Mars,
the Outermost Terrestrial Planet
Notes compiled
p
ffor Astronomyy 1001
by
Paul Woodward
Univ. of Minnesota
Dept. of Astronomy
The orbit of Mars is just barely visibly elliptical, and this is why it
was the planet that drove Kepler to try ellipses rather than circles.
The spin axis of Mars is tilted, like the earth’s, and therefore Mars has
seasons.
Fig. 9.5: Interior
structure of a
generic
terrestrial
planet.
1
Mars is intermediate between the earth and Venus on the one hand
and the moon and Mercury on the other.
It is large enough to have had significant heat generation in its core
through radioactive decay, leading to volcanic activity in its early
history, and it is small enough to have cooled well before the present
era, developing a thick, rigid lithosphere and a solid metallic core.
Mars is therefore an intermediate case from the point of view of its
internal str
structure.
ct re
It is also an intermediate case from the point of view of its
atmosphere.
Mars is an intermediate case between the earth and Venus, on one
hand, and Mercury and the Moon on the other. Due to its smaller size,
Mars has had time to cool, so that its rigid lithosphere goes to great
depths, and any earlier tectonic motions have ceased long ago.
Viewed from the earth, Mars is a reddish object.
Its blood-like color inspired the Greeks and later the Romans to
name it after their gods of war. The name of the Roman god, Mars,
has persisted.
Mars, seen with a small telescope with attached video camera by a
student, Rolf Karlstad.
Mars is massive enough to hold an atmosphere, unlike Mercury and
the moon, but it is not massive enough to have retained a dense
atmosphere over the entire age of the solar system, like Venus or the
earth.
Mars, seen with
the 100-inch
telescope on
Mt. Wilson
Karlstad observing
equipment setup.
A video camera
records the images at
a rate of 60 per sec.
in a digital format
saved on video tape.
The camera can take
pictures in infrared
light.
The images are later
aligned and
composited on a PC.
This gives a manual
kind of adaptive
optics, which,
together with the
patience and
persistence of the
observer, explains the
image clarity.
2
When viewed through a powerful telescope, surface markings
are visible, as well as polar caps, from which the rotation of the
planet and the alternation of its seasons can be observed.
Two views of Mars, showing the rotation of the planet.
An image of Mars
taken with the
Hubble Space
Telescope
8/24/03.
4 views of Mars with the 100-inch telescope. A,B,C in red light, and D in blue light.
This is the
sharpest
color
picture
ever taken
of Mars
from Earth.
Mars, due to its proximity and similarity to the earth, has
continually inspired theories that it harbors intelligent life.
Seasonal changes were attributed to the growth and retreat of
vegetation, and linear markings on the planet, real and imagined,
were attributed to irrigation canals.
A map of Mars drawn by Italian astronomer Giovanni Schiaparelli,
who in 1877 reported the discovery of strange markings on Mars.
Some thought the long straight markings evidence for intelligent life
on Mars, while still others could not find these markings at all
in their telescopes.
(from Mars, p. 35)
3
Maps of Mars drawn by Italian astronomer Giovanni Schiaparelli,
who in 1877 reported the discovery of strange markings on Mars.
Some thought the long straight markings evidence for intelligent life
on Mars, while still others could not find these markings at all
in their telescopes.
(from Mars, p. 35)
Lowell’s observations, apparently much enhanced by his active
imagination, helped to fuel interest in the possibility of life on Mars.
(from Mars, p. 191)
The Mars
Global
Surveyor
“settled” the
face-onMars
controversy
with new,
high
resolution
i
images
off
the Cydonia
region of
Mars in
April, 1998.
(from Mars, p. 190)
The Mars Global
Surveyor “settled”
the face-on-Mars
controversy with
new, high
resolution images
of the Cydonia
region of Mars in
April, 1998.
Here is an image
from April,
April 2001
2001,
with a resolution
of
2 meters per pixel.
(from Mars, p. 211)
The
“face” on
Mars.
If you
want life
on Mars
enough, it
is easy to
believe
that this is
a
sculpture,
a message
from a
Marian
Michelangelo
Evidence of a dense Martian atmosphere, certainly a
prerequisite, many believed, for life as we know it, was visible
from the earth.
Storms are visible on Mars from the earth.
The different appearance of the planet in light of different
wavelengths gives evidence for an atmosphere as well.
4
A storm on Mars.
3 views from the Lowell Observatory, 1973.
Before storm (left), first day of storm (center), and
eighth day of storm (right).
The composition and density of the Martian atmosphere has been
determined by space probes.
Previous estimates by Lowell and others set the density of the
atmosphere more than 10 times too high.
The CO2 in the atmosphere was detected by its absorption of
reflected sunlight from the surface, and was known to be 30
times the amount in the earth’s atmosphere.
Water on the surface of Mars would immediately either freeze or
evaporate. The present Martian atmosphere does not permit
liquid water to exist on the surface.
However, the atmosphere of Mars may have been quite different
very long ago.
This helped to fuel the idea of a dense Martian atmosphere,
dense enough for the ice caps to be water ice.
In its fly-by visit to Mars, Mariner-4 transmitted its radio signals
through the atmosphere, revealing its surface pressure to be only
about 1% that of the earth’s atmosphere.
This, together with the measured amount of CO2 and the -125°C
temperature of the polar caps, meant dry ice, not ice.
The density and temperature profiles (with altitude) of
the atmospheres of the earth and Mars are compared
5
This Viking orbiter
view of the edge of
Mars’ uplifted and
rough Argyre
impact basin
shows the Martian
atmosphere,
composed
predominantly of
CO2 and with a
surface pressure
only 1% of that of
the earth’s
atmosphere.
Before sending space probes to Mars, scientists thought they had bits of Mars
that had fallen to earth.
These were 12 rocks, 4 of which were seen to fall from the sky (in France, in
Egypt, in Nigeria, and in India) and the other 8 of which were found in
Antarctica (6), in Indiana, and in Brazil.
All are similar to terrestrial basalt. (Basalt forms from melted mantle that
rises to the surface and freezes. Iron and magnesium, not silicon. Dense.)
All 12 much younger than all other meteorites (which are about 4.6 billion yrs
old).
old)
Most show evidence of being shocked in an impact event.
All formed in oxidizing conditions, and some in hydrous conditions. All have
similar, distinct oxygen isotopes different from all earth rocks.
A couple have gas trapped in glassy nodules that matches composition of
Martian atmosphere.
Mars only possible source.
(from Mars, p. 87)
Mars does not appear to have continents, like the earth.
However, it has a pronounced north-south asymmetry.
The southern hemisphere is mostly covered with heavily cratered
terrain. The many craters indicate that this terrain is ancient,
probably over 3.8 billion years old.
The heavily cratered terrain is mostly elevated 1-4 km above the
datum. The datum is like sea level on earth. It is defined as
“datum.”
the altitude at which the Martian atmosphere has an average
pressure of 6.1 mbar (6.1 thousandths of the pressure of the
earth’s atmosphere at sea level).
The northern hemisphere of Mars is covered mostly by sparsely
cratered plains that, presumably, formed after the decline in
impact events, which we believe to have occurred about 3.8
billion years ago.
1997 rock with apparent fossilized bacteria. (Convenient???)
Because the boundary of the relatively smooth plains is roughly
circular, one theory of their formation is that they resulted from a
gigantic impact which occurred near the end of the accretion of
the planet.
The plains at high northern latitudes are mostly low lying,
averaging about 1-2 km below the datum.
The eccentricity of Mars’ orbit about the sun, together with the
inclination of its rotation axis, produces extremely warm
southern summers and frigid winters.
For this reason, the northern polar cap, which includes both
water and CO2 ice, is larger than the southern polar cap, which
consists mostly of CO2 and changes dramatically in size with the
seasons.
South polar
hemisphere
of Mars.
This
terrain,
which is
heavily
cratered, is
believed
more
ancient
than the
smoother
plains of
the
northern
hemisphere
6
North polar
hemisphere
of Mars.
The ice cap
consists of a
stack of finely
layered
deposits a few
km thick. The
relative
l i lack
l k off
impact craters
indicates that
this polar
region is
young
(perhaps only a
few hundred
million years).
Western
equatorial
hemisphere
of Mars.
The Tharsis
bulge,
centered
on the
equato ,
equator,
is 5000 km
across and
10 km high.
It has been
the site of
volcanic
activity for
much of the
planet’s
history.
Map of Mars’
eastern
hemisphere.
The Viking-2
landing site is
marked in the
upper right, in
Utopia Planitia.
To the south of it
are the 3 large
volcanoes in
Elysium.
Eastern
equatorial
hemisphere
of Mars.
The dark
areas are
relatively
dust free
and are
perhaps
covered
with rocks
or sand.
Map of Mars’
western
hemisphere.
The Tharsis
bulge is
centered around
the 4 large
volcanoes.
The Viking-1
and Mars
Pathfinder
landing sites are
marked in
Chryse Planitia.
We have learned an enormous amount about Mars from the Mars
Global Surveyor, which has taken high resolution images of
Mars and also used a laser altimeter to map its topography.
The following maps were made from the laser altimeter data,
using a color scale where blue denotes low regions, green
regions of intermediate (average) altitude, and as the altitude
increases, the colors go from yellow to red to white.
These color choices make the lowest regions look like oceans and
lakes, but of course there is now no liquid water on the surface.
The following black and white images are less suggestive but
equally detailed. Like maps of the earth, they give a flat
representation that distorts regions near the poles.
7
Global view
of Mars,
with the
Hellas
impact
basin just
below and
to the left
of center.
Global view
of Mars,
with Utopia
Planitia
near the
center.
Color
shows
t
topography
h
based upon
laser
altimeter
data.
Color
shows
topography
based upon
laser
altimeter
data.
Here laser altimeter data are combined with color images.
Map of the Earth, drawn for comparison with Venus and Mars
(from Sky & Telescope, Feb., 1982)
8
Map of Venus, drawn for comparison with Earth and Mars
Map of the Mars, drawn for comparison with Venus and Earth
(from Sky & Telescope, Feb., 1982)
(from Sky & Telescope, Feb., 1982)
The Tharsis bulge, centered on the equator, is 5000 km across
and 10 km high.
The smaller Elysium bulge is 2000 km across and 4 km high.
Both are the sites of large shield volcanoes (like the volcanoes of
Hawaii, with large, broad bases from the eruption of fluid,
basaltic lava).
The lack of motion of crustal plates (plate tectonics) on Mars has
kept these volcanoes active in the same sites for long periods,
accounting for their immense size.
The small number of impact craters on the large Martian
volcanoes indicates that they may have gone dormant about
1 billion years ago.
The most recent thinking is that these bulges are the result of the
accumulation over long periods of time of many layers of
volcanic rocks.
However, evidence from meteorites that came from Mars shows
that
h fresh
f h volcanic
l i lavas
l
were produced
d d muchh more recently.
l
This is supported by crater counts on some lava flows imaged by
the Mars Global Surveyor.
The largest volcano, Olympus Mons, is 550 km across at the
base and is 27 km high. In contrast, Mauna Loa in Hawaii is
120 km across and rises 9 km above the ocean floor.
On the earth, motion of the Pacific plate carries the Hawaiian
volcanoes away from their lava sources, and causes them to go
extinct within a few hundred thousand years.
Olympus Mons,
Mars’ largest
volcano,
viewed by the
Mars Global
Surveyor.
This volcano, the
largest
g in the solar
system, measures
550 km across and
is 27 km high.
Bluish water ice
clouds are visible
in the shadow at
the right.
Mariner-9’s first image
g of Mars,,
at the left, revealed 4 spots poking up
through a layer of dust from a global
dust storm.
As the dust eventually cleared, these
were revealed to be giant volcanoes.
The largest of these, Olympus Mons,
is shown in the image above.
9
Mars’ largest volcano, Olympus Mons
The Tharsis bulge has stressed the Martian crust, giving rise to a
vast system of roughly radial fractures, indicating tensional
deformation, and to an array of circumferentially oriented
wrinkle ridges, thought to have formed from compression.
Along a series of the radial faults are huge canyons which show
evidence of erosion from landslides and development of side
canyons cut into the adjacent plateau.
The largest of these canyons is the Valles Marineris.
Topography of the Tharsis region of Mars.
Mosaic
of the
northern
hemisphere
of Mars.
Note the size of the
Vallis Marineris
relative to the
planet.
Mars’ Valles Marineris,
a depression 600 km across and, in places, 6 km deep
(from Planets, p. 95)
10
Evidence for the action of liquid water on Mars:
Liquid water is fundamental to life as we know it.
Therefore, the presence of liquid water on Mars, either now or in
the past, is an essential prerequisite for life on Mars, either now
or in the past.
The first pictures of Mars, from the Mariner-4 spacecraft in 1965
dashed the hopes of many that Mars could support life
life.
One of the
21 images
of the
Martian
surface
sent back
to earth by
Mariner-4
in July,
July
1965.
The Mariner-4 spacecraft flew by Mars in July, 1965,
providing the first close-up views of Mars in 21 images it sent back.
(from Mars, p. 55)
(from Mars, p. 55)
Hopes for
life on
Mars sank
as a result
of these
moon-like
scenes.
11