AST 101: Introduction to Astronomy

There are 4 (5?) terrestrial planets:

Mercury Venus Earth and Moon Mars
Mercury Venus Terra and Luna Mars


Barsoom is the native name for Mars in the science fiction tales of Edgar Rice Burroughs.

This is classic science fiction featuring the valiant Earthman, John Carter, who rescues princesses, fights the monsters, and pacifies the warring races of Barsoom.


Mars has always held a fascination for mankind. It becomes quite bright at opposition, when it is about 0.5 AU from Earth. It is red - the color of blood - which may explain its association with war gods in many cultures.

Telescopic observations showed that Mars was a dynamic place. Its white polar caps waxed and waned with the seasons. The reddish surface was mottled with dark, apparently green (the color of vegetation), regions, and their shapes too changed with time. In 1877 Giovanni Schiaparelli published drawings of Mars as seen through his telescope. He identified canali, dark linear features on the surface. The tern canali is Italian; it translates most directly as channels.

Schiaparelli's map

Percival Lowell, who was to found the Lowell Observatory on Mars Hill in Flagstaff, AZ, mistranslated this as canals.
Canals imply intelligent life, and deliberate construction. Lowell built up a mythos of Mars as a dying planet, whose intelligent denizens built canals to collect the melting polar ice, and to distribute the water among the oases (dark regions at the intersections of the canali.

Part of Lowell's map

Surface of Mars
Chesley Bonestell Surface of Mars
© Bonestell Space Art

We now know that this mythos arose from two optical illusions. The first is that the brain, when confronted by a disordered pattern of dots, attempts to provide order. When these dots are blurred as seen through a telescope, the human brain tends to connect the dots, which results in apparent linear features. The second illusion is that dark grey, when viewed next to red, appears green. Mars is not a red planet with green vegetation outlining water canals, it is a red planet with dark grey regions. The brain is not an objective observer!

As Seen from Earth

Dreams Smashed

Mars approached by Mariner 7 in 1969

Mariner 4 flew b at a distance of 9800 km in 1967. The Mariner 6 and 7 spacecraft flew within 4000 km of Mars in 1969. In 1971, Mariner 9 went into orbit around Mars and returned thousands of images. Our view of Mars changed radically.

m04_05b m04_09d
Craters seen by Mariner 4 (1965)

m04_05b m04_09d
Meridiani Sinus and Deucalionis Regio Rilles in Mare Sirenum


Phobos Phobos Deimos
Phobos Phobos from MGS Diemos

Global Views

Schiaparelli hemisphere

Springtime at the north pole.


Olympus Mons: the biggest volcano in the Solar System.

Elysium Mons and Albor Tholus, in the Tharsis ridge.


Ice clouds seen from the surface

Frost in Lowell Crater

A Martian Dust Devil

tracks of dust devils

Dunes and dust devil tracks

Dust devil in Gusev crater

mist in Valles Marineris

a frosty morning at Utopia Planitia

Running Water?

Nirgal Vallis system. 10 miles wide

Valles Marineris detail

Valles Marineris detail 6 miles wide

North pole

CO2 frost, south polar region

layerings due to sedimentation?

500 meter snow drifts

The Surface

Spirit and Opportunity on Mars

Important Results from the Mars Rovers

Fossil Water?

Frozen Icebergs?

Radar mapping by the Mars Express shows H2O ice under the south polar cap of Mars. The amount of water would cover the planet to a depth of 11 meters. Press report (3/16/07)

Life on Mars? - Fiction

The Face on Mars - from the Viking orbiters in 1976. Evidence for an early civilization? Richard Hoagland thinks so. Phil Plait rebuts this malarkey here, as does Gary Posner here.

The Face on Mars - images with better resolution. It is a weathered mesa.

Another view of the Face on Mars, with different lighting conditions, from the MGS.

Our brains are hard-wired to look for recognizable patterns. It would be surprising if we did not see them. Witness these:

Happy Face Heart
Happy Face crater The Heart of Mars

Brain Crater

A giant worm, a glass tube, or aeolian (wind-blown) dunes?

Life on Mars? - Fact

There is no macroscopic life on Mars today.

Is there microscopic life on Mars?
The Viking Landers undertook three biochemical tests, with inconclusive results - but any life is likely to be buried down where the subsurface water is.

Was there life on Mars?

The meteorite ALH84001 (so-named because it was found in the Allen Hills of Antarctica in 1984) is of Martian origin. Microscopic analysis of the rock has found evidence for ancient life - maybe.

Are these micro-fossils?

In the Summer of 1996, David McKay and his co-workers made quite a splash when they announced that they may have found evidence of life in a Martian meteorite. ALH84001 is about 4.5 billion years old; about 3.6 billion years ago water (or some other liquid) flowed through the rock, depositing carbonate globules wherein was found the purported evidence for life. The meteorite was ejected from Mars during a meteorite impact about 16 million years ago. It orbited the Sun for 16 million years, and fell to Earth in Antarctica about 13,000 years ago. It was picked up by an expedition looking for Antarctic meteorites in 1984.

ALH84001 is of 30 known SNC meteorites. These are undoubtably of Martian origin, because their compositions and isotopic abundances match those of Mars (isotopic abundances are like fingerprints: each planet has a unique pattern).

The evidence consists of a number of separate items, none of which independently provides striong evidence, but which together look fairly strong. There is evidence for

A more complete discussion of the evidence is given here.

The data are intriguing, but like most controversial issues in science, the data are inadequate to confirm the finding beyond any reasonable doubt. McKay and colleagues take the holistic approach: that the simplest explanation of all the data involves biologic origins, and hence life on Mars about 3.6 billion years ago. Critics often take the reductionist approach, showing that all of the evidence can be created by inorganic processes, though there is no single process that can create all the data.


Using the infrared spectrometer on the Mars Express orbiter, scientists have confirmed the presence of methane in the Martian atmosphere (press release), at a level of 10 parts in a billion. Methane is oxidized under the influence of solar UV radiation, and should disappear in a few hundred years.

The methane is concentrated along equator in regions where water vapor is also concentrated.

Are we the Martians?

It has been suggested that perhaps life on Earth arose on Mars. After all, Mars was more hospitable to life early on, and it did have a significant atmosphere and running water. The Earth took longer to develop a solid surface (large planets retain heat longer).

Some consider it a problem that there is evidence of microbial life on Earth a mere 200 million years after the Earth's surface is thought to have solidified. They think it more likely that life evolved on Mars, and was carried to Earth in the same way the ALH84001 got here.


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