In this chapter, you will discover…In this chapter, you will discover…

Mercury, a Sun-scorched planet with dormant volcanoes, Mercury, a Sun-scorched planet with dormant volcanoes, a heavily cratered surface, and a substantial iron corea heavily cratered surface, and a substantial iron core

Venus, perpetually shrouded in thick, poisonous clouds Venus, perpetually shrouded in thick, poisonous clouds and mostly covered by gently rolling hills with few cratersand mostly covered by gently rolling hills with few craters

Mars, a red, dusty planet that once had running water on Mars, a red, dusty planet that once had running water on its surface and may still have liquid water undergroundits surface and may still have liquid water underground

Mercury and Our Moon

Mercury and our Moon are shown here to the same scale. Mercury’s radius is 2439 km and the Moon’s is 1738 km. For comparison, the distance from New York to Los Angeles is 3944 km (2451 mi). Mercury’s surface is more uniformly cratered than that of the Moon. Daytime temperatures at the equator on Mercury reach 700 K (800°F), hot enough to melt lead or tin.

Mercury may look like our Moon at first glance but it has a variety of volcanic craters which poured out large amounts of lava filling smaller impact craters and creating smooth plains between the larger ones. It also lacks maria and craterless regions.

This view of Mercury’s northern hemisphere was taken by Messenger as it sped past the planet in 2009. Numerous craters on the top left and bottom half of the image are separated by a broad intercrater plain created by lava flow from now extinct volcanoes. The image is about 880 km (550 mi) across.

Mercury’s Craters and Plains

Major Impacts on Mercury: The Caloris Basin

• Messenger sent back this view of a huge impact basin on Mercury’s equator. It very likely shook the entire planet and the side directly opposite it has a jumbled, hilly surface probably caused by the force of the impact.

• The basin is the entire orange-colored portion of the figure. This image has been color enhanced to show the different surface compositions of Mercury. The orange regions around the edge of the basin are believed to be volcanic features.

Major Impacts on Mercury

(b) Only about half the Caloris Basin appears here because it happened to lie on the terminator when the Mariner 10 spacecraft sped past the planet. Although the center of the impact basin is hidden in the shadows (just beyond the left side of the picture), several semicircular rings of mountains reveal its extent.

(c) What look like tiny, fine-grained wrinkles on this picture are actually closely spaced hills, part of a jumbled terrain that covers nearly 500,000 square km on the opposite side of Mercury from the Caloris Basin. The large, smooth-floored crater, Petrarch, has a diameter of 170 km (106 mi). This impact crater was produced more recently than Caloris Basin.

Major Impacts on Mercury

Possible Volcanic Vent

The central indentation in this Messenger image from 2009 is believed to be a caldera (sunken vent) of an explosive volcano on Mercury. It is unlikely to be an impact crater, as it completely lacks a raised crater wall. (Compare to impact crater SW of it.)

Scarps on Mercury

A long, meandering cliff, called Santa Maria Rupes, runs from north to south across this Mariner 10 image of a region near Mercury’s equator. This cliff, called scarp by geologists, is more than 1 km high and runs for several hundred kilometers and probably formed as Mercury cooled. Note how the crater in the center of the image was distorted vertically when the scarp formed.

Geology on Mercury

This Messenger image shows a variety of interesting features, including lava-filled craters, secondary craters created by debris that splashed out of an impact crater, a scarp cutting through an old crater, and as-yet unidentified, light-colored rock at the bottom of a crater.

The Interiors of Earth and Mercury

Even though Mercury’s surface has a low iron content it has the highest percentage of iron of any planet in the solar system. Consequently, its iron core occupies an exceptionally large fraction of its interior (75% of its diameter).

Mercury’s Magnetosphere

In 2008, the Messenger spacecraft revealed that Mercury had a liquid iron core explaining the source of its magnetic field. Mercury’s magnetic field is about 1% as strong as Earth’s field, just strong enough to deflect the solar wind. The slow rate of Mercury’s rotation explains why it is so weak.

The Stripping of Mercury’s Mantle

To account for Mercury’s high iron content, one theory proposes that a collision with a massive planetesimal stripped Mercury of most of its rocky mantle. (a) These three images show a computer simulation of a nearly head-on collision between proto-Mercury and a body one-sixth its mass. (b) Both worlds are shattered by the impact, which vaporizes much of their rocky mantles. (c) Mercury eventually re-forms from the remaining iron-rich debris. The rest of the original Mercury and the impactor left this area of the solar system. This theory could help explain Mercury’s significantly elliptical orbit

3-to-2 Spin-Orbit Coupling

Mercury undergoes three sidereal rotations (respect the stars not the Sun) every two years while undergoing two revolutions around the Sun. When Mercury formed, the Sun’s gravitational force locked in two regions of high tide on the planet. Its highly eccentric orbit ended up with the 3-to-2 spin-orbit coupling instead of a synchronous orbit like our Moon. The result, is a sidereal year 88 Earth days long and a solar day 176 Earth days long.

Mercury’s atmosphere is the thinnest of all planets. Its Mercury’s atmosphere is the thinnest of all planets. Its gravity force is too weak to hold a permanent atmosphere. gravity force is too weak to hold a permanent atmosphere. but traces of seven different gases have been detected but traces of seven different gases have been detected around it.around it.

Hydrogen and helium probably come from the Sun. but Hydrogen and helium probably come from the Sun. but sodium, magnesium, calcium, and potassium gas escape sodium, magnesium, calcium, and potassium gas escape from rocks inside the planet. Oxygen may come from polar from rocks inside the planet. Oxygen may come from polar ice sublimating (changing from ice to gas). ice sublimating (changing from ice to gas).

All of these gases drift into space but are continually All of these gases drift into space but are continually replenished in the atmosphere.replenished in the atmosphere.

The solar wind pushes much of the gases outward. creating The solar wind pushes much of the gases outward. creating a tail similar to one formed by comets.a tail similar to one formed by comets.

Mercury’s Atmosphere

Evening or Morning Star or UFO?No, It Is Venus!

Venus is bright and is often seen near the horizon where rising and sinking gases in Earth’s atmosphere make it appear to move and change color, like alleged UFOs.

Temperature and Pressure in the Venusian Atmosphere

The pressure at the Venusian surface is a crushing 92 atm (1296 lb/in2). Above the surface, atmospheric pressure decreases smoothly with increasing altitude. The temperature decreases from a maximum of nearly 750 K (900°F) on the ground to a minimum of about 173 K (−150°F) at an altitude of 100 km.

The Venusian Surface

The horizon and cloudy, carbon dioxide−rich sky of Venus imaged by the Venera 7 lander. Large quantities of short-lived sulfur compounds suggest the presence of active volcanoes on Venus. Russian scientist believe this region was covered with lava that cooled and formed the interlocking shapes seen in the photograph.

• The Venus Express orbiter has detected short-lived magnetic fields on Venus that may be caused by lightning discharge.

• This photograph of lightning generated by the Eyjafjallajökull volcano in Iceland in 2010 shows the phenomena that may have recently been observed on Venus.

Lightning on Earth from Volcanic Eruptions

The Greenhouse EffectA portion of the sunlight penetrates through the clouds and atmosphere of Venus, heating its surface. The surface in turn emits infrared radiation, much of which is absorbed by carbon dioxide (and to a much lesser degree, water vapor). The trapped radiation helps increase the average temperatures of the surface and atmosphere. Some infrared radiation does penetrate the atmosphere and leaks into space. In a state of equilibrium, the rate at which the planet loses energy to space in this way is equal to the rate at which it absorbs energy from the Sun. This is why Venus has a higher surface temperature than Mercury, even on the side facing away from the Sun.

Color Correcting Light on Venus’s Surface

(a) This color photograph, taken by a Soviet spacecraft, shows rocks that appear orange because the light was filtered through the thick, sulfur-rich clouds. (b) By comparing the apparent color of the spacecraft to the color it was known to be, computers can correct for the sulfurous light. The actual color of the rocks is gray. In this view, the rocky plates that cover the ground may be fractured segments of a thin layer of lava called basalt. The toothed wheel in each image is part of the landing mechanism that keeps the spherical spacecraft from rolling.

A Venusian Landscape

A computer combined radio images to yield this perspective view of Venus as you would see it from an altitude of 4 km (2.5 mi). The color results from light being filtered through Venus’s thick clouds. The brighter color of the extensive lava flows indicates that they reflect radio waves more strongly. The vertical scale has been exaggerated 10 times to show the gentle slopes of Sapas Mons and Maat Mons, volcanoes named for ancient Phoenician and Egyptian goddesses, respectively.

A Map of Venus

This false-color radar map of Venus, analogous to a topographic map of Earth, shows the large-scale surface features of the planet. The equator extends horizontally across the middle of the map. Color indicates elevation—red for highest, followed by orange, yellow, green, and blue for lowest. The planet’s highest mountain is Maxwell Montes on Ishtar Terra. Scorpion-shaped Aphrodite Terra, a continent-like highland, contains several spectacular volcanoes. Do not confuse the blue and green for oceans and land.

A “Global” View of Venus

A computer using numerous Magellan images creates a simulated globe. Color is used to enhance small-scale structures. Extensive lava flows and lava plains cover about 80% of Venus’s relatively flat surface. The bright band running almost east-west is the continent-like highland region Aphrodite Terra.

Radar images indicate approximately 1600 Radar images indicate approximately 1600 major volcanic features and fewer than major volcanic features and fewer than 1000 impact craters.1000 impact craters.

Venus’s thick atmosphere heats and Venus’s thick atmosphere heats and vaporizes, much of the material that would vaporizes, much of the material that would create craters. Present theories suggest create craters. Present theories suggest some type of crustal melting occurs as some type of crustal melting occurs as heat is released to the surface, followed by heat is released to the surface, followed by a cooling period when the crust solidifies a cooling period when the crust solidifies and thickens. and thickens.

Volcanic Features and Impact Craters

Craters on Venus

These three impact craters, with extensive ejecta surrounding each, are located on Venus’s southern hemisphere. They were imaged using radar by the Magellan spacecraft. The colors are based on the Venera images.

Venus exhibits retrograde rotation that is Venus exhibits retrograde rotation that is opposite of the rest of the inner planets. (Uranus opposite of the rest of the inner planets. (Uranus is the only other planet to display this is the only other planet to display this phenomenon.)phenomenon.)

This is determined by viewing the direction of This is determined by viewing the direction of rotation as seen far above the north pole. rotation as seen far above the north pole.

Earth and most other planets rotate Earth and most other planets rotate counterclockwise, but Venus rotates clockwise.counterclockwise, but Venus rotates clockwise.

Venus’s Retrograde Rotation

The Illusion of Martian Canals

Giovanni Schiaparelli examined Mars through a 20-cm-diameter (8-in.) telescope, the same size used by many amateur astronomers today. His drawings of the red planet showed features perceived by Percival Lowell and others as irrigation canals. Higher-resolution images from Earth and spacecraft visiting Mars failed to show the same features.

The Topography of Mars

The color coding on this map of Mars shows elevations above (positive numbers) or below (negative numbers) the planet’s average radius. To produce this map, an instrument on board Mars Global Surveyor fired pulses of laser light at the planet’s surface, then measured how long it took each reflected pulse to return to the spacecraft. The Viking 1 lander (VL1), Viking 2 lander (VL2), Mars Pathfinder (MP), Opportunity, and Spirit landing sites are each marked with an X.

Martian Terrain

This high-altitude photograph shows a variety of the features on Mars, including enormous volcanoes (left) on the highland, called Tharsis Bulge; impact craters (upper right); and vast, windswept plains. The enormous Valles Marineris canyon system crosses horizontally just below the center of the image. Inset: Details of the Valles Marineris, which is about 200 km (120 mi) wide. The canyon floor has two major levels. The northern (upper) canyon floor is 8 km (5 mi) beneath the surrounding plateau, whereas the southern canyon floor is only 5 km (3 mi) below the plateau.

Craters on Mars

This image taken by the Hubble Space Telescope was made during the opposition of 2003. Arabia Terra looks like the Arabian peninsula on Earth. It is an old, highly eroded region dotted with numerous flat-bottom craters. Lava-covered Syrtis Major was first identified by Christiaan Huygens in 1659. A single impact carved out Hellas Planitia, which is five times the size of Texas. Inset: This mosaic of images from the Viking 1 and 2 orbiter spacecraft shows an extensively cratered region located south of the Martian equator. Note how worn down these craters are compared to those on Mercury and our Moon.

Volcanoes on Mars

Most of the volcanoes on Mars are located in the northern hemisphere. The largest volcano in the solar system, Olympus Mons covers an area as big as the state of Missouri and raise 26 km (16 mi) above the surrounding plains. Clouds of water vapor surround the volcanoes in this image.

Volcanoes on Mars

These cones on Mars may have been created when lava from Olympus Mons heated underground ice, causing the resulting water and vapor to expand, raise the planet’s surface, and burst out.

In the Eye of the Beholder

These two images of the same site on Mars, taken 22 years apart, show how the apparent face in (a) changed to a more “natural-looking” feature in (b). This transformation was due to weathering of the site, improved camera technology, and the change in angle at which the photograph was taken.

In the Eye of the Beholder

(c) In the same region of Mars, other erosion features also appear to be pyramids and skulls. (d) The Galle crater and its interior features combine to give the impression of a “happy face.”

The Local Magnetic Fields on MarsThe Local Magnetic Fields on Mars

Unlike Earth, with its global magnetic field, Mars has only local magnetic fields, shown here in the regions of yellow and orange. They are believed to emanate from magnetized rock under the planet’s surface. The magnetism probably occurred when Mars was young, had a strong magnetic field, and before tectonic activity ceased.

Changing Seasons on Mars

During the Martian winter, the temperature decreases so much that carbon dioxide freezes out of the Martian atmosphere. A thin coating of carbon dioxide frost covers a broad region around Mars’s north pole. During the summer in the northern hemisphere, the range of this north polar carbon dioxide cap decreases dramatically. During the summer, a ring of dark sand dunes is exposed around Mars’s north pole.

The Atmosphere of Mars

When Mars’s sky is relatively free of dust, it appears similar in color to our sky, as shown in this sunset photo taken by the rover Opportunity. The darker, brown color in which the Sun is immersed is due to lingering dust in the sky. When less dust is present, the Sun looks almost white during Martian sunsets.

Mars’s atmosphere is mostly carbon dioxide. The Mars’s atmosphere is mostly carbon dioxide. The rest is nitrogen, argon, water vapor, and traces of rest is nitrogen, argon, water vapor, and traces of oxygen.oxygen.

Its gravitational force is strong enough to keep all Its gravitational force is strong enough to keep all of the gases attracted to its surface but water of the gases attracted to its surface but water vapor.vapor.

In the past, the Martian atmosphere may have In the past, the Martian atmosphere may have been thicker allowing liquid water to collect on its been thicker allowing liquid water to collect on its surface. As the density of the atmosphere surface. As the density of the atmosphere decreased, much of the surface water evaporated decreased, much of the surface water evaporated and drifted into space.and drifted into space.

What was left of the surface water turned to ice.What was left of the surface water turned to ice.

The Martian Atmosphere

The Atmosphere of Mars

Most images we have of Mars’s sky show colors like those seen here. Taken by the Mars Pathfinder, this photograph shows the Sojourner rover snuggled against a rock named Moe on the Ares Vallis to run tests on the rock. At the top of the image, the pink color of the Martian sky is evident.

Martian Dust Devil

This Spirit image is one in a movie sequence of a dust devil moving left to right across the surface of Mars. The rovers have filmed several such events.

Martian Dust Devil

These dark streaks are the paths of dust devils on the Argyre Planitia of Mars. The tracks cross hills, sand dunes, and boulder fields, among other features on the planet’s surface.

Crater Endurance

Photographed by the rover Opportunity, this crater on Mars is about 130 m (430 ft) across. Rocks are visible in the crater and in vertical cliffs along its walls. By studying such rocks, astronomershope to understand more of the history of water on Mars.

Rivers on Mars and Earth

(a) Winding canyons on Mars, such as the one in this Viking 1 orbiter image, appear to be formed by sustained water flow. This theory is supported by the terraces seen on the canyon walls in high-resolution Mars Orbiter images. Long periods of water flow require that the planet’s atmosphere was once thicker and its climate more Earthlike. (b) This is the Yangtze River near Chongqing, China. Typical of rivers on Earth, it shows the same snakelike curve as the river channels on Mars.

Evidence of Water on Mars

(a) This dry Martian lake, photographed by the Mars Global Surveyor with a resolution of 1.5 m, is an excellent example of how geology and astronomy overlap. The features of this dry lake are consistent with those found on lakebeds on Earth. (b) This network of dry riverbeds is located on Mars’s cratered southern hemisphere.

Ancient Oceans and Lakes on Mars

This Mars Global Surveyor image of a portion of Valles Marineris reveals terrain with “stair-step layers.” Such terrain is likely to have been created by sedimentation at the bottom of an ancient body of water.

Ancient Oceans and Lakes on Mars

This image of Burns Cliff, photographed by rover Opportunity, shows a close-up of layers of rock laid down on a body of water on Mars that went through wet and dry periods.

Layers of Rock Laid Down by Water

This close-up image taken by the rover Opportunity shows a small section of rock layers in a location, called The Dells. The angled and curved layering seen here is created only on Earth by water flow, strongly suggesting that this sediment was also deposited by water. The nearly spherical rocks, called “blueberries,” because they are dark, have been chemically identified as hematite, an iron-rich mineral that is usually formed in water. The rovers have found blueberries strewn in a wide variety of locations.

Layers of Rock Laid Down by Water

This iron meteorite, dubbed the “Heat Shield Rock” because it was discovered behind the Mars rover Sojourner’s heat shield, is surrounded by hematite blueberries.

Martian Gullies

Two images of the same southern hemisphere crater on Mars taken six years apart. Whether the new deposit was initiated by ground vibration or by a short period of liquid water flow from inside Mars is still under investigation.

Martian Gullies

Polygonal cracks are visible in this Opportunity image of Escher Rock. They were believed to have formed when this area was flooded. Water seeped into the rock, which cracked as the water froze and expanded, then evaporated away.

An Active Avalanche on Mars

This image, taken by the Mars Reconnaissance Orbiter, shows an active landslide of falling ice, dust, and rock. This debris first hit the cliff wall about halfway down and then continued falling down the less steep slope, as seen in the bottom of the image. The cloud of debris near the bottom is about 180 m (590 ft) wide.

A Piece of Mars on Earth

• Mars’s thin atmosphere does little to protect it from impacts. Some of the debris ejected from impact craters there apparently traveled to Earth. This is an SNC meteorite recovered in Antarctica. The 4.1- billion-year-old meteorite, designated ALH84001, shows strong evidence of having been exposed to liquid water on Mars, perhaps for hundreds of years.

• At least 110 meteorites from Mars have been identified.

A Piece of Mars on Earth

Possible fossil remains of primitive bacterial life on Mars, although theories of nonbiological origins have also been presented.

Regolith of Mars

• The rover Spirit’s wheels churned up the regolith of Mars, revealing sulfur-based salts just below the surface.

• The Martian regolith has a large amount of oxygen in it tied up in unstable chemicals called peroxides and superoxides that break down in the presence of water.

• The chemical reactivity comes from the Sun’s ultraviolet radiation and electric activity inside dust devils.

Phobos and Deimos

Phobos, the larger of Mars’s two moons, is potato-shaped and measures approximately 28 23 20 km. It is dominated by crater Stickney (on the right of the image). Deimos is less cratered than Phobos and measures roughly 16 12 10 km.

Summary of Key IdeasSummary of Key Ideas

Mercury Even at its greatest orbital elongations, Mercury can be seen Even at its greatest orbital elongations, Mercury can be seen

from Earth only briefly after sunset or before sunrise.from Earth only briefly after sunset or before sunrise. The Mercurian surface is pocked with craters like the Moon’s, The Mercurian surface is pocked with craters like the Moon’s,

but extensive, smooth plains lie between these craters. Long but extensive, smooth plains lie between these craters. Long cliffs meander across the surface of Mercury. These scarps cliffs meander across the surface of Mercury. These scarps probably formed as the planet cooled, solidified, and shrank.probably formed as the planet cooled, solidified, and shrank.

The long-ago impact of a large object formed the huge Caloris The long-ago impact of a large object formed the huge Caloris Basin on Mercury and shoved up jumbled hills on the opposite Basin on Mercury and shoved up jumbled hills on the opposite side of the planet. Several similar, but smaller, impact features side of the planet. Several similar, but smaller, impact features also exist on our Moon.also exist on our Moon.

Mercury has an iron core, which fills more of its interior than Mercury has an iron core, which fills more of its interior than Earth’s core fills Earth.Earth’s core fills Earth.

Mercury has a weak, global magnetic field that partially shields it Mercury has a weak, global magnetic field that partially shields it from the solar wind.from the solar wind.

Venus Venus is similar to Earth in size, mass, and average Venus is similar to Earth in size, mass, and average

density, but it is covered by unbroken, highly reflective density, but it is covered by unbroken, highly reflective clouds that conceal its other features from observers clouds that conceal its other features from observers using visible-light telescopes.using visible-light telescopes.

Although most of Venus’s atmosphere is carbon dioxide, Although most of Venus’s atmosphere is carbon dioxide, its dense clouds contain droplets of concentrated sulfuric its dense clouds contain droplets of concentrated sulfuric acid mixed with yellowish sulfur dust. Active volcanoes acid mixed with yellowish sulfur dust. Active volcanoes on Venus may be a constant source of this sulfurous on Venus may be a constant source of this sulfurous veil.veil.

Venus Venus’s exceptionally high temperature is caused by the Venus’s exceptionally high temperature is caused by the

greenhouse effect, as the dense carbon dioxide greenhouse effect, as the dense carbon dioxide atmosphere traps and retains heat emitted by the planet. atmosphere traps and retains heat emitted by the planet. The surface pressure on Venus is 92 atm, and the The surface pressure on Venus is 92 atm, and the surface temperature is 750 K. Both temperature and surface temperature is 750 K. Both temperature and pressure decrease as altitude increases.pressure decrease as altitude increases.

The surface of Venus is surprisingly flat and mostly The surface of Venus is surprisingly flat and mostly covered with gently rolling hills. There are two major covered with gently rolling hills. There are two major ““continentscontinents”” and several large volcanoes. The surface of and several large volcanoes. The surface of Venus shows evidence of local tectonic activity but not Venus shows evidence of local tectonic activity but not the large-scale motions that play a major role in the large-scale motions that play a major role in continually reshaping Earth’s surface.continually reshaping Earth’s surface.

Mars Earth-based observers found that the Martian solar day is nearly Earth-based observers found that the Martian solar day is nearly

the same as that of Earth, that Mars has polar ice caps of carbon the same as that of Earth, that Mars has polar ice caps of carbon dioxide snow that expand and shrink with the seasons, and that dioxide snow that expand and shrink with the seasons, and that the Martian surface undergoes seasonal color changes.the Martian surface undergoes seasonal color changes.

A century ago, observers reported networks of linear features A century ago, observers reported networks of linear features that many perceived as canals. These observations led to that many perceived as canals. These observations led to speculation about self-aware life on Mars.speculation about self-aware life on Mars.

The Martian surface has many flat-bottomed craters, several The Martian surface has many flat-bottomed craters, several huge volcanoes, a vast equatorial canyon, and dried-up huge volcanoes, a vast equatorial canyon, and dried-up riverbeds—but no canals formed by intelligent life. River deltas riverbeds—but no canals formed by intelligent life. River deltas and dry riverbeds on the Martian surface indicate that large and dry riverbeds on the Martian surface indicate that large amounts of water once flowed there.amounts of water once flowed there.

Mars Liquid water would quickly boil away in Mars’s thin present-day Liquid water would quickly boil away in Mars’s thin present-day

atmosphere, but the planet’s polar ice caps contain significant atmosphere, but the planet’s polar ice caps contain significant quantities of frozen water, and a layer of permafrost exists beneath quantities of frozen water, and a layer of permafrost exists beneath parts of the regolith.parts of the regolith.

The Martian atmosphere is composed mostly of carbon dioxide. The The Martian atmosphere is composed mostly of carbon dioxide. The surface pressure is less than 0.01 atm.surface pressure is less than 0.01 atm.

Chemical reactions in the regolith, together with ultraviolet radiation Chemical reactions in the regolith, together with ultraviolet radiation from the Sun, apparently act to sterilize the Martian surface.from the Sun, apparently act to sterilize the Martian surface.

Mars has no global magnetic fields, but local fields pierce its surface Mars has no global magnetic fields, but local fields pierce its surface in many places. in many places.

Mars has two irregularly shaped moons, Phobos and Deimos. Both Mars has two irregularly shaped moons, Phobos and Deimos. Both are in synchronous rotation with Mars. How they came into orbit is are in synchronous rotation with Mars. How they came into orbit is still under investigation.still under investigation.

Key TermsKey Terms

3-to-2 spin-orbit couplingcalderadust devilgreenhouse effecthot-spot volcanismnorthern vastness (northern devastation)retrograde rotationsouthern highlands