Inner Planets

Part A (Worlds & Light)
➤ Week-3: The Planets (Inner & Outer)
➤ Topic-7: Inner Planets (Mercury, Venus, Earth, Mars)

1. Summary

The surfaces of all five terrestrial worlds (Mercury, Venus, Earth, the Moon, and Mars) must have looked quite similar when they were young.

All five were made of rocky material that had condensed in the solar nebula, and all five were subjected early on to the impacts of the heavy bombardment.

The great differences in their present-day appearance must therefore be the result of changes that have occurred through time.

Ultimately, these changes must be traceable to fundamental properties of the planets.

Why do Mercury and Venus have virtually no moons, while Earth has one and Mars has two small ones?

2. Mercury (#1)

Phases of Mercury can be seen best when Mercury is at its maximum elongation.

Mercury was long thought to be tidally locked to the Sun; measurements in 1965 showed this to be false.

Rather, Mercury’s day and year are in a 3:2 resonance; Mercury rotates three times while going around the Sun twice.

Start counting the rotation at "Day 0" and continue with Day 59, Day 118 and Day 176.
Revolution counts are at Day 88 (Year 1) and Day 176 (Year 2).

Scarp (cliff), several hundred kilometers long and up to 3 km high.

Caloris Basin, very large impact feature; weird terrain on opposite side of planet

Weird terrain is thought to result from focusing of seismic waves.

Formation

Formed about 4.6 billion years ago
Melted due to bombardment, cooled slowly
It's crust is shrank and crumpling.

Why do Mercury and the Moon have extreme temperature variations between day and night?

3. Venus (#2)

Venus is much brighter than Mercury, and can be seen farther from the Sun. Called morning or evening star, as it is still “tied” to Sun.

It is the brightest object in the sky, after Sun and Moon

Apparent brightness of Venus varies, due to changes in phase and distance from Earth.

Venus has a slow, retrograde rotation.

It results in a very long Venus day:

1 Venus solar day is 117 Earth days
1 Venus year takes 225 Earth days.

Therefore, 1 Venus year has almost 2 Venus days.

Why does Venus rotate retrograde compared to most planets?

Dense atmosphere and thick clouds make surface impossible to see. Surface temperature is about 730 K (hotter than Mercury!). Even probes flying near Venus, using ultraviolet or infrared, can see only a little deeper into the clouds.

Surface is relatively smooth.
Two continent-like features: Ishtar Terra and Aphrodite Terra.
No plate tectonics.
No magnetic field, probably because rotation is so slow.

Mountains, a few craters, many volcanoes and large lava flows.
Venus resembles a young Earth (1 billion years) - no asthenosphere (soft sphere), thin crust.

(a) Radar map of the surface of Venus, based on Pioneer Venus data. Color represents elevation, with white the highest areas and blue the lowest.

(b) A similar map of Earth, at the same spatial resolution.

Landing on Venus

(a) The first direct view of the surface of Venus, radioed back to Earth from the Soviet Venera 9 spacecraft, which made a soft landing on the planet in 1975.

The amount of sunlight penetrating Venus’s cloud cover is about the same as that reaching Earth’s surface on a heavily overcast day.

(b) Another view of Venus, in true color, from Venera 14. Flat rocks like those visible in part (a) are seen among many smaller rocks and even fine soil on the surface. This landing site is not far from the Venera 9 site shown in (a).

The peculiar filtering effects of whatever light does penetrate the clouds make Venus’s air and ground appear peach colored—in reality, they are most likely gray, like rocks on Earth.

A Venera orbiter image of a plateau known as Lakshmi Planum in Ishtar Terra.

The Maxwell Montes mountain range (red) lies on the western margin of the plain, near the right-hand edge of the image.
A meteor crater named Cleopatra is visible on the western slope of the Maxwell range.
Note the two larger craters in the center of the plain itself.

A Magellan image of Cleopatra showing a double-ringed structure that identifies the feature to geologists as an impact crater.

A Magellan image of Ovda Regio, part of Aphrodite Terra.

The intersecting ridges indicate repeated compression and buckling of the surface.
The dark areas represent regions that have been flooded by lava up-welling from cracks like those shown in the right panel.

This lava channel in Venus’s south polar region, known as Lada Terra, extends for nearly 200 km.

Lava Flows. These cracks have allowed lava to reach the surface and flood the surrounding terrain.

The dark regions are smooth lava flows. The network of fissures visible here is about 50 km long.

Two larger volcanoes, known as Sif Mons (left) and Gula Mons (right), appear in this Magellan image.

Color indicates height above a nominal planetary radius of 6052 km and ranges from purple (1 km, the level of the surrounding plain) to orange (corresponding to an altitude of about 4 km).
The two volcanic calderas at the summits are about 100 km across.

A computer-generated view of Sif Mons, as seen from ground level.

Gula Mons, as seen from ground level.

In (b) and (c), the colors are based on data returned from Soviet landers, and the vertical scales have been greatly exaggerated (by about a factor of 40).

So these mountains look much taller relative to their widths than they actually are; Venus is actually a remarkably flat place.

Lava Domes. These dome-shaped structures resulted when viscous molten rock bulged out of the ground and then retreated, leaving behind a thin, solid crust that subsequently cracked and subsided.

A computer-generated view of four of the domes toward the right from near the center of the image in part (a). Colors are based on data returned by Soviet Venera landers.

This corona is called Aine, lies in the plains south of Aphrodite Terra and is about 300 km across.

Coronae probably result from up-welling mantle material, causing the surface to bulge outward. Note the pancake-shaped lava domes at top, the many fractures in the crust around the corona, and the large impact craters with their surrounding white (rough) ejecta blankets that stud the region.

Atmosphere

Venus’s atmosphere is very dense. Solid cloud bank 50–70 km above surface.

It is mostly carbon dioxide; clouds are sulfuric acid. Upper atmosphere has high winds, but atmosphere near surface is almost calm. There are also permanent vortices at the poles; the origin of the double-lobed structure is a mystery.

Runaway Greenhouse Effect

Because Venus’s atmosphere is much deeper and denser than Earth’s, a much smaller fraction of the infrared radiation leaving the planet’s surface escapes into space. The result is a much stronger greenhouse effect than on Earth and a correspondingly hotter planet.

The outgoing infrared radiation is not absorbed at a single point in the atmosphere; instead, absorption occurs at all atmospheric levels.

The arrows indicate only that absorption occurs, not that it occurs at one specific level; the arrow thickness is proportional to the amount of radiation moving in and out.

Why is Venus hotter than Mercury even though Mercury is closer to the Sun?

4. Earth (#3)

What geological processes make Earth more geologically active than Mercury or Mars?

5. Mars (#4)

Radius = 3400 km
Mars day = 24.6 hours (similar to Earth's)
Least eccentric orbit (circular than ellipse)
Two captured small moons(?): Phobos, Deimos

Only the polar caps can be seen from the Earth. They grow and shrink with the seasons. Polar caps contain frozen carbon dioxide. Water ice is permanently frozen under the surface.

(a) Taken in 1991. (b) A visible-light HST image taken in 2003. (c) Taken from Viking in 1976.

Viking 1 view. The fine-grained soil and the reddish rock-strewn terrain contains substantial amounts of iron ore; the surface of Mars is literally rusting away. The sky is a pale pink color, the result of airborne dust.

Viking 2 view. This one rock strewn and flat, as seen through the camera aboard the Viking 2 robot that soft-landed on the northern Utopian plains. The discarded canister is about 20 cm long. The 0.5-m scars in the dirt were made by the robot’s shovel.

Opportunity Rover. A panoramic view of the terrain near where NASA’s Opportunity rover landed on Mars in 2004. This is Endurance crater, roughly 130 m across

Main Surface Features

Dust cover on the surface shifts regularly. Creating frequent dust storms with high winds.
The major feature: Tharsis bulge (size ~ North America; 10 km above surroundings)
There is no evidence of plate tectonics

Hemispheres

Northern: rolling volcanic terrain

Southern: heavily cratered highlands (on average ~5 km above Northern hemisphere)

Probably Northern is younger: it mush have been lowered in elevation and then flooded with lava.

Valles Marineris

Huge canyon which is created by crustal forces.

- 4000 km long
- 120 km wide (max)
- 7 km deep

(On the left) complexity of valley walls and dry branches. (On the top right) Comparison with Grand Canyon in USA

Olympus Mons

The largest volcano in the solar system. Volcano is extinct for several 100s millions of year.

700 km base diameter
80 km caldera diameter (fallen in part of volcano)
25 km high

Comparison with Earth:

The largest is in Hawaii: Mauna Loa
120 km across
9 km above ocean base

How do surface conditions on Mars today hint at a once wetter climate?

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