Being a member of a science team of a planetary mission is like being a starter on a major league baseball team—you’re in the game. That’s how I felt as a member of the MESSENGER mission to Mercury. During the final months of MESSENGER’s time in orbit, before the fuel on the spacecraft was expended and crashed on Mercury’s surface, a decision had to be made—keep the spacecraft in its nominal mapping orbit as long as possible or let the spacecraft altitude drift lower to get as close to the planet as possible. It was an easy vote for me because a lower altitude made it possible to image the surface at much higher resolution than had been possible in the mapping orbit. Those low-altitude images would open a whole new window on Mercury, revealing small landforms and details of larger landforms that were beyond the limits of the previous orbital images. I was hoping to find a very specific landform, one I had become very familiar with on another object much closer to home, the Moon.
The topography of the northern hemisphere shows the highs and lows of Mercury. The closest approach of the MESSENGER spacecraft to Mercury was in the northern hemisphere. During the low-altitude campaign the small, young fault scarps were found in highest resolution images of that region.
The first images of Mercury from the flybys of Mariner 10 in the mid-1970s showed large fault scarps, cliff-like landforms that look like a giant stair-step in the landscape. MESSENGER confirmed that these large fault scarps were evidence that Mercury had experienced global contraction. As Mercury’s interior cooled, it caused the crust to shrink and be pushed together, break, and thrust upward along faults, making cliffs up to hundreds of kilometers long and over a kilometer high. We knew from MESSENGER images that Mercury had contracted over a long time, but what we didn’t know was if the planet was still shrinking. What would tell us if Mercury was still shrinking would be to find very small fault scarps, scarps so small that they’d have to be very young because small landforms cannot survive the steady meteoroid bombardment. These small scarps would be the tectonic saplings that, with enough contraction, could grow in size to become the giant redwoods of Mercury’s fault scarps.
Sure enough, after MESSENGER’s altitude drifted lower and lower, and the spatial resolution of the images got higher and higher, the first small fault scarps began to be detected. It turns out that Mercury’s small scarps are virtually identical to small, young fault scarps we’ve found on the Moon in images from the Lunar Reconnaissance Orbiter Camera that are evidence the Moon is shrinking. This discovery means that Mercury joins Earth as a tectonically active planet in our solar system and that Mercury’s interior, like Earth’s, is still slowly cooling.
MESSENGER confirmed that the contraction of Mercury resulted in a global array of lobate scarps, tectonic landforms that are the surface expression of thrust faults. Enterprise Rupes is the Goliath of lobate scarps on Mercury (top, white arrows). At about 1000 km long, comparable in length to Earth’s San Andreas fault, and with over 3 km of relief it is the largest lobate scarp on Mercury. Images obtained after MESSENGER’s altitude was lowered during the last phase of its mission have revealed a population of small fault scarps (bottom, white arrows) that can be more than an order of magnitude smaller in size than their larger counterparts. These small scarps are less than 10 km in length and have only tens of meters of relief. They are comparable in size and morphology to small fault scarps imaged on the Moon by the Lunar Reconnaissance Orbiter.
Something else that goes hand-in-hand with tectonic activity is seismicity. On Earth, slip events on active faults are directly connected to earthquakes; so, slip events on newly formed small faults and reactivated older large faults on Mercury will create "mercuryquakes." If seismometers are deployed on Mercury in future missions, from movement on active faults should be detected.
I was indeed fortunate to be a player on the MESSENGER team. A tectonically active Mercury is just the latest in a long list of discoveries made possible by the mission, and you can expect many more to come.