The Solar System:

Mercury

“Mercurius” (“Mercury”) by Giambologna (1529-1608 CE; Flemish), circa 1580.

Credit: Giambologna (National Museum of Bargello, Florence, Italy) [link

Colorful (?) Mercury

This colorful view of both sides of Mercury was produced by using images from the color base map imaging campaign during the space probe MESSENGER's primary mission to Mercury. These colors are not what Mercury would look like to the human eye, but rather the colors enhance the chemical, mineralogical, and physical differences between the rocks that make up Mercury's surface. Young crater rays, extending radially from fresh impact craters, appear light blue or white. Medium- and dark-blue areas are a geologic unit of Mercury's crust known as the "low-reflectance material", thought to be rich in a dark, opaque mineral. Tan areas are plains formed by the eruption of highly fluid lavas. The giant Caloris basin is the large circular tan feature located just to the upper right of center of the upper image. The crater in the upper right of the lower image, whose rays stretch across the planet, is Hokusai.

Credit (image and some text): NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington [link] [link]

The Mercury Dual Imaging System (MDIS) on the MESSENGER space probe had 11 narrow-band spectral filters covering visible and near-infrared wavelengths (400 to 1050 nanometers). The specific colors of the filters were selected to discriminate among common minerals. Three-color images (480 nm, 560 nm, 630 nm) were combined to produce an approximation of Mercury's true color as might be seen by the human eye (left). From this rendition of Mercury it is obvious that color differences on the surface are small. Statistical methods that utilize all 11 filters in the visible and near-infrared highlight subtle color differences (right) and aid geologists in mapping regions of different composition.

Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington [link]

Watch This! Crash Course Astronomy #13: Mercury

Many (big) craters!

Mercury has more craters on its surface than any other planet in our solar system.

There is effectively no atmosphere on Mercury, so meteors don’t burn up before impact, and there is no erosion of craters by wind.
There have been no recent volcanos or lava flows on Mercury and, hence, no “repaving” of its surface.
Even though Mercury is the smallest of the terrestrial planets and has the weakest gravity, its fast orbital velocity around the Sun means that meteors hit the surface harder than on other planets, making bigger craters.

Mercury's cratered southern hemisphere.

Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington [link]

Surface cracks

There are numerous large cracks in Mercury's surface called rupes (“cliff” in Latin).
The rupes are belived to have resulted when Mercury's core shrank slightly (by 0.1-0.3% in diameter) as it cooled, causing the solid surface to crack.
The rupes range in height from a few hundred meters to about three kilometers and can extend for hundreds of kilometers (the Grand Canyon on Earth is 1.9 km deep and 450 km long).

Victoria Rupes on Mercury in an image from the MESSENGER space probe. Rupes on Mercury are named for ships of discovery, and Victoria Rupes is named for the Victoria that formed part of Ferdinand Magellan's fleet in his 1519-1522 effort to circumnavigate Earth. The crater near image center that covers part of the rupes has a diameter of 42 km.

Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington [link]

Mercury has a very large iron core

Comparison of the internal structures of Earth and Mercury based on data from the MESSENGER mission show that Mercury’s interior has a larger ratio of metallic core material to silicate rock material than Earth. Mercury also appears to have a solid layer of iron sulfide that lies at the top of the core. The presence of this solid layer places important constraints on the temperatures within Mercury’s interior and may influence the generation of the planet’s magnetic field. The inset shows a comparison of the relative radial sizes of Earth and Mercury. Earth's core volume is 17% of its total volume, whereas Mercury's core volume is 40% of its total volume.

Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington [link]

Mercury has a tail!

Material blasted off Mercury’s surface by the solar wind trails behind it in orbit (like the tail of a comet).

(top) An image of Mercury’s tail obtained from combining a full day of data from a camera onboard the STEREO-A spacecraft orbiting the Sun. The sunlight reflected off the planet's surface results in a type of over-exposure that causes Mercury to appear much larger than its actual size. The tail-like structure extending anti-sunward from the planet spans an angular size exceeding that of the full Moon in Earth's night sky.

(bottom) A schematic representation of the viewing geometry that allows the STEREO-A satellite's camera systems to make observations of Mercury’s tail.

Credit: This work was sponsored by grants from NASA to Boston University and by research funds from its Center for Space Physics, and was conducted in collaboration with colleagues at the Rutherford Appleton Laboratory in England and the University of Adelaide in Australia. [link]

There is water on Mercury

Water ice on Mercury is found in the bottoms of deep craters at the planet's poles that are always in shadow.
The water was likely delivered to Mercury via past asteroid impacts (like much of Earth's water).

Mercury's north pole from MESSENGER data. Red denotes areas that are always in shadow. Yellow shows the locations of bright (i.e., highly reflective) polar deposits (identified as water ice) imaged by Earth-based radar.

Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington/National Astronomy and Ionosphere Center, Arecibo Observatory [link]

Mercury's unique orbit

Mercury has the most elliptical orbit of any planet in our solar system.
Mercury is tidally locked in a 3:2 ratio of rotation period to orbital period.
- Mercury rotation = 59 days
- Mercury orbit (“year”) = 88 days
- 3 Mercury rotations = 2 Mercury orbits
- If you were standing on Mercury's surface, it would take the Sun 2 Mercury years (176 Earth days) to go around the sky once (what we call a day on Earth).
Earth’s Moon is tidally locked in its orbit around Earth in a different way: with “day” and “year” of equal lengths (1:1 ratio).

Orbits of three objects in our solar system: Earth, Mercury, and Comet Halley. Mercury's orbit has the largest eccentricity of all of the planets in our Solar System, yet its orbit is still very circular (the red dashed lines show only a small vertical deformation in the shape of Mercury's orbit compared to Earth's nearly circular orbit). But because the foci of Mercury's orbit have a significant separation compared to Earth's, Mercury's aphelion (furthest distance from the Sun; 0.47 AU) and perihelion (closest distance to the Sun; 0.31 AU) are significantly different. Comet Halley has a highly eccentric orbit, and spends most of the time far away from the Sun.

Credit: D. W. Hoard (2018)

Evidence supporting Einstein's General Relativity

Precession of Mercury’s orbit was the first evidence that Einstein’s Theory of General Relativity was correct. General Relativity describes how objects move close to a massive object (like the Sun) due to the gravitational warping of spacetime. Einstein's theory predicts slightly different behavior than the classical (Newtonian) Law of Gravity. The observed precession of Mercury's orbit matches the prediction of General Relativity.

Mercury orbits the Sun in an ellipse that gradually rotates (precesses) over time.

Credit: E. Otwell (ScienceNews) [link]

References

Surface cracks

Choi, C. Q. 2015 (June 10), Space.com, "Mystery on Mercury: Strange Pattern of Huge Cliffs Defy Explanation"

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