ASTR 1210 (O'Connell) Study Guide 16
Crescent Mars during Viking 2 approach 1976.
Clouds trail downwind of volcano Ascraeus Mons.
Mars is the most intriguing planet. From Earth, it exhibits the
largest brightness variations of any planet and has the most
distinctive color (red/pink). Telescopes have revealed it to be
the most Earth-like planet, with a transparent atmosphere,
varied terrain, polar caps, and seasonal changes.
Although Mars is smaller than Earth, it has no oceans, so its land
area is comparable to Earth's. Space missions have revealed fantastic
topography, including the largest canyon and the largest mountain in
the solar system.
Finally, for over 100 years, Mars has been the favorite candidate
for another biosphere. Claims by some astronomers
that there were "canals" on Mars had tremendous impact on popular
culture. The canals were optical illusions, but recent
evidence for possible fossil lifeforms and for abundant water on its
surface in the past finally have given real credence to speculations
about life on Mars. Consequently, Mars is now under intense scrutiny
by spacecraft for evidence of a favorable habitat (now or in the
A. Mars: Introduction
Small: 50% Diam(Earth); 10% Mass(Earth)
Orbit: Semi-major axis 1.5 AU. Orbital period 1.88 yr.
"Oppositions" occur every 2.1
Atmosphere: thin. Mass ~1% Earth's. Mainly CO2;
some H2O vapor.
Surface: easily visible since CO2 is transparent. Has been
explored in ever increasing detail by Earth-bound telescopes and by
spacecraft, including orbiters, landers and rovers.
- Mars' orbital ellipticity (10%) implies a large variation in
distance & brightness at opposition, depending on where that occurs in
its orbit. See diagram at right. (Click for a different
- When brightest, Mars is a very conspicuous, red-pink object (hence
its association with the God of War). Mars can be brighter than
- Mars' red color is caused by iron oxide compounds such as
hematite on its surface. This is equivalent to rust(!). Other
conspicuous markings include white polar caps and large dark areas,
some appearing greenish to the eye in a telescope.
- Mars is distant enough even at opposition that telescopes on
Earth yield relatively poor resolution. This led to a long and
controversial history over whether or not there was evidence for
"canals" or other artificial features on its surface. (See
Section C below.)
- The image below was taken from Earth orbit by the Hubble Space Telescope and shows the
main kinds of features visible from Earth.
Click here to see a global Mars video composited from multiple
HST data frames.
B. Major Spacecraft Missions to Mars
Mars has been the target of over 40
robotic spacecraft missions, many of which (especially the USSR's) failed
(wow! incompetence or
All types: flybys, orbiters, landers, rovers. Successful missions
have mapped nearly its entire surface and have sampled its atmosphere
and soil and even subsurface layers using special techniques.
Important Earlier Missions:
Missions: 3 flybys, 1 orbiter, 1964-71; preliminary imaging
reconnaissance of Martian surface
Missions: 2 orbiters/2 landers. 1976. Sampled soil and searched
for chemical signatures of lifeforms.
Pathfinder: a lander with the first rover. Sampled soil and
Mars Global Surveyor: orbiter. Mapped surface at high
resolution with a camera and the MOLA laser altimeter. 1998--2006.
- 2001 Mars
Odyssey: orbiter. Studying mineralogy, elemental abundances
(including hydrogen, a water tracer). 2001-- .
- Mars Express: first European mission to Mars. Lander
failed (2003), orbiter (2004-- ) studying surface features,
mineralogy, atmosphere. Carries a stereoscopic camera and the MARSIS
radar instrument for probing subsurface materials.
- Mars Exploration Rovers Mission: orbiter plus two
landers and rovers ("Spirit" and "Opportunity"), on opposite sides of
planet in areas thought to be associated with large water flows in
past. Geological analysis ongoing. 2004-- .
Reconnaissance Orbiter: Entered Mars orbit in March 2006.
Instrumentation includes the HIRISE camera for high resolution (3 ft on
the Martian surface) imaging and experiments for mineral mapping and
subsurface analysis by radar.
- The Mars Science Laboratory,
successfully landed its "Curiosity" rover in August 2012 in
Gale Crater, a
large, ancient impact crater which contains a mountain of sedimentary
C. Percival Lowell and Canals on Mars
"What goes on upon all those distant globes? Are they
worlds, or are they mere masses of matter? Are physical forces alone
at work there, or has evolution begotten something more complex,
something not unakin to what we know on Earth as life? It is in this
that lies the peculiar interest of Mars."
--- Percival Lowell (1895)
Lowell devoted his career & his observatory (ca. 1890-1915) to
study of features on Mars' surface which he believed to be
artificial canals engineered by an advanced civilization for
survival on a desert planet. He made numerous sketches using a
medium-sized telescope (at left; an example of a Lowell sketch is at
right). Another, much more detailed, Lowell map can be
Lowell was not the only astronomer claiming to have seen the canals.
The visibility of any such features is strongly affected by the
blurring effects ("seeing"---see Study Guide
14) of Earth's atmosphere. The canal enthusiasts claimed to have
seen the sharp, straight-line features crossing the planet's face
emerge during brief moments of atmospheric stability. They believed
they saw systematic changes that implied the Martians were improving
their canal system and greening of the surface indicating vegetation
spreading in the springtime. However, many careful observers were never
able to see the canals at all.
the idea of civilizations on Mars had a great impact on the public
imagination. They were the stimulus for H.
G. Wells' War of the
Worlds, the prototype story of alien invasion, and a
tidal wave of subsequent science fiction and fantasy stories
(see Study Guide 18).
But the canals are optical illusions!, created by the tendency
of the human brain to link threshold markings together. "Pattern
recognition" by the eye-brain system is an important survival
adaptation for human beings: those who could see the tiger lurking in
the forest shadows survived longer than those who could not. However,
in many situations (e.g. at the telescope or in sudden emergencies) it
can produce misleading memories of what was seen. The canals were
never photographed even with the largest Earth-bound telescopes (see
A photograph (left) and drawing of Mars made on the same
Click for an enlarged comparison of an HST image to
The "canals" are an object lesson in dealing with marginal evidence,
a not uncommon situation in science:
- Beginning in the 1960's, close-up images from spacecraft proved
there to be no artificial structures on Mars.
- "Green" markings are likewise an optical illusion, caused by the color
contrast between reddish and grey-brown areas.
There is a great temptation to overinterpret marginal data and
to try to force them to conform to preconceived ideas. Good
scientists will resist this. They will honestly assess the
uncertainties in the situation and will withhold judgement until the
It turns out that few of the real topological features on
Mars---such as the enormous mountains, canyons, and craters revealed
by spacecraft imaging---are even visible in Earth-based telescopes
despite their scale. Instead, the patterns conspicuous from Earth
are large-scale differences in albedo (reflectivity) caused by
surface dust and mineral deposits. The pink/orange dust is very fine.
Winds carry it easily across the surface, causing continuous small
changes in appearance.
An example of the best maps of Mars produced by Earth-based telescopes
before the first spacecraft imaging was obtained in the 1960's is
shown here. It bears little
resemblance to modern topographic maps
produced by orbiting spacecraft (and it shows no evidence of the
multiple canals claimed by Lowell).
There is good evidence for water and possibly primitive
lifeforms on Mars in the distant past (discussed below and in
Guide 17)---but not for
Topographic charts of Mars, color-coded for altitude,
main features identified (from the MGS MOLA Altimiter)
D. Martian Topography
Mars features an amazing landscape. Martian topography has been
surveyed by many spacecraft (see above), now reaching an accuracy
of about one meter.
1210 Mars Images Page. For illustrations, click here or on
highlighted items below.
- The dual-hemisphere image above shows the main topographic
features of Mars.
- The image at right illustrates the red color, craters, mountains,
plains, and dust-laden atmosphere of Mars. Click for a larger
- Impact Craters: are widely distributed with more in the
southern hemisphere, which is therefore older. Craters are weathered
and often partially dust-filled. The Hellas Basin is the largest impact crater.
- Volcanos: There are 5 large
"shield" volcanos plus a
number of smaller ones. All are dormant now.
- Olympus Mons is the
largest mountain in the solar system (85,000 ft altitude).
- Olympus Mons and the other volcanos in its vicinity are thought
to have originated from the upwelling of a warm mantle
plume, 1-2 Byr ago
- The plume's effect was concentrated into a huge uplift region
(the "Tharsis bulge")
because there was minimal motion of the crust past the plume.
- This qualifies as proto-tectonic activity. But Mars' surface
is not broken into tectonic plates, and mantle activity has been
dormant for most of the last 1 Byr.
- So, neither Mars nor Venus have widespread tectonic plate
structures like those on Earth. Mars, however, has not
experienced a violent resurfacing episode like that 500 Myr ago on
- Valles Marineris is
the largest canyon in solar system. It was not produced by
water (though some "tributary" canyons may have been). Instead, VM is
a surface rift, like
Rift Valley, created during the Tharsis upwelling.
- Polar caps: the caps
in winter are a mixture of frozen CO2 and H2O.
They melt and refreeze with seasonal change. Water ice melts at a
higher temperature than CO2 ice, so any ice visible
between the two melting points must be water ice.
E. Evidence for Water on Mars
Click for illustrations.
There are no open bodies of water on Mars now, but there is extensive
evidence for the presence of frozen water at the poles and in the soil
and for liquid water on the surface in the past:
- Water ice has long been known to be present at the poles. There
is a trace of water vapor in the atmosphere [in percent, 30 times smaller
than on Earth but much larger than on Venus]
- By mapping gamma-ray emission, Mars Odyssey (2001+) provided
evidence for widespread deposition of (frozen) water molecules in the
upper 1-meter of the surface, especially the poles.
- Layered terrain is
evidence for sedimentary rocks, deposited by lakes and oceans.
- Erosion channels are evidence of
massive floods at some time in the past.
- Evidence for a huge ocean
basin in the northern lowlands.
- Radar evidence for water ice layers buried thousands of feet deep under
the Martian poles.
- Lander missions (5 to date, see Study
Guide 17) have found extensive evidence for water molecules bound
within Martian minerals. The Phoenix lander (2008), with a
soil-sampling scoop, detected subsurface ice crystals; its images of
changing droplets are possible evidence for liquid (saline) water.
Data from the Curiosity lander (2013) indicate that the mean water
content of soil minerals on Mars is about 2%, or (if extracted) 2 pints
of water per cubic foot of soil.
- All the evidence points to large amounts of liquid water in
the past, perhaps one billion years ago. Water ice is widely
distributed in the soil and rocks now. It is likely that there are
large permafrost reservoirs of water under the surface. In the past,
these could have been melted by volcanic activity, inducing
widespread, possibly cyclic, floods.
- Independent sources of evidence concerning water are the
SNC meteorites (see Guide
Discussion of Mars is continued on
Study Guide 17.
Reading for this lecture:
Study Guide 16
Bennett textbook, p. 206, Sec. 9.4.
Reading for next lecture:
Study Guide 17
Bennett textbook, p. 206, Sec. 9.4.
November 2017 by rwo
"Red Mars" image © 1997 by Calvin J. Hamilton. Mars
orbit graphic by A. Huffman. Text copyright © 1998-2017 Robert W.
O'Connell. All rights reserved. These notes are intended for the
private, noncommercial use of students enrolled in Astronomy 1210 at
the University of Virginia.