The Solar System

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The Terrestrial Planets

Copyright (c) The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

The Terrestrial Planets

• The four terrestrial planets – Mercury, Venus, Earth, and Mars – have similar sizes and structure

• These rocky worlds orbit in the inner part of the Solar System, too small and too warm to have captured massive hydrogen atmospheres like the Jovian giants

• They have very few possessions – the Earth has the relatively large Moon and Mars has two small captured asteroids as moons

Terrestrial Planet Overview

• Mercury – smallest terrestrial planet, looks like Moon (gray, bare, cratered), essentially no atmosphere

• Venus – covered with deep sulfuric acid clouds in a dense CO2 atmosphere, hottest planet, immense volcanic peaks tower over desolate plains

Terrestrial Planet Overview

• Mars – polar caps of ice and CO2, vast red deserts with craters and dunes, canyons, and dry river beds, ancient volcanoes, thin CO2 atmosphere

• Earth – blue seas, white clouds and ice caps, red deserts, green jungles, mountains

Terrestrial Planet Overview

• Planetary size coupled with distance from Sun is the cause for these differences!

Mercury

• Mercury’s radius is 1/3 and its mass 1/20 that of Earth

• Circular craters cover the surface with the largest one being Caloris Basin with a diameter of 1300 km

• Unlike the Moon where they are found almost exclusively in maria, congealed lava flows are found in many of Mercury’s old craters and pave much of its surface

Scarps

• Enormous scarps (cliffs), formed as Mercury cooled, and shrank, wrinkling like a dried apple

“Weird” Terrain

• “Weird terrain” feature opposite side of planet from Caloris Basin possibly caused by seismic waves generated by impact that created Caloris

Mercury’s Temperature

• Mercury’s noon temperature at the equator (about 700 K = 800° F) and nighttime temperature (100 K = -280° F) are near the Solar System’s surface extremes

• These extremes result from Mercury’s proximity to the Sun and its lack of atmosphere

Mercury’s Atmosphere?

• Its low mass and proximity to the Sun do not allow Mercury to retain an atmosphere of any significance

• Its lack of volcanoes suggests that Mercury never had a significant atmosphere

Ice at the Poles?

• Mercury’s poles are always very cold, enough so that small ice caps exist there - perhaps the result of a comet impact that created gaseous water that drifted to the poles and froze out

Note to editor: This image is From 3rd ed. Newer images Are available in the literature

Mercury’s Interior

• Mercury’s very high average density suggests that its interior is iron-rich with only a thin rock (silicate) mantle

• Two possible reasons for a thin silicate surface:– Silicates did not condense

as easily as iron in the hot inner solar nebula where Mercury formed

– Rocky crust was blasted off by an enormous impact

Another Large Impact Hypothesis

Mercury’s Magnetic Field

• Mercury’s very weak magnetic field probably due to:– Small molten core– Slow rotation rate– The field may be simply

that its solid iron-nickel core is a huge permanent magnet, but one weaker than the dynamo-created magnetic field of Earth

Mercury’s Rotation

• Mercury spins very slowly with a sidereal rotation period of 58.646 Earth days, exactly 2/3 its orbital period around the Sun of 87.969 Earth days

• Consequently, Mercury spins 3 times for every 2 trips around the Sun

Mercury’s Rotation

• Such a ratio of periods is called a resonance– Mercury’s resonance is the result of the Sun’s

tidal force on Mercury and its very elliptical orbit – the Sun cannot lock Mercury into a synchronous 1:1 rotation because of the high eccentricity of Mercury

• Mercury’s solar day is 176 Earth days, longer than its year!

Venus

• Venus has a mass and diameter very close to that of Earth

• However, the two planets have radically different surfaces and atmospheres

The Atmosphere of Venus

• Reflected spectra and spacecraft measurements show the Venusian atmosphere is 96% CO2, 3.5% N2, and small amounts of H2O and other gases

The Atmosphere of Venus

• The clouds of Venus are sulfuric acid droplets with traces of water– The clouds are very high

and thick, ranging from 30 km to 60 km above the surface

– Surface cannot be seen through clouds

– Some sunlight penetrates to surface and appears as tinged orange due to clouds absorbing blue wavelengths

The Atmosphere of Venus

• The atmosphere is extremely dense, reaching pressures about 100 times that of Earth’s

• The lower atmosphere is very hot with temperatures of 750 K (900° F) at the surface, enough to melt lead

• Spacecraft have landed on Venus, but do not survive long

The Greenhouse Effect on Venus

• Large amounts of CO2 in the Venusian atmosphere create an extremely strong greenhouse effect

• The effect is so strong Venus’s surface is hotter than Mercury’s although Venus is farther from the Sun

• The high temperature and density of Venus then create the high Venusian atmospheric pressure

The Surface of Venus• Ground features can be

mapped with radar from Earth and spacecraft orbiting Venus since radar can penetrate the Venusian clouds

• Venus’s surface is less mountainous and rugged than Earth, with most of its surface low, gently rolling plains

The Surface of Venus• Only two major highlands, Ishtar Terra and Aphrodite

Terra and about 8% of the surface, rise above the plains to form land masses similar to terrestrial continents

The Surface of Venus

• Ishtar Terra is about the size of Greenland and is studded with volcanic peaks – Maxwell Montes, the highest, is at 11 km above the average level of the planet (the equivalent “sea level” reference)

Surface Features• Radar maps have shown

many puzzling surface features (or lack thereof)– Few plate tectonic features:

continental blocks, crustal rifts, trenches at plate boundaries

– A few distorted impact craters and crumbled mountains

– Volcanic landforms dominate: peaks with immense lava flows, “blisters of uplifted rock, grids of long narrow faults, peculiar lumpy terrain

Surface Features

• These features indicate a young and active surface– Venus’s original surface

has been destroyed by volcanic activity

– The current surface is not more than 500 million years old (much younger than Earth’s) with some regions less than 10 million

Active Surface?

• Volcanic eruptions have not been directly observed– Some lava flows appear

fresh– Electrical discharges on

Venus indicative of eruptions

– Brief increases in atmospheric sulfur content also indicative of eruptions

Active Surface?

• Numerous volcanic peaks, domes, and uplifted regions suggest that heat flows less uniformly within Venus than Earth – “hot spot” generation of volcanoes dominate on Venus, which is not the case on Earth

Venus is not Earth’s twin!

• Venus still evolving into the smooth heat flow patterns found on Earth

• Earth rocks have more trapped water in them, making Earth rocks “runnier” than Venusian rocks and the Earth crust thinner (which will allow easier cracking of the crust into plates for tectonic movement)

Interior of Venus probably very similar to Earth – iron core and rock mantle

First Image from Venus

• Pictures from the Russian Venera landers show a barren surface covered with flat, broken rocks lit by the pale orange sunlight – sampling also indicated the rocks are volcanic

Rotation of Venus

• Radar measurements show Venus is the slowest rotating planet, taking 243 Earth days to rotate once, and its spin is retrograde (“backward”)

• Two possible causes of this slow retrograde rotation:– Venus was struck shortly after its birth by a huge

planetesimal– Tidal forces from the Sun and perhaps Earth may have

shifted its spin axis over time

• Solar day on Venus is 117 Earth days• Venus rotates too slowly to generate a magnetic

field

Mars• Although its diameter

is 1/2 and its mass 1/10 that of Earth, Mars is the planet that most resembles the Earth

• Mars extensively photographed by the Mariner, Viking, and Mars Global Surveyor spacecraft

Mars• On a warm day, the

temperature hits about 50° F (10° C)

• Winds sweep dust and patchy ice crystal clouds through a sky that generally is clear enough for its surface to be seen from Earth

• Sparkling white polar caps contrast with the reddish color of most of the planet

Vallis Marineris

• A rift running along the equator stretching 1000 km long, 100 km wide, and 10 km deep

• This canyon, named after Mariner, dwarfs the Grand Canyon and would span the U.S.

Polar Ice Caps• Change in size with

seasons (Mars tilt similar to Earth’s)

• Thin atmosphere creates more severe extremes in the seasons leading to large ice cap size variations

• Southern cap is frozen CO2 (dry ice) and its diameter varies from 5900 km in winter to 350 km in summer

Polar Ice Caps• Northern cap shrinks to

about 1000 km, has surface layer of CO2, but is primarily water ice and has separate layers indicative of climate cycles (including “ice ages”)

• Water contained in Mars caps is far less than that in Earth’s caps

Dune Fields

• Martian poles are bordered by immense deserts with dunes blown by winds into parallel ridges

The Tharsis Bulge• At midlatitudes, there

is the huge uplands called the Tharsis bulge– Dotted with volcanic

peaks including Olympus Mons, which rises 25 km above its surroundings (3 times higher than Mt. Everest on Earth)

The Tharsis Bulge• Believed formed as hot

material rose from the deep interior and forced the surface upward

• Scarcity of impact craters put its age at no older than 250 million years

• May have created gigantic Valles Marineris

Largest Mountain in the Solar System

Water on Ancient Mars• From winding nature of

features that often contain “islands”, it is inferred that water once flowed on Mars

• No surface liquid is now present

• Huge lakes and small oceans thought to have once existed – evidence comes from smooth traces that look like old beaches around edges of craters and basins

Morning Frost

Ancient Lake?

Splash Craters

The Atmosphere of Mars• Clouds and wind blown

dust are visible evidence that Mars has an atmosphere

• Spectra show the atmosphere is mainly CO2 (95%) with traces of N2 (3%), oxygen and water

• The atmosphere’s density is about 1% that of the Earth’s

The Atmosphere of Mars• The lack of atmospheric

density and Mars distance from the Sun make the planet very cold– Noon temperatures at the

equator reach a bit above the freezing point of water

– Night temperatures drop to a frigid 218 K (-67° F)

– Thus, most water is frozen, locked up either below the surface as permafrost or in the polar caps as solid ice

The Atmosphere of Mars• Clouds, generally made

of dry ice and water-ice crystals, are carried by the winds

• As on Earth, the winds arise from warm air that rises at the equator, moves toward the poles, and is deflected by the Coriolis effect

• Winds are generally gentle, but can strengthen and carry lots of dust!

Not a drop of rain…

• No rain falls, despite clouds– Atmosphere is too

cold and dry– Fog seen in valleys

and ground frost has been observed

– CO2 “snow” falls on poles during winter

Ancient Atmosphere of Mars• Dry river beds indicate

liquid water flowed in Mars’s past

• This implies that Mars had to have a denser atmosphere (higher pressure) to prevent the fast vaporization of surface water into the atmosphere

• Cratering indicates that this thicker atmosphere disappeared about 3 billion years ago

Where did the atmosphere go?

• 2 ways Mars lost its thick atmosphere– Mars was struck by a huge asteroid that blasted

the atmosphere into space– Mars’s low gravity coupled with low volcanic

activity produced a net loss of gas molecules into space over the first 1-2 billion years of its existence, decreasing the effectiveness of the greenhouse effect to maintain a warm atmosphere

The Martian Interior

• Differentiated like the Earth’s interior into a crust, mantle, and iron core

• Having a mass between that of dead Mercury and lively Earth/Venus implies Mars should be intermediate in tectonic activity– Numerous volcanic peaks and uplifted

highlands exist– Olympus Mons and other volcanoes do not

show any craters on their slopes indicating they may still occasionally erupt

The Martian Moons

• Phobos and Deimos are about 20 km across and are probably captured asteroids

• Their small size prevents gravity from pulling them into spherical shapes

• Both are cratered, implying bombardment by smaller objects

Life on Mars?

• Interest in life on Mars grew enormously with the misinterpretation of observations made by astronomer Giovonni Schiaparelli in 1877, who called certain straight-line features on Mars “canalli” meaning “channels”– English-speaking countries interpreted this as

“canals” and the search for intelligent life on Mars began

– Spacecraft photos later revealed features on Mars to be natural land structures

Life on Mars?• Viking spacecraft

landed on Mars to search for life up closer – no evidence found

• In 1996, a meteorite was found on Earth with a Mars origin– Certain meteorite

structures suggested Martian bacteria

– Most scientists today are unconvinced

Exploring Mars

• Twin rovers, Spirit and Opportunity, have landed on the surface of Mars and have returned an amazing amount of data!

Exploring Mars

• Rock outcropping at the Opportunity landing site. Thought to be material deposited at the bottom of an ancient ocean

Exploring Mars

• Closeup image of rock at the Opportunity landing site

• Possibly formed from sediment in flowing water

Exploring Mars

• Image from Mars Global Surveyor, a Mars orbiter that ended its mission in 2007

• A flat-topped mesa

Exploring Mars

• A view of what appears to be a dried-up river delta

The Outer Planets

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The Outer Worlds…• Beyond the orbit of Mars, the low temperatures of the solar

nebula allowed condensing bodies there to capture hydrogen and hydrogen-rich gases

• This, together with the vast amount of material in the outer Solar System, lead to the creation of the four large Jovian planets – Jupiter, Saturn, Uranus, and Neptune

• Composed mainly of gaseous and liquid hydrogen and its compounds, these planets lack solid surfaces and may have cores of molten rock

• Pluto is an exception to these rules resembling the ice and rock makeup of the giant planets’ larger moons

• The moons of the outer planets form families of miniature solar systems, although individually each moon presents a unique combination of size, structure, and appearance

Jupiter

• Jupiter is the largest planet both in diameter and mass: more than10× Earth’s diameter and 300× the mass!

• Dense, richly colored parallel cloud bands cloak the planet

• Atmosphere is mainly H, He, CH4, NH3, and H2O

Jupiter• Clouds appear to be

particles of water, ice, and ammonia compounds

• Bright colors of clouds may come from complex organic molecules or compounds of sulfur or phosphorous

• Jupiter rotates once about every 10 hours with this fast rotation leading to a significant equatorial bulge

Jupiter’s Interior

• Jupiter’s average density is 1.3 g/cm3 – indicates an interior composed of very light elements

• Interior becomes increasingly dense with depth, gas turning to liquid hydrogen about 10,000 km down

• Deeper still, liquid hydrogen compresses into liquid metallic hydrogen, a material scientists only recently created in tiny high-pressure chambers

• An iron rocky core, a few times bigger than the Earth, probably resides at the center

Jupiter’s Interior

• Jupiter, with a core temperature of about 30,000 K, emits more energy than it receives– Possibly due to heat left

over from its creation

– Planet may still be shrinking in size converting gravitational energy into heat

Jupiter’s Atmosphere

• General convection pattern:– Heat within Jupiter

carries gas to the top of the atmosphere

– High altitude gas radiates into space, cools and sinks

Jupiter’s Atmosphere

• Coriolis effect turns rising and sinking gases into powerful jet streams (about 300 km/hr) that are seen as cloud belts

Jupiter’s Atmosphere

• Adjacent belts, with different relative speeds, create vortices of various colors, the largest being the Great Red Spot, which has persisted for over 300 years

The Great Red Spot

Jupiter’s Magnetic Field

• Convection in the deep metallic liquid hydrogen layer coupled with Jupiter’s rapid rotation creates a powerful magnetic field– 20,000× stronger than the

Earth’s field, it is the largest planetary magnetic field

– Jupiter’s auroral activity and intense radio emissions are indicative of its magnetic field

Jupiter’s Magnetic Field

• Magnetic field also traps charged particles far above the planet in regions resembling the Earth’s Van Allen radiation belts

• Lightning in clouds has been observed

Jupiter’s Ring• Solar radiation and

collisions with charged particles trapped in Jupiter’s magnetic field exert a friction on the ring dust that will eventually cause the dust to drift into the atmosphere

• To maintain the ring, new dust must be provided – possibly from collision fragments ejected from the Jovian moons

Jupiter has a thin ring made of tiny particles of rock dust and held in orbit by Jupiter’s gravity

The Moons of Jupiter

• Jupiter currently has 63 natural satellites or moons

• Number changes frequently as more are discovered

• Four innermost moons are called the Galilean Moons

The Moons of Jupiter

• Except for Europa, all are larger than the Moon• Ganymede is the largest Moon in the Solar System,

and has an intrinsic magnetic field!• Formed in a process similar to the formation of the

Solar System – the density of these satellites decreases with distance from Jupiter

Io

• Gravitational tidal forces induced from Jupiter and Europa keeps Io’s interior hot

• Volcanic plumes and lava flows are the result

Europa• Very few craters indicate

interior heating by Jupiter and some radioactive decay

• Surface looks like a cracked egg indicating a “flow” similar to glaciers on Earth

• Heating may be enough to keep a layer of water melted below the crust

Liquid Water Ocean on Europa?

Ganymede and Callisto

• Look like Moon with grayish brown color and covered with craters

• However, their surfaces are mostly ice – whitish craters a very good indication of this

• Callisto may have subsurface liquid water

• Ganymede is less cratered than Callisto indicating maria-type formations although tectonic movement cannot be ruled out

Other Observations

• Galilean average densities indicate their interiors to be composed mainly of rocky material

• Differentiation may have allowed iron to sink to core

• Rest of Jupiter’s moons are much smaller than the Galilean satellites and they are cratered

• Outermost moons have orbits that have high inclinations suggesting that they are captured asteroids

Saturn• Saturn is the

second largest planet, 10× Earth’s diameter and 95 × Earth’s mass

• Its average density of 0.7 g/cm3 is less than that of water

• Low density, like Jupiter, suggests a composition mostly of hydrogen and its compounds

Saturn looks different from Jupiter – temperature is low enough for ammonia gas to freeze into cloud particles that veil its atmosphere’s deeper layers

Interior of Saturn

• Saturn radiates more energy than it receives, but unlike Jupiter, this energy probably comes from the

conversion of gravitational energy from falling helium droplets as they condense in Saturn’s interior

The Rings of Saturn• Rings are wide but thin

– Main band extends from about 30,000 km above its atmosphere to about twice Saturn’s radius (136,000 km)

– Faint rings can be seen closer to Saturn as well as farther away

– Thickness of rings: a few hundred meters

– Visible A, B and C rings, from outside in

Ring Structure

• Rings not solid, but made of a swarm of individual bodies– Sizes range from

centimeters to meters– Composition mainly

water, ice, and carbon compounds and is not uniform across rings

Ring Structure

• Large gaps due to resonances with Saturn’s moons located beyond the rings

• Narrow gaps due to complex interaction between ring particles and tiny moons in the rings

The Roche Limit

• Any object held together solely by gravity will break apart by tidal forces if it gets too close to the planet.

• Distance of breakup is called the Roche limit and is 2.44 planetary radii if object and planet have the same density

• All planetary rings lie near their planet’s Roche limit• Existence of side-by-side ringlets of different

compositions indicates rings supplied by varied comets and asteroids

• Objects bonded together chemically will survive Roche limit

The Roche Limit

Saturn’s Moons• Saturn has several large moons and many more smaller

ones• Like Jupiter, most of the moons form a mini-solar

system, but unlike Jupiter, Saturn’s moons are of similar densities indicating that they were not heated by Saturn as they formed

• Saturn’s moons have a smaller density than those of Jupiter indicating interiors must be mostly ice

• Most moons are inundated with craters, many of which are surrounded by white markings of shattered ice

• The moons also have several surface features that have yet to be explained

Saturn’s Moons

Titan

• Saturn’s largest moon

• Larger than Mercury

• Mostly nitrogen atmosphere

• Solid surface with liquid oceans of methane

• The Huygens Probe landed on the surface

Images from Titan’s Surface

Uranus• Uranus was not discovered

until 1781 by Sir William Herschel

• While small relative to Jupiter/Saturn, Uranus is 4× larger in diameter than Earth and has 15× the mass

• At 19 AU, Uranus is difficult to study from Earth, but even close up images from Voyager reveal a rather featureless object

Atmosphere of Uranus

• Atmosphere is rich in hydrogen and methane

• Methane gas and ice are responsible for the blue color of Uranus’s atmosphere

Interior of Uranus• With a density of 1.2 g/cm3 and smaller size, Uranus

must contain proportionally fewer light elements than Jupiter/Saturn

• Density is too low for it to contain much rock or iron• Uranus’s interior probably contains water, methane,

and ammonia• Size of equatorial bulge supports the idea that the

interior is mostly water and other hydrogen-rich molecules and that it may have a rock/iron core

• It is currently not known if the core formed first and attracted lighter gases that condensed on it, or the core formed by differentiation after the planet formed.

Interior of Uranus

Rings of Uranus• Uranus is encircled by a

set of narrow rings composed of meter-sized objects

• These objects are very dark, implying they are rich in carbon particles or organic-like materials

• The extremely narrow rings may be held in place by shepherding satellites

Moons of Uranus• Uranus has 5 large

moons and several small ones that form a regular system

• Moons probably composed of ice and rock and many show heavy cratering

• Miranda is very unique in that it appears to have been torn apart and reassembled

Uranus’s Odd Tilt

• Uranus’s spin axis is tipped so that it nearly lies in its orbital plane

• The orbits of Uranus’s moons are similarly tilted

• Uranus may have been struck during its formation and splashed out material to form the moons, or gravitational forces may have tipped it

Neptune• Neptune is similar in size to

Uranus• Deep blue world with cloud

bands and vortex structures – the Great “Dark” Spot being, at one time, the most prominent feature

• Neptune was discovered from predictions made by John C. Adams and Urbain Leverrie, who calculated its orbit based on disturbances in Uranus’s orbit

Interior of Neptune

• Neptune’s interior is probably similar to Uranus’s – mostly ordinary water surrounded by a thin atmosphere rich in hydrogen and its compounds and probably has a rock/iron core

Neptune’s Atmosphere

• Neptune’s blue, like Uranus, comes from methane in its atmosphere

• Unlike Uranus, Neptune has cloud belts– Like Jupiter/Saturn, Neptune

radiates more energy than it gains from the Sun

– The deep interior heat source drives convective currents which then lead, via the Coriolis effect, to the visible atmospheric belts

Seasons on Neptune???

• Tilted 29 deg• Hubble showed inc.

in brightness at S. Pole• Small rate of seasonal

change due to 165year journey aroundthe Sun! Spring-40 yrs.!

Rings of Neptune• Neptune, like the other Neptune, like the other

giant planets, has ringsgiant planets, has rings• They are probably debris They are probably debris

from satellites or comets from satellites or comets that have broken upthat have broken up

• They contain more dust They contain more dust than the Saturn/Uranus than the Saturn/Uranus ringsrings

• The rings are not The rings are not distributed uniformly distributed uniformly around the ring around the ring indicating they are indicating they are relatively newrelatively new

Triton• Triton’s orbit is “backwards” and is highly tilted with

respect to Neptune’s equator – Triton is perhaps a captured planetesimal from the Kuiper belt

• Triton is large enough and far enough from the planet to retain an atmosphere

• Triton has some craters with dark steaks extending from them – at least one of which originates from a geyser caught in eruption by the passing Voyager II

• The material in the geyser is thought to be a mixture of nitrogen, ice, and carbon compounds heated beneath the surface by sunlight until it expands and bursts to the surface

Triton

Pluto• Discovered by Clyde

Tombaugh in 1930 by scanning millions of star images over the course of a year

• Pluto’s large distance and very small size make it difficult to study, even in the largest telescopes

• In 1978, James Christy discovered Charon, Pluto’s moon

Orbit of Pluto

Pluto and Charon• The orbiting

combination of Pluto and Charon allows an accurate measurement of their masses – Pluto is the least massive planet

• Charon’s steeply tilted orbit implies that Pluto is highly tilted as well– Charon takes 6.4 days to

orbit Pluto once– Pluto rotates with the

same period of 6.4 days

Pluto and Charon• The recent eclipses of

Pluto with Charon have allowed the radii of both objects to be determined– Pluto is 1/5 the diameter

of Earth– Charon is relatively large

being about 1/2 Pluto’s diameter

• From these masses and diameters, Pluto’s density is 2.1 g/cm3,

suggesting an object of water, ice, and rock

Mystery Planet!

• Very little is known of Pluto’s surface, but computer analysis of eclipse images suggests a bright south pole, perhaps a frozen methane cap

• Pluto also has a tenuous atmosphere of N2, CO, and traces of CH4

The Dwarf Planets