The Planet Mercury Has Been Shrinking—More Than Scientists Expected

Mercury's tortured surface obscures some signs of its past. —Jpl/NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington Mercury is nobody’s idea of a big cosmic deal.

The solar system’s smallest planet, it measures just 3,023 miles across —or barely 3.6% of the 88,846 mi . diameter of Jupiter , the solar system’s largest.

Saturn’s moon Titan and Jupiter’s Ganymede both also have greater diameters than Mercury .

And according to a new study in Geophysical Research Letters , Mercury has actually put in the work to get as small as it is, spending part of the past 4.5 billion years shrinking—significantly more than planetologists once thought.

That matters.

Figuring out the history of Mercury provides clues to how all rocky worlds like the moon and Mars formed.

And what happens in our solar system surely happens in ones much farther afield in space.

How did scientists measure Mercury’s shrinking? The findings come courtesy of data gathered by the Mercury MESSENGER spacecraft —a ship whose name was derived from the decidedly clumsy acronym “MErcury Surface, Space ENvironment, GEochemistry, and Ranging.” MESSENGER launched in August 2004; reached Mercury in March 2011; and completed 4,105 orbits of the planet before running out of fuel and crashing into the surface on April 30, 2015.

In those four-plus years on the job, MESSENGER did a lot of work, studying Mercury with seven different scientific instruments —including its Mercury Laser Altimeter, a ranging tool that could measure surface features to within one meter ; and a dual imaging system that used two cameras to map the surface in wide- and narrow-angle and black and white and color.

Scientists have known for a long time that Mercury has gotten steadily smaller since its birth.

The planet was formed when dust, gas, rocks, and asteroids left over from the formation of the sun collided and collected, coalescing into a discrete body and generating enormous amounts of heat in the process.

Over the eons much of that heat dissipated, causing Mercury to contract by what astronomers estimated to be 2.5 miles to 10 miles in diameter.

To confirm that conclusion, MESSENGER scientists used the spacecraft’s laser altimeter and dual-imager to scan the planet looking for so-called shortening structures, which are distortions in the surface made up of wrinkles, ridges, and what are known as lobate scarps—cliff-like structures that form when the crust pushes rock upward along a fault.

Shortening structures form when a planet contracts; the greater the number of those surface features, the greater the planetary shrinkage has been.