Messenger discovered that the planet Hema is contracting

An extraordinary number - 11 articles - were devoted this weekend in the journal Science to the discoveries made during the passage of Messenger near the planet in January of this year. Volcanic chimneys were discovered on the sides of a large crater, and that the craters are flatter than their lunar counterparts

Scientists have argued among themselves about the origin of the smooth planes of the planet Mercury and the origin of the planet's magnetic field for over 30 years. Now analysis of data from the Mercury Messenger flyby in January 2008 showed that volcanoes were involved in the formation of these plains and suggested that the magnetic field was actively generated in the planet's core.

The scientists also observed for the first time the chemical composition of the surface of the planet Mercury. The tiny spacecraft probed the composition of Mercury's thin atmosphere, sampled ionized particles near the planet, and demonstrated the connections between the two series of observations and homers on the surface. The results were published in a series of 11 papers in a special issue of the journal Science on Friday, July 4.

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The debate about the origin of the flat plains on the planet Mercury began with the Apollo 16 mission to the moon in 1972. The data collected by the astronauts showed that lunar plains come from materials ejected from large meteorite impacts and created smooth "pools". When Mariner 10 photographed similar formations on the planet Mercury in 1975, some scientists believed that a similar process was occurring there as well. Others have argued that the material that formed the flat plains originated from lava, but due to the absence of volcanic chimneys or other volcanic formations in the images from this mission, it was impossible to decide the question.

Six of the papers in Science deal with the analysis of the planet's surface using its light reflectance and color variation, surface chemistry, high-resolution photographs at different wavelengths, and height measurements. The researchers found evidence of volcanic vents along the rim of Caloris Basin, one of the youngest impact craters in the Solar System. They also found that Caloris has a much more complicated geological history than previously thought. Altimeter data from all the spacecraft that visited Mercury also found that the craters on the planet are on average half as shallow as those on the Moon. These measurements also proved that the geological history of Mercury is complex.

The core of Mercury occupies about 60% of its mass, twice that of the other terrestrial planets. The approach flight revealed that the magnetic field created in the outer part of the core and driven by the cooling of the core, causes dynamic and complex interactions between the interior of the planet, its surface, the exosphere and the magnetosphere.

Commenting on the importance of the core to the geological structures on the surface, MESSENGER's principal investigator, Sean Solomon of the Carnegie Institution in Washington, says: "The dominant tectonic influence on the planet Hema, including areas photographed for the first time by MESSENGER, are formations known as lobate scarps - Huge cliffs that mark the heads of defects in the planet's crust that are formed during the contraction of the areas surrounding them. They tell us how important the core ceiling was to the evolution of the surface. After the end of the great bombardment period at the beginning of the days of the solar system, the cooling of the planets started the magnetic dynamo, causing the entire planet to contract. The data from the approach flight also indicate that the rate of contraction is at least a third greater than what we believed so far."

The flyby also provided the first observation of the ionized particles in the unique exosphere of the planet Hema. The exosphere is an extremely thin atmosphere where the molecules are so far apart that they are more likely to collide with the surface than with each other. The planet Hema has an extremely elliptical orbit, and it slows down the rate of its rotation and the interactions of the particles with the magnetosphere, the interstellar medium and with the solar wind. This result causes seasonal differences and the difference between day and night in the way the particles behave.

For information on the NASA website