The innermost planet is shrinking faster than first calculated

Mar 18, 2014 12:40 GMT  ·  By

A group of investigators based at the Carnegie Institution for Science (CIS) argue in a new study that Mercury, the innermost planet in our solar system, is losing mass at a very fast pace, significantly higher than what other studies have determined. The fact that the small world is shrinking was discovered long ago, but uncertainties still persisted as to the rate at which this process occurs.

The data used in this investigation was collected by the NASA MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft, which has been orbiting Mercury since March 18, 2011. The probe was launched into space on August 3, 2004, aboard a Delta II rocket, from Space Launch Complex 17B at the Cape Canaveral Air Force Station, in Florida.

According to data returned to Earth by the orbiter, Mercury's crust has contracted by more than 7 kilometers (4.4 miles) as it cooled, a value higher than previous estimates. This discovery helps scientists settle a decade-long debate related to what happened to the innermost planet billions of years ago.

“These new results resolved a decades-long paradox between thermal history models and estimates of Mercury's contractions,” says CIS investigator Paul Byrne, the lead author of the new study. Details of the analysis appear in the March 16 issue of the top journal Nature Geoscience, Space reports.

It is important to keep in mind that the geology of Mercury is very different from that of Earth. Whereas our planet features 12 large tectonic plates sliding over an ocean of molten magma, the inner planet features a single tectonic plate, which makes up its entire surface. The mantle and crust on Mercury have a collective thickness of just 420 kilometers (260 miles).

On the other hand, the iron core at the center of the plant is roughly 4,040 kilometers (2,500 miles) in diameter. By comparison, Earth's mantle alone is 2,900 kilometers (1,800 miles) thick, with the crust contributing an additional 40 kilometers (25 miles). Despite its small size, Mercury continues to shrink.

The first signs of shrinkage were recorded in the 1970s by the NASA Mariner 10 mission. This was made obvious by structures called wrinkle ridges, which develop in the crust as the planet cools and its molten iron core solidifies. Since MESSENGER is orbiting Mercury, not just flying past, Byrne's team had access to many more sample images.

A total of 5,935 wrinkle ridges were analyzed for this investigation. While Mariner 10 data suggested Mercury had lost 1 to 2 kilometers (0.6 to 1.24 miles) of its radius since forming, MESSENGER images painted a much direr picture, where the planet lost 7 kilometers (4.4 miles) of its diameter.

“The discrepancy between theory and observation, a major puzzle for four decades, has finally been resolved. It is wonderfully affirming to see that our theoretical understanding is at last matched by geological evidence,” explains Sean Solomon, the principal investigator for the MESSENGER mission.