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Terrestrial Planets

–Begin With

Mercury

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Guiding Questions

1. What makes Mercury such a difficult planet to see?

2. What is unique about Mercury’s rotation?3. How do the surface features on Mercury

differ from those on the Moon?4. Is Mercury’s internal structure more like

that of the Earth or the Moon?

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88 days

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Earth-based optical observations of Mercury are difficult

• At its greatest eastern and western elongation, Mercury is never more than 28° from the Sun

• It can be seen for only brief periods just after sunset or before sunrise

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Solar Transit

There was a transit on November 8, 2006

Transits occur about twelve times a century when the sun, Earth and Mercury are aligned

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Earth-based Views of Mercury

Difficulties observing Mercury from Earth led early astronomers to incorrectly decide that Mercury always kept the same face towards the sun in synchronous orbit

Note phases like the moon

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Mercury rotates slowly and has an unusualspin-orbiting coupling

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Radio telescope observations from sites such as Arecibo gave evidence of a non-synchronous orbit

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• Mercury spins 1 ½times on its axis for every complete orbit

• Mercury spins three times during every two orbits 12

• Strong tidal effects, Mercury’s slightly elongated shape and its very eccentric orbit cause this strange 3-to-2 orbit

• A “day” of solar light on Mercury would be 88 earth days

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Images from Mariner 10 revealed Mercury’s heavilycratered surface

• Most of our detailed information about Mercury’s surface is from the Mariner 10 flyby mission in 1974/1975.

• Mariner only saw one side of the planet.• The MESSENGER mission spacecraft is now

at Mercury.– It is adjusting its flight path for an ultimate orbit

of Mercury– 3 flybys to date– Will achieve orbit in 2011

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MESSENGER Spacecraft

Image from Flyby #3 - 2009

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• Heavily cratered surface

• Less dense cratering than moon

• Gently rolling plains

• Scarps• No

evidence of tectonics

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Note how much more densely the craters occur on the Moon’s surface.

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Scarps are cliffs

This one is more than a km high

They probably formed as the planet cooled and shrank

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• The CalorisBasin is evidence of a large impact

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The seismic waves from the impact that caused the CalorisBasin caused this deformation on the opposite side of Mercury

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Mercury’s North Pole. 22

Mercury has an iron core and a surprisingmagnetic field

• Most iron-rich planet in the solar system with a core that is 75% of the diameter

• The earth’s core is 55% of its diameter and the moon’s core is 20% of its diameter

• Highest density for the planets• Weak magnetic field indicating part of the

core is liquid• Magnetic field causes a magnetosphere

similar to Earth’s but weaker

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The magnetosphere blocks the solar wind from reaching the surface of the planet

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Cloud-Covered Venus

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Guiding Questions

1. What makes Venus such a brilliant “morning star” or “evening star”?

2. What is strange about the rotation of Venus?3. In what ways does Venus’s atmosphere differ

radically from our own?4. Why do astronomers suspect that there are active

volcanoes on Venus?5. Why is there almost no water on Venus today? Why

do astronomers think that water was once very common on Venus?

6. Does Venus have the same kind of active surface geology as the Earth?

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• At its greatest eastern and western elongations, Venus is about 47° from the Sun

• It can be seen for several hours after sunset or before sunrise

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The surface of Venus is hidden beneath a thick,highly reflective cloud cover

• Venus is similar to the Earth in its size, mass, average density, and surface gravity

• It is covered by unbroken, highly reflective clouds that conceal its other features from Earth-based observers

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In 1962 the unmanned U.S. spacecraft Mariner 2 made the first close flyby of Venus

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Venus’s rotation is slow and “retrograde”

• Venus rotates slowly in a retrograde direction with a solar day of 117 Earth days and a rotation period of 243 Earth days

• There are approximately two Venusian solar days in a Venusian year.34

Venus has a hot, dense atmosphere and corrosive cloud layers

• Spacecraft measurements reveal that 96.5% of the Venusian atmosphere is carbon dioxide

• Most of the balance of the atmosphere is nitrogen.

• Venus’s clouds consist of droplets of concentrated sulfuric acid.

• The surface pressure on Venus is 90 atm, and the surface temperature is 460°C

• Both temperature and pressure decrease as altitude increases

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The upper cloud layers of the Venusianatmosphere move rapidly around the planet in a retrograde direction, with a period of only about 4 Earth days

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The circulation of the Venusian atmosphere is dominated by two huge convection currents in the cloud layers, one in the northern hemisphere and one in the southern hemisphere

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Volcanic eruptions are probably responsible for Venus’s clouds

• Venus’s clouds consist of droplets of concentrated sulfuric acid

• Active volcanoes on Venus may be a continual source of this sulfurous material

The density of craters suggests that the entire surface of Venus is no more than a few hundred million years old.

According to the equilibrium resurfacing hypothesis, this happens because old craters are erased by ongoing volcanic eruptions

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The climate on Venus followed a differentevolutionary path from that on Earth

• Venus’s high temperature is caused by the greenhouse effect, as the dense carbon dioxide atmosphere traps and retains energy from sunlight.

• The early atmosphere of Venus contained substantial amounts of water vapor

• This caused a runaway greenhouse effect that evaporated Venus’s oceans and drove carbon dioxide out of the rocks and into the atmosphere

• Almost all of the water vapor was eventually lost by the action of ultraviolet radiation on the upper atmosphere.

• The Earth has roughly as much carbon dioxide as Venus, but it has been dissolved in the Earth’s oceans and chemically bound into its rocks

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The surface of Venus shows no evidenceof plate tectonics

• The surface of Venus is surprisingly flat, mostly covered with gently rolling hills

• There are a few major highlands and several large volcanoes

• The surface of Venus shows no evidence of the motion of large crustal plates, which plays a major role in shaping the Earth’s surface

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Venusian Surfaces

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Mars

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What I’ll Talk About

• Some history– a view at the start of the 20th century

• Mariners to Mars• Viking Mission

– in search of life of Mars• A meteorite

– in search of life in a rock• Some latest views from Mars• Conclusions

– keeping it simple

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The High Hopes

• “The planet Mars, on the other hand, exhibits in the clearest manner the traces of adaptation to the wants of living beings such as we are acquainted with. Processes are at work out yonder in space which appear utterly useless, a real waste of Nature’s energies, unless, like their correlatives on earth, they subserve the wants of organized beings.” [Richard Proctor, 1902]

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From Schiaparelli…

• As seen by telescopes from Earth– An orange-red orb, with

some darker patches and bright polar caps sometimes visible

• Giovanni VirginioSchiaparelli (1835-1910)– 1876 announced discovery

of “canali” (channels) on Mars

– misreported as canals (artificial) by the press

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To Percival Lowell

• Percival Lowell (1855-1916)– appointed MIT

astronomy professor in 1902

– published books• Mars (1895)• Mars and its Canals

(1906)• Mars as the Abode

of Life (1908)

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Lowell’s Observations and Explanation

• No canals• human brain tendencies

• connect unrelated points together by lines

• Recent theory• Lowell’s telescope acted as an ophthalmoscope

• caused Lowell to see the reflection of the radial pattern of his own retinal blood vessels

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More Historical Background

• At the turn of the 20th century:– publication offered a reward for anyone

coming forth with proof of life on another planet or anywhere in space EXCEPTING Mars

– just about every major observatory had released hand paintings of Mars and some were even releasing photographs as astrophotography was in its infancy

• no two drawings could agree on the formations on the planet's surface

• they showed a Mars with a varied surface possessing darker and lighter areas, as well as the polar caps

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Mariner 4, 6 and 7

• Mariner 4– Mars flyby mission– closest approach came on July 15, 1965– pictures from this mission showed no canals and a

surface that was disappointingly looking like that of the moon, quite LIFELESS

• In 1969 the United States launched Mariner 6 (February) and Mariner 7 (March)

• At closest approach (July for Mariner 6 and August for Mariner 7) both craft were at a distance of approximately 3400 kilometers 56

Mariner 4 Photographs

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Mariners 6 and 7

• The Mariners (6 & 7) contained:– narrow and wide angle cameras– infra-red radiometer– infra-red spectrometer– ultra-violet spectrometer

• Temperature, pressure and atmospheric constituents were analyzed

• Pictures were still anything but spectacular

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A Time to Fail and Succeed

• In 1969– two unsuccessful attempts by the Russians

• In 1971– both Americans and Russians had unsuccessful

missions to Mars– Russian Mars 2 and Mars 3

• both equipped with lander modules but neither lander was successful

– Americans Mariner 9• reached Mars during a global dust storm

– the storm did eventually subside and the mission was enough of a success so as to provide pictures for the choosing of a site for landing the upcoming Viking missions

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Mariner’s Atmosphere

• First look provided by Mariner spacecraft– Mariner 9 specifically

• faced presence of a global dust storm• illustrated the progress of a feature that looked very

much like a terrestrial cold front, visible as a bright band extending across many of the images

• saw evidence of dust storm associated with strong winds

• saw large crater rim produce wave clouds, believed to be composed of water ice (resembling "sonic boom shock wave”) produced by strong low level winds passing over the crater

• saw day-to-day variations indicative of day-to-day weather changes and frontal systems 60

Mariner 9 Photographs

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A Prelude to Viking

• First approved in December of 1968 for a 1973 launch

• Launch date postponed due to Congressional funding cutbacks

• Idea was to launch the craft in 1975 for a landing to take place on Independence Day in 1976

• Viking 1 was to be launched on August 11, 1975 but was postponed due to a malfunction

• While fashioning repairs for the spacecraft, the twin unit was substituted and so Viking 2 became Viking 1 and vice versa 62

Viking Liftoff

• Viking 1 launched August 20, 1975• Viking 2 launched September 9, 1975• Each Viking orbiter consisted of:

– television camera system– an atmospheric water detector– an infra-red thermal mapper

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Viking Instruments

• Each Viking lander contained:– television camera system– gas chromatograph mass spectrometer– x-ray fluorescence spectrometer– seismometer– biology lab– weather station– sampler arm

• Each aeroshell contained:– a retarding potential analyzer– upper-atmosphere mass spectrometer 64

Arrival at Mars

• Viking 1 arrived at Mars on June 19,1976– took pictures to aid in the choice of a

landing site for the lander• caused a delay in the landing beyond its

Independence Day rendezvous

• Using the latest pictures, the western slopes of Chryse Planitia were selected for the landing of Viking Lander 1

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Another Giant Leap for Mankind

• On July 20, 1976 (seven years after a man had taken his first steps on the moon)– Viking Lander I successfully descended

upon the soil of Mars• immediately after successful touchdown, the

lander had instructions for taking pictures with its camera (there was actually a concern that the lander might sink into the soil, and so at least a picture was desired before it conceivably had sunken)

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The Viking Look

• The Viking cameras– not cameras in the conventional sense– each consisted of:

• a nodding mirror• a rotating turret which caused the images to

be reflected down to the photodiode, which built up a picture as a series of pixels from each scan of the mirror and rotation of the turret

– criticized for its inability to detect any moving objects (some still felt it possible that there might be macroscopic creatures on the planet)

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Viking Orbiter Photograph

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The Face on Mars

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The Face on Mars - Caption

• The picture shows eroded mesa-like landforms. The huge rock formation in the center, which resembles a human head, is formed by shadows giving the illusion of eyes, nose and mouth. The feature is 1.5 kilometers (one mile) across, with the sun angle at approximately 20 degrees. The speckled appearance of the image is due to bit errors, emphasized by enlargement of the photo. The picture was taken on July 25 from a range of 1873 kilometers (1162 miles). Viking 2 will arrive in Mars orbit next Saturday (August 7) with a landing scheduled for early September.

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The Changing Face

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Viking Lander Photograph

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Reach Out and Touch

• On July 22, 1976 the sampler arm was to be deployed– however, there were difficulties

• overcome by ingenious engineers

• The sampler arm was finally deployed on July 28

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First Results from Soil Sample

• X-ray fluorescence spectrometer (to determine the inorganic composition of the soil sample)– 15-30 percent silicon– 12-16 percent iron– 3-8 percent calcium– 2-7 percent aluminum

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A Mass Disappointment

• Gas chromatograph mass spectrometer results– indication of carbon dioxide– little water– NO organic compounds

• The beginning of a controversy– this negative result conflicted with

results from the biology experiments• indicative of the existence of microbial life

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Looking for Life

• The biology laboratory– approximately a single cubic foot of volume– consisted of:

• pyrolytic release experiment• labeled release experiment• gas exchange experiment

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Pyrolytic Release Experiment

• PI was Norman Horowitz• Basis of experiment

– ability of an organism to metabolize carbon dioxide and produce some product (reverse process of Levin's experiment)

– soil sample placed in test chamber for five days and incubated with/without light

– if soil had fixed or metabolized the carbon dioxide (carbon-14 tagged) then pyrolysis of the sample would allow detection of labeled carbon in the chamber’s gas

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Gas Exchange Experiment

• PI was Vance Oyama• Basis of experiment

– evidence of metabolism by noting changes in the gaseous environment of the sample

– sample would be introduced into the chamber and the chamber's atmosphere analyzed• after a period of incubation, the gas would be

re-examined and a comparison is made between this analysis and the initial analysis 78

Labeled Release Experiment

• PI was Gilbert Levin• Basis for experiment

– property of microorganisms to metabolize organic compounds in a nutrient broth

– organics in broth tagged with carbon 14– If organisms in the sample were

metabolizing the nutrient, the carbon-14 would appear in the chamber's gas by the appearance of tagged carbon monoxide or carbon dioxide

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Biology Experiment Results

• All three biology experiments registered results which were indicative of some very active samples, and if these results were obtained on earth there would be no doubt that organisms were responsible

• Doubt of the biological results once the GCMS had failed to detect any organics within the soil sample

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Explaining Biology Away

• Theories dealing with superoxides, peroxides and superperoxides to explain apparent positive results away the results of

• Only hold-out for the possibility that the biology experiments still might indicate the existence of life on Mars was Gilbert Levin [only science team member that still maintains belief that evidence of life was found]

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Levin’s View 25 Years Later

• “After 25 years, the Mars LR data still excite attempts at a chemical explanation, three within the last year. This indicates that none of the 30 non-biological explanations offered to date has been completely convincing. New findings concerning the existence of liquid water on the surface of Mars, and extremophile microorganisms on Earth, are consistent with my conclusion that the LR detected living microorganisms in the soil of Mars (Levin 1997), which may explain the difficulties with the non-biological theories.”

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Viking’s View of Atmosphere

• Viking Lander meteorological instruments– at end of boom that deployed after landing

• contained thermocouple units to measure the atmospheric temperature and wind speed

– an atmospheric pressure sensor which was not on the boom so as to be shielded from winds

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First Mars Weather Report

• Seymour Hess stated:– "Light winds from the east in the late

afternoon, changing to light winds from the southwest after midnight. Maximum winds were 15 miles per hour. Temperature ranged from minus 122 degrees Fahrenheit just after dawn to minus 22 degrees Fahrenheit. Pressure steady at 7.7 millibars."

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Viking Looks at Climate

• Long term data available–from Viking Lander 1 through Novermber 5, 1982

–from Viking Lander 2 through April 11, 1980

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Viking Climate Conclusions

• discovered nature of surface pressure variations over the seasons and the cycling of the atmosphere between the polar caps– minimum in the pressure cycle occurs during the

southern winter when the carbon dioxide mass condensing onto the south polar cap is a maximum

– as the seasonal carbon dioxide sublimes out of the south polar cap, the pressure rises until the north polar cap starts to form

– process reverses seasonally and carbon dioxide reforms at the south polar cap 86

More on Atmospheric Findings

• Other characteristics of Martian atmosphere– difference in pressures between the two

landers• attributed to the difference in elevations

between the two sites– there was also much noise on the pressure curves,

which, in the end, was determined NOT to be noise, but associated with traveling cyclones of the kind that had been speculated upon based on images from Mariner of the dust storms

» these cyclones occurred only during the winter

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A Little Pressure

• Pressure variations detected– linked to optical depth computations and

demonstrated the presence of what meteorologists call atmospheric tides• atmospheric tides should not to be confused

with gravitational tides– wind and pressure variations that are produced by

the daily cycle of heating over the whole atmosphere• what results from the daily loading cycle,

among other things, are traveling waves that follow the sun and have both diurnal and semidiurnal periods 88

Meridional Circulation[Say What?]

• Landers helped produce charts of meridionalcirculation– on Earth we have the familiar pattern of rising

motion in the tropics and a descending motion in the subtropics with a connecting meridional flow pattern

– on Mars, there is a strong seasonal varying circulation rather than one centered about the equator

– in summer the air rises near the subsolar point in the southern hemisphere subtropics and crosses the equator to a point where it can descend [more like a one-cell circulation with a strong descending motion in the winter hemisphere]

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A Little Mars Geology

• Viking Orbiter images– largest volcano in solar system, Olympus Mons– large canyon, Valles Marineris– a global appearance roughly organized latitudinally

• equatorial belt is somewhat darker than the mean albedoand very changeable over time

• northern and southern mid-latitude regions are brighter, due probably to the deposits of very fine, bright material

• a dark collar around the north polar region• polar regions with the very bright polar caps

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More Beautiful Pictures

• High resolution images from Viking Orbiters– contributed to better understanding the surface– indication that the darker areas are where the silicates

are somewhat more reduced and richer in ferrous rather than ferric silicates

– areas that were originally considered for landing were found to be too hilly

– surprised to find that the Lander was actually in a field strewn with rocks (e.g. Little Joe) large enough so that if the Lander had landed on one of them the mission would have failed

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Map: UCAR

Summary of Mars Landing Sites

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Image credit: NASA/JPL

Pathfinder at Ares Vallis

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Sojourner

• Sojourner weighed 10 kg and spent 3 months roaming on the surface

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Mars Global Surveyor

• Orbiting Mars from 1996 to the present– evidence of

“recent”subsurface water

95Image credit: NASA/JPL/MSSS

Mars Global Surveyor

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Odyssey 2001

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Spirit Rover

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Opportunity Rover

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• Mars core• FeS (iron sulfide),

• FeS has a lower density compared to the Earth’s Fe and Ni

• diameter 40% of Mars

• similar proportion to the Earth’s core/diameter

Figure credit: Albert T Hsui, Univ. Ill

Mars Interior

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•The core is solid, not liquid• do not expect a strong magnetic field

• Magnetometers on MGS have discovered a weak magnetic field over certain regions of the planet• Mars once had a liquid core and magnetic dynamo in the past, and this has permanently magnetized some rocks.• These magnetic rocks are very old, suggesting the field was only ‘on’ for the first few hundred million years of Mars’ history.• Mars is differentiated

• Mantle and Crust

Mars Interior

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• Largest of the four great Tharsis volcanoes first seen by Mariner 9

• Largest volcano in the entire solar system

• About 27 km high and 700 km wide at the base

Figure credit: NASA

Olympus Mons

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Valles Marineris

• A giant canyon system discovered by Mariner 9

• named after the spacecraft!

• Stretches more than 4000 km in length, 500 km wide, and up to 8 km deep

Figure credit: NASA/USGS

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• Tectonic in origin• Huge cracks in the crust widened and shaped by erosion

Figure credit: NASA/JPL. Viking mosiac of Western Candor Chasma

Valles Marineris

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• Largest impact basin on Mars; rim of mountains showing much erosion• Approximately 2000 km across; 5 km below mean Martian surface level• Clouds sometimes found in interior region• Impact occurred during Late Heavy Bombardment stage of solar system formation, approximately 3.9 Gyr ago

Figure credits: (left) NASA/JPL (right) MGS/MOLA

Hellas Basin

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• Compare Olympus Mons with Everest (fold mountain) and Mauna Loa(shield volcano) on Earth.

• Mountains on Earth and Venus can only rise 10-15 km before the rock begins to deform under its own weight

• Why can mountains on Mars get so big?

• Hint: Martian gravity is about 40% that of the Earth

Figure credit: Universiity of North Dakota

Terrain Comparison

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A massive uplifted region

• 10 km above its surroundings• one of the least cratered terrains on Mars• Area equal to North America

Figure credit: NGDC/USGS

The Tharsis Bulge

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• Evidence of “mass wasting”

Figure credit: NASA/JPL. Vikingimage of Western Candor Chasma

Canyon Widening Evidence

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• Ejecta patterns differ from the lunar impact craters• Craters on Mars display a more fluid ejecta pattern

• Consider what may have caused differences

Figure credit: NASA ARC/CMEX

Impact Craters

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Real Dunes

• This image is of ‘cemented’ sand dunes in the Herschel crater of the Terra Cimmeriataken by Mars Global Surveyor• Image credit to MSSS/NASA/JPL

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Channels

Image credit: NASA/JPL

• Three major types of channels1. Runoff channels2. Outflow channels3. Gullies

• Runoff channels• similar to terrestrial dry

river beds• often seen on the steep

sides of crater walls• as old as the cratered

highlands• Evidence for a thicker,

warmer atmosphere in the past

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Outflow Channels

• Larger and less common than runoff channels• Caused by flooding• Evidenced by teardrop islands, terraced walls, and sandbars

• carved by flood waters rushing over original terrain

Image credit: NASA/JPL

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Meteorite from Mars

• ALH84001– possible

evidence of fossil microbes from Mars

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Statement from Daniel S. Goldin, NASA Administrator

• "NASA has made a startling discovery that points to the possibility that a primitive form of microscopic life may have existed on Mars more than three billion years ago. The research is based on a sophisticated examination of an ancient Martian meteorite that landed on Earth some 13,000 years ago.

• “The evidence is exciting, even compelling, but not conclusive. It is a discovery that demands further scientific investigation. NASA is ready to assist the process of rigorous scientific investigation and lively scientific debate that will follow this discovery.

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Goldin Statement (August 6, 1996)

• “I want everyone to understand that we are not talking about 'little green men.' These are extremely small, single- cell structures that somewhat resemble bacteria on Earth. There is no evidence or suggestion that any higher life form ever existed on Mars.

• “The NASA scientists and researchers who made this discovery will be available at a news conference tomorrow to discuss their findings. They will outline the step-by-step 'detective story' that explains how the meteorite arrived here from Mars, and how they set about looking for evidence of long-ago life in this ancient rock. They will also release some fascinating images documenting their research."

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Science Paper by McKay et al.• “In examining the Martian meteorite ALH84001 we have found that the

following evidence is compatible with the existence of past life on Mars: (i) an igneous Mars rock (of unknown geologic context) that was penetrated by a fluid along fractures and pore spaces, which then became the sites of secondary mineral formation and possible biogenic activity; (ii) a formation age for the carbonate globules younger than the age of the igneous rock; (iii) SEM and TEM images of carbonate globules and features resembling terrestrial microorganisms, terrestrial biogenic carbonate structures, or microfossils; (iv)magnetite and Fe-sulfide particles that could have resulted from oxidation and reduction reactions known to be important in terrestrial microbial systems; and (v) the presence of PAHs associated with surfaces rich in carbonate globules. None of these observations is in itself conclusive for the existence of past life. Although there are alternative explanations for each of these phenomena taken individually, when they are considered collectively, particularly in view of their spatial association, we conclude that they are evidence for primitive life on early Mars.”

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Paper by Scott et al.

• “In an electrifying paper published in August, 1996 in the journal Science, David McKay (NASA Johnson Space Center) and his colleagues suggested there were fossils of martian organisms associated with carbonate minerals in martian meteorite ALH84001. How these carbonate minerals formed (biologic origin or not) and the temperature at which they formed (low or high) are hotly debated questions. We have proposed an entirely different origin: the carbonates in ALH84001 formed in seconds at high temperatures (>1000oC) from melts produced during a large impact on Mars 4.0 billion years ago (Scott and others, 1997). We infer that it is unlikely that the carbonates or any minerals in them contain mineralogical evidence for ancient martian life.”

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Paper by Scott and Barber

• “Magnetic minerals in Martian meteorite ALH 84001 formed as a result of impact heating and decomposition of carbonate; they were never used as compasses by Martian microorganisms.”

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A Quick Review of Mars

• Has been of interest for a century– originally felt to show evidence of life

• Has been targeted for study– numerous missions - some fail, some

succeed• Has been suggested as source of

microbes• Will be studied in future• Future life may well be human

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Simplified Conclusions re Mars

• Did Viking find life on Mars?– Nope, but it’s considered uncertain and

controversial• Did Viking find ruins of an ancient

civilization?– Nope

• Does ALH84001 contain microfossils?– Nope

• Do we know that there is no life on Mars?– Nope