ASTR 121, O'CONNELL. Study Guide 15 [Spring 2008]

ASTR 121 (O'Connell) Study Guide

15. MERCURY & VENUS

Radar map of Venus' surface, from the Magellan Mission. The red color is artificial,
intended to represent the effects of Venus' thick clouds. Click for enlargement.

A. THE "INFERIOR" PLANETS

Mercury and Venus are called "inner" or "inferior" planets because they are closer to the Sun than is Earth

Both revolve around the Sun in shorter times than the Earth (88 and 225 days, respectively).

Planet-Sun configurations

Elongation is the angular distance of a planet from the Sun as viewed from Earth. The term "configurations" refers to the various characteristic elongations possible for planets as shown in the figure above.
See the illustration above. As viewed from the Earth, the two planets inside the Earth's orbit can never appear at large angles from the Sun. Mercury and Venus always stay within 27^o and 48^o, respectively, of the Sun. These are their "maximal elongations."
- Copernicus showed that in his heliocentric model, the sizes of the orbits of the planets (with respect to Earth's orbit) could be deduced from these angles. In the Ptolemaic model, there was no simple geometric method for determining the sizes of the planetary orbits.
Consequently, Venus and Mercury are visible in the sky only near sunset or sunrise. Venus is the most common evening or morning "star."
Because Venus' orbital period is similar to Earth's, it tends to linger in the sky near the horizon for many weeks at a time. [Recall the planetarium simulations shown during our discussion of the Maya obsession with Venus.]
Because of its proximity to Earth and the high albedo (~70%) produced by its thick cloud layers, Venus is the brightest object in the sky other than the Sun or the Moon. Its intense brightness and white color make it look artificial.
The planets outside Earth's orbit ("superior" planets), starting with Mars, can be seen at up to 180^o from the Sun. At that point they are highest in the sky at midnight and are said to be at "opposition" with respect to the Sun.
- As the figure shows, when a planet is at opposition, it is also nearest the Earth and therefore brightest. It will also be undergoing its fastest "retrograde motion" at that point.

B. MERCURY

Hard to observe from Earth because of brief periods above the nighttime horizon.

Less well-studied than most other planets (only 2 spacecraft visits so far, both flybys, in contrast to Venus, which has been a major destination of space missions, including orbiters and landers).

flybys

MESSENGER

Average density: 5.4 grams/cc, like Earth, but mass (& therefore gravity compression) is smaller ===> richer in heavy elements than Earth

Surface? like Moon, with differences (e.g. shallower craters) due to slower cooling and higher gravity

Important test of General Relativity, the revised interpretation of gravity by Einstein. Mercury's perihelion shifts 43 arc-seconds/century more than in Newton's theory [1/10 millionth of orbital motion per orbit]; predicted exactly by Einstein's GR theory (1916).

Venusian Clouds from Mariner Spacecraft

C. VENUS: INTRODUCTION

Near "twin" of Earth: diameter (95%); mass (82%); distance from Sun (0.7 AU)

But unlike Earth, thick cloud layers completely obscure surface. See image above (click for enlargement).

The surface of the planet CANNOT be studied from outside atmosphere at optical wavelengths.
Clouds in planetary atmospheres are composed of liquid droplets or ice crystals and are distinct from the atmosphere (gas) in which they are embedded
Therefore, we can't determine cloud composition by spectroscopy (easy only for vapors).
Originally thought to be water clouds, suggesting wet, jungle-like planet.

USSR & USA space missions to Venus: orbiters, atmospheric probes, and short-survival landers.

Above is a wide angle color image of Venus' surface returned by the USSR Venera 13 lander (1982). It shows a lava-strewn plain, extending to the horizon at right. Color is produced by the thick cloud layer. Click for enlargement.

Best mapping technique: use radar to penetrate clouds

Radar systems

burst

Radar Map of Venus (Pioneer, 1981)

D. VENUS: SURFACE/TOPOGRAPHY

Determined by radar mapping. Image above is a relief radar map of Venus from the Pioneer mission. Best coverage: Magellan (radar orbiter, 1990-94).

Overall topography flatter than Earth. Only two "continent"-like features (Ishtar and Aphrodite in map above).

tectonic

Vast lava flows cover 85% of surface, but are neither impact basins (like the Moon's marias) nor tectonic-related (like Earth ocean beds). Most are smooth. Little current eruptive activity.

Many volcanoes, from 500 km diameter to tiny vents; 3000 over 20 km diameter; 100,000 altogether!

Above are four overlapping volcanic domes. They average about 16 miles in diameter with maximum heights of 2,500 feet. They were produced by eruptions of thick lava coming from a vent on the relatively level ground, allowing the lava to flow in an even lateral pattern. Click for enlargement.

Many impact craters, but fewer per square mile than Moon, Mercury. This implies a younger surface than those planets, ~500 million years old (as opposed to 4 billion).

Shown at right is a radar image of a 30-mile diameter impact crater surrounded by a bright "splash blanket" of ejecta. Click for a larger view.

Surprisingly, Venus shows a uniform distribution of craters across its surface, which is unique in the solar system.

sudden catastrophic melting & resurfacing

Venus' surface history will be discussed in the video "Venus Unveiled"

E. VENUS: ATMOSPHERE

Dense, hot, dry, corrosive [no tropical paradise!]

Mainly carbon dioxide (CO₂)

H₂O vapor only 1/10000 of abundance on Earth, and no water on surface. Dessicated planet/atmosphere.

We will find later that the absence of water is a key to the bizarre properties of the Venusian atmosphere.

Clouds = sulfuric acid(!!) droplets

From volcanic outgassing in absence of rainfall

See atmospheric profile chart at right:

Remarkable difference from Earth atmosphere
Temperatures and pressures like those at Earth's surface occur at an altitude of 50 km in Venus atmosphere. Below that, pressures and temperatures are much higher than on Earth.
Surface Temp ~ 750^oK (480^oC)!
[Hotter than Mercury despite larger distance from Sun.]
Surface Pressure = 90x Earth's. Implies Venus' atmosphere is 90x more massive than Earth's!

The Greenhouse Effect

The high temperature is produced by the Greenhouse Effect

Main heat input to atmosphere is from the Sun (true for all planets, including Earth). Occurs mainly at visible wavelengths, where Sun is brightest.
Cooling is by radiation to space. Occurs at infrared wavelengths.
Final temperature is determined by the equilibrium point where the heating rate balances the cooling rate.
Certain gases (CO₂, H₂O) act like a blanket. Partially prevent radiative cooling (at infrared wavelengths).
===> 30^o rise on Earth but 400^o rise on Venus because of massive atmosphere.

F. VENUS AND EARTH

The incredible differences between the terrestrial and the Venusian atmospheres were a great shock to astronomers. How can Venus and Earth, despite similarities in size, mass, and distance from Sun, be so different? The seemingly small difference in distance to the Sun (30%) probably is the culprit, as we will see in Study Guide 19.

Spaceman Spiff zooms past Venus on his way into deepest space.

Reading for this lecture:

Reading for next lecture:

Web links:

Mercury Info at Views of the Solar System

Venus Info at Views of the Solar System (includes many images, animations, links)

The Magellan Project

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Last modified March 2008 by rwo

Venus images copyright © 1997, Calvin J. Hamilton. Atmosphere profile copyright © Harcourt, Inc. Text copyright © 1998-2008 Robert W. O'Connell. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 121 at the University of Virginia.