Section 10 - Deserts and Wind, Planetary Geology

Section 10
  1. Deserts and Wind
  2. Planetary Geology

Deserts and Wind - Lecture Notes

What is a desert?

A desert is a geographical location where there is less than 25 cm (10 inches) of precipitation per year.  The average annual precipitation in Menomonie, WI, is about 31 inches.  The cause of this low rainfall may be the result of many influences. 
These influences include:
1. Global Air Circulation - At about +/- 30o latitude, a high pressure of dry air moving downwards inhibits cloud formation.  (See figures 13.2 and 13.3 in Tarbuck and Lutgens.)  The Sahara Desert is largely governed by this effect and a great distance from the ocean.
2. Rain Shadow effect - As prevailing winds approach large mountain ranges, the air is forced to rise and cool.  This produces an abundance of cloud formation and high precipitation.  After having moved over the mountain range, the air has lost most of its moisture.  It also descends and warms which prevents cloud formation.  The Sierra Nevada mountain range in California produces large regions of dry land East of the range in Nevada and Arizona.

Composite of three pictures showing the rain shadow effect.  Prevailing winds are moving from right to left and are going over the cascade mountains of the Pacific Northwest (US).  The clouds are seen in the right but not the left.  This picture was taken aboard a commercial airliner at an altitude of about 30,000 feet above Washington state looking South.  The mountain in the background is Mt. Rainier.

3. Distance from Large Water Body - As a general rule, the farther the air has to travel, the more likely it will have already lost its moisture.
4. Proximity to Cold Ocean Currents - Cold moist air moving onto land will many times warm up and make rainfall less likely.

Important Meteorological Information

When we refer to the weather as being dry or humid, we are referring to the amount of water vapor present in the air.  If the water vapor content reaches saturation, water vapor condenses out of the air and forms tiny, liquid droplets of water (such that, a fog or a cloud is formed).  The Relative Humidity is defined as the ratio of the amount of water vapor present divided by the amount of water vapor needed for saturation, such that:

(Relative Humidity) = ((amount of water vapor present)/(amount needed for saturation at that temperature) ) x 100%.

In general, warmer air can contain more water vapor than cooler air before reaching saturation.  When air is cooled it reaches a point where the amount of water vapor present is equal to the amount needed for saturation.  The temperature required to reach this point of 100% relative humidity is called the Dew Point.  When warm air is cooled, the water vapor condenses out and forms clouds which result in precipitation.  When cool air is warmed, liquid water droplets evaporate into water vapor.  Thus preventing any cloud formation or precipitation from falling.

Cloud Chamber Video - This video shows clouds being formed from alcohol vapor above dry ice.  I (Dr. Scott) made the cloud chamber during the fall semester of '01.  One can also see "whispy condensation tracks" formed in the wake of charged particles moving through the chamber.  I placed two radioactive objects into the chamber to watch these tracks.  You can see them by looking carefully.  Cosmic rays will also produce tracks that can be seen in the chamber.


Common Misconceptions About Deserts

  • Deserts are always hot and dry.  Deserts are dry (on average) but they may not be hot.  The Gobi Desert in Mongolia has an average high temperature of -19oC (note: 0oC is freezing).  One of the earth's most unique deserts is Taylor Valley in Antarctica.
  • Deserts are mostly shaped by the wind.  Yes there is wind and it does some shaping of the land but the predominant influence that shapes landforms is water.  When water comes to a desert, a large proportion of it forms runoff and erodes the land.
  • Deserts are lifeless, barren wastelands.  The amount of biological activity is significantly less than in a tropical rainforest, but it does have a significant amount of animal and plant life that has adapted to the dry environment.  Aesthetically, deserts can be quite beautiful - consider the Painted Desert in the American Southwest.  One creature that has adapted well to sandy, desert-like environments is the antlion.





Mechanical weathering is a predominant influence in a desert.  Thus, larger rocks are constantly being made into smaller rocks by means of abrasion, falls, freeze/thaw, etc.  The soils are usually pedacols and in many cases are immature and transported.  Most deserts are rocky or covered with "desert pavement".  Sand dunes are not the predominant landforms in most deserts!  In fact, only 1/5 of the desert areas in the world are covered with sand dunes.  About 1/10 of the Sahara Desert is covered with sand.

Deserts are prone to flash flooding.  The sparse rain that does come, usually takes the form of a short duration, heavy down-pour.  "Rivers" in desert regions are usually dry and are called Dry Washes.  The water that enters dry washes evaporates and infiltrates fast, causes a significant amount of erosion when flowing, and most die-out before reaching a larger river that would eventually flow into the ocean.  The rivers are said to be "ephemeral".

Wind Erosion and Landforms

Desert pavement is created by wind erosion through a process called deflation.  This process is characterized by small grain particles getting eroded away by the wind leaving coarse gravel/stones.  The resulting land surface is lowered.

Mesas - Many desert terrains contain landforms that were created from horizontal rock layers getting eroded away around the base of a flat, weathering resistant rock layer.  These mesas look like hills with flat "tops".

Sand Dunes

Sandy landforms that are produced predominantly by wind are called sand dunes.  They need (1) arid conditions to thrive (and move), (2) good supply of sand from surrounding weathered rock, and (3) a an environment frequented by winds.  The shape is governed by laminar and turbulent fluid flow (i.e. air as a fluid) around obstacles and this flow's ability to move the sand particles.  [Sand drifts are usually the beginnings of  sand dunes.]  Small perturbation in the laminar flow is what initiates sand drifts, then the drifts themselves become the obstacle.

As a general rule, the gently sloping side of the dune faces into the prevailing wind direction.  The entire sand dune moves downwind slowly as a result of erosion happening on the side facing the wind and deposition happening on the downwind side.

Types of Sand Dunes:
1. Barchan dunes - These are crescent shaped dunes with the points directed downwind.  Produced by constant wind direction.
2. Transverse dunes - Dunes that form long ridges that are perpendicular to the direction of the wind.  Sometimes the transverse and barchan dunes can combine to form barchanoid dunes.  Usually occur where the sand supply is abundant.
3. Blowout (or parabolic) dunes - Crescent shaped dunes with points toward the wind (i.e. opposite to the Barchan dune).  Usually associated with deflation on the interior of the crescent.
4. Longitudinal dunes - Dunes that form long ridges of sand oriented parallel to the prevailing wind direction.  Usually occur where the sand supply is limited.
5. Star dunes - Resembles a star shape and is produced by shifting winds.

Much of the soils in the midwest can be characterized as wind blown silt from glacial debris called loess.

Deserts and the Global Perspective

In many parts of the world, in particular - Northern Africa, a process called desertification is at work.  The regions that surround deserts can be very sensitive to human intervention.  These semi-arid lands usually support a significant amount of vegetation.  However, a problem develops if there is a period of extended drought-like conditions which is compounded by a rapid exploitation of the land for farming.  This combination reduces the amount of native vegetation, kills the planted crops, and the land is subsequently exposed to intense erosion.  This region progresses into a more desert-like environment.  This further exhacerbates the human conditions in the area - more starvation/malnutrition and a lessened soil quality for producing crops.  Population increases further complicates the problem in these areas.  Some scientist estimate that about 35% of the Earth's surface is potentially threatened by desertification.  [Source of much of this information: F. Press, R. Siever, Understanding Earth, 2nd Ed. (1994), p. 362] 
Aral Sea is a prime example of how human influence has adversely impacted the ecology of a region.

dustbowl The great Dust Bowl of the 1930's.

Deserts and Wind - Related Web Links

Wind Erosion Research Unit (Kansas State University) - lots of pictures of wind erosion, on-line movies of the dust bowl and dune evolution within a wind tunnel.
Death Valley National Park
Mohave Desert (Mohave National Preserve)
Links to Desert Information and Tourism
Glossary of desert and geological terms
Pictures (1,2) of desert landforms
Lecture Outline of deserts and landforms, W.K. Fletcher, University of British Columbia
Precipitation Map of the US
Google - Search for Deserts, Geology
- Search for Sand Dunes
- Search for Desert Landforms

Planetary Geology - Lecture Notes

The general approach to science is to ask what do we know and how do we know it?  In presenting the topic of planetary geology I would like to reverse this process and first present the "modes" to which observations were made.  Then proceed to discuss how these observations have formed the basis for our view of the geology of these distant celestial objects.

How can we learn about distant worlds?  Such that, how can we measure the characteristics of celestial objects and/or planets?

Method #1:  Spectroscopic Analysis - Visible light that our eyes respond to is only a small part of what is called the Electromagnetic Spectrum.  Every object in the universe (including ourselves) emit, absorb, and reflect light in a specific way.  By analyzing the light spectrum that is coming from distant objects in the universe, we can examine the chemical composition and the motion of that object.  Spectroscopic analysis is a very powerful tool in science.
Emission and Absorption Spectra of the chemical elements.

Method #2:  Optical Imaging (visible and non-visible light) - Telescopes can use mirrors and lenses to create an image of a distant object.  These images can give us information about the structure, position, and motion of distant objects.  [Optical imaging is sometimes combined with spectroscopic analysis instruments.]  These instruments may use visible light but can also image non-visible light that is being emitted from an object.  Here is a nice web page that presents the different types of telescopes.  The Hubble Space Telescope is one of the most prominent telescopes.

Method #3:  Space Probes (with numerous measuring capabilities) - Space probes have the advantage of actually going to the distant celestial object and orbit near it and/or land onto it.  Some landing probes can take direct measurements of the planet's surface geology.  Things that space probes can measure include: biological activity, spectroscopic analysis, magnetic field, charged particles, optical imaging (sometimes 3-D), temperature, pressure, soil conditions.

Space Probe
(most recent to older)



Titan - a moon of Saturn (Arrival date 2004)


Launched in Feb. '99, Intended to fly into Comet Wild-2's tail to collect cometary dust in early 2004.

Mars Exploration Rovers

Two rover missions launched in 2003 toward Mars.  Landing is expected January 3 and 24, 2004.  Rover's can travel far and contain spectroscopic tools, cameras, microscopes, and a rock abrasion tool.  The rover's are named Spirit and Opportunity.   

Mars Express

This mission contains an orbiter and lander called Beagle II.  The lander will touch down on December 25, 2003.  Contains a mass spectrometer (can radiometrically date the rocks), cameras, microscope, wind, pressure, temperature, and a mole-like device for probing the soil.  (A mission by the European Space Agency.)


A Japanese space probe intended for Mars.  It was damaged by a Solar Flare.  It is not expected to complete its mission.

Mars Polar Lander

Believed to have crashed onto the planets surface due to a mis-understanding between English and Metric units.  Mission lost. (Dec. '99)

Mars Odyssey

Launched in 2001.  Reached Mars in the same year of 2001.  The instruments include a high resolution camera and spectroscopic instruments.

Mars Global Surveyor

Launched Nov. '96, started optically mapping the surface of Mars in April '99.  Highest resolution is 1.4 m/pixel.


Jupiter and Jupiter's Moon (arrived in 1995)


Swing by a near earth asteroid named Eros (Feb. '99)


Orbiter, lander, and rover on Mars (1998), The rover named Sojourner was a "six-wheeled geologist" exploring the surface of Mars.


Performed a detailed radar mapping of the surface of Venus ('90-'94), burned up in the atmosphere of Venus at end of mission.

Voyager 1&2

Explored the Jovian planets of Jupiter, Saturn, Uranus, and Neptune between '77-'89.

Viking 1&2

Mars orbiter and landers (1976), sent back a wealth of information about Mars.


Former Soviet Union sent landing probes to Venus (Venera 9 & 10) in 1972.  The probe lived for about 1 hour sending back pictures and data before most of its circuitry was melted from the seering heat.

Pioneer 10&11

Studied Jupiter and Saturn back in 1974.

Mariner 10

Flew by Venus and Mercury sending back pictures in 1973.


Missions to the Moon, One mission involved a moon rover to travel around on the surface.  First moon landing was Apolla

Future missions that are being planned.

[Long list of space probes and satellites.]

Apollo 17 astronaut Harrison Schmitt standing next to a large Moon boulder.  Photo courtesy of the NSSDC Photo Gallery.

Now that we know how the observations were made, lets look at how these observations have shaped our understanding of the geology of these places.

Summary of Geological Characteristics
[Note: 1 Astronomical Unit (AU) = 1.49x1011 meters = average distance the earth is away from the sun.  To convey a good feeling for how far away the planets are from earth, I've expressed their distance in terms of how long it would take the Space Shuttle flying at 17,500 miles per hour (direct course) to get to the planet or object.]

Sun - 350,000 times more massive than earth; 10,000,000 times more voluminous; surface temperature 5,800 oK (note that room temperature is 293 oK, rocks melt at about 1400 oK), inner core temperature of 15,600,000 oK.
Composition - Hot gaseous object with mostly helium and hydrogen.
Fuel - Nuclear fusion (in the core) is the tremendous source of the Sun's energy.
Space Shuttle travel days to get there from Earth =  222 days

Terestrial Planets - planets that are "rocky", relatively small, and close to the sun.

Mercury - no atmosphere, small magnetic field, long "days", large temperature fluctuations
Landforms - Scarps (or cliffs) have been formed possibly from the cooling of the planet, weird terrain can be found opposite of a large asteroid impact crater (Caloris Basin), high density, and has been geologically dead for a long time (such that, no volcanic or plate tectonic activity) 
Size - little bigger than the Earth's moon, 6/100 of the Earth's mass
Space Shuttle travel days = 135 days, 0.39 AU from the Sun

Venus - thick atmoshere, 96% CO2, 90 times the pressure of earth's atmosphere, temperatures remain about constant at 600 oK (hot enough to melt lead), no magnetic field, no plate tectonics, volcanic activity has been indirectly observed.
Landforms - thousands of volcanic structures, shield volcano eruptive style, volcanic domes
Size - 8/10 of the Earth's mass
Space Shuttle travel days = 62 days, 0.72 AU from the Sun

Earth - (Already have learned about the Earth and its landforms.)
Size - (Earth)
Space Shuttle travel days = 0 days, 1.0 AU from the Sun

Mars - thin atmosphere, 95% CO2, no magnetic field, no plate tectonics, volcanic activity in geologically recent past, 7/1000 the earth's pressure
Summary of Mars Geology From the Pathfinder Mission
Landforms - Largest known volcanoes in the Solar System, Northern hemisphere contains lowlands, Southern hemisphere contains the highlands, polar regions have ice caps (water and carbon dioxide), temperatures fluctuate around -40oC, Valles Marineris (AVI Movie) is the "Grand Canyon" of Mars (4,000 km long, 120 km wide, and 7 km deep), has evidence for surface water in the geologic past - run-off channels and outflow channels have been observed, no liquid water has been observed, may have abundant frozen water in the form of permafrost, wind storms and landslides are present on the surface.
Size - 1/10 of the Earth's mass
Space Shuttle travel days = 111 days, 1.5 AU from the Sun

Jovian Planets - planets that are gaseous, relatively large, and further from the sun.  Average densities are significantly less than the Earth's however the cores of the jovian planets can be very dense.

Jupiter - The "surface" features are governed by fluid dynamics, composed of 86% hydrogen and 14% helium, emits more radiant energy than it absorbs, light bands in atmosphere represent ascending gases, dark colored bands are descending gases, the "Red Spot" is a huge hurricane-like storm that has been swirling for at least 300 years
Results of Galileo's planetary probe that was shot into Jupiter's  atmosphere
Size - 318 times the Earth's mass
Space Shuttle travel years = 2.6 years, 5.2 AU from the Sun
Jupiter's Moons are some of the most fascinating places in the Solar System - Io, Europa, Ganymede, Callisto

Saturn - Planet with the prominent rings (note that all the jovian planets have rings but some are quite faint), 92% hydrogen, 7.4% helium, surface is very dynamic, 1500 km/hr winds (compare to 160 km/hr winds in a hurricane on earth), contains large cyclonic storms similar to the "Red Spot" on Jupiter, emits more radiant energy than it absorbs
Size - 95 times the Earth's mass
Space Shuttle travel years = 5.2 years, 9.5 AU from the Sun

Uranus - atmospheric composition is very similar to Jupiter and Saturn, does not emit more radiant energy than it absorbs, inner solid core has a strange rotation - the axis of rotation is parallel with its orbital plane (the Earth's is perpendicular), appearance is somewhat hazy and atmosphere features are difficult to identify.
Size - 14 times the Earth's mass
Space Shuttle travel years = 11.1 years, 19.2 AU from the Sun

Neptune - very similar to Uranus, Voyager probes did identify a clear "Blue Spot" hurricane like storm similar to the giant Red Spot on Jupiter, winds exceed 1000 km/hr, 
Size - 17 times the Earth's mass
Space Shuttle travel years = 17.6 years, 30 AU from the Sun

Pluto (Dwarf Planet) and other objects - Not much is known about Pluto other than it is quite small and cold, temperatures are estimated at -210oC, some have argued that Pluto should not be classified as a planet and is more like a large asteroid.  A newly opened science museum doesn't even put Pluto into its mosaic of the planets.  Some theories suggest Pluto may have once been a satellite of Neptune that experienced a massive collision to put it into orbit around the Sun instead of Neptune.
Size (Pluto) - 3/1000 of the Earth's mass
Space Shuttle travel years = 23.4 years, 40 AU from the Sun

Nearest Star (Alpha Centauri) Space Shuttle travel time = 168,000 years
Nearest Galaxy (Andromeda Galaxy) Space Shuttle travel time = 88 billion years*
  [The Clouds of Megallan galaxy are a actually closer to the Milky Way than the Andromeda Galaxy.]
Edge of the known Universe Space Shuttle travel time = 14 trillion years*

(*These numbers make little sense because the Universe is believed to be expanding much faster than the Shuttle can fly.  The Andromeda Galaxy and the edge of the Universe are moving away from us much faster than the shuttle can fly.   And the number of years is older than the Universe itself!  But they do give you a "feeling" for the size of the Universe )

Geological Evolution of Planets

According to the Big Bang theory, almost all the atoms of the universe began as hydrogen or helium.  Atoms that are heavier, such as carbon, oxygen, nitrogen, iron, uranium, etc., are believed to have formed in fusion reactions within stars.  Thus, you, I, and most of the earth are composed mainly of "star dust". 

It is believed that our solar system formed from the gravitational collapse and coalescence of a "cloud" of interstellar star dust.  Most of the matter collapsed into the center to form what is called a protosun at the center and protoplanets encircling the protosun.  The matter that formed the sun was sufficiently large to produce nuclear fusion reactions at the core.  [Jupiter came close to being big enough to produce fusion reactions but, lucky for us, didn't.]  After the collapsing matter sufficiently cooled, the protoplanets became planets.  Upon cooling chemical differentiation was happening where heavy elements sunk closer to the core (like iron) and the lighter elements went to the surface.

The Moon has an interesting evolution.  The theories presented to explain its evolution include 1) coformation, 2) capture, 3) daughter or fission, and 4) impact.  Many scientist favor the impact theory which suggests an object flying through the early solar system collided with the earth and the pieces from the collision formed the resulting moon.

The atmospheres of the early planets Venus, Earth, and Mars, are believed to have evolved via a primary and secondary atmosphere.  All three planets had the same primary atmosphere of light gases of hydrogen, helium, and methane (CH4).  The primary atmosphere of the planets "evaporated" into space and was lost.  A secondary atmosphere developed via outgassing of volcanic activity composed of CO2, SO2, and nitrogen compounds.   On the earth, just the right amount of greenhouse gases (mainly CO2) dissolved or chemically combined into the surface.  This caused the right temperatures to let the water vapor condense and form oceans.  On Venus, the greenhouse gases never left the atmosphere, producing more heat trapping conditions and preventing water in the form of vapor from condensing.  This is commonly referred to as the "runaway greenhouse effect".  On Mars the reverse is true.  So much greenhouse gases "leaked" into space or chemically combined with the surface that Mars got quite cold.  The early planet of Mars probably had a warmer atmosphere, blue sky, and liquid water.  Mars experienced a "reverse greenhouse effect".

Life Elsewhere?

Sigmund Feud once said "...great revolutions in the history of science have but one common, and ironic, feature: they knock human arrogance off one pedestal after another of our previous conviction about our own self-importance."  The scientific revolutions to which he is referring are mainly the Copernican Revolution (Earth is not the center of the Solar System), Darwin Evolutionism, and Unconscious/Sociobiology.  This last category was, to a certain degree, showing the personal bias of Feud.

So why do I present this statement in this category?  (I'll let you answer this question.)

"Are we alone?" is one of the most fascinating and philosophically important questions of all time.  To approach this question scientifically, we need to (i.) examine the definition of "life", (ii.) the current evidence that might suggest an answer, and (iii.) the probability of life in the Universe existing even if we haven't directly observed evidence for such.

In the October, 1994, issue of Scientific American, Carl Sagan wrote a very informative exposition on this topic.  The title of his article was "The Search For Extraterrestrial Life: The earth remains the only inhabited world known so far, but scientists are finding that the universe abounds with the chemistry of life."

On the definition of life Sagan writes

" 'I'll know it when I see it.' is an insufficient might identify life as anything that ingests, metabolizes, and excretes, but this description applies to my car or to a candle flame...Biochemical definitions -- for example, defining life in terms of nucleic acids, proteins and other molecules -- are clearly chauvinistic.  Would we declare an organism that can do everything a bacterium can dead if it was made of very different molecules?  The definition I like the best -- life is any system capable of reproduction, mutation and reproduction of its mutations."

Most likely places within our Solar System to find signs of life (or past life) are in fossilized records on Mars and/or underneath Europa's icy surface which may lie a vast liquid water ocean.  Magnetic data, recently gathered by the Galileo Space Probe, suggests a pattern consistent with Europa having a liquid ocean beneath a thick crust of ice.

Meteorite ALH84001 - So what's the big fuss over this rock?  Many highly respected scientist have argued that when examined closely it shows evidence for life on Mars (several billion years ago) from microscopic fossils contained in the rock!  A tantilizing possibility but at this point it is still being debated.

Greenbank Equation (or the Drake Equation)

The Greenbank Equation represents an effort to place a numerical probability on the possibility that life exists out there somewhere.  Some components of this equation are well known and some are meer speculation.  To fully appreciate this equation, one needs to be familiar with concepts in Astronomy, Biology, and little bit of Geology.

In essence, it attempts to put a numerical estimate on the number of technological civilizations present in our galaxy - the Milky Way.  The formula tries to establish numbers on questions that factor into making such an estimate.  These factors include: rate of star formation, probability of the star having planets, planets with the right conditions necessary for life, will life emerge if given the right conditions, probability of life evolving intelligence, intelligence producing technologically advanced civilization, average lifetime for a technologically advanced civilization.

 Some scientist are becoming somewhat pessimistic about finding intelligent life elsewhere. 
International Conference on "Astrobiology" - "I don't think there is anything out there at all except ourselves..." states British paleontologist Simon Conway Morris.  "Microbial life is probably widespread in the universe...However, complex life - animals and higher plants - is likely to be far more rare than is commonly assumed." exclaims Peter Ward, geologist at the University of Washington in Seattle.   Source of information: Pioneer Press article (4/16/00) with the title Dreams of extraterrestrial civilizations are fading.

Planetary Geology - Related Web Links

Mars Exploration Rover Missions - Scheduled for launch on May 30, 2003 and June 25, 2003.

Powers of 10 animation.  Fly in a ship that goes from the outer reaches of space to the inner reaches of the atom.
Mars Landscape
- Computer enhanced images of the Martian landscape.  Gives you the feeling you are actually there.
Origin of Life
- NPR program May 14, 1999 (requires a free RealPlayer)
NASA's Planetary Photojournal

Solar System - A multimedia tour of the Solar System (excellent site)
History of Space Exploration - A very nice site with just about all the information you would need.
The Planets - CalTech site on the planets in the Solar System.
Solar System Simulator - Jet Propulsion Laboratory (JPL)
Exploring the Solar System - A web site sponsored by the New York Times
Web Pages Related To The Search For Extraterrestrial Life
Drake Equation
Extra-Solar Planets - Yahoo web links
Planetary Society
Harvard's SETI Home Page
UC-Berkeley's SETI

Google - Search for Planetary Geology
Google - Search for Planetary Evolution
Google - Search for Exobiology or Astrobiology