-5
Image: Realistic space background with all planets (Freepik).
