Surface Features of the Terrestrial Planets

The surface features of Terrestrial planets may be young or old based upon whether the planet shows ongoing active geology. Young surfaces are geologically active while old surfaces are not. In the latter case, when active geology disappeared determines what processes determine the appearance of the current surface of the planet.

Today, the appearances of the Terrestrial planets differ strongly. We look at the Earth and the Moon in detail but, this time, we will also look at Venus, and Mars as well.
In terms of their surfaces, the Terrestrial planets may be divided into two groups: (1) Venus and Earth; and (2) Mercury and the moons. Mars is intermediate between the two groups. The division is based on the amount of ongoing evolution and appearance of the surface features of the planets (which is determined by the size of the planet).

Group 1: the larger planets have hot interiors which leads to significant heat flow from their centers to their surfaces. The heat flow drives geology (crustal motions) which has the dominant effect on the evolution of the surface features of the planets. The geologically active Earth has a few rocks almost 4 billion years old (with perhaps some as old as 4.4 billion years) on its continents, but there are much younger features such as the oceanic basins which are less than a few hundred million years old, and other features such as mountain ranges and volcanos as young millions of years to tens of millions of years old. The Earth has a young surface.
Group 2: the smaller planets have cold interiors and geologically inactive. The geologically dead Moon has surface features as old as 4.1 billion years with nearly all features older than 3 billion years old. The Moon has an old surface. Its surface has been shaped primarily by impacts.

I. THE MOON (GROUP 2)

A. Surface Features
(1) A neat animation shows the rotating Moon put together using Clementine images. (2) Two pairs of images are shown below. The top two are photographs of the Moon, one of the near side (the Earth-facing side) and one of the far side (the side never presented to us). The Moon rotates at the same rate as it orbits the Earth so that it always presents the same face to the Earth. The second pair of images are relief maps (topographic maps) for the near side and far side of the Moon.

The Moon is divided into two types of terrain:

maria, the lightly cratered, dark-colored lowland basins which cover 15 % of the lunar surface. 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 cratering. The largest impact basin on the Moon is the South Pole-Aitken basin. It is 2,500 kilometers in diameter! The circumference of the Moon is 11,000 kilometers! The South Pole-Aitken basin is 4.2-4.3 billion years old.

highlands, the light-colored, heavily cratered regions which cover 85 % of the lunar surface. Because of their heavy cratering, the highland regions are inferred to be ancient while the lightly-cratered maria are inferred to be young (younger). The lunar mountains in the highland regios are produced by the large impacts uplifting the terrain. They are not formed as are mountains on Earth, that is, they are not formed through crutal motions driven by geologic forces ( e.g., on Earth driven by motion of lithospheric plates).

Features similar to those found on the Moon are also seen on Mars and Mercury.

The far side of the Moon is similar to the near side of the Moon in terms of cratering rate. There are some differences in cratering though. (1) There are fewer maria on the far side of the Moon, probably because of the asymmetry in the crutal thickness of the Moon. (2) Recent studies find that impact basins are smaller in size on the far side of the Moon than those on the Earth-facing side of the Moon. What are possible implications of this result?
Crater Origin: The craters on the Moon and on most other Terrestrial planets (see Mercury) were produced by impacts and not by volcanism. The Lunar craters are low-lying, below sea-level, not like the caldera of volcanos. The primary factors that affect crater size are the mass of the impacting object, the speed with which the objects strikes the ground, and the local geology (type of rock).

Copernicus (93 km) and Tycho (85 km) are fairly young craters on the Moon, 800 million years and 108 million years old, respectively. Suppose the objects which produced Copernicus and Tycho on the Moon had actually struck the Earth, would the craters on Earth be larger or smaller than Copernicus and Tycho?

B. Lunar Chronology

The relative ages of different regions on the Moon are easy to determine. If we count the number of crators in different regions, we infer

The higher the crater density, the older the region
The lower the crater density, the younger the region
So, for the Moon, we knew early on that the heavily cratered highland regions were older than the lightly cratered maria.
After we went to the Moon, we could take the next step. We could find the chronology for the Moon, in that we could determine actual ages in terms of years for features, not just whether they were younger or older. We could do this because we have been there, seen it. This allowed us to gather rock samples from various terrains on the Moon and determine their ages using radioactive age dating.
This allowed us to set a firm timeline for the evolution of the lunar surface features and to set the chronology for lunar cratering. This is valuable because the Lunar cratering history is, presumably, the same as the the inner Solar System planets cratering history. The cratering density has, so far, been the most useful way to judge ages of features on Mars.

Lunar Chronology
Using radioactive age dating, Moon rocks were found to be:

Mare Basalt: - 3.1 to 3.8 billion years old

Highland Breccia: - 3.8 to 4.0 billion years old

Highland Anorthosite - proved difficult for absolute dating; but are estimated to be 4.4 billion years or older.
Given these ages, can construct the lunar chronology shown to the right. The abbreviation, Ga, is 1 billion years.

C. Cratering Rate
We think about how this works below but first, let me point out that because the Moon has an old surface, and so the cratering history goes back further than it does on the Earth.

The left figure shows the cratering density for the Moon and how it has changed over the years. The first step is to simply count the numbers of craters in some region, say a mare. For mare, we find that there are relatively few craters, mare are definitely younger than the highland regions. Next, find the ages of the craters in the mare. The ages measured for the craters then tell us the rate at which craters were formed in the mare. In this way we found that mare fairly old, between 3.2 and 3.8 billion years or so, despite being lightly cratered. This shows us that the cratering rate for the Moon was low over the lifetime of the mare. Looking at other regions on the Moon, we can get the cratering rate for nearly the entire lifetime of the Moon. The crater counting is consistent with a cratering rate on the Moon that has been slow and steady for the past 3 billion or so years. Around 4 billion years ago, there isn't a jump, but it was around this time the the large impact basins that formed the maria were made. Apparently an increased rate of large impactors happened at this time, the so-called Late Heavy Bombardment. Not apparent in the plot, but there was also a jump in the cratering rate around 290 million years ago when the rate jumped by around a factor of 3.
We can do a similar exercise for the Earth. Surprisingly, studies suggest that the cratering history for land masses on Earth is complete for craters larger than 6 km over the last billion years or so. This result says that we have found nearly all of the craters on the Earth with diameters larger than 6 km over this time. Craters smaller than 6 km in diameter are destroyed by erosion and geological effects. Older craters suffer erosion and affects due to geology. So, although this result is great, it could be better in that it doesn't reach back to Late Heavy Bombardment times. The cratering rate for the Earth is then
The shape of the plot is similar to that for the Moon suggesting we share a common source for the impactors. A dinosaur killer occurs roughly every 100 million years or so. The dinosaur killer produced the Chicxulub crater 65 million years ago.

II. GEOLOGIC ACTIVITY ON EARTH (GROUP 1)
The most massive Terrestrial planets, Earth and Venus, are likely to show active geologies. Mars may show current or recent geological activity as well. Mercury and the Moon are probably dead from a geological standpoint. The sorts of geology we expect are volcanism and crustal motion, both manifestations of the fact that the interiors of the planets are hot.
A consequence of geological activity (and wind and water erosion on the Earth) is to produce a relatively young surface. 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 Lunar Chronology where the youngest features, the maria, are older than 3 billion years.
Geologic Activity on the Terrestrial Planets? The Earth shows plate tectonics. In addition to showing vertical crustal motion, the Earth has a segmented lithosphere that moves horizontally. This proves to be crucial for how our atmosphere evolves. It is thus of interest to know if other Terrestrial planets show Plate Tectonics.

Earth

Venus

Mars

Venus and Mars both show plenty of geologic activity but neither shows compelling evidence for Plate Tectonics. For some reason both planets do not have segmented lithospheres, their lithospheres are single solid pieces. (1) Venus's lithosphere may not be brittle as Earth's because Venus is hot. Even if Venus had a segmented lithosphere it might still not have plate tectonics because it is so dry. On Earth, convection drives plate tectonics and water acts as lubrication in subduction zones. (2) Mars's lithosphere is thicker than is the Earth's (likely because Mars is smaller and thus cools faster than does Earth). Consequently, Mars's lithosphere may not break as easily as does the Earth's.