Lecture #19: Mars
The Main Point
• Mars: Overview
•General properties
•Telescopic
observations
•Space missions
•Atmospheric
Characteristics
Changes in the Martian surface and
atmosphere have been studied for
centuries by telescopes, and more
recently even more details have been
discovered by spacecraft missions.
• Reading:
– Chapters 7.1 (Mars), 9.4, 10.4
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Basic Properties of Mars
• Recall: Mars sometimes has a retrograde motion
• Earth has a "close encounter" with Mars about once
every 26 months
– Distance varies because Mars' orbit is eccentric
• Surface area of Mars ≈ Land area of Earth's continents
• During these times, Mars is a prominent bright "star"
in the evening sky
Density = 3.9 g/cm3 (little metal; some ice? (Earth: 5.5))
Surface Gravity = 3.7 m/sec2 (38% of Earth's)
Thin CO2-rich atmosphere; Surface pressure ~1% Earth's
Average Surface Temperature: -60°C (-76°F); ΔT ~100°C
Complex surface geologic processes at work...
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Observations
• Average Distance from Sun: 227,900,000 km
(a=1.52 AU)
• Orbital period: 687 days
• Period of Spin around axis: 24 hours, 37 minutes (= 1 Sol )
• Tilt of Martian spin axis: 25.2° (Mars has seasons)
• Mass: 6.7x1023 kg = 0.11 ME ; Radius: 3397 km = 0.53 RE
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– It is noticeably red!
– Hence its historic association with blood, war
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Telescopic Observations
Limitations on Resolution
• Mars usually has a small apparent angular
diameter in telescopes: 4" to 6"
• Every 26 months or so, Mars' apparent size
increases above 10" for a few months and astronomers can
observe surface details
• Atmospheric turbulence (seeing) limits the
resolution of features on Mars to a few hundred km
(~size of Arizona) from ground based telescopes
• Hubble Space Telescope can get to about 20 km
And oh what details
they have observed!
Percival Lowell
and the "canals"
of Mars
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Mt. Wilson Obs (1956)
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Telescopic Observations
HST (1999)
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Telescopic Observations
• The surface of Mars is obviously RED
– Caused by the presence of heavily
oxidized ("rusted") surface minerals (iron oxides, other
silicates)
• Three main kinds of areas:
– Bright regions: more heavily oxidized, fine grained
– Dark regions: less oxidized, coarser grained
– Polar caps: very bright, consist of H2O and/or CO2 ice
– These surface markings observed to change over time
• Clouds can also be seen on Mars
– White clouds composed of condensed H2O and/or CO2
– Yellow clouds from regional and global dust storms
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Lowell Obs. (1986)
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• Mars has also been intensely studied with radio
telescopes from the Earth
Mars radar images (1988)
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• In these images, radio signals
were transmitted from Goldstone
in California, bounced off Mars,
and were received by the VLA
radio telescope in New Mexico
• Many surface features visible,
including prominent polar caps
• Some areas show no radar return
- (dubbed "Stealth" regions)
- explanation unknown!
- maybe "fluffy" dust deposits?
- ash? (occur near volcanoes)
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Space
Missions
17 robotic space missions have either flown by, orbited, or landed
successfully on Mars (out of ~ 37 attempts)
Mission Name
Mariner 4
Mariner 6 & 7
Mariner 9
Mars 2 & 3
Mars 4 & 5
Viking 1 & 2
Phobos 2
Mars Pathfinder
Mars Global Surveyor
Dates
1965
1969
1971
1971
1974
1976-1982
1989
1997
1996-2006
Mars Odyssey
Mars Express
20012003-
Mars Exploration Rovers
2003-
Mars Reconnaissance Orbiter 2005PhoenixAstro 102/104
2007-
Goals and Results
Mars flyby; photography
Mars flybys; photography, spectroscopy
Mars orbiter; photography, spectroscopy
Mars orbiters and lander (no data); photography
Mars orbiter; photography, polarimetry
2 orbiters and 2 landers; imaging, soil analyses
Mars orbiter; CCD imaging, imaging spectroscopy
Mars lander & rover, CCD imaging, geochemistry
Mars orbiter; CCD imaging, IR spectroscopy,
laser altimetry, gravity & magnetic fields
Mars orbiter; spectroscopy & IR imaging
Mars orbiter; CCD imaging, IR & ultraviolet
spectroscopy, particle sensors, subsurface radar
Mars rovers; CCD imaging, IR, Moessbauer, and
Alpha Particle X-ray spectroscopy, rock abrasion
Mars orbiter; telescopic cameras, spectroscopy, radiometry
Mars lander; soil analysis, meteorology
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The Atmosphere
of Mars
• Because the Martian orbit is eccentric
(e = 0.09), the seasons have unequal lengths
– Southern summer (northern winter)
• Short: 154 Martian days
• Occurs near perihelion: a(1-e) = 1.52(1-0.09) = 1.38 AU
The Atmospheres of Mars and Earth
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Mars Has Seasons!
• Composition determined by spectroscopy from ground
based telescopes and direct sampling by surface landers
Gas
Carbon Dioxide (CO2)
Nitrogen (N2)
Argon (Ar)
Oxygen (O2)
Water (H2O)
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Mars
95.3%
2.7
1.6
0.13
0.03
– Northern summer (southern winter)
Earth
0.03%
78.1
0.93
21.0
~2
• Long: 178 Martian days
• Occurs near aphelion: a(1+e) = 1.66 AU
2
• Solar heat input ratio = (1.66 / 1.38) = 1.45
– Almost 50% more heat input near perihelion!
– This has a strong effect on the weather patterns!
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Local storm (Viking)
Size ~200 km
Dust Storms!
Local storm (HST)
Size ~500 km
• Occur primarily during times of strongest solar
heating (southern summer)
• Clouds of micron-sized (smoke-sized) oxidized
dust lifted and transported by the winds
– Wind speeds in excess of 100-150 mph common!
– BUT: not much force, since pressure ≤ 10 millibars
Global dust storm (1971)
• Surface features completely
wiped out from view!
• Last major global dust
storms: 1973, 2001, 2007!
• Mini-storms observed by the
Viking and Pathfinder landers
• Dust clouds have a strong influence on the
weather, since they strongly absorb sunlight and
prevent much of it from reaching the surface
– Important climate implications for Mars and Earth
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Mars has Polar Caps
• Mars winters are so cold (-120°C or about
-200°F) that CO2 condenses out onto the surface!
– 25% of the atmosphere "snows out" onto the ground!
– Analogy for Earth: at -320°F, N2 would snow out
• A seasonal CO2 polar cap a few meters thick forms
• In spring, the CO2 sublimes back to the atmosphere
• There are permanent polar caps underneath
– Composed primarily of water ice! (only minor CO2 ice)
– May be 1-2 km thick! (depth uncertain...)
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North Polar Cap
Size ~ 1000 km
Martian Satellites
South Polar Cap
Size ~ 350 km
Stored in the polar
caps are layers of
bright and dark
deposits
• Discovered in 1877 by
Asaph Hall (U.S. Naval
Obs., Washington DC)
• Tiny, dark objects
- Phobos ~ 21 km diam.
- Deimos ~ 13 km diam.
• Captured asteroids?
Origin unknown...
Phobos
Evidence for
climate change on
Mars???
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Deimos
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Summary
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Next Lecture...
• Mars is a small rocky planet with a thin atmosphere
• Telescopic observations reveal changing surface
features (polar caps, dust storms, dark features)
• The Martian atmosphere is almost entirely CO2, and
the surface pressure is only ~1% of Earth's
• Mars has seasons, and the planet's eccentric orbit
results in big differences in seasonal weather
• Mars has two small, asteroid-like Moons
• Mars has been extensively studied by spacecraft
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Phobos
Deimos
• Surface of Mars
•Geology
•Composition
•Evolution
• Reading:
– Chapter 9.4
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