The planet Mercury has many mysteries of which the new spacecraft MESSENGER hopes to solve, but one of those mysteries is of particular interest to scientists. There are many eccentric features of the small planet closest to the Sun, but one theory suggests that the planet is shrinking. Slowly contracting in on itself as its core freezes, the planet Mercury appears to be indeed getting smaller.

This theory is based on pictures taken from the only spacecraft to visit Mercury, NASA’s Mariner 10, which visited the planet in the 1970s and took photos of approximately 40% of the planet’s surface. The pictures show places where the surface of the planet appears to have buckled from within, leaving randomly strewn scarps across half of the planet. Scientists hope that new images from the MESSENGER mission will shed new light on the surface as well as the core of this rocky planet. Scientists and researchers will look for more signs of buckling on the surface where the Mariner 10 was not able to get photos. Also surface samples will be collected to examine composition data on material that may have once come from the interior of Mercury.

The MESSENGER named as such for Mercury Surface, Space Environment, and Geochemistry and Ranging, launched its mission on August 3rd of this year and plans to reach Mercury in March of 2011. MESSENGER will pass the planet three times before entering orbit.

One of the scarps discovered on the surface of Mercury that helped scientists discover Mercury’s shrinkage, is Discovery Rupes, which cuts a mile into the crust of the rocky planet as it weaves across the planet’s surface. Scarps such as this one would resemble features like fault lines on Earth. However on Mercury, these lines are randomly distributed and don’t appear to have formed due to the filling in processes similar to what can be found on the surface of the Moon.

The theory behind Mercury’s shrinkage comes from scientists’ belief that Mercury’s crust was formed over a gigantic molten core. The core cooled and that is what led to the volume change in the planet. As Mercury’s core cooled and condensed, the surface buckled and broke, giving scientists the view of those large scarps on the outer crust. Most materials, including those on Mercury’s outer layers, contract as they cool; this is unlike water, which expands. Scientists have estimated that the dense and rocky planet has actually shrunk inward anywhere between less than one kilometer and three kilometers, which is not insignificant.

The lack of understanding and information about Mercury’s core is part of the reason scientists can only speculate about what is happening on Mercury. They know that the planet has a very high uncompressed density for its size, and that it has a large metallic, most likely iron core, but what it unknown is far greater. Researchers don’t know exactly how large the core is, or whether any of it is molten. They also don’t know if it rotates to power Mercury’s strong magnetic field.

MESSENGER will give scientists and researchers a better view, which should conform, or at least strengthen their shrinking theories. Unlike the Mariner 10’s 45 % scan of Mercury, MESSENGER will spend a year mapping the entire planet. The clarity of this scan will show enormous amounts of detail previously unseen with the former mission to the tiny planet. The sophisticated cameras on MESSENGER will show details down to 60 feet across, compared to one mile with Mariner10.

To help determine the state and composition of the core, the spacecraft has a Mercury Laser Altimeter instrument that can track how the planet wobbles on its axis. Also, tools will be used to scan the surface and test the composition of Mercury’s ancient lava flows, which will help scientists determine the composition of the mantle.

More than half of the planet Mercury is a mystery. Hopefully the mystery of Mercury’s shrinking size will be solved after MESSENGER visits the planet in 2010.

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