Lecture
Mercury

Mercury is an oft-forgotten planet when it comes to solar system studies; or, at the most, it gets maybe half of a lecture. It is a small, dense, hot piece of rock that orbits our Sun about 40% of the distance to the Earth. A smelter could probably have not done such a fine job of isolating the metals in the solar system--Mercury is close to 70% metal (iron and nickel).

Objectives

1. Use the nebular theory (condensation sequence) to explain why Mercury has the composition it has today.
2. State one reason why scientists believe Mercury may have experienced a giant impact in the past.
3. Summarize the results that are coming about due to a re-examination of data from fly-by mission.
4. Compare and contrast features found on Mercury to those found on the Moon.
5. List a couple of engimatic features found on the surface of Mercury and give the current theories for their formation.
6. Explain the 3:2 spin-orbit coupling of Mercury and how tidal forces lead to this kind of resonance.
7. Describe the questions arising from the presence of a weak magnetic field around Mercury.

Moon and Mercury: a comparison

First, similarities:

• Heavily cratered--saturated in many regions

• Contains large impact basins

• Many lava flows

• Except for occasional cratering, a dead planet.

• May have water ice at the poles (north for Mercury, south for Moon)

Mercury, however, has some notable differences from the Moon:

• No huge maria

• Much, much larger core

• Formation was through condensation, not collision

• Mercury has huge curved cliffs, or lobate scarps, probably from cooling

• Moon is tidally locked; Mercury is in a 3:2 (rotation-revolution) resonance.

Other notable characteristics:

• Has one of the most eccentric orbits of the planets

• Temperature range is from 427 to -173 degrees Celsius

• Never gets more than 28 degrees away from the Sun (thus hard to view)

• Barely 50% of its surface has been mapped; NASA plans to return in 2004 with the Messenger Mission

Exploration of Mercury:

(now in orbit about the Sun) is the only space craft to visit Mercury. "After the Venus flyby, Mariner's trajectory was bent in toward the Sun to accelerate and fling it out of Venus's gravitational field and onward to Mercury. Mariner 10 reached Mercury on March 29, 1974, passing over the planet at 705 kilometers (438 miles) above the surface. A second encounter with Mercury occurred on September 21, 1974, at an altitude of about 47,000 kilometers (29,200 miles). The sunlit side of the planet and the south polar region were photographed. A third and last Mercury encounter, at an altitude of 327 kilometers (203 miles), occurred on March 16, 1975. About 300 additional photographs were obtained along with magnetic field measurements. Photographs of the planet reveal an intensely cratered, Moon-like surface and a faint atmosphere of mostly helium, resulting from solar wind bombardment. Engineering tests were continued until March 24, 1975, when the supply of attitude-control gas was depleted and the mission was terminated." (From Solar Views.)

Take a close look at the maps at the Mercury Map Images and see if you can identify the features noted on the Moon's surface. Look for simple craters, complex craters, ghost craters, impact basins, lava flows, rays, chain craters, straight and sinuous rilles, saturation cratering.

Start your exploration with this image of a region inside the Caloris Basin. How many different features can you find?

Don't forget that the story of the Moon and of Mercury is the story of impact cratering in the inner solar system. If you cannot remember the series of events that go into forming a complex crater and the different types of craters, then you should review Caring about Cratering (cratering in the solar system).

Take the Quiz!

Relevant Links:

• Solar System Collisions: enter your target, and projectile composition, diameter, and velocity and "Let'r Rip" with a KABOOM! This is an on-line calculatory that approximates what would happen if an asteroid or comet hits.

• The animations shown in class of the formation of Mercury and the evolution of its surface can be found at Animations of Mercury.

• JPL's Mercury Page

• "Mercury, the second smallest planet and the closest one to the Sun, may appear to some as a drab, colorless, heavily-cratered world. Not so. New analysis of data returned by the Mariner 10 mission in 1974 and 1975 reveals a surface with lava flows and deposits from explosive volcanic eruptions, variations in composition across its surface and into its crust, and a different chemical composition from the other inner planets. These discoveries were made by Mark Robinson at the United States Geological Survey in Flagstaff, Arizona (he is now at Northwestern University) and Paul Lucey of the University of Hawai'i. Using improvements in computer and image-processing technologies, and a better understanding of how light reflects off planetary surfaces than was available in the mid-1970s, Robinson and Lucey manipulated the original data and produced a color image of Mercury that depicts compositional differences across its stark surface (Robinson, Mark S. and Lucey, Paul G., 1997, Recalibrated Mariner 10 Color Mosaics: Implications for Mercurian Volcanism, Science, vol. 275, p.197-200.)" From Mercury Unveiled by G. Jeffrey Taylor.

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