The Other Terrestrial Planets

Mercury

View from Earth

Doppler Effect

Rotation and

Revolution

Surface Features

History

Earth Mercury

Semi-major Axis 1 A.U. 0.387 A.U.

Inclination 0° 7°

Orbital period 1.000 tropical year 87.97 days

Orbital eccentricity 0.017 0.206

Rotational period 23 h 56 min 4.1 s 58.65 days

Tilt 23° 27’ 0°

Radius 6378 km 2439 km

Mass 5.97 x1024 kg 3.30 x 1023 kg

Bulk density 5.52 g/cm3 5.43 g/cm3

Atmosphere N2, O2 trace Na, K, H2, He

Albedo 0.40 0.06

Surface temperature 250-300 K 100-700 K

Escape speed 11.2 km/s 4.3 km/s

Magnetic moment (equator) 8 x 1010 G.km3 4.8 x 107 G.km3

Phases of Mercury can be seen best when

Mercury is at its maximum elongation

Viewing Mercury from Earth

Measuring Mercury’s Rotation:

the Doppler Effect

Blue Shift (to higher

frequencies) Red Shift (to lower

frequencies)

The light of a moving

source is blue/red

shifted by

Dl/l0 = vr/c

l0 = actual

wavelength emitted

by the source

Dl = l - l0

wavelength change

from the Doppler

effect

vr = radial velocity

vr

Example 1 of the Doppler Effect Earth’s orbital motion around the Sun causes a

radial velocity towards (or away from) any star.

Example 2 of the Doppler Effect Take λ0 of the Hα (Balmer alpha) line:

λ0 = 656 nm

Assume, we observe a star’s spectrum with the Hα line at λ = 658 nm. Then,

Δ λ = 2 nm.

We find Δλ / λ0 = 0.003 = 3 x 10-3

Thus,

vr/c = 0.003,

or

vr = c Δλ / λ0 = 300,000 km/s x 0.003 = 900 km/s

Because the value is positive, the line is red shifted, so the star is receding from us with a radial velocity

of 900 km/s.

Doppler

Effect:

Planetary

Rotation Radar is bounced

off the two sides of

the planet. The

Doppler shift is

used to determine

the rotation velocity.

With the

measurement of the

circumference of the

planet, the rotation

period can be

calculated.

Rotation and Revolution of Mercury Like Earth’s Moon (tidally locked to revolution around Earth),

Mercury’s rotation has been altered by the Sun’s tidal forces,

but it has not locked into a 1:1 spin-orbit resonance. It is in a

3:2 resonance. Revolution period = 3/2 times rotation period.

Revolution: ≈ 88 days, Rotation: ≈ 59 days

Extreme day-night temperature contrast: 100 K (-173 oC) – 600 K (330 oC)

Spacecraft Exploration of Mercury

Mariner 10: flew by

Mercury, 1974–75

Spacecraft Exploration of Mercury

The Messenger

spacecraft was

launched in

August 2004. It

flew by Mercury

in January and

October, 2008

and September,

2009. It was put

in orbit around

Mercury in

March, 2011.

Mercury is very similar to the Moon in several ways:

• Small; no atmosphere

• Lowlands flooded by ancient lava flows

• Heavily cratered surfaces

Mercury

Lobate Scarps

Curved cliffs, several hundred km long and up to 3

km high, probably formed when Mercury shrunk

while cooling down

Mariner 10

Messenger

The Caloris

Basin, is a very

large impact

feature that is

not as

completely

flooded as the

mare of the

Moon

Caloris Basin

The Surface of Mercury Weird terrain was found on the opposite side of the planet from the

Caloris Basin . Weird terrain was subsequently found on the far

side of the Moon opposite large basins.

History of Mercury

1) Formed about 4.6 million years ago

2) It melted from impact energy and cooled

slowly

3) Differentiation to form metallic core and

rocky mantle

4) Major impact might have melted and

ejected much of the mantle

5) Interior shrank, crumpling the solid crust

6) Massive meteorite bombardment

Cratering; lava flows

Venus

Rotation

Atmosphere

Surface Features

History

Earth Venus

Semi-major Axis 1 A.U. 0.723 A.U.

Inclination 0° 3° 23’

Orbital period 1.000 tropical year 224.7 days

Orbital eccentricity 0.017 0.007

Rotational period 23 h 56 min 4.1 s 243.01 days—retro

Tilt 23° 27’ 117° 18’

Radius 6378 km 6052 km

Mass 5.97 x1024 kg 4.87 x 1024 kg

Bulk density 5.52 g/cm3 5.24 g/cm3

Atmosphere N2, O2 CO2, N2

Albedo 0.40 0.76

Surface temperature 250-300 K 700 K

Escape speed 11.2 km/s 10.4 km/s

Magnetic moment (equator) 8 x 1010 G.km3 >4 x 106 G.km3

The Rotation of Venus

Almost all planets rotate

counterclockwise (prograde

rotation), i.e. in the same sense as

orbital motion.

Exceptions: Venus, Uranus and

Pluto

Venus rotates clockwise

(retrograde rotation), with a period

slightly longer than the orbital

period.

Possible reasons:

Tidal forces of the Sun on a

molten core

Off-center collision with a

massive protoplanet

Long-Distance Observations of Venus

Dense atmosphere and

thick clouds make the

surface impossible to

see

The surface temperature

is about 730 K – hotter

than Mercury!

Spacecraft Exploration of Venus Soviet Venera probes landed on Venus from

1970–1978

The Magellan spacecraft mapped Venus using

synthetic aperture radar from 1990-1994

The Atmosphere of Venus

Venus’s atmosphere

is very dense (~100 x

pressure of Earth’s)

Solid cloud bank 50–

70 km above surface

hiding it from view

Atmosphere is

mostly carbon

dioxide; clouds are

sulfuric acid

The Atmosphere of Venus Venus is the victim of a runaway greenhouse

effect – it just kept getting hotter and hotter as

infrared radiation was reabsorbed

The Surface of Venus

Beta Regio

Alpha Regio

Aphrodite Terra

Chasma region

0° 90° 180° 270°

Ishtar Terra

Lakshmi Planum Maxwell Montes

North The surface is relatively smooth: 60% of

the terrain lies within 500 m of the mean

planetary radius

Two continent-like features: Ishtar Terra

and Aphrodite Terra

No plate tectonics

Mountains, craters, many volcanoes and

large lava flows

The Surface of Venus

Venera 13

photograph of

surface of Venus:

Colors modified

by clouds in

Venus’

atmosphere

The only direct surface information we have about Venus came from

a few Venera spacecraft

Synthetic aperture radar images made by the Magellan spacecraft

give us the greatest detailed information about surface features

After correction for

atmospheric color

effect:

The Surface of Venus Volcanoes on Venus

Above: Sif Mons

Right: Gula Mons

Volcanic Features

on Venus

Baltis Vallis: 6800 km long

lava flow channel (longest

in the solar system!)

Coronae: Circular bulges formed by

volcanic activity

Aine Corona

Lava flows Pancake domes:

Associated with

volcanic

activity forming

coronae

Some lava flows

collapsed after

molten lava drained

away

Craters on Venus

There are nearly

1000 impact

craters on Venus’

surface

Surface not very

old.

There is no water

on the surface; it

has a thick, dense

atmosphere

No erosion

Craters appear

sharp and fresh

Craters of Venus

Venus’ largest

impact crater,

named after

Margaret Mead

Venus’ Magnetic Field and Internal

Structure

There are no measurements available that

would give clues to internal structure.

However, because it is similar to Earth in size

and density, it is reasonable to assume that it

has a similar internal structure.

There is no magnetic field, probably because

its rotation is so slow. This would be

consistent with the magnetohydrodynamic

model.

A History of Venus

Complicated history; still

poorly understood.

Solar wind interacts

directly with the

atmosphere, forming a

bow shock and a long

ion tail.

CO2 produced during

outgassing remained in

the atmosphere (on

Earth: it dissolved in

water).

Any water present on the surface rapidly evaporated

feedback through the enhancement of the greenhouse effect

Heat transport from core mainly through magma flows close to the

surface ( coronae, pancake domes, etc.)

Mars

Earth

Observations

Surface

Features

Volcanism

Craters

Water

Polar Caps

Atmosphere

Satellites

Earth Mars

Semi-major Axis 1 A.U. 1.524 A.U.

Inclination 0° 1° 51’

Orbital period 1.000 tropical year 1.881 tropical year

Orbital eccentricity 0.017 0.094

Rotational period 23 h 56 min 4.1 s 24 h 37 min

Tilt 23° 27’ 25° 12’

Radius 6378 km 3397 km

Mass 5.97 x1024 kg 6.42 x 1023 kg

Bulk density 5.52 g/cm3 3.94 g/cm3

Atmosphere N2, O2 CO2, N2,H2O

Albedo 0.40 0.16

Surface temperature 250-300 K 210-300 K

Escape speed 11.2 km/s 5.0 km/s

Magnetic moment (equator) 8 x 1010 G.km3 2.5 x 107 G.km3

Earth Observations of Mars

Can see polar ice caps that grow and shrink

with the seasons.

Earth Observations of Mars

• Changing polar ice caps are frozen carbon

dioxide (dry ice); water ice is permanently

frozen

• Shifting dust cover makes the surface look

like it is changing

• Frequent dust storms, with high winds

• We did not understand that dust and winds

were responsible for changing features

before spacecraft observations

Orbital Exploration of Mars

1971—Mariner 9

1976—Viking 1 and 2

1993—Mars Observer (failed)

1997—Mars Global Surveyor

2001—Mars Odyssey

2003—Mars Express (ESA)

2006—Mars Reconnaissance Orbiter

No seismic studies have been done

From the behavior of the crust, it is estimated

to be 100 km thick

There is no magnetic field, so the core is

probably not metallic, or not liquid, or neither

liquid nor metallic

Interior of Mars

Surface

Features

Olympus Mons

Tharsis Montes

Elysium Mons

Valles Marineris

Hellas Basin

Chryse Planitia

Argyre Basin

Utopia Planitia

Landing Sites

Curiosity

Phoenix

Surface Spacecraft Exploration

of Mars Viking landers arrived at Mars in 1976.

The Surface of Mars

Both Viking landers landed in low-latitude

northern plains

They

photographed a

rocky surface

The red color is

caused by

hydrated iron

oxides

Viking 1 image

The Surface of Mars Viking 2 image

Shows

frost which

quickly

evaporates

after

sunrise

The Surface of Mars Sojourner rover was deployed on Mars in 1997

during the Pathfinder mission.

The Surface of Mars

Spirit and Opportunity rovers were deployed

on Mars in 2004.

Below is a 360° panoramic image of Spirit’s

landing site

The Surface of Mars

Phoenix landed on

Mars in late May, 2008

and explored the soil

where it landed for ~5

months. It landed

farther north than any

previous Mars

mission. The white

material is water ice.

The Surface of

Mars

Curiosity landed on

August 6, 2012 in 96

mile wide Gale crater.

It will explore the

crater’s central peak,

Aeolis Mons (Mount

Sharp). It is a small

robotic laboratory that

will examine geology

and look for signs of

life.

The Geology of Mars

Giant volcanoes

Valleys

Vallis

Marineris

Impact craters

Reddish deserts of

broken rock,

probably smashed by

meteorite impacts.

The Geology of Mars • Northern hemisphere (left) is rolling volcanic terrain

• Southern hemisphere (right) is heavily cratered

highlands; average altitude 5 km above northern

• Assumption is that northern surface is younger than

southern

• Means that northern hemisphere must have been

lowered in elevation and then flooded with lava

Geology of Mars Northern Lowlands: Free of craters; probably re-surfaced a

few billion years ago. Possibly once filled with water.

Southern Highlands: Heavily cratered;

probably 2 – 3 billion years old.

Volcanism on Mars

Tharsis rise

(volcanic bulge):

Nearly as large

as the U.S.

Rises ~10 km

above mean

radius of Mars.

Rising magma

has repeatedly

broken through

crust to form

volcanoes.

Volcanism on Mars

Volcanoes on

Mars are

shield

volcanoes.

Olympus Mons

Highest and

largest volcano

in the solar

system.

• 700 km diameter

at base

• 25 km high

• Caldera is 80 km

in diameter

The Surface of Mars Impact craters less than 5 km across have mostly been eroded

away

Analysis of areal crater densities allows us to estimate the age of

the surface

The crater on the right may have penetrated a layer of permafrost

that released water and formed the lobate ejecta blankets by

means of fluidized flow.

The Surface of Mars There is evidence of slumping in the wall of this

crater indicating that the surface is probably

covered with unconsolidated material (regolith).

The Surface of Mars Valles Marineris: huge canyon, created by crustal forces

(tectonics)

Grand Canyon on same scale

• 4000 km long

• Maximum 120 km wide, 7 km deep

The Surface of Mars Was there running water on Mars?

Runoff channels

resemble those

on Earth.

Left: Mars

Right: Louisiana

The Surface of Mars

No evidence of a

connected river

system; these features

are probably the result

of flash floods

The Surface of Mars

This may be an ancient

Martian river delta (or

not)

The Surface of Mars

Much of

northern

hemisphere

may have

been ocean

The Surface of Mars Recently, gullies have

been seen that seem to

indicate the presence

of liquid water;

interpretation is still in

doubt

More intriguing, this pair of

images appears to show that

gully formation is ongoing

Hidden Water on Mars

No liquid water permanently on the

surface:

Would evaporate due to low pressure.

But evidence for liquid water in the past:

Outflow channels from sudden,

massive floods

Collapsed structures after withdrawal

of sub-surface water

Splash craters and valleys resembling

meandering river beds

Gullies, possibly from debris flows

Central channel in a valley suggests

long-term flowing water

Evidence for

Water on Mars

Hematite concretions

(spheres)

photographed by Mars

rover Opportunity:

Probably crystals

grown in the

presence of water.

Layered rocks:

Evidence for

sedimentation

Ice in the Polar Caps

Polar caps contains

mostly CO2 ice, but

also water. The water

ice is permanent

Multiple ice regions

separated by valleys

free of ice.

Boundaries of polar

caps reveal

multiple layers of

dust, left behind by

repeated growth

and melting of

polar-cap regions.

The Martian Atmosphere

Martian atmosphere

is mostly carbon

dioxide, and very

thin (~1% of Earth’s

pressure)

95% carbon dioxide

Too thin to retain

much heat;

temperature drops

sharply at night

The Martian Atmosphere

Fog can form in low-lying areas, as sunlight

strikes.

History of Mars’ Atmosphere

Atmosphere probably

initially produced through

outgassing.

Loss of gases from a

planet’s atmosphere:

Compare escape velocity

(red dots) to typical velocity

of gas molecules (blue lines)

The planet was probably

hotter in the past which

would shift the red dots to

the right

Escape velocity less than

gas molecule velocity gas

escapes into space. Mars has lost all lighter gases retaining

only heavier gases (CO2, N2, H2O).

Mars may be victim of an inverse greenhouse.

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.

History of Mars’ Atmosphere

History of Mars’ Atmosphere As a result, Mars may have had a thicker atmosphere

and liquid water in the past, but they are now gone

Life on Mars?

Viking landers looked for

evidence of living organisms,

did not find anything

conclusive. Failing to find

carbon using a mass

spectrometer, it was concluded

that the ambiguous results of

three other experiments did

not indicate life.

Life on Mars?

In the mid 1980’s it became evident that some meteorites originated on

Mars. In 1996, scientists claimed to have found evidence for life in an

Antarctic meteorite from Mars. Most planetary scientists are not

convinced that observed features prove that there was life on Mars, but

most believe that conditions on Mars might have supported life at some

point in time (probably not now). We are still looking.

ALH84001

The Satellites of Mars

Phobos (28 km x

20 km)

Deimos (16 km x 10 km)

Mars has two small satellites:

Phobos and Deimos.

They are too small to pull

themselves into a spherical

shape.

Typical of small, rocky, solar

system bodies: cratered,

dusty, dark grey, low density.

Very close to Mars; orbits

around Mars faster than Mars’

rotation.

Both were probably captured from the

inner asteroid belt.