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.


  • Formed about 4.6 billion years ago

  • Melted due to bombardment, cooled slowly

  • It's crust is shrank and crumpling.

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

  • Brightest object in the sky, after Sun and Moon

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.

  • Surface is relatively smooth

  • Two continent-like features: Ishtar Terra and Aphrodite Terra

  • No plate tectonics

  • Mountains, a few craters, many volcanoes and large lava flows

(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.

    • 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.

(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.

    • 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.

(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.

    • 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.

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

(c) 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 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

  • This corona, 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


  • Venus’s atmosphere is very dense

  • Solid cloud bank 50–70 km above surface

  • Atmosphere is mostly carbon dioxide; clouds are sulfuric acid

  • Upper atmosphere of Venus 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

  • No magnetic field, probably because rotation is so slow

  • No evidence for plate tectonics

  • Venus resembles a young Earth (1 billion years)—no asthenosphere (soft sphere), thin crust

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.

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)


  • Surface Temperature = 290 K

  • Inclination = 23.45 degree

  • Escape Velocity = 11.2 km/s

  • Earth = 99.9%: Differentiated Layers + 0.1%: Atmosphere+Magnetosphere


  • Nitrogen = 78 %

  • Oxygen = 21 %

  • Argon = 0.9 %

  • Carbon dioxide = 0.03 %

  • Water vapor = 0.1 - 0.3 %

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


  • The layer where the convection is the energy transport mechanism

    • A constant motion of warm air and the concurrent downward flow of cooler air to take its place.

    • Therefore convection is the process that physically transfers heat from a lower (hotter) to a higher (cooler) level.

  • Convection creates a circulation pattern

  • This pattern is named as convection cells

➤ rise/fall of air

➤ surface winds


Layers of the Atmosphere


  • Absorbs UV radiation

  • Contains Oxygen, Ozone (O+O+O), Nitrogen


  • Top most part of the atmosphere is ionized by the Sun's radiation

(molecules ➤ atoms ➤ ions)

  • As the elevation increases, ionization increases

    • ions ➤ conductor ➤ reflective to certain wavelengths

Surface Heating

  • Most of the Sun's radiation manages to penetrate Earth's atmosphere:

    • Stage-1: Both reflected and absorbed by the clouds

    • Stage-2: Absorbed by Earth's surface

      • Therefore the surface is heated during the day

    • Stage-3: The surface is reradiated the absorbed energy (a blackbody curve)

      • Therefore increase in surface Temperature causes much more increase in the released energy

      • The balance is at -23 C degree; and due to Wien's Law, reradiated energy will in IR (heat)

    • Stage-4: IR is partially blocked (due to water vapor and carbon dioxide)

      • Only small amount escapes

      • trapped radiation causes temperature in the layer to increase

Greenhouse Effect

Origin of Earth's Atmosphere

Primary Atmosphere

  • Common to whole solar system

  • H, He, methane, ammonia, water vapor

  • H, He escaped 1/2 billion years ago

Secondary Atmosphere

  • Outgased from planet's interior (e.g volcanic activity)

    • Volcanic gases are rich in water vapor, methane, CO2, SO2, compounds

  • UV radiation from the Sun decomposes the lighter and H-rich gases allowing H to escape

  • Temperature drops ➤ Water vapor condenses ➤ oceans are created

  • Much of CO2 and SO2 become dissolved in the oceans or combined with the rocks

  • Oxygen was removed as quickly as it is formed (it is a reactive gas)

  • Nitrogen dominated atmosphere is formed


  • It appeared in oceans 3.5 billion years ago

  • Organism eventually began to produce atmospheric oxygen

  • Ozone layer formed

  • Life spread to the land

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

Earth's Interiors

  • One cannot drill beyond 10 km depth because of the pressure.

  • So, to study the interiors of the Earth Seismic Waves are used

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.

  • They carry information (EM radiation is carried with waves)

  • This information can be detected and recorded

  • The devices used to record these signals are called seismographs

Types of Seismic Waves

P-waves (pressure)

  • Alternately expand and compress the material medium through which they move

  • Their speed is around 5-6 km/s

  • They can travel through both liquids and solids

S-waves (Shear)

  • They come shortly after P-waves

  • Unlike P-waves they cause side-to-side motion

  • Example: waves in a guitar string

  • The speed of the wave depend on the density of the matter

  • They don't travel in straight lines

    • The velocity varies with depth

    • Waves bend as they move through the interior

  • Earthquakes

    • P + S waves

    • They create shadow zones

    • Observing seismic wave propagation and surface rocks one can model the interiors

From the Analysis of Seismic Waves

  • Mantle ~ 3000 km thick (density: 5000 km/m3)

  • Crust ~ 15 km (8 km under ocean; 20-50 km under continents with a density: 3000 km/m3)

  • Core: Ni, Fe and some other lighter elements)

    • It is liquid

    • However pressure near the center forces the material into solid state (even T is high)


  • Earth is not homogeneous; it has a layered structure.

  • Density and compositions are different for different layers:

    • CRUST - Low Density

    • MANTLE - Medium Density

    • CORE - High Density


  • Earth was molten at some time.

  • Higher-density parts sank to the core

  • Lower-density parts displaced toward the surface

  • Therefore Earth's central temperature become hot like the surface of the Sun.

What heated the Earth's center?

  • Earth grew by capturing material from its surroundings

  • This increased its gravitational field and therefore speed of material striking to surface increased

  • Therefore generated heat made the Earth molten

    • Earth began to differentiate.

    • As heavy material sank to the center;

    • More gravitational energy released,

➤ which caused interior temperature to increase further.

  • Bombardment continued:

➤ a thin layer of surface continue to be molten


  • Heavy elements like Uranium, Thorium and Plutonium release energy as they decay (break up) into lighter elements.

  • This energy also heated the Earth

  • Released energy from the decay is very small. However:

    • Large amount of these elements stored in Earth.

    • Earth had enough time to accumulate this energy

    • Created energy at interior parts couldn't leak into space because rocky surface was opaque to the heat (rock is a poor conductor)

  • Earth's crust solidified around 700 million years after Earth's formation

  • Heating continued however since radioactive heating is one directional, it gets lower and lower through time.

  • So, the Earth has been cooling for the past 4 billion years.

    • Cooling is from the outside in like a hot potato

    • Regions closest to the surface will release more energy into space.

  • Therefore this cooling also created current differentiated layer structure.

Earth's Magnetosphere

  • It is discovered by artificial satellites in late 1950s

  • It is created by the planet's magnetic field.

  • Our magnetosphere extends far beyond the atmosphere.

  • It will look like a bar magnet when observed close to the Earth.

  • The magnetic field lines runs from South to North and they intersect the Earth's surface vertically.

  • The poles are roughly aligned with Earth's spin axis.

  • The poles are not fixed and they drift with 10 km/year

    • North Pole is at 80 degree North (northern Canada)

    • South Pole is at 60 degree South (off the coast of Antarctica)

Van Allen Belts

  • The magnetosphere contains two doughnut-shape zones of high energetic charged particles above the surface

    1. Located about 3 000 km - catching heavier protons

    2. Located about 20 000 km - catching mostly electrons

  • They are named as belts because

    • their efficiency are the highest at the equator

    • they completely surround the planet

  • The particles in the belts originated from the Sun and they are carried by the solar wind.

    • Neutral particles and EM radiation are unaffected by the Earth's magnetism

    • But charged particles (electrons and protons) are strongly influenced

    • They spiral around the magnetic field lines and they are trapped in the Earth's magnetic field creating the zones (aka. belts).

  • Therefore, the belts creates an invisible shield to protect the planet surface from energetic charged particles.


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

  • Particles collide with air molecules

  • They fall back to their ground states

  • Their energy re-emit in visible spectrum; named as Aurora

  • This event are observed mostly in higher latitude regions.

  • The event can be observed on other planets too.

Shape of Magnetosphere

  • It is not symmetric

  • It looks like a tear-drop

  • Sunlit side is compressed by the solar wind;

  • It extends to 10 Earth radii from the Earth; named as magnetopause

The tides

  • Ocean level fluctuates in daily base.

  • This is due to gravitational influence of the Moon and the Sun on Earth

  • This deforms the Earth's surface depending on the distance separating any two objects

    • The Moon's gravitational attraction is greater on the side of Earth that faces the Moon than on the opposite side.

    • Earth becomes slightly elongated with the long axis of the distortion pointing towards the Moon.

    • Earth's oceans undergo the greatest deformation because liquid can most easily move around on Earth.

    • The ocean becomes:

      • a little deeper in some places (along the equator)

      • shallower in others (closer to the poles)

    • Adding the Sun's effect:

      • Sun is farther away but its mass is so much greater.

      • Its tidal influence is about half of Moon's tidal influence

Tidal Force

  • Average gravitational interaction determines the object's orbit

  • Adding tidal force to the gravitational influence deforms both bodies

  • This deformation can be estimated as 1 / r3

Earth's Rotation

  • Earth's spin is slowing due to tidal bulge offset

    • The rate of slow down is 1.5 msec / year

  • Therefore the Moon is spiraling slowly away from the Earth by 4 cm / year

  • This slow down will stop when the offset becomes zero

    • then Earth's rotation period will become 47 days

    • and the distance to the Moon will become 550 000 km.

Why the sky is blue

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

  • radiation is absorbed

  • and then re-radiated by the material in air

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

  • Blue light is much more easily scattered than the red light.

    • Blue light (~400 nm) frequency ~ Size of air molecules

    • Not red light

  • Scattering ∝ 1 / lambda^4

When the Sun is high:

  • Blue component of light will be scattered so blue light will be removed from our line of sight.

  • Red or yellow components will be little scattered and arrives along the line of sight

direction of the Sun is reddened slightly

away from the Sun appears blue

When the Sun is low

  • Blue component is gone completely

  • Red diminished in intensity

➤ The Sun becomes orange in color

5. Mars (#4)

  • Radius = 3400 km

  • Two moons(?) : Phobos, Deimos - very small compared to the Moon

  • Mars day = 24.6 hours (similar to Earth's)

  • Least eccentric orbit (circular than ellipse)

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

  • 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


  • 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

  • 700 km base diameter

  • 80 km caldera diameter (fallen in part of volcano)

  • 25 km high

  • Volcano is extinct for several 100s millions of year

  • Comparison with Earth:

    • The largest is in Hawaii: Mauna Loa

    • 120 km across

    • 9 km above ocean base

Water on Mars

  • Runoff channels resemble those on Earth

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

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,

  • Mars became more and more reflective

  • and its atmosphere thinner and thinner,

  • freezing more and more water

  • and eventually carbon dioxide as well.

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.