Mercury Is Tectonically Active & Shrinking

Mercury is a fascinating planet. As our Suns’ closest orbiting body, it experiences extremes of heat and cold, has the most eccentric orbit of any Solar planet, and an orbital resonance that makes a single day last as long as two years. But since the arrival of the MESSENGER probe, we have learned some new and interesting things about the planet’s geological history as well.

For example, images that were recently obtained by the NASA spacecraft revealed previously undetected landforms – small fault scarps – that appear to be geologically young. The presence of these features have led scientists to conclude that Mercury is still contracting over time, which means that – like Earth – it is tectonically active.

In geology, fault scarps refer to small step-like formations in the surface of a planet, where one side of a fault has moved vertically relative to the other. Previously, scientists believed that Mercury was tectonically dead, and that all major geological activity had taken place in the planet’s early history.

Small graben, or narrow linear troughs, have been found associated with small fault scarps (lower white arrows) on Mercury, and on Earth’s moon. The small troughs, only tens of meters wide (inset box and upper white arrows), likely resulted from the bending of the crust as it was uplifted, and must be very young to survive continuous meteoroid bombardment. Credits: NASA/JHUAPL/Carnegie Institution of Washington/Smithsonian Institution
Images showing small fault scarps and trough (lower and upper white arrows) found on Mercury;s surface. Credits: NASA/JHUAPL/Carnegie Institution of Washington/Smithsonian Institution

This was evidenced by features spotted by the MESSENGER and Mariner 10 probes, both of which found evidence of large wrinkle ridges and fault scarps on the surface. The features were reasoned to be the result of Mercury contacting as it cooled early in its history (i.e. billion of years ago).

This action caused the planet’s crust to break, forming cliffs up to a kilometer and a half (about 1 mile) in height and hundreds of kilometers long. However, as the MESSENGER team noted, these small scarps were considerably younger, dating to about 50 million years of age.

They concluded that the scarps would have to be this young in order to survive bombardment by comets and meteoroids, a common occurrence on Mercury. They also noted their resemblance to similar features on the Moon, which also has young scarps that are the result of recent contraction.

The team’s findings were reported in a paper titled “Recent Tectonic Activity on Mercury Revealed by Small Thrust Fault Scarps“, which appeared in the October issue of Nature Geoscience.

The MESSENGER spacecraft has been in orbit around Mercury since March 2011. Image Credit: NASA/JHU APL/Carnegie Institution of Washington
The MESSENGER spacecraft has been in orbit around Mercury since March 2011. Credit: NASA/JHU APL/Carnegie Institution of Washington

As Tom Watters, the Smithsonian senior scientist at the National Air and Space Museum and the lead author of the paper, stated in a NASA press release:

“The young age of the small scarps means that Mercury joins Earth as a tectonically active planet, with new faults likely forming today as Mercury’s interior continues to cool and the planet contracts.”

The findings were made during the last 18 months of the MESSENGER mission, during which time the probe lowered its altitude to get higher-resolution images of the planet’s surface. The findings are also consistent with recent findings about Mercury’s global magnetic field, which appears to be powered by the planet’s slowly-cooling outer core.

As Jim Green, NASA’s Planetary Science Director, said of the discovery:

“This is why we explore. For years, scientists believed that Mercury’s tectonic activity was in the distant past. It’s exciting to consider that this small planet – not much larger than Earth’s moon – is active even today.”

All told, these findings have let scientists know that the planet is still alive, in the geological sense. It also means that that there is likely such as thing as Mercury-quakes, something which NASA is sure to follow up on if and when a lander mission (equipped with seismology instruments) is dispatched to the surface of the planet.

Further Reading: NASA, Nature Geoscience

Mercury Shrinking: the First Rock from the Sun Contracted More than Once Thought

Whatever Mercury’s did to trim down its waistline has worked better than anyone thought — the innermost planet in our Solar System has reduced its radius* by about 7 kilometers (4.4 miles), over double the amount once estimated by scientists.

Of course you wouldn’t want to rush to begin the Mercury diet — its planetary contraction has taken place over the course of 3.8 billion years, since the end of the Late Heavy Bombardment. Still — lookin’ good, Mercury!

These findings come thanks to the MESSENGER spacecraft, in orbit around Mercury since 2011. Now that MESSENGER has successfully mapped literally all of Mercury’s surface, detailed measurements of more than 5,900 landforms created by cooling and contraction of the planet’s crust have allowed researchers to more precisely determine its geologic history and answer some decades-old questions raised by Mariner 10 images.

“This discrepancy between theory and observation, a major puzzle for four decades, has finally been resolved,” said MESSENGER Principal Investigator Sean Solomon. “It is wonderfully affirming to see that our theoretical understanding is at last matched by geological evidence.”

This image shows a long collection of ridges and scarps on the planet Mercury called a fold-and-thrust belt. The belt stretches over 336 miles (540 km). The colors correspond to elevation—yellow-green is high and blue is low. Image courtesy NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.
This image shows a fold-and-thrust belt stretching over 540 km on Mercury. The colors correspond to elevation— yellow/green is high and blue is low. (Courtesy NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.)

Using high-definition images acquired with MESSENGER’s MDIS (Mercury Dual Imaging System) instrument, planetary geologist at the Carnegie Institution of Washington and study lead author Paul Byrne and his colleagues identified 5,934 lobate scarps and wrinkle ridges on Mercury that are the result of contraction. From measurements of these features, the team determined that the planet’s radial contraction was much more than that estimated by models based on incomplete imaging from NASA’s Mariner 10 mission — the very first spacecraft to visit (but not orbit) Mercury.

Watch: Fly Across Mercury with MESSENGER!

“These new results resolved a decades-old paradox between thermal history models and estimates of Mercury’s contraction,” said Byrne. “Now the history of heat production and loss and global contraction are consistent.

“Interestingly, our findings are also reminiscent of now-obsolete models for how large-scale geological deformation occurred on Earth when the scientific community thought that the Earth only had one tectonic plate,” Byrne said. “Those models were developed to explain mountain building and tectonic activity in the nineteenth century, before plate tectonics theory.”

Unlike Earth, Mercury has only one global tectonic plate.

The findings were published in the Sunday, March 16 edition of the journal Nature Geoscience.

Source: MESSENGER press release. Read more about tectonic features on Mercury here.

*Mercury’s current radius is  2,440 kilometers (1,516 miles).