Mercury, Smallest Planet in the Solar System, Is Still Shrinking and Getting More Wrinkles

Astronomers have long known that Mercury has been shrinking for billions of years. Although it is the closest planet to the Sun, its interior has been cooling down as the internal heat is released, suggesting that the rock it is made of has slightly contracted in volume.

It is unknown, however, to what extent Mercury is still shrinking today, and if so, for how long it is likely to continue. A new study conducted by experts provides fresh insight into this phenomenon.

A Shrinking World

The first evidence of Mercury's shrinking condition was observed in 1974 when the Mariner 10 mission sent photographs of scarps snaking their way across the terrain of the minor planet. The Messenger space probe, which orbited Mercury from 2011 to 2015, showed more of these scarps in all parts of the planet.

Because of the shrinking of Mercury's interior, its crust has less area to cover. As a response, the surface develops 'thrust faults,' or the areas where a tract of terrain gets pushed over the nearby terrain. This resembles the wrinkles created on an apple's surface as it ages, except that an apple shrinks because it gets dried out. In contrast, Mercury shrinks because of the thermal contraction of its interior.

From these observations, it was deduced that thrusts, or gently dipping geological faults, approach the surface below each scarp. They are also attributed to the shrinking of Mercury's radius by 7 kilometers.

Astronomers also tried to find out when this phenomenon happened. The accepted method to determine the age of Mercury is by counting the density of impact craters, where the older surface indicates more craters. However, this method is tricky since the rate of impacts that generate the crater was much greater in the past.

It was accepted that Mercury's scarps must be fairly ancient because even if they cut through older craters, a few younger craters are superimposed upon the scarps. The consensus view is that the scarps on the planet are mostly about 3 billion years old.

Search for More Clues

Evidence has been sparse until now. A team of researchers studying this event discovered ambiguous signs that many scarps have continued to move in geologically recent times even if they started billions of years ago.

Researcher Ben Man noticed that some scarps contain small fractures lying on their stretched upper surfaces. He interpreted them as "grabens," a geological term referring to a strip of ground dropped between two parallel faults.

This event normally happens when the crust is stretched. The overall crust is being compressed in Mercury, so stretching may seem surprising. However, Man realized that these graben happen if a thrust slice of the crust has been bent as it is pushed over the nearby terrain. This is like a piece of toast getting cracked when it is bent.

The grabens are not more than one kilometer wide and less than 100 meters deep. Such small features are assumed to be younger than the ancient structures on which they sit. Otherwise, they would be erased from view by the impacts that tossed materials across the surface in a process called "impact gardening."

Based on the blurring rate that results from impact gardening, it was calculated that most grabens are less than 300 million years old. This suggests that the latest movement along the crust of Mercury must have happened recently.