ASTR 1210, O'CONNELL. Study Guide 15 [Spring 2013]

ASTR 1210 (O'Connell) Study Guide

15. MERCURY AND 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 Mercury and Venus (relative 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.

Image of the Caloris Basin on Mercury taken by the MESSENGER spacecraft.
Color coding is for different mineral types.

B. Mercury

Mercury is hard to observe from Earth because it is above the nighttime horizon only for brief periods.

It has been less well studied than most other planets. Until 2007 there had been only 2 spacecraft visits, both flybys, in contrast to Venus, which has been a major destination of space missions. Here is hemispheric view of Mercury from Mariner 10 (1974).

MESSENGER

flybys

Mercury has a high average density of 5.4 grams/cc, like Earth, but Mercury's mass (& therefore gravity compression) is smaller. That implies Mercury is more rich in heavy elements than Earth

Mercury's surface is similar to Earth's Moon (impact-driven terrain), but with important differences (e.g. shallower craters) due to slower cooling and higher gravity. See image at right and compare to Earth's Moon topography here.

Mercury's orbit is an important test of General Relativity, the revised interpretation of gravity proposed by Einstein in 1916. Mercury's perihelion (the closest point to the Sun in its elliptical orbit) shifts 43 arc-seconds/century more than predicted for Newtonian gravity. This is only 1/10 millionth of its total orbital motion per orbit, but it can be measured highly accurately over many orbits. The extra shift is predicted exactly by Einstein's GR theory (1916).

Venusian cloud layers in UV/optical bands (image from Mariner 10, 1974)

C. Venus: Introduction

Venus is a near "twin" of Earth in global properties: diameter (95%); mass (82%); distance from Sun (0.7 AU)

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

The surface of Venus cannot be studied from outside its 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).
The naive presumption, giving Venus' appearance and overall similarity to the Earth, was that the clouds were made of water and that the planet probably hosted a flourishing, wet, jungle-like biosphere.

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

Results from these missions, as well as Earth-based radio-wave observations, quickly demolished the notion of a Venusian tropical paradise:

surface temperature was almost 500^o C (900^o F) and the lower atmosphere was crushingly dense

bleak, lava-covered surface

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.

D. Venus: Surface/Topography

For Venus, the only feasible mapping technique was to use radar to penetrate the thick clouds.

Radar systems

burst of radio waves

Radar map of Venus (Pioneer Mission, 1981)

The image above is a relief map of Venus derived from radar observations with the Pioneer mission. Best mapping coverage was from the Magellan Mission (radar orbiter, 1990-94).

The overall topography is flatter than Earth's. There are only two "continent"-like features (Ishtar and Aphrodite in the map above).

modest tectonic

much less conspicuous

no oceans

Vast lava flows cover 85% of the surface, but there are no large basins, neither impact (like the Moon's marias) nor tectonic-related (like Earth ocean beds). Most flow regions are smooth. There is little current eruptive activity.

There are many dormant volcanoes, from 500 km diameter to tiny vents; 3000 over 20 km diameter; 100,000 altogether! Over 160 larger than the largest volcano on Earth (Hawaii).

This radar image shows 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 vents
on the relatively level ground, allowing the lava to flow in an even lateral pattern.
Click for enlargement.

There are also many impact craters, but fewer per unit area than on the Moon or Mercury. This implies a younger surface than those planets.

Shown at right is a radar image of a 30-mile diameter impact crater surrounded by a bright "splash blanket" of ejecta. Lighter-toned regions on radar images are rougher; darker-toned are smoother. Click for a larger view.

Surprisingly, Venus shows a uniform distribution of craters across its surface.

unique in the Solar System

whole surface formed at one time

only about 500 Myr old

sudden catastrophic melting & resurfacing

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

E. Venus: Atmosphere

Venus' atmosphere is dense, hot, dry, and corrosive. It is entirely hostile to Earth-like life.

The bulk of the atmosphere is carbon dioxide (CO₂)
H₂O vapor has only 1/10000 of its abundance on Earth, and there is no liquid water on the surface. A dessicated planet/atmosphere.
- We will find later (Study Guide 19) that the absence of water is a key to the bizarre properties of the Venusian atmosphere.
- Lack of liquid water, which on Earth is a lubricant for the outer layers of the interior, may also help to inhibit tectonic activity on Venus.
The Venusian cloud decks? The clouds are sulfuric acid(!!) droplets
- They originate from volcanic outgassing in the absence of rainfall
See the atmospheric profile chart at right:
- Remarkable differences from Earth's 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)!
- Venus' surface is hotter than Mercury's, despite its larger distance from the Sun!
- Surface Pressure = 90x Earth's. This implies Venus' atmosphere is 90x more massive than Earth's!

Despite the dense and corrosive atmosphere, there is little weathering of surface features on Venus because windspeeds are very low (and the sulfuric acid rain evaporates at high altitude before reaching the ground).

The Greenhouse Effect

Venus would be warmer than the Earth simply because it is nearer the Sun. But the extraordinarily high Venusian temperature is not caused by higher solar input. Instead, it is produced by the Greenhouse Effect, an atmospheric process which inhibits surface cooling.

The Greenhouse Effect was first recognized in the 1820's, and the first quantitative discussion was published by Arrhenius in 1896.
The main heat input to any planetary atmosphere (including Earth's) is from the Sun. This occurs mainly at visible wavelengths, where the Sun is brightest.
Cooling from the surface is by radiation to space. Because the temperature of planetary surfaces is (fortunately for us!) much lower than the Sun's, this occurs not at visible but instead at infrared wavelengths.
The final temperature is determined by the equilibrium point, where the heating rate balances the cooling rate.
Certain "Greenhouse gases" (H₂O, CO₂, CH₄) act like a blanket to "trap the heat." They preferentially absorb infrared radiation and reflect it back to the surface, thereby reducing radiative cooling. See sketch above right (click for enlargement).
This causes a significant temperature rise to the point where the surface can radiate as much energy to space (through the Greenhouse blocking) as it receives from the Sun. The situation is like the level of a lake adjusting to the increased height of its outlet dam.
Because all of the surface cooling must take place in the infrared, any gas that can impede IR radiation is an effective Greenhouse agent. Even the tiny amounts of Greenhouse gases in the Earth's atmosphere can have a big effect.
On Earth, where the Greenhouse gases are only "trace" constituents of the atmosphere (CO₂ totals only 0.04% of the atmosphere's mass), the Greenhouse temperature increase is a modest 30^o C (or 54^o F), which is just enough to keep Earth's surface "comfortable" by human standards and prevent the oceans from freezing over.
But on Venus, where the atmosphere is almost pure CO₂ and massive enough to block large regions of the infrared spectrum, the temperature rise is 400^o C.

F. Venus and Earth

Venus is a sobering lesson in comparative planetology.

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

two

global warming

Spaceman Spiff zooms past Venus on his way to Mars --- next lecture.

Reading for this lecture:

Reading for next lecture:

Web links:

Mercury Info at Views of the Solar System

Mercury MESSENGER Mission

Venus Info at Views of the Solar System

The Venus Magellan Mission

Interactive Simulation of the Greenhouse Effect

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Guide Index

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Last modified March 2013 by rwo

Venus images copyright © 1997, Calvin J. Hamilton. Atmosphere profile copyright © Harcourt, Inc. Greenhouse effect drawing copyright © Toby Smith. Text copyright © 1998-2013 Robert W. O'Connell. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 1210 at the University of Virginia.