A German scientist believes he has resolved a three-decade mystery about why Mercury, the closest planet to the Sun, has such a weak magnetic field.
Mercury is believed to generate its magnetism in the same way as Earth – by a dynamo, caused by the rotation of molten iron at its core.
But the question is why Mercury’s magnetic field is so puny, as it amounts to just 1% of the strength of Earth’s. If the dynamo theory is right, the planet’s magnetism should be 30 times stronger than it is.
Ulrich Christensen of the Max Planck Institute for Solar System Research in Katienburg-Lindau, Germany, believes the answer lies in the structure of Mercury’s core. The core’s outer layers are “stably stratified,” which means they are largely insulated from the heat of the swirling inner core.
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As a result, only the inner core rotates effectively to generate the magnetic field. This braking effect is relatively important because Mercury has a very slow rotation, which also affects the dynamo’s power. The planet takes 59 Earth days to complete one spin on its axis, the slowest rotation in the solar system except for that of Venus.
Christensen says this theory can be tested by the two scheduled missions to Mercury – NASA’s Messenger, which arrives at the solar system’s innermost planet in 2008, and the European Space Agency’s BepiColombo, due for launch in 2013.
The only previous trip to Mercury was made by the US spacecraft Mariner 10, which recorded the planet’s unusual magnetic field in flybys in 1974 and 1975.
Journal reference: Nature (vol 444, p 1056)
![Astronomers have long known that understanding how star clusters come to be is key to unlocking other secrets of galactic evolution. Stars form in clusters, created when clouds of gas collapse under gravity. As more and more stars are born in a collapsing cloud, strong stellar winds, harsh ultraviolet radiation and the supernova explosions of massive stars eventually disperse the cloud, and their light can bear down on other star-forming regions in the galaxy. This process is called stellar feedback, and it means that most of the gas in a galaxy never gets used for star formation. Researching how star clusters develop can answer questions about star formation at a galactic scale. Now, the state of the art has been further developed with both Hubble and Webb working together to provide a broad-spectrum view of thousands of young star clusters. An international team of astronomers has pored over images of four nearby galaxies from the FEAST observing programme (#1783), trying to solve this mystery. Their results show that it is the most massive star clusters that clear away their gaseous shroud the fastest, and begin lighting their galaxy the earliest. The team identified nearly 9000 star clusters in the four galaxies in different evolutionary stages: young clusters just starting to emerge from their natal clouds of gas, clusters that had partially dispersed the gas (both from Webb images), and fully unobstructed clusters visible in optical light (found in Hubble images). With Webb???s ability to peer inside the gas clouds, they were able to then estimate the mass and age of each cluster from its light spectrum. This image shows a section of one of the spiral arms of Messier 51 (M51), one of the four galaxies studied in this work, as seen by Webb???s Near-Infrared Camera (NIRCam). The thick clumps of star-forming gas are shown here in red and orange, representing infrared light emitted by ionised gas, dust grains, and complex molecules such as polycyclic aromatic hydrocarbons (PAHs). Within these gas complexes, each tens or hundreds of light years across, Webb reveals the dense, extremely bright clusters of massive stars that have just recently formed. The countless stars strewn across the arm of the galaxy, many of which would be invisible to our eyes behind layers of dust, are also laid bare in infrared light. [Image description: A large, long portion of one of the spiral arms in galaxy M51. Red-orange, clumpy filaments of gas and dust that stretch in a chain from left to right comprise the arm. Shining cyan bubbles light up parts of the gas clouds from within, and gaps expose bright star clusters in these bubbles as glowing white dots. The whole image is dotted with small stars. A faint blue glow around the arm colours the otherwise dark background.]](https://images.newscientist.com/wp-content/uploads/2026/05/13114322/SEI_296271016.jpg)
