1. Concepts 2. Solar System Origin 3. Planetary Processes 4. Earth Processes 5. Meteorites
6. Our Moon 7. Remote Sensing 8. Mercury 9. Mars 10. Venus, Our Twin
11. Jupiter & Jovian Moons 12. Saturn, Rings & Moons 13. Uranus 14. Neptune 15. Pluto, Charon & Comets


by Scott Hughes

Reading: Chapter 7, Mercury, and Chapter 12, Surfaces of the Terrestrial Planets, by James W. Head, III


Mercury is the closest planet to the sun. As you might imagine, this proximity to our star would make Mercury a very harsh place to live. Temperatures range from 90K in the nighttime to over 700K in the day. This is the most extreme temperature range in the whole solar system. Mercury is the second smallest planet, larger only than Pluto the farthest planet from our sun. Mercury was named for the "winged messenger of the gods" in Greek mythology because it moves so quickly across the sky as viewed from the Earth.

Mariner 10 is the only spacecraft to have visited Mercury.

In 1973 and 1974, Mariner 10 flew by the planet three times and photographed almost all of the lighted half of the planet. Since no rock samples were taken during this mission, what we know of Mercury has been gleaned from the Mariner 10 photographs and the knowledge of the basic formation of our solar system. Mercury is most closely related to the Earth's Moon because the surface is heavily cratered and very old, but unlike the Moon, it has an internal metallic core that may be partially molten.



Physical Characteristics of Mercury

Mercury's large metallic core makes it much denser than the Moon and it is the most iron-rich planet in the solar system.Surrounding the metallic core is a rigid outer shell, or lithospheric mantle, that is about 500-600km thick. This lithosphere probably has an anorthosite composition which is a rock type made of Ca-rich feldspar, one of many common silicate minerals found on the Earth and on the Moon. Mercury has virtually no atmosphere, with only slight traces of He, K, O2,and Na vapors, and experiences no plate tectonic forces.

Orbital and Physical characteristics of the planet Mercury:

Mean Distance from the Sun relative to Earth 0.387
Diameter at the Equator 4,880 km
Mass relative to Earth 0.055
Volume relative to Earth 0.06
Density 5.4 g/cm3
Gravity at the Surface relative to Earth 0.37
Period of Revolution 88 days
Period of Rotation 59 days
Axis Inclination 28°
Mercury rotates 3 times in 2 of its years! This is the only planet in our solar system with an orbital:rotational resonance ratio of 2:3.

Landforms of Mercury

Mercury's major landforms include impact craters which dominate the landscape, cratered terranes, widespread intercrater plains, multiring basins, and some smooth plains that may have been formed by flood basalts. Though an absolute age is not available now, scientists assume Mercury's heavily cratered regions probably formed during the same period of intense bombardment that formed the lunar highlands. Try to locate each of these features on the mosaic image of Mercury.

Mercurian craters appear to have smaller ejecta blankets than those seen on the Moon, however. This is probably due to the fact the Mercury's surface gravity is almost 2 times that of the Moon. The stronger gravity pulls the ejecta down to the surface faster than on the Moon. Many craters show evidence of bombardment while the lithosphere was still warm and semi-plastic. Some crater bottoms have undergone isostatic rebound after impact or have been filled in by basaltic flows and appear only as ghost craters.

Large multiring basins are seen on both Mercury and the Moon. Most of the multiring basins on Mercury are 200-600 km in diameter. Surely the largest single feature on the face of this planet is the Caloris Basin. Within the 1300 km diameter basin can be seen many smaller craters, indicating a relative age relationship of the older Caloris Basin compared to the younger craters on top of it. This multiring basin was most likely formed by a very large impact of an asteroid early in Mercury's development. In fact, the impact was so large that it affected the opposite side of the planet! When the asteroid hit Mercury, a compression wave rippled though the middle of the planet and caused deformation of the rocks exactly opposite of the Caloris Basin! This antipode consists of hills and linear valleys that measure 500 km across.

Age relations between the intercrater plains and the craters seen on Mercury are somewhat unclear. Some craters appear younger than the plains, but the plains may represent a period of widespread volcanism in Mercury's early history that wiped out many of the older craters and resurfaced much of the planet.

Discovery Quadrangle of Mercury

Mariner 10 photomosaic of the Discovery quadrangle of Mercury. This mosaic was made from images taken during the first and second flybys. The large flat-floored crater just above and to the left of the image center is Schubert, with a diameter of 160 km. Above and to the left of that is the double crater Chekhov. A number of scarps can be seen trending SW to NE (North is up), just below and to the right of the center. The largest of these is the Discovery scarp, or Discovery Rupes. Image and caption provided by NASA NSSDC.

The smooth plains seen on Mercury resemble lunar maria and are likewise, sparsely cratered, but may have formed from different processes than those on the Moon. Mercury's smooth plains might have been caused by large amounts of ejecta from the impacts that cratered the surface and formed the Caloris Basin. On the other hand, a volcanic origin is possible and would liken Mercury's plains to the Moon's maria even more. Evidence for volcanic origin can be found in your textbook. The origin of the smooth plains of Mercury will continue to be a mystery until further discoveries are made. A recent proposal to send two more spacecraft to Mercury in the year 1999 would help to answer some of the many questions about this interesting planet.  Also, collecting rock samples from the varies surficial features would certainly help to determine their origin.

As noted before, Mercury is not currently undergoing any plate tectonic activity; however, there is some evidence on the surface that compression of the crust did once play a part in Mercury's development. Large faults can be seen trending north-south across much of the surface. These faults are known as thrust faults, and are caused by one block of rock being pushed up on top of another.

Scarp on Mercury

Mariner 10 image showing a scarp on the plains of Mercury. The SW-NE trending scarp, which may have been formed by compressional stresses, is radial to the Caloris Basin, to the southwest (North is up). The image is about 240 km across. Image and caption provided by NASA NSSDC.


Thrust faults occur on other planets, including the Earth. An example of a place on Earth where you would see thrust faults is in the Himalayas, where India is colliding with Asia and pushing tons of rock upward, making the Himalayan mountains the highest on our planet. Since the faults on Mercury are seen over the entire planet, scientists believe that at one time Mercury underwent a global contraction. This could have been caused by despinning, or, more likely, by cooling of the planet's interior. Envision how contraction and cooling might have caused these thrust faults.

Compare to ice on the Moon: "Ice on Mercury" http://nssdc.gsfc.nasa.gov/planetary/ice/ice_mercury.html

WEB SITES to learn more about Mercury:

The Nine Planets: http://www.nineplanets.org/mercury.html

Also try:http://starchild.gsfc.nasa.gov/docs/StarChild/solar_system_level2/mercury.html

NSSDC Image Catalog: http://nssdc.gsfc.nasa.gov/planetary/planets/mercurypage.html

Planet Quest: http://planetquest.jpl.nasa.gov/

NASA Jet Propulsion Lab

Also try: http://pds.jpl.nasa.gov/planets/welcome/mercury.htm

and: http://www.cv.nrao.edu/vla/upgrade/node34.html

Greek Astronomy: Greek Astronomy

Caloris Basin and Antipode pix: http://www.solarviews.com/cap/merc/caloris.htm

Vocabulary List:

spin-orbit coupling/orbital resonance
Mariner 10
large iron core
cratered terrain
intercrater plains
Caloris Basin
antipode/hills of Mercury
compression wave
smooth plains: volcanic or ejecta?
ghost crater
Giovanni Schiaparelli
Eugenios Antoniadi
Gordon H. Pettengil
Richard B. Dyce
hot poles
secondary craters
Discovery Rupes
impact craters and basins
multiring basins
crater degradation
thrust faults/scarps/despinning
five-stage geologic evolution of Mercury
Answer these questions as completely as possible, but no more than a few sentences each.

1. How does Mercury's lack of a substantial atmosphere relate to the early development of the solar system?

2. False color images of the Mariner 10 Mercury mosaics as analysed by Robinson and Lucey show new evidence supporting a volcanic origin for the smooth plains. Look up the website PSR Discoveries provided by Jeff Taylor at the University of Hawaii and describe two reasons these scientists believe in this volcanism on Mercury.

3. Also from the PSR Discoveries website: How does the FeO content suggest that the planetesimals that originally accreted to form Mercury came from a larger region than the planets current orbit?

4. Why do we only see Mercury for a brief time in the pre-dawn and post-dusk sky with the naked eye? You can look at a standard chart of planetary orbits and the figure below to help you answer this one.

Mercury can only be observed from Earth for a brief time in the pre-dawn and post-dusk sky. The orbits of the inner planets shown in the chart above explain why we cannot see them at night. Note that Mercury will always be located in the sky no more than 28 degrees from the Sun, and Venus is always within 47 degrees.. When the sun is visible, then we cannot see planets or stars with the naked eye, but we can see them when the Sun is just below the horizon either before sunrise or after sunset.

5. Radar images taken from Earth suggest that ice is at both the north and south poles of Mercury. From where did this ice come? Also, if the temperature on Mercury gets up to 700K, how could this ice stay frozen?

HINT: https://stardate.org/astro-guide/ssguide

6. Compare and contrast the following aspects of Mercury and the Moon:

density -
intercrater plains -
size -
scarps -

7.  What factors may have caused Mercury's orbital/rotational resonance to be different than the rest of the planets in our solar system?  Has Mercury's orbital/rotational resonance always been like it is now?   How does this affect the amount and extent of data gathered on the Mariner 10 mission?

8.  Use the internet addresses provided and any others you might find to locate an example of a ghost crater on Mercury. Provide a short description of the features you see and the website.

9.  Are there any examples of large craters on the Earth? Give two examples and their locations.

10.  What are the "hot poles?"  How hot are they?  How do they compare to places on other terrestrial planets?

End Of The Module
On to Module 9