Humans have developed the ability to detect rocky planets in the habitable zones of distant stars. The day will come when we have to make some very expensive decisions about which planets are worth visiting to either colonize or search for life.
How do we make those decisions? New research into the geology of the planet Mercury could help. We finally have something else to compare to Earth's active geology—and maybe, a system that could teach us more about the conditions necessary for life.
Mercury turns out to be currently tectonically active. Other than Earth, it is the only rocky planet in this solar system that is still slowly thrusting up parts of its crust and changing the surface over time. This means that we finally have something else to compare Earth's active geology with.
“Together with the tectonic history, it paints a whole new picture of what Mercury's history must have been like,” says Thomas Watters, senior scientist of the Smithsonian's Center for Earth and Planetary Studies at the National Air and Space Museum and lead author of a new paper on Mercury's geology. “It puts Mercury very close to Earth in terms of very slow cooling that allows the outside to remain cool and the inside hot.”
Mercury is a tough little planet to study. Bigger than our moon but much smaller than the Earth, it orbits tightly around the sun. Temperatures range from 800 degrees to -280 degrees Fahrenheit, but it is a rocky planet made of similar stuff as the Earth. Mercury is a long way away and its close proximity to the sun means that there is a lot of gravity to fight against. It takes more fuel to visit Mercury than it does to leave the solar system. NASA visited for the first time when the Mariner 10 spacecraft flew past it in 1974.
“Mariner 10 imaged less than a full hemisphere, but a good chunk” of Mercury's surface in low-resolution, says Watters. “Big thrust fault scarps that indicate that the crust had been fused together and contracted was evident in those images.”
The Mariner 10 mission showed us that Mercury had been active billions of years ago. Scientists could look at long cliff-like escarpments, or “scarps,” and see where the surface of the planet had been thrust upwards. The density of craters from meteor impacts allowed them to work backwards and figure out roughly how long ago those scarps had formed. The mission also found that Mercury had at least the remnants of a weak magnetic field.
But was all that in the distant past? A more recent mission to orbit Mercury using the MESSENGER spacecraft was launched in 2004 and gathered data until it crashed in 2015. It was data from the end of the decaying orbit, as the spacecraft was on its way to add a new crater to the surface of the planet, that allowed Watters and his colleagues to understand what is still happening on Mercury.
Originally, MESSENGER was supposed to map the surface from a very high orbit right up until it ran out of fuel and would crash. But NASA changed plans along the way. The life of the mission was already limited by the close gravitational influence of the sun, so they took a small risk.
Because of the force of the solar tides, says Watters, “there is no way you could keep a spacecraft in an orbit around Mercury for long.”
NASA decided to send MESSENGER into a terminally low orbit that would allow them to get closeups of part of the surface before the end. It worked.
“When we lowered the altitude we got [camera resolution of the surface] down to one to two meters per pixel in some places,” says Watters. “It was like a new mission. It meant that the spacecraft was doomed, but that was going to happen anyway... The big news finding in these low altitude final campaign MESSENGER images is that we found very small versions of these big scarps that we've known were on Mercury since Mariner 10.”
The small scarps are clearly recently formed (with minimal impacts from meteors) and they show that the surface of Mercury has continued to change relatively recently, on a scale of millions of years rather than billions. The data proved that Mercury's formation and ongoing geology are a lot like that of Earth. It has an ongoing plate tectonic system, but with a key difference versus ours.
“Earth's shell is broken up among about a dozen plates that cause most of the tectonic activity on Earth,” says Watters. “On Mercury, we don't have any evidence for a series of plates. Mercury seems to be a one-plate planet. That shell is uniformly contracting. We don't really understand why the Earth developed this mosaic of plates. But it's what keeps the Earth from contracting.”
Mercury still has a molten core, like Earth does. As Mercury's core slowly cools, the density of that core increases and it gets slightly smaller. When it shrinks, the cooler, rocky outer crust collapses slightly, creating the scarps and causing the planet to slightly contract. The contractions have probably removed one to two kilometers from Mercury's diameter in the last 3.9 billion years.
Mars, the closest thing to another habitable planet in our solar system, is also a rocky planet made up of similar material as Mercury, Venus and Earth. But it seems to have a core that is only partially molten. It has no active tectonic plate system. Long ago, Mars had both a magnetic field and an atmosphere. When the field disappeared, the atmosphere gassed off into space.
Could there be a relationship between molten cores, plate tectonics, and the magnetic fields that allow a dense atmosphere to exist?
“What we've found now from Mercury is that there's no other planet we know of that is tectonically active,” says Watters. “Trying to understand how rocky planets evolve in this solar system. . . . what is the spectrum of evolution on a rocky body? Is plate tectonics a necessary element of developing life on a rocky planet? There are some really important things to learn about.”