Lecture 22: Mars
Percival Lowell
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Martian Water Mystery
•Lots of interesting connections to this course:
–Comparative planetology of Earth/Mars
–Insights into the origin of life
–Application of the scientific method
–Examination of the Copernican Principle
–Study of crater production and meteorite orbits
Mars Rovers
•Evidence for Martian water!
•Mars Rover Website
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Martian Landers
Martian Topography
•Mars Orbiting Laser Altimeter
•Mars Global Surveyor Images
Water Erosion on Mars
3
Geological Variety on Mars
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Orbit of Mars
•The semi-major axis of the Martian orbit is a = 1.52 AU
Mars
Sun
a
•The eccentricity of Mar’s orbit is e = 0.093
Orbit of Mars
Mars
Sun
a
•Due to the rather large eccentricity, the distance between Mars
and the Sun varies substantially during the Martian year:
™ D = 1.38 AU (perihelion)
™ D = 1.66 AU (aphelion)
•The ratio of these distances is about 1.20
Orbit of Mars
•Since the ratio of the perihelion to aphelion distances is about
1.20 Mars is much closer to the Sun during perihelion
•The solar flux therefore varies by about 45% during the Martian
year
•This causes complications in the Martian seasons and climate
Mars
(perihelion)
Sun
Mars
(aphelion)
Earth
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Orbit of Mars
•At favorable opposition, the Earth-Mars distance is only about
34.65 million miles
•This happened in 2004, for the first time in 60,000 years
Mars
Mars
(favorable
(opposition)
opposition)
Earth
Earth
Sun
Earth
Mars
(conjunction)
Orbit of Mars
•The minimum Earth-Sun distance at (favorable) opposition is
1.38 AU – 1 AU = 0.38 AU = 35 million miles
•The maximum Earth-Sun distance at (unfavorable) opposition is
1.66 AU – 1 AU = 0.66 AU = 61 million miles
•Hence Mars is about 1.7 times closer during favorable opposition
Orbit of Mars
•Kepler’s third law relates the semi-major axis a to the orbital
period P
2
⎛ P ⎞ ⎛ a ⎞
⎜⎜
⎟⎟ = ⎜
⎟
⎝ years ⎠ ⎝ AU ⎠
3
•Solving for the period P yields
⎛ P ⎞ ⎛ a ⎞
⎜⎜
⎟⎟ = ⎜
⎟
⎝ years ⎠ ⎝ AU ⎠
3/ 2
•Since a = 1.52 AU for Mars, we obtain P = 1.87 Earth years or
P = 682 Earth days
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Bulk Properties of Mars
•We have for the radius and mass of Mars
™Rmars = 3,397 km = 0.53 Rearth
™Mmars = 6.4 x 1026 g = 0.11 Mearth
•The volume of Mars is given by
Vmars =
4
π R 3mars
3
•Hence the volume of Mars is Vmars = 1.6 x 1026 cm3
Bulk Properties of Mars
•The average density of Mars is therefore
ρ mars =
M mars
6.4 × 10 26 g
=
Vmars 1.6 ×10 26 cm 3
•We obtain
ρ mars = 3.9 g cm −3
•This is much lower than the density of the Earth
•Hence Mars probably does not contain a great deal of iron, or a
large, dense core
•The lack of a dense core is consistent with the absence of a
magnetic field
Surface of Mars
•The surface of Mars appears quite reddish
•This indicates that much of the iron remains at the surface, and
the color is due to iron oxide
•Hence, very little differentiation occurred, and the iron did not
sink to the center of the planet
•Mars must have solidified before differentiation could be
completed
•This is probably because the planet is so small that it cooled
despite the radioactive heating
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Rotation of Mars
•The surface of Mars is visible because the atmosphere is very
thin
•Hence the rotation of Mars can be measured by tracking surface
features:
•The sidereal day is 24.6 hours long (vs. 24 hours for Earth)
•The rotation axis is tilted by 25.2o with respect to the orbital
plane (the angle is 23.4o for Earth)
Highlands and Lowlands
•There are large differences between the terrain in the Northern
and Southern hemispheres
North:
Lava-filled lowlands;
3 x 109 years old
South:
Heavily cratered highlands;
4 x 109 years old
Surface of Mars
•The surface of Mars has many interesting features, such as
™Volcanoes
™Canyons
™Dunes
™Evidence for water erosion
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Surface of Mars
•The lava flows probably occurred about 3 x 109 years ago
•There was little plate tectonic activity, because the planet is
small and cooled quickly
•The only “continent” is the Tharsis bulge on the equator
•The height of Tharsis is 10 km, and it’s about 2-3 billion years
old
Formation of Tharsis
•How did Tharsis form? Probably due to upward convection in
the mantle, which ended soon after Tharsis was uplifted
Valles Marineris
•Valles Marineris, located on the Tharsis bulge, is the largest
canyon in the solar system
•This canyon is 3,000 km long, 600 km wide, and 8 km deep
(over 4 times larger than the Grand Canyon on Earth)
•Valles Marineris is a tectonic crack formed during the uplifting of
the Tharsis bulge (unlike the Grand Canyon, which was formed
by water erosion)
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Martian Topography
•Martian Topography Movie
Martian Volcanism
•The largest volcano is Olympus Mons:
™Height = 25 km (3 times taller than Everest)
™Base = 700 km (about the Texas)
•There is no evidence for ongoing plate tectonics
•The Martial volcanoes are shield volcanoes sitting on top of hot
spots in the mantle
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Seasons on Mars
•The seasons on Mars are complicated by the large eccentricity
of the orbit, which causes substantial variations in the solar flux
on the surface
•Mars is closest to the Sun during summer in the Southern
hemisphere
Mars
(perihelion)
Sun
Earth
Seasons on Mars
•Mars is farthest from the Sun during winter in the Southern
hemisphere
•Mars is closest to the Sun during summer in the Southern
hemisphere
•Therefore, the seasons in the South are extreme and the
seasons in the North are mild
Mars
(perihelion)
Sun
Mars
(aphelion)
Earth
Seasonal Variation of Polar Caps
carbon dioxide and
water ice at polar caps
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Atmosphere of Mars
•The surface temperature on Mars was measured by instruments
on board the Viking landers
•They indicate a low, variable temperature
•This is due to the thin atmosphere
Atmosphere of Mars
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Atmosphere of Mars
•Atmospheric composition:
•Venus:
™Carbon Dioxide -- 95.3%
™Carbon Dioxide: 96.5%
™Nitrogen -- 2.7%
™Nitrogen: 3.5%
™Argon
-- 1.6%
™Trace gases: < 0.01%
™Oxygen
-- 0.13%
™Almost no oxygen
™CO
-- 0.07%
™Practically zero water
™H2O
-- 0.03%
™Clouds are sulfuric acid
™Water ice clouds
•Though somewhat similar in composition, the atmosphere of
Mars is about 10,000 times thinner than Venus’ (150 times
thinner than Earth’s)
•Mars is much colder (250 K) than Venus (750 K)
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Water on Mars
•Liquid water once existed on the surface of Mars
•We see runoff channels and outflow channels
•There was a huge flood in the distant past from south to north
•The flow rate was equivalent to 100 Amazon Rivers!
•All of this water is probably now locked up in the soil as
permafrost
•Mars Rover Opportunity finds Evidence for Ancient Sea on Mars
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Martian Topography
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Martian Atmosphere
•The primary atmosphere (mostly hydrogen) was lost to space
due to the low mass of Mars
•When the secondary atmosphere formed, water and CO2 were
out-gassed from volcanoes
•As the planet cooled, the water froze out of the atmosphere
onto the surface
•About 109 years later, volcanoes melted the ice, creating a
gigantic flood of water and lava from south to north
Martian Atmosphere
•The flood ran from the highlands in the South to the lowlands in
the North
•The volcanic activity stopped, and the CO2 was absorbed by the
water
•This was followed by a period of strong cooling, and the
development of the permafrost
•We call this process the “Reverse Runaway Greenhouse Effect”
•Mars is now in a permanent deep-freeze, but the atmosphere
could rise again if heating occurs…
Life on Mars?
•Many potato-sized meteorites from Mars have been found in
Antarctica
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Life on Mars?
•Gases trapped in bubbles in the rock are identical in
composition to samples obtained by Viking in 1976
•Hence the meteorites are almost certainly from Mars
Life on Mars?
•The rocks contain organic polycyclic aromatic hydrocarbons
(PAH’s), often found in decaying microbes on Earth
•There are also small structures in the meteorites that look like
micro-fossils…
Astrobiology Website
Scenario for Life on Mars
•Microscopic life began on Mars when the atmosphere was thick,
warm, and dense
•Liquid water was still abundant on (or just under) the surface
•Some of the microbes became fossilized in rock
•A large meteor or asteroid impact on Mars launched the rock
into space
•The meteorite traveled through the solar system for 15,000,000
years
•It landed in Antarctica about 13,000 years ago
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Questions
•Is the rock really from Mars?
¾Probably, based on the composition of the trapped gas
•Could the rock have been contaminated with organic materials
while on Earth?
¾Probably not, since the concentration of the materials
increases towards the center of the sample
•Were the polycyclic aromatic hydrocarbons produced in space
via radiation exposure?
¾Probably not, because the composition is different from
chemicals seen in other meteorites
Questions
•Why are the fossils so small?
¾We’re not sure
•Why do we see similar structures in lunar rock samples brought
back by the Apollo astronauts?
¾This is a strong argument against evidence for life
•The debate continues
¾Several new missions are planned or already on their way
to Mars to try to answer these questions
•There may also be other places in the solar system where we
should look for life (perhaps Saturn’s moon Titan?)
Martian Moons
•Mars has two very small moons, Phobos (“fear”) and Deimos
(“panic”)
™Phobos is 28 km long
™Deimos is 16 km long
•Both are probably captured asteroids
•They orbit very close to Mars:
™Phobos at distance 9,378 km from Mars
™Deimos at distance 23,459 km from Mars
™Recall that Rmars = 3,397 km
•Phobos is so close to Mars that it travels faster than Mar’s spin
rate
•Hence Phobos rises in the West and sets in the East!
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