PES 105        Fall 2001 Previous Lecture General Astronomy I Next Lecture

Lecture Notes:

Mercury

text: , Chapter 8: Sections 8.1 and 8.2

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Introduction and General Properties

Here is a good link for information about the mercury: http://seds.lpl.arizona.edu/billa/tnp/mercury.html

Here is another good link for information, pictures and movies about Mercury: http://www.solarviews.com/eng/mercury.htm

Here is a link with pictures of Mercury: http://antwrp.gsfc.nasa.gov/apod/index/Mercury.html

Mercury is hard to observe since it is always close to the Sun.

only see it in morning or evening

albedo = 0.10 . . . . similar to Moon

density = 5.5 g/cm³ . . . similar to Earth

size: slightly larger than Moon

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Motions

orbit:

• quite elliptical
  • distance from Sun varies => speed varies
• plane inclined about 7 degrees from ecliptic
• period = 88 days
  • shortest "year"

rotation:

• once every 59 days
  • one "Mercury day" = 59 "Earth days"
• rotation period = 2/3 orbital period

spin - orbit coupling

= simple relationship between rotation and orbital period

example: Synchronous rotation

Why? tidal forces from Sun (strong for close planet)

like the Moon, one side of Mercury is somewhat more dense

The rotation of Mercury is also shown in Figure 8.9. [Link to Figure 8.9]

CD: 2nd edition: See the animation in the Chapter 8 gallery, p. 1. [Link to web animation. SLOW DOWNLOAD]

OPTIONAL MATERIAL: Combination of slow rotation and fast orbit when closest to Sun causes Sun to appear to turn around in the sky and go backward !!!! (like retrograde motion)

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Interior

observe weak magnetic field and very slow rotation

=> liquid in core

molten => differentiation

Figure 8.7 also shows the interior. [Link to Figure 8.7]

Why thin mantle ?

1. too hot for lighter elements to collect
2. collision with large planetisimal knocked mantle away
3. very powerful solar wind blew mantle away

No obvious volcanoes or earthquakes . . . but could be

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Surface

looks like moon at first glance

Figure 8.2 shows the similarity of Mercury and Moon.

• rolling hills
  • not found on Moon
• cliffs (scarps)
  • from cracking and settling ........
  • Figure 8.4 shows a scarp [Link to Figure 8.4]
  • Here is another picture of a scarp: http://www.solarviews.com/r/merc/fault.jpg
• craters
  • more separated than on moon
    • stronger surface gravity keeps debris closer
  • 
• flat lava areas
  • like maria on moon
• wavy region
  • directly opposite large crater
    • Here is a photo of the crater area: http://pds.jpl.nasa.gov/planets/captions/mercury/merccal.htm
  • from seismic waves of collision
  • 
    • Here is a photo of the wavy region: http://pds.jpl.nasa.gov/planets/captions/mercury/mercter.htm
• ice cap at N pole ?
  • radar reflection data suggests it
  • ice might be below surface

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Atmosphere

almost none

weak gravity and very hot => gasses escape

some sodium, potassium from rocks outgassing

some hydrogen and helium from Sun

no weather => very little erosion => old surface

temperature during a "day" ranges from 100 to 700 K

from slow rotation and lack of atmosphere

(largest spread in solar system)

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History:

Here is a NASA movie on the early formation of Mercury: http://www.solarviews.com/cap/merc/merc1.htm

Here is a continuation of the NASA movie showing the later stages of the formation of Mercury: http://www.solarviews.com/cap/merc/merc2.htm

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