Secrets Hidden Underneath Mercury’s Surface

FROM THE LECTURE SERIES: A Field Guide to the Planets

By Sabine Stanley, Ph.D., John Hopkins University

What do the craters and the holes reveal about Mercury? Could there still be water on the surface of this small planet? What other secrets are hidden underneath the surface of Mercury? Let’s explore!

A 3D illustration of the surface of Mercury under the dark sky.
A 3D equirectangular projection displaying craters and holes present on the surface of Mercury. (Image: Jurik Peter/Shutterstock )

What would we witness on the surface of Mercury? If we look more closely at Mercury’s gray surface, we notice that some spots on Mercury are visibly brighter and some spots are darker. The brighter spots, looking gray or even white, are mostly associated with recent impact craters. When a meteor hits Mercury, it excavates a hole and brings new material up to the surface from depth. This fresh material hasn’t had as much exposure to space weathering and can appear brighter than the older material.

But there are also some regions that are anomalously dark on Mercury. Initially, it was thought that these regions might have higher iron content since iron is what makes part of the Moon darker. But the last days of the MESSENGER mission, as it came closest to Mercury, revealed that these regions are enriched in graphite. That’s right, pencil lead.

Carbon deposits are likely to have floated to the top of Mercury’s original crust, back when Mercury’s magma ocean solidified around 4.5 billion years ago. Those impacts may also be exposing the graphite crust. In addition, a lot of Mercury’s surface carbon may have come from carbon meteorites.

One estimate suggested Mercury has experienced 50 times more impacts from carbon meteorites than the Moon. When you’re so close to the Sun, you’re much more likely to get hit by stuff falling towards the center of the solar system.

The Craters of Mercury

Any way you look at it, Mercury is densely covered with craters. Over 30,000 craters with diameters larger than 5 kilometers have been cataloged from the images of the planet. Almost 400 of the largest craters are named after musicians, writers, and artists.

Mercury’s largest crater, Caloris, was formed about 3.9 billion years ago. At 1550 kilometers in diameter, it is one of the largest impact craters in the solar system. For comparison, the biggest known crater on Earth, Vredefort crater in South Africa, is only around 300 kilometers in diameter. That’s 5 times smaller than Caloris.

An image of Caloris, the biggest crater on the surface of Mercury.
MESSENGER spacecraft displaying Caloris, the biggest crater present on the surface of Mercury. (Image: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington/Public domain)

Caloris was formed by an impacting object at least 100 kilometers in diameter. This impact was so devastating that it left signatures, both near and far. In addition to the crater created by the impact, the impact’s energy resulted in volcanic lava that filled the crater as well as surrounding regions, forming smooth volcanic plains. That was the near effect.

This is a transcript from the video series A Field Guide to the Planets. Watch it now, on The Great Courses Plus.

Secondary Craters on Mercury’s Surface

But even if we look as far away as possible on the planet, we see unusually hilly terrain. This region may have been shaped by shock waves that traveled through the whole planet and concentrated at the antipodal point. Or ejecta from the impact itself may have traveled all the way around to the other side of the planet and converged at this point.

An enhanced image of Munch, Sander and Poe craters amid volcanic plains.
Secondary craters are caused by the impactful release of material from the primary craters. (Image: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington/Public domain)

Because Mercury is so close to the Sun, objects that impact Mercury is usually moving faster than objects that impact the other planets. All those fast-moving objects striking Mercury’s surface mean that Mercury has more secondary craters than other planets. A secondary crater occurs when material that was ejected from Mercury’s surface during a meteor hit re-impacts the surface with enough energy to cause another, secondary crater.

Learn more about Earth: how plate tectonics sets up life.

What Do Craters Tell about Mercury’s Age?

The sheer number of primary craters also tells us that Mercury’s surface is very old. Older surfaces have more craters because they’ve been around longer and so have been hit by more meteors.

Scientists use this method of crater counting to distinguish the relative ages of different areas on a single planet and also to compare the ages of planet surfaces to each other. Mercury, the Moon, and Mars all have very old surfaces, with some visible features dating back over 4 billion years. What we see on the surface of Venus and Earth, by contrast, is much younger.

Learn more about exploring the Earth-Moon system.

The Mystery of Contraction Scarps and Mercury Hollows

Like all planets, Mercury has cooled over time, but it has preserved evidence of this cooling in a quite dramatic way. Close-up pictures of Mercury reveal some large cliffs or ridges, known as contraction scarps. These are long, wavy cliffs that formed as Mercury cooled and contracted, thereby shrinking its volume. But as its volume shrank, its surface area remained the same, resulting in the wrinkling up of the surface.

These wrinkle ridges are up to 7 kilometers high on Mercury. So the surface of Mercury is not only very old. It’s also very wrinkled. What’s it like to be on a planet with no atmosphere and intense solar radiation? One unusual surface feature giving us clues are the Mercury hollows. These are bright holes that occur in some places on the surface and have a very strange pattern. There are thousands of them on Mercury, and they appear to be quite fresh.

They are mysterious because they haven’t been seen on any other planets and because they don’t have the right shape to be craters or volcanic pits, which are much more circular.

The leading explanation for these features is that they result from sublimation. Some materials in these regions, possibly magnesium and calcium sulfides, are vaporizing from the solid phase. We don’t fully understand all the details of how this is happening, but Mercury has intense solar radiation and no protection from an atmosphere. So it’s possible that material buried slightly under the surface, where it is somewhat protected, can behave very differently if it gets suddenly exposed to the hot vacuum of Mercury’s surface.

Does Mercury Have Water?

But the best surface feature is also the most surprising: Mercury has ice! Yes, Mercury is very close to the Sun, so the daytime surface temperature can get really hot: over 400°C, 750°F. If the planet ever had any water in its past, it would have evaporated from the surface long ago.

And yet, radar images have found really bright radar spots in certain regions near Mercury’s poles that are best explained by the presence of water ice.

But how can we have water ice on Mercury? The answer lies in the unique locations of these radar spots. If we superimpose the radar spots on any image of Mercury, we notice that they are found in portions of crater floors. But the spots don’t cover the whole crater floor. Scientists have realized that these spots are found in the regions of crater floors that are in permanent shadow. This can happen in craters in the polar regions because the Sun is never high in the sky.

Remember, Mercury’s axis of rotation has almost no tilt. So sunlight never hits the same polar spots, all year round. Hence, these permanently shadowed regions experience much colder temperatures than the rest of the planet. This is because Mercury has no atmosphere to transport heat around the planet. So if the Sun doesn’t shine on a particular spot, it’s going to remain extremely cold, say, −170°C. Shadowed Craters on Earth’s Moon also drop to similar temperatures.

Some comets and asteroids contain water. If a water-rich comet or asteroid impacts in the polar regions, then some of the water may remain on the surface, buried in the permanently shadowed regions of polar craters. And we’re not just talking trace amounts, the ice might be 20 meters deep in places and total 100 billion tons or more.

This finding, although cool in its own right, also has important implications for future space travel and the habitability of other planets. Having access to water is considered a crucial resource for astronauts who want to spend a significant amount of time off the Earth.

Mercury has demonstrated that there is water even in the most unlikely of planets. Earth’s moon has similar polar ice deposits, also found in permanently shadowed craters, making future lunar exploration more feasible.

Common Questions about Mercury’s Surface

Q: What is the surface of Mercury like?

The planet Mercury is a little bit like Earth’s moon. Its surface is covered with craters caused by the impacts of fast-moving objects.

Q: What is the surface of Mercury made of?

Mercury has a large liquid metal surrounded by a mantle of silica and a solid outer crust. Mercury’s core accounts for 42% of the planet,

Q: Is there any water on Mercury?

Yes, there are water ice deposits present in Mercury’s shadow craters located near the poles.

Q: What is the color of mercury?

Mercury has a dark gray colored rocky surface that is covered with a thick layer of dust.

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