Scholars Online Astronomy - Chapter 11: The Terrestrial Planets
Reading: Astronomy, Chapter 11: Mercury, Venus, and Mars: Earthlike yet Unique
Study Notes: notes on your assigned reading from the text
- 11.1: Earth-based observations of Mercury and Venus are difficult because both planets, while visible to the naked eye, spend much of their orbital period close to the sun (in angular distance as seen from earth). Venus' surface is obscured by its dense cloud cover. In contrast, Mars is a superior planet and at opposition, many surface features are visible.
- 11.2: Mercury's rotation is synchronized to its orbital period: it rotates on its axis 3 times to every 2 revolutions around the sun, a condition which minimizes angular momentum. This created tidal forces resulting in a slight elongation of the planet along an axis through its equatorial plane. Mercury would have a 1:1 spin coupling with respect to the sun (as the moon does with earth) if its orbit were more nearly circular; because it is highly eccentric, solar tidal forces vary, creating variation in the spin rate.
Venus' rotational direction is opposite to its revolutionary direction. Seen from a point high above Earth's north pole and the solar system plane, Earth both rotates counterclockwise and moves around the sun in a counterclockwise direction, as do most of the planets. Venus and Uranus, however, rotate in a clockwise direction seen from the same vantage point. Possible explanations include rotational drag from Venus' atmosphere, gravitational influence from Earth, and impact or near-collision with another massive body during solar system formation.
- 11.3: Mercury is very much like Earth's moon in appearance: it has a barely-detectable atmosphere of helium and a highly cratered surface. A key feature is the Caloris basin, the result of a massive impact event that also created hilly regions on the opposite side of the planet. Mercury's mass indicates a highly dense iron core, its magnetic field indicates that this core is partially molten. Our models for planets the size of Mercury do not adequately account for these phenomena.
- 11.4: Interplanetary expeditions provide our key information about Mercury and Venus, given the difficulty of Earth-based observations. Mercury expeditions include Mariner 10 (1973-1975) and the MESSENGER mission (2004-2015). Expeditions to Venus met with a number of issues: Mariner 2 could only fly by the planet and measure radiation; early Soviet Venera missions which descended into the atmosphere were crushed by the pressure. Magellan (1990-1994) eventually mapped the Vesuvian surface.
Because it is actually more hospitable (and therefore more likely a candidate for colonization), a number of missions have been sent to Mars, beginning with Soviet Marsnik 1 and 2 in 1960, NASA Marinar 3 and 4 flyby probes in 1964, and Mariner 6 and 7 in 1969. These were followed by orbiter and lander probes (Viking 1 and 2) in the 1970s, and by lower-cost lander and orbiter programs, including Mars Rovers (Pathfinder, Spirit, and Opportunity). These expeditions have mapped the Martian surface and performed detailed chemical analysis of the Martian atmosphere and soil.
- 11.5: While Mercury exhibits no recent geological activity, both Venus and Mars have recognizable volcanoes; Vesuvian volcanoes are currently active, while Martian volcanoes appear to be dormant. Both planets have crustal dichotomies, with highland mountainous regions and plateaus concentrated in one hemisphere and more plain-like lowlands in the other. The low number of impact craters on both planets compared to Mercury and the Earth's Moon indicate that the surfaces of Mars and Venus are younger, a sign of geological activity where lava flow obscures craters. While Mars appears to have a cold, thick, stable crust that floats above a fluid mantle, Venus has a thin crust that apparently remains in position. Neither has separated plates that move against each other, as does Earth, so their volcanoes are hot-spot volcanoes (similar to the Yellowstone Caldera).
- 11.6: Mercury's atmosphere is at most a slightly denser concentration of helium temporarily captured from the solar wind. Despite their difference in density, Venus and Mars have atmospheres that are primarily CO2, with smaller amounts of nitrogen N2, and trace amounts of sulfur dioxide (probably as a result of outgassing from volcanic activity). Neither planet has oxygen in any concentration. Venus has two main convection currents driven by its slow rotation. The Martian atmosphere varies in pressure as CO2 freezes out of the air in winter and evaporates back into the air in spring.
- 11.7: Earth scientists assume that the early atmospheres of Earth, Venus, and Mars were similar: high concentrations of CO2, sulfur dioxide, water vapor, and nitrogen. On Earth, water could exist as a liquid and wash dissolved CO2 out of the atmosphere and into soil, where it was captured and "fixed" in soil deposits. The development of photosynthetic life created the oxygen stores now present in Earth's atmosphere. In contrast, the temperatures on Venus led to a runaway greenhouse situation in which the water boiled away. The "washing out" effect on CO2 and SO2 ceased, leading to higher concentrations of these gases in Venus' atmosphere. Because of the lower temperature on Mars, CO2 froze out of the atmosphere, and there was no heat trap as on Venus, which led to a runaway "icehouse" effect.
- 11.8: The availability of water is key to finding life-as-we-know-it on any celestial object, and also to colonization, since transporting water from Earth is difficult (it's bulky and heavy). Surface features and soil studies by rovers on Mars indicate liquid water may have been present in the past, but in small, isolated lakes. This water may still exist as subsurface permafrost or under the carbon dioxide polar caps.
- 11.9: Mercury and Venus have no moons, but Mars has two sizeable non-spherical moons, which are probably captured asteroids.
Key Formulae to Know
- There are no key formulae for this chapter.
Read the following weblecture before chat: The Terrestrial Planets and Plate Tectonics
Planetarium Program: Use the features of your planetarium program to view Mercury, Venus, and Mars from space.
- View each planet from high above its north pole through two revolutions about the Sun, and note how it rotates.
- Explore the surface of each planet. Identify any features you can find on one of the planets but on not the other two.
Early Missions to the Planets and the Moon
List of lunar missions, contains pictures and information from US, European, Japanese, and Chinese missions to Earth's Moon.
List of Mercury missions, contains pictures and information from US missions to Mercury.
List of Venus missions, contains pictures and information from US and Soviet missions to Venus.
List of Venus missions, contains pictures and information from US, European, Japanese, and Chinese missions to Mars.
Viking Lander Catalogue of pictures (Mars).
Chat Preparation Activities
- Essay question: The Moodle forum for the session will assign a specific study question for you to prepare for chat. You need to read this question and post your answer before chat starts for this session.
- Go over the list of Key Words and Key Ideas at the end of the chapter. If you don't remember the definition of the key word, review its use (the page number on which it is explained is given).
- Read through the Review Questions and be prepared to discuss them in class. If any of them confuses you, ask about it!
- Mastery Exercise: The Moodle Mastery exercise for the chapter will contain sections related to our chat topic. Try to complete these before the chat starts, so that you can ask questions.
- Required: Complete the Mastery exercise with a passing score of 85% or better.
- Go to the Moodle and take the quiz for this chat session to see how much you already know about astronomy!
Read through the lab for this week; bring questions to chat on any aspect of the lab, whether you intend not perform it or not. If you decide to perform the lab, be sure to submit your report by the posted due date.
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