How heat and cold affect Mercury
Recent research findings from the smallest planet in the Solar System
Mercury is the smallest of the eight planets in the Solar System, the closest to the Sun, and unusually, it has no atmosphere. Together, these factors make it a unique case in planetary research. The German Aerospace Center (Deutsche Zentrum für Luft- und Raumfahrt; DLR), the Technische Universität Berlin, the Karlsruhe Institute of Technology and Charles University in Prague have investigated various geophysical aspects related to Mercury’s thermal conditions and its interior. Their findings have now been published in three articles in the scientific journal Geophysical Research Letters. The results are surprising, but also important for the investigation of Mercury with the European-Japanese Mercury mission BepiColombo, which is set to reach its orbit around the planet in late 2026.
Mercury orbits the Sun at just under 60 million kilometres from the star’s 5500-degree-Celsius surface. As a result, the sunlit side of Mercury is intensely hot, while extremely low temperatures prevail on the night side. This large difference is also due to the lack of a heat-storing atmosphere on the planet – as heat is radiated directly into space after sunset. Mercury's proximity to the Sun, combined with its surface characteristics, its composition from innermost core to crust and its varying gravitational fields, sets it apart from the other planets in the Solar System. It is possible that the sum of these extremes may have led to changes in the planet’s rotation and orbit around the Sun in the past – but this is only one of several possible explanations.
The recent three scientific papers include measurement data from NASA's MESSENGER mission, which observed Mercury from orbit between 2011 and 2015. They also include modelling based on known parameters, which were used to simulate Mercury's structure and evolutionary processes in space and time. Like Venus, Mars and Earth, Mercury is a rocky planet. Similar to Earth, it has a magnetic field due to its liquid metal core, but it has no atmosphere.
Just like the Moon, Mercury has too little mass to hold onto the volatile molecules of a gas envelope. This fact alone has a significant impact on the properties and processes related to solar radiation. Its structure also differs considerably from that of other Earth-like bodies, with a disproportionately large metal core making up 80 percent of the planet's radius, leaving an overlying rocky mantle only 400 kilometres thick. The reason for this remains one of the great mysteries of planetary science.
Rocky crust reveals insights into planetary evolution – the greater the porosity, the lower the heat transfer
Adrien Broquet from the DLR Institute of Planetary Research in Berlin-Adlershof and his team found that Mercury's cratered crust has a porosity of 9 to 18 percent. This suggests an average rock density of just over 2.5 tonnes per cubic metre – which is comparable to the rocks in the lighter parts of the Moon's crust, known as anorthosites. These are aluminosilicates rich in feldspar and calcium.
These cavities are formed by either the cooling and crystallisation of molten rocks or by the shattering of the crust during large asteroid impacts. It is therefore no coincidence that the regions with the highest measured porosity values were found around the 1500-kilometre-wide Caloris basin. The model underlying these results derives the thickness of the planet's crust from high-resolution gravity and topography data collected by NASA's MESSENGER orbiter.
The porosity of surface rocks influences the transport of heat, which is generated inside the planet, rises, and 'wants' to be radiated into space. The surface of a rocky planet not only absorbs the energy radiated by the Sun and releases it back into space in the darkness of night, but also acts as a thermal barrier for the heat generated by the decay of radioactive elements in the planet's interior and remains stored from the time of its formation – known as accretional heat. This heat rises and, depending on the properties of the crust, is radiated into space. Through this process, the planet cools over the course of billions of years – and the smaller the planetary body, the faster it loses heat. Understanding Mercury's crustal structure is therefore of crucial importance for deciphering the geological history an Earth-like body.
Strong temperature contrasts influence internal dynamics
Mercury's orbit around the Sun and its spherical shape result in some regions receiving more solar radiation than others. Mercury currently has what's called a '3:2 spin-orbit resonance', meaning that it rotates three times on its axis for every two orbits around the Sun. Additionally, its axis of rotation is almost perpendicular to its orbital plane, which has led to a surface temperature pattern that is unique in the Solar System. Hot regions around the equator have temperatures of up to 430 degrees Celsius during the day, while the polar regions and colder areas – created by the 3:2 resonance – reach minus 170 degrees Celsius. It is thought that ice may even exist in the deep craters at Mercury's poles, where no sunlight ever penetrates. These extreme temperatures and the distinctive surface temperature pattern have a major impact not only on the surface of Mercury, but its interior.
Geophysicist Aymeric Fleury also from the DLR Institute of Planetary Research and his team discovered how Mercury's surface temperature variations influence temperatures in the deeper layers of the planet. These variations also affect the surface heat flow and show how Mercury loses the heat produced in its interior. Besides influencing the heat flow on the planet's surface, temperature differences also impact the boundary between the rocky mantle and metallic core 400 kilometres below. Heat currents resulting from these temperature differences could therefore influence the generation of magnetic fields.
This remarkable observation will be tested further using magnetic field models of the core and, from 2027 onwards, will be increasingly measured and analysed with the MPO-Mag experiment developed by Technische Universität Braunschweig onboard the BepiColombo planetary orbiter.
Did Mercury once orbit the Sun differently?
Studying Mercury's large impact basins also provides insights into structures that lie beneath the surface, hidden from the cameras. Asteroid impacts in the planet's early days created dozens of craters with a diameter of over 100 kilometres. This resulted in the redistribution of huge masses of rock, leading to variations in the gravitational field. After an impact, the ejection of lighter crustal material and the ascent of denser mantle material from below, the gravitational pull becomes higher at such points than in surrounding areas. However, contrasts in the gravitational field usually even out again over the course of millions of years, as the material that has been pushed to the side slowly fills the depression again. This process, known as viscous flow, occurs more quickly in warm or hot material than in brittle, cold rock. As a result, differences in the gravitational field level out again.
The crustal structure of these large impact basins thus provides valuable insights into the geological history of planets like Mercury. Geophysicists Claudia Szczech and Jürgen Oberst of the Technische Universität Berlin worked with a five-person team from the DLR Institute of Planetary Research to investigate gravitational field differences that can still be measured after more than three billion years. They studied 36 impact basins with diameters of more than 300 kilometres, and their Bouguer contrasts – named after French polymath Pierre Bouguer (1698–1758) – as indicators of viscoelastic relaxation.
The team used thermal development models based on Mercury's current 3:2 resonance to predict crust temperatures. The study shows that the expected correlation between zones with a warm crust and low Bouguer contrast (little relaxation) was not observed in the available data. This could mean that crust temperatures in the past were different to what previous models had assumed. Possible reasons for this could include a change in Mercury's orbit around the Sun or a major volcanic event associated with the formation of the extensive smooth plains in the planet's northern hemisphere.
BepiColombo set to reach Mercury's orbit in November 2026
On 8 January 2025, the BepiColombo mission, a joint project of the European Space Agency (ESA) and the Japanese Aerospace Exploration Agency (JAXA), completed its sixth and final flyby of Mercury, passing just 295 kilometres above its surface. This manoeuvre positioned the spacecraft duo – the Mercury Planetary Orbiter (MPO, ESA) and the Mercury Magnetospheric Orbiter (MMO, JAXA), is now on a perfect trajectory for the BepiColombo probe to enter the planet's orbit in November 2026. Due to its proximity to the Sun, with its enormous gravitational pull, six flybys were carried out to allow the probe to approach the planet on the ideal path. For the MPO, DLR has provided the BELA laser altimeter and the MERTIS spectrometer, which is operated jointly with the University of Münster.
Related links
- Publication in Geophysical Research Letters: Mercury's Crustal Porosity as Constrained by the Planet's Bombardment History. Adrien Broquet et al. (2024)
- Publication in Geophysical Research Letters: Variations of Heat Flux and Elastic Thickness of Mercury From 3‐D Thermal Evolution Modeling. Aymeric Fleury et al. (2024)
- Publication in Geophysical Research Letters: Relaxation States of Large Impact Basins on Mercury Based on MESSENGER Data. Claudia Szczech et al. (2024)
- DLR press release of 9 December 2024 on the fifth flyby of Mercury by BepiColombo: Planet Mercury seen ‘in a different light’ for the first time
- DLR Institute of Planetary Research
- NASA Mission MESSENGER website – MErcury Surface, Space ENvironment, GEochemistry, and Ranging
Contact
German Aerospace Center (DLR)
Philipp Burtscheidt
DLR media relations
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www.dlr.de
Ulrich Köhler
Institute of Planetary Research
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Press release DLR, 12 February 2025