Exploration of the Solar System

Topic 10, Week 7

Mercury and Venus

Announcements

Today we will start talking about Venus and Mercury. Venus dominates the morning sky. Get up just before dawn twilight blots out all of the stars. The brightest object in the sky is high in the east. That is Venus. Mercury will be visible in the evening sky later in the Fall. It is harder to find, but I’ll give you directions.

We spent a lot of time talking about Mars, which is the terrestrial planet with the most current interest. Now let’s discuss the other two (besides the Earth), Mercury and Venus. First, let’s see where these planets are relative to the Earth. Both of these planets are so-called inferior planets, meaning the are closer to the Sun than we are.

Just the facts, Ma’am

Let start with the fundamental properties of these innermost terrestrial planets, compared with the Earth.

Quantity	Mercury	Venus	Earth
A (semimajor axis)	0.387	0.723	1
E (eccentricity)	0.206	0.007	0.017
Diameter (km)	4878	12,104	12756
Mass (units of Earth)	0.055	0.815	1.0
Density (g/cc)	5.4	5.2	5.5

Numbers always tell a story. Some of the things to note from this table are the following.

1.The orbit of Mercury is unusually eccentric for a major planet

2.Mercury is really fairly small (mass and diameter) relative to the Earth.

3.Venus is remarkably similar to Earth as far as size and mass (look at Venus globe)

4.The density of Mercury is about the same as that of the Earth. (Question: what does that mean?)

We can also see how they stack up by reference to figure 10.1 of the textbook. We can see by this time in the course, we have discussed several of these objects.

Mercury

Mercury is close to the Sun, and therefore is never seen high in the sky when it is dark. You will see that for yourself in a few weeks. For this reason, relatively little was known about it before the “space age”, beginning about 40 – 50 years ago.

One thing we do know from Kepler’s 3^rd law is its period, given the semimajor axis. Since a=0.387 au, the period is 0.2408 years, or 88 days. This is the length of Mercury’s year.

The tidal forces exerted by the Sun on Mercury are extremely large, and so it was expected that Mercury should show synchronous rotation, in which it would rotate once on its axis in 88 days, thus showing the same face to the Sun, just like the Moon shows the same face to the Earth. There was even some observational evidence that seemed to suggest that this was true.

However, in about 1965, radar signals bounced off Mercury showed that it actually rotated in 58.6 days, or exactly 2/3 or a Mercurian year. In astrophysics we call this a 2/3 resonance. So it is not synchronized so that one rotation period equals one revolution period, but rather 3 rotation periods = 2 revolution periods.

How this works is shown in Figure 10.3 of the book.

This 2/3 rotation/revolution law, plus the highly eccentric figure of Mercury’s orbit, has some interesting consequences.

1. The day (time in which the Sun is above the horizon) lasts 88 days, and the night lasts 88 days.

2. There are two longitudes for which the noon always occurs at perihelion (time when the planet is closest to the Sun), and two longitudes when noon always occurs at aphelion (time when the planet is furthest from the Sun).

3. Since the orbit is so eccentric, there is a big difference between the perihelion distance and aphelion distance (see diagram above) there is a big difference in the solar heating at those longitudes.

4. Since the rotation rate is so long, and related to the revolution period by a 2/3 resonance, watching the motion of the Sun in the sky at Mercury would be a weird experience. For some longitudes (those for which noon occurs at perihelion) the Sun would relatively rapidly rise in the sky to transit, then linger there for a long time before setting (relatively) rapidly.

5. For other longitudes (noon at aphelion) the Sun would linger for a long time near the horizon at sunrise, then zip across the sky before lingering a long time near the western horizon before sunset.

Mercury is a place of extreme temperatures. As mentioned above, daytime lasts 88 days. In addition, Mercury is only 0.387 times the distance of the Earth from the Sun, so the intensity of sunlight is 1/(0.387)² = 6.7 times stronger.

On the other hand, during the 88 day night, there is no atmosphere to hold in the heat and keep it from radiating away to the black of space. As a result, the temperature of Mercury varies from 700 K (700 degrees centigrade above absolute zero) on the daytime side to 100K ( only 23 degrees centigrade above the temperature of liquid nitrogen). The temperatures on the daytime size are hot enough to melt tin and lead!

These extreme temperatures, combined with the lack of an atmosphere and the total lack of water, make Mercury a hostile and alien place.

The structure of Mercury. As noted above, Mercury has a density nearly the same as the Earth. This means it must have a largely metallic composition. The inferred internal structure of Mercury is shown in Figure 10.13 of the book, where is shows that about half the volume, and most of the mass of Mercury must be iron.

This largely iron composition accounts for the fact that Mercury has a stronger magnetic field than either Venus or Mars. This “metal planet” nature also presents something of a mystery in terms of how it was formed. See p214 and 215 of the book.

The Surface of Mercury

Until the space age, the only views of the surface of Mercury were meager telescopic observations that showed only a couple of dark and light areas. Even now, we have had only one good look at the surface of Mercury, and that was due to the Mariner 10 spacecraft flyby in 1974. Most of what we know about Mercury came from that mission.

An overall view of Mercury from the Mariner photos is shown in Figure 10.4 of the textbook.

As we have already discussed, this shows a heavily cratered object like the Moon, so its surface has not changed much since the age of bombardment. For four and a half billion years, the arid plains of Mercury have seen nothing except the rising and setting of the blinding Sun.

Mercury: A Curiosity

For all the reasons given above, Mercury would seem to be the last place in the solar system to look for water. Nonetheless, about 10 years ago a strange discovery was made.

Radar observations of Mercury had been made since the 1960s, with powerful radars bouncing signals off the planet. By the 1980s, these radars had developed to the point where they could image the reflected radar signal, thus making a radar map of Mercury.

The picture of Mercury in radar is shown below, the work of John Harmon at the Arecibo Observatory.

The brighter tones correspond to higher radar reflectivity. The bright radar reflection from the north pole of Mercury was a surprise. This corresponds to the bright red dot at the top of the image of Mercury. Eventually, Harmon and colleagues concluded that it was reflection from ice (cold ice is a good reflector of radio waves) that must be in craters at the north pole, shaded from the brutal sunlight. The water that made up this probably came from an impacting comet, and may have been an object similar to those comets that brought the ocean water to the early Earth.

The exploration of Mercury is entering a new phase, too. This past summer, the Messenger spacecraft was launched. It will eventually go into orbit around Mercury, and map the entire surface of the planet in high resolution. Since the surface of Mercury has changed very little since the earliest days of the solar system, it might preserve clues to the origins of our solar system. Messenger is on a complicated trajectory to Mercury, and so will not arrive there until 2011.

Venus

Now let’s move on to Venus. The fascination of Venus is that, on one hand, it is so similar to the Earth in its overall physical properties, and on the other is so unbelievably different.

First look at the table at the start of these notes to see the similarity between the Earth and Venus.

Since Venus comes the closest to Earth of any major astronomical object other than the Moon, you would think we would have known a lot about the surface of Venus for a long time, at least a couple of centuries. But that is not the case.

Below shows a picture of Venus taken with the Hubble Space Telescope, but you can get about as much information from the eyepiece of a relatively small telescope.

The problem is that from Earth, all we see is the top of a layer of clouds. It is always overcast on Venus, everywhere on the planet.