Celestial Twins – What a Wonder!

Celestial Twins

Take a close look at the images above, do you recognize anything? Do you think the images are of the same object? You may be surprised to find out that these images are in fact of two different objects, one a satellite and one a planet.

On the right we see the familiar face of our Moon, on the left we see what would be reasonable to assume is the back side of the Moon, but it is actually Mercury. How can a planet and our Moon look so similar? Today we will explore the similarities and differences between our Moon and tis fraternal twin, Mercury


Looking at the surfaces of these object they could be mistaken as twins. Both grey with craters covering their surfaces from bombardments of meteorites at their formation and throughout their lives. A difference to note between the craters we see on Mercury and those on the Moon are the rays surrounding the craters.

Crater rays are formed from dust and other crushed material that is thrown onto the surface from the impact sight. These light streaks exploding out from the central crater are much more prominent on Mercury’s surface than on the lunar surface. This suggest that these rays are mainly made up of finer-grained material.

One explanation for this is that the rayed craters on Mercury are younger and therefore less eroded by space than the ones we see on the Moon

We see so many impact craters on their surfaces because neither Mercury nor the Moon have thick atmospheres, they actually have very thin unprotective atmospheres, called exospheres, leaving their surfaces unprotected from the ravages of space.

Exploring more of their surfaces we find even more similarities between the Moon and Mercury with very similar surface composition. Both object’s crusts contain large amounts of oxygen and silicon that form mineral silicates. Traces of elements such as aluminum and titanium give the Moon’s surface a reflective quality, and we find this to be true as well for Mercury

While there are many similarities between the surfaces of these bodies, as we dig deeper, we find many characteristics that distinguish these objects. Both the Moon and Mercury appear grey to the naked eye, but with advanced telescopes astronomers can digitally enhance their natural color.

When this is done to Mercury, we see the planet come to life with blues, yellows, and bits of orange.  

Surfaces (2)

Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

However, when this enhancement process is done to the Moon we still see mainly greys and browns. 


Image Credit: NASA

The color of any object, a planet, a rock, or a vegetable, is determined by what it is made of. Though the surfaces of these objects have some similar composition, their true colors reveal that their overall compositions are more different than alike. 

Volcanic Activity

Volcanic Activity

During its formation the solar system was a hot and volatile place. Many celestial bodies still have or show evidence of volcanic activity. Both the surfaces of the Moon and Mercury indicate previous volcanic activity. This can be seen in what appears to be more craters but are actually believed to be dormant volcanoes.

Unlike the mountainous volcanoes we see on Earth, many of the volcanic structures found on Mercury are likely from lava oozing out of vents in the crust. This lava spread from the vents and settled over Mercury’s surface. On the far side of the Moon, we see steep sided domes that indicate similar volcanic activity may have occurred here as well. Moreover, the familiar dark regions on the face of the Moon are vast plains of once liquid rock and lava. 

Water Ice

Water Ice

While it is clear neither the Moon nor Mercury have liquid water, there is evidence of ice on both their surfaces. NASA’s MESSENGER (Mercury Surface, Space Environment, Geochemistry and Ranging) mission discovered, that despite the heat on Mercury, there is hydrogen in amounts consistent with frozen water at Mercury’s North and South poles.

These ice deposits exist in perpetual shadows where sunlight never reaches. Recent investigations of the Moon’s Shackleton Crater by NASA’s Lunar Reconnaissance Orbiter suggest that there may be ice on the floor of the crater, also only surviving in perpetual shadows. Ironically, despite the Moon being colder, Mercury has larger stores of ice on its surface. 

Key Differences

Key Differences

Other key differences between these fraternal twins comes down to the basics. Mercury is larger with a diameter of 4,879 km while the Moon’s diameter is 3,474 km. Mercury is much heavier than the Moon weighing in at 3.285 x 1023 kg and the Moon at 7.35 x 1022 kg.

While both the Moon and Mercury have a central iron core, the Moon’s core does not generate a magnetic field. The Moon’s core is not large enough to have a convective force to produce a magnetic field, however Mercury’s core is and does produce a magnetic field, though not a very strong one. Mercury’s magnetic field is approximately 1% the strength of Earth’s magnetic field. 

While at surface level the Moon and Mercury are seemingly identical, they have many key differences. Studying their differences and similarities give us great insights into the formation of planets and other objects in our solar system. 


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Cassie Hatcher

Cassie Hatcher

Cassie is a lifelong learner with a passion for communicating high level science in a conversational matter. She holds a B.S. and M.S. in physics and has written two astronomy theses, one of which is published. She earned an internship at NASA’s Goddard Space Flight Center in 2016 and got the chance to see the James Webb Space Telescope while it was being built.

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