Astounding Proof of Geologic Activity on Mercury You Wont Believe What We Found
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. NASA/Johns Hopkins' Applied Physics Laboratory
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.
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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.
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The NASA spacecraft MESSENGER sent back high-resolution images of the surface of Mercury that confirmed not only evidence of tectonic activity (arrows show faults and other surface landforms), but that the planet is still geologically active. NASA/Johns Hopkins' Applied Physics Laboratory
“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.
Chapter 5. Mercury
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.”
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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.
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“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.
We Just Found Out That Mercury Is Geologically Active
“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.”
Jackson Landers is an author, science writer and adventurer based out of Charlottesville, Virginia, specializing in wildlife out of place. His most recent book,
Chronicles a year and a half spent hunting and fishing for invasive species and finding out whether we can eat our way out of some ecological disasters.Perspective view looking NW over the Caloris Basin. Pantheon Fossae, radial dike swarm in the foreground. The Impact crater just offset to the right of the swarm is 41 km in diameter. Red and white are higher topography; blues are lower. Total vertical difference is 4 km. Image courtesy NASA/Johns Hopkins University Applied Physics Laboratory (JHU / APL) / Carnegie Institution of Washington/Goddard Space Flight Center
Researchers Trace Mercury's Origins To Rare Meteorite
I did a post four years ago entitled Volcanoes as Heat Engines that touched on the observation that we see things that look like volcanoes on every reasonably sized body with a solid surface in the solar system. In this context, a volcano is simply a location where hot material escapes to a region that is less hot. On the larger bodies, these manifest themselves as volcanoes. We see them on all the planets (other than the gas giants), the largest asteroid Ceres, and several of the larger moons. The smaller moons and comets eject their hot material into space via geysers (Enceladus, Triton) and jets (comets).
Between our former and current blog home, a couple writers (mainly Albert and myself) have written posts about volcanic activity on most of the major bodies in the solar system. Today’s post will look at what looks like volcanic activity on the innermost planet in the solar system, Mercury.
Mercury is the smallest of the terrestrial planets and closest to the sun. It is both the hottest and one of the coldest places in the entire system. It might even have ice in permanently shadowed impact craters at its poles.
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Mercury is 4, 880 km in diameter, smaller than the two largest moons in the system, Titan and Ganymede. It has the most eccentric orbit of any planet, varying from 0.308 – 0.467 AU (AU is Astronomic Unit, the average distance from earth to the sun). It orbits the sun in just under 88 days. Long thought to be tidally locked to the sun, it is now known as having a 3:2 resonance. This means that it rotates three times on its axis every two trips around the sun.
It is close enough to the sun for the sun’s gravity to have a small relativistic effect on its orbit. This may impact its long-term orbital stability. A small percentage of long term orbital simulations done for billions (Ga) of years in the future
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