06 Inner Planets

1. Summary of Terrestrial Planets

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. 

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.

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.


3. Venus (#2)

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

Slow, retrograde rotation of Venus results in large difference between solar day (117 Earth days) and sidereal day (243 Earth days); both are large compared to the Venus year (225 Earth days)  .

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.

(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. (c) Another version of (a), with major surface features labeled.

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

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

(a) A Magellan image of Ovda Regio, part of Aphrodite Terra.

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

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

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

(c) Gula Mons, as seen from ground level.

Lava Dome

(a) 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. Magellan found features like these in several locations on Venus.

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

Venus Corona


Runaway Greenhouse Effect

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. 

4. Earth (#3)



The content of the atmosphere, excluding Nitrogen, makes the Earth's atmosphere very distinct from the others


➤ rise/fall of air

➤ surface winds


Layers of the Atmosphere



(molecules ➤ atoms ➤ ions)

Surface Heating

Greenhouse Effect

Origin of Earth's Atmosphere

Primary Atmosphere

Secondary Atmosphere


So, oxygen containing atmosphere is due to the evolution of life on Earth

Earth's Interiors

Earthquake cause the entire planet to vibrate a little. These vibrations are not random. They are systematic waves called seismic waves. They move outward from the site of the quake.

Types of Seismic Waves

P-waves (pressure)

S-waves (Shear)

From the Analysis of Seismic Waves



What heated the Earth's center?

➤ which caused interior temperature to increase further.

➤ a thin layer of surface continue to be molten


Earth's Magnetosphere

Van Allen Belts


The particles that can escape from Van Allen belts penetrates into lower layers of the atmosphere.

Shape of Magnetosphere

The tides

Tidal Force

Earth's Rotation

Why the sky is blue

It is due to scattering of the sunlight by the air molecules and dust particles

This is called Rayleigh scattering. The scattering is wavelength dependent.

When the Sun is high:

direction of the Sun is reddened slightly

away from the Sun appears blue

When the Sun is low

➤ The Sun becomes orange in color

5. Mars (#4)

Mars from Earth

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

Main Surface Features


Valles Marineris

Olympus Mons

Water on Mars

Left: Mars, Right: Mississippi River

(a) An outflow channel near the Martian equator bears witness to a catastrophic flood that occurred about 3 billion years ago. (b) The onrushing water that carved out the outflow channels was responsible for forming these oddly shaped “islands” as the flow encountered obstacles—impact craters—in its path. Each “island” is about 40 km long. 

This makes us to think that "Open water once existed on Mars".

This may be an ancient Martian river delta.

(Upper panel) Much of northern hemisphere may have been ocean. Blue color indicates lower elevations. (Lower panel) Evidence of erosion by standing water in the crater's floor (140 km across)

Impact craters less than 5 km across have mostly been eroded away. Analysis of craters allows estimation of age of surface.

(a) The large lunar impact crater Copernicus is typical of those found on Earth’s Moon. Its ejecta blanket appears to be composed of dry, powdery material. (b) The ejecta from Mars’s crater Yuty (18 km in diameter) evidently was once liquid. This type of crater is sometimes called a "splosh crater".

(a) This high-resolution Mars Global Surveyor view (left) of a crater wall (right) near the Mariner Valley shows evidence of “gullies” apparently formed by running water in the relatively recent past.

Recent Martian Outflow This comparison between two Mars Global Surveyor images, taken in 1999 and 2005, of a Martian impact crater shows that something—the white streak (lower right), possibly water—flowed across the surface during that 6-year period: the activity is ongoing!

Martian Polar Caps

The southern (a) and northern (b) polar caps of Mars are shown to scale in these mosaics of Mariner 9 images. These are the residual caps, seen here during their respective summers half a Martian year apart.

The southern cap is some 350 km across and is made up mostly of frozen carbon dioxide.

The northern cap is about 1000 km across and is composed mostly of water ice. The inset shows greater detail in the southern cap.

Exploration of Mars

Viking 1 This is the view from the Viking 1 spacecraft now parked on the surface of Mars. The fine-grained soil and the reddish rock-strewn terrain stretching toward the horizon 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 Another view of the Martian surface, 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) 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 

©2024 NASA. NASA’s Perseverance Mars rover captured this mosaic showing the Ingenuity Mars Helicopter at its final airfield on Feb. 4, 2024. 

Mars Atmosphere

The troposphere, which rises to an altitude of about 30 km in the daytime, occasionally contains clouds of water ice or, more frequently, dust during the planet wide dust storms that occur each year. But it mostly contains carbon dioxide and it is very thin.

Above the troposphere lies the stratosphere.

Note the absence of a higher temperature zone in the stratosphere, indicating the absence of an ozone layer.

Fog can form in low-lying areas as sunlight strikes

Atmospheric Change. Mars may be victim of runaway greenhouse effect in the opposite sense of Venus’s:

As water ice froze,

As a result, Mars may have had a thicker atmosphere and liquid water in the past, but they are now gone.

Martian Moons

(a) A Mars Express photograph of the potato-shaped Phobos, not much larger than Manhattan Island. The prominent crater (called Stickney) at left is about 10 km across.

(b) Like Phobos, the smaller moon, Deimos, has a composition unlike that of Mars.

Both moons are probably captured asteroids. This close-up photograph of Deimos was taken by a Viking orbiter. The field of view is only 2 km across, and most of the boulders shown are about the size of a house.