Today, the appearances of the Terrestrial planets are grossly different. Here, we will again look at the Earth and the Moon in detail but, this time, we will also look at Mercury, Venus, and Mars as well.

The Terrestrial planets are divided into two groups: (1) Venus and Earth; and (2) Mercury and the moons. Mars is intermediate between the groups. The division is based on the amount of ongoing evolution of the surface features of the planets (which is determined by the size of the planet).

Recall that the larger planets are hot which leads to significant heat flow from their centers to their surfaces. The heat flow drives geology (in particular, tectonic activity) which has a significant effect on the evolution of the surfaces of the planets. For example, the geologically dead Moon has surface features as old as 4.1 billion years and most features are older than 3 billion years while the geologically active Earth has a few rocks almost 4 billion years old (and perhaps as old as 4.4 billion years) on the continental plates with oceanic plates which are less than a few hundred million years old.

THE MOON

A neat animation shows the rotating Moon put together using Clementine images. Also, see some pictures of the Moon, Mercury, and Mars which exhibit the various features in which we are interested.

The Moon is divided into two types of terrain: maria, the dark-colored lowland basins which cover 15 % of the lunar surface the light-colored heavily cratered highland regions which cover 85 % of the lunar surface. The highland regions are heavily cratered and inferred to be ancient while the maria are much less cratered and inferred to be young (younger). Similar types of features are seen on Mars and Mercury. Crater Origin: The craters on the Moon and most on other Terrestrial planets (see Mercury) were produced by impacts and not by volcanism.

The maria are large basins (old impact craters) which have been filled in by large lava flows. Their surfaces are dark and smooth with little evidence of cratring.

The highland regions are the ancient lunar regions; the lunar mountains are produced by the large impacts uplifting the terrain. They are not formed as are the mountains on Earth, that is, they are not formed through the collisions of lithospheric plates.

The backside of the Moon is very similar to the near side of the Moon in terms of cratering,

There is, however, a lack of maria on the far side of the Moon. (Perhaps due to the asymmetry in the crutal thickness of the Moon.)

LUNAR CHRONOLOGY

The chronology for the Moon is very good because we have been there, seen it. This has allowed the gathering of rock samples from various terrains on the Moon. The importance of this is that the rocks could be age-dated using the technique of radioactive age dating which allowed us to set a firm chronology for the evolution of the lunar surface features. This is important not only for the Moon but also for the inner Solar System in general as, presumably, the chronology of the Moon roughly follows the timeline as the rest of the inner Solar System (that is, it tells what the cratering rate was for all of the inner Solar System bodies.)

Here, we present the chronology of the Moon's evolution and the cratering rate for the inner Solar System over the last 4 billion or so years.

TECTONIC ACTIVITY

The most massive Terrestrial planets, Earth and Venus, are likely to show active geologies. Mars may show geological activity. Mercury and the Moon are probably dead from a geological standpoint. The sorts of geology we expect are volcanism and tectonic activity, both manifestations of the fact that the interiors of the planets are hot.

A consequnce of geological activity (and wind and water erosion on the Earth) is to produce a relatively young surface. (By young, we mean that the surface features were formed recently.) The surface of Venus is around 300 - 800 million years old, while the majority of the Earth's surface features are much younger. For example,

• Ocean basins are < couple hundred million years old
• Hawaiian Islands, island chain, ~ millions of years old
• Grand Canyon (cut by the Colorado River) < 10 million years old
• Himalayas, Andes mountain ranges < tens of millions of years old

These ages are to be compared to the ages of the planets and Solar System of 4.6 billion years.

• Tectonic Activity on the Terrestrial Planets?
  • Earth
  • Venus
  • Mars