Transcript Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
Mars Facts and Figures
• Mars orbits the Sun at a distance of 228 million km, or 1.52 AU.
 Can you calculate it orbital period in Earth years? Use Kepler’s
third law?
• Mars is a medium-sized terrestrial planet. With a diameter of 6787 km,
Mars is half the size of the Earth, and falls in between the larger Earth
and Venus and the smaller Moon and Mercury.
• Its mass is 11% of the Earth’s mass, or nine times as much as the
Moon.
 Can you calculate the density?
• (Answers: 1.88 Earth Years, 3.9 g/cm3)
Figure credit: Albert T Hsui, Univ. Ill
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Lecture 16:
Mars I
Mars historical perspective
Picture credit: NASA/JPL - Viking
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Overview
• Mars certainly has the most magnetic appeal for most Earthlings, due
to countless fictionalizations in novels, comic books, TV shows, radio
dramatizations, and of course, movies.
• As with the Earth, a single lecture barely begins to scratch the surface
of what we now know about Mars.
• In this lecture we will concentrate on:
• Exploration of Mars, from the 19th century
canal-watchers, to 21st century rovers.
• The major terrain types on Mars.
• Volcanism and tectonics; soil and rock.
• Atmosphere.
Picture credit: NASA
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Telescopic Observations Of Mars
• As seen by most telescopes from
the Earth, Mars is an orange-red
orb, with some darker patches and
bright polar caps normally visible.
• Seen through the Hubble Space
Telescope (HST) – right – the
shapes of the major terrains and
the largest geographical features
begin to appear.
• (Compare to the Viking image on
the first slide).
• In the 19th century however, this
level of detail would not have been
visible.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
The Mars Saga: 1877-1920s
• The Mars controversy began in 1877 with the
observations and notes of the Italian astronomer
Giovanni Schiaparelli (1835-1910).
• Schiaparelli claimed sightings of faint dark linear
markings on the surface, which he referred to as
‘canali’, the Italian word for ‘channels’.
• In English translation of course, these quickly
became ‘canals’, with all the consequent
connotations of industry by intelligent beings.
• The most famous proponent of the Martian canals
was the American Percival Lowell (1855-1916), who
took up astronomy after reading Schiaparelli, and
founded an observatory in Flagstaff, AZ to study the
canals.
Picture credits: SPL/Photo Researchers. Lowell Observatory
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Lowell’s Contribution
• Lowell, with his 24 in refractor
was able to see a great many
canals, intersecting at junctions he
referred to as ‘oases’.
• Lowell published 3 books of
drawings describing the canals,
and even made similar claims
regarding Venus!
• Many astronomers doubted
Lowell however: most could not
see the canals. Perhaps a warning
sign was that observers using
smaller telescopes were better able
to see the features.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
No canals, but…
• We now attribute Lowell’s sightings to a trick of the mind,
connecting unrelated points together by lines, like a
picture outline of dots - remember what constellations
are?
• In the case of Venus, a recent theory is that Lowell’s
particular telescope settings acted as an ophthalmoscope,
allowing him to see the radial pattern of blood vessels in
his own retina, backlit by the bright Venus.
• However, as the 20th century progressed, darker areas
on Mars were definitely seen to change shape over the
Martian year: proof some said of seasonal growth of
vegetation.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Early Spacecraft visits
• Of the over 40 attempts to send spacecraft to Mars, starting in 1960, only
around a third have had real success. Many early missions failed before
leaving the Earth, as their rockets exploded or didn’t ignite.
• Mariner 4 (1965) was the first successful US attempt, sending back the
blurry TV pictures of craters (rather than lush vegetation), and determining
the surface pressure of the atmosphere to be around 0.01 bar.
Picture credits: NASA/NSSDC
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Mariner 9
• After Mariner’s 4, 6 and 7 which saw
only craters, the prevailing view of Mars
was of a geologically inactive, heavily
cratered world like the Moon. By pure
bad luck, the most interesting features
had been entirely missed!
• When Mariner 9 arrived at Mars in
1971, the planet was encompassed in
one of its trademark global dust storms.
However, as the dust storm finally
subsided, four giant ‘craters’ began to
emerge. Soon, it became clear that
these were not surface impact features,
but calderas on top of immense
mountains: the first volcanoes
discovered outside the Earth.
Picture credits: NASA/NSSDC
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Vikings 1 & 2
• One of the most ambitious and costly ($1bn) unmanned planetary
missions ever was also one of the most successful.
• The dual Viking orbiters/landers (2 of each) arrived at Mars in 1976.
• The huge landers (600 kg each, and the size of a subcompact car)
contained entire weather stations which remained active for 6 years
(Viking 1) and 4 years (Viking 2), much longer than designed for.
Picture credits: NASA/NSSDC
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Mars Density and Composition
• The density is 3.9 g/cm3, or 3.8 g/cm3 uncompressed. Compare this to
the Earth (4.5 g/cm3) and we expect a lower ratio of iron to silicates
(rocks).
• We do believe Mars has a
core of FeS (iron sulfide),
with a diameter 40% of
Mars: a similar proportion to
the Earth’s core.
• However, the lower
density of FeS compared to
the Earth’s Fe and Ni leads
to a lower overall density.
Figure credit: Albert T Hsui, Univ. Ill
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Interior and Core
• Our calculations predict that the core is solid, not liquid, so we do not
expect a strong magnetic field, which requires a spinning liquid core.
• However, magnetometers have discovered a
weak magnetic field over certain regions of the
planet.
• We guess that Mars did in fact have a liquid
core and magnetic dynamo in the past, and that
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 of course differentiated, with a mantle
and crust: we do not know much about them for
certain.
Figure: PSRD Hawaii, Brook Bays
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Quick Tour 1: Olympus Mons
• Olympus Mons (Mount Olympus) is the largest of the four great
volcanoes, seen by Mariner 9.
• Aptly named after the mythological seat of the Greek gods, Olympus
Mons is the largest volcano in the entire solar system.
• Olympus Mons is nearly 27 km high and 700 km wide at the base!
Figure credit: NASA
Dr Conor Nixon
Fall 2006
ASTR 330: The Solar System
Quick Tour 2:
Valles Marineris
• In a rare event, the
giant canyon system
discovered by
Mariner 9 was
named after the
spacecraft!
• The ‘Mariner
Valleys’ stretch more
than 4000 km in
length, 500 km wide,
and up to 8 km deep:
this would swallow
up the Grand
Canyon many times
over.
Figure credit: NASA/USGS
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Quick Tour 3: Hellas
• Hellas is the largest impact basin on Mars, about 2000 km across and 5
km below the average Martian surface level. Hellas often collects clouds in
its interior.
• Hellas was produced by … you’ve guessed it, a giant impact during the
Late Heavy Bombardment stage of the solar system formation, 3.9 Gyr ago.
• Hellas has a
relatively simple
form, with a
single rim of
mountains, parts
of which are
missing, eroded.
Figure credits: (left) NASA/JPL (right) MGS/MOLA
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Scale of Terrain on Mars
• We have just seen that topography on Mars can be on a huge scale.
• The figure below compares Olympus Mons with Everest (fold mountain)
and Mauna Loa (shield volcano,wrongly labeled) on Earth.
• Calculations show that on Earth, and Venus, mountains can only rise 1015 km before the rock begins to deform under its own weight.
• Now can you guess
why mountains on
Mars can get so big?
• Answer: the Martian
gravity is only 2/5
(40%) that of the
Earth.
Figure credit: Universiity of North Dakota
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Mars: North and South
• The northern and southern hemispheres of Mars are very different:
• Elevation: the north is much lower than the south, by about 6 km.
• Roughness: the northern hemisphere is fairly flat and smooth: the
southern hemisphere is rougher.
• Color: the southern hemisphere is darker.
• Cratering: the southern hemisphere is more cratered, probably older.
• If we believe that the southern hemisphere accurately reflects older,
original terrain, then what happened to lower the north so much? And how
did the north become flatter?
• This is one of the greatest riddles of Mars.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Craters on Mars
• Craters on Mars look much like the
ones on airless planets, with raised
and terraced rims, flat floors, and
central peaks. However, the ejecta
patterns are quite different from the
lunar variety. Lunar craters have a
rough, hilly blanket close to the rim,
surrounded by radial streaks.
• Craters on Mars however display a more fluid
ejecta pattern, such as the ‘flower’ form (crater
Yuty, 18 km, left) at lower latitudes, or the
‘pancake’ form (crater Arandas, 28 km, above)
closer to the poles. The explanation is that the
Martian ejecta flowed along the surface rather
than being flung through the air, probably due
to melting of crustal ice.
Figure credit: NASA ARC/CMEX
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Argyre
• At the opposite end of the crater
size scale, we have huge impact
basins such as Hellas and
Argyre.
• Argyre is some 700 km wide,
including a 300-km wide smooth
central plain surrounded by a 200km thick rugged rim.
• This picture is a cleverly shaded
altimetry map, not a real image.
• Note the Uzboi valley entering
or exiting to the north (top):
evidence of ancient inflow or
outflow.
Figure credit: MOLA Science Team and G. Shirah, NASA GSFC
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Craters, Basins and Ages
• Mars has many more craters than the Earth or Venus, so the highland
terrain is fairly old: at least 3.6 Gyr.
• Mars has fewer of the large impact basins than the Moon, despite its
larger surface area. This is a fact we should explain.
• Our theories suggest that the crust of Mars probably stabilized
(geologically) later than the Lunar surface.
• Mars is larger then the Moon and therefore took longer to cool, and
so remained geologically active until near the end of the Late Heavy
Bombardment.
• This allowed some of the basins which had formed before that time to
be erased.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Martian Timeline from Crater Counts
• Craters on Mars were caused by a similar population of impactors as
craters on the Moon.
• By counting craters on Mars and comparing the numbers to the areal
density of lunar craters, we can derive the age of various terrains.
• We must
remember to take
account of some
factors which may
differ between
planets, such as
gravity, which
affects the number
and speeds of
impactors.
Feature
Olympus Mons
Arsia Mons
Tharsis Plains
Elyisum Plains
Chryse Planitia
Alba Mons
Hellas basin
Cratered Uplands
Table: Morrison and Owen
Crater density Crater retention
relative to
age (billion
Lunar Maria
years)
0.1
0.1
0.5
0.7
1.1
1.8
1.8
10.0
0.2
0.2
1.6
2.6
3.2
3.5
3.5
4.0
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
The Volcanoes
• The three central Tharsis volcanoes are each about 400 km across
and 25 km in height. Even more modest shield volcanoes elsewhere
are 100 km across.
• Each volcano has a caldera,
formed when the magma retreats
and the peak partially collapses.
• The caldera of Olympus Mons
(left) is 80 km across, shows multiple
episodes of collapse, but no erosion.
• Note the lava flow channels.
Animation of Tharsis Caldera
Figure credit: ESA/DLR/FU Berlin (Neukum)
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
The Volcanoes (contd.)
• The shield volcanoes are predicted to be basaltic, like
those in Hawaii.
• On the flanks of the volcanoes, lava channels about
100m in width carve the shallow broad slopes.
• The slope of 4° indicates a low viscosity and a large
volume of outflow. However we do not see lava rivers as
on Venus.
• Some volcanoes have steeper sides, indicating more
viscous outflow, but we do not see the ‘pancake domes’ of
Venus.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Tharsis Bulge
• Tharsis is a massive uplifted region the size of North America, right
between the northern plains and southern uplands.
• The Tharsis area bulges 10 km above its
surroundings (figure right) and is one of the
least cratered (youngest) terrains on Mars.
• In Tharsis are
3 of the 4 great
volcanoes, and
also the Valles
Marineris.
• Olympus
Mons is offset
on the NW
slope.
Figure credit: NGDC/USGS
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Tectonics and Tharsis
• What is the cause of the Tharsis bulge?
• Tharsis appears to be due to two reasons: firstly, an actual bulge of the
crust due to a mantle plume, and second, a build-up of layers of lava.
• How do we know? Extensive fracturing of the crust occurs in a radial
pattern like spokes of a wheel. The fractures can be 100s of km in length
and several km in width.
• We believe that the uplift began about 3 Gyr ago, and continued to 1 Gyr
ago, before the formation of the actual volcanoes.
• On Earth and Venus, compressional forces produce uplifted terrain, such
as Tibet and Lakshmi, with high mountains. On Mars however, the highest
features are volcanic, not compressional in origin.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Canyons On Mars
• What is the origin of the great canyon systems on Mars, including the
Valles Marineris? Were they carved by rivers like the Grand Canyon?
• These features are in fact tectonic in origin: originally huge cracks in the
crust, which were later widened and shaped by erosion.
Figure credit: NASA/JPL. Viking mosaic of Western Candor Chasma
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Canyon Widening
• This image shows a closer view of the white box region on the previous
slide. Here, the edges of the canyons, which appear similar to landslides,
are clearly visible.
• We believe that
landslides, possibly
lubricated by undercutting
water springs or melted
ice, took the main role in
widening the canyons.
• But where has the
material gone to? Possible
explanations include dust
removal by wind, and ice
itself which has run off and
perhaps evaporated.
Figure credit: NASA/JPL. Viking image of
Western Candor Chasma
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Channels
• The Martian channels are quite distinct from the canyons. The runoff
channels are positively caused by running water.
• The channels caused great
excitement when discovered: nowhere else in the solar system
other than the Earth has
evidence for running water been
found.
• This Viking orbiter image shows
an area between the Lunae
Planum and Chryse Planitia, just
west of the Viking 1 lander site.
The image is 300 km across. The
channels here are due to outflow.
Image credit: NASA/JPL
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Channel Morphology and Origin
•
The water channels can be split into three types:
1. Runoff channels
2. Outflow channels.
3. Gullies.
• Runoff channels are similar to terrestrial dry river beds, found only in the
cratered uplands of the southern hemisphere.
• They are often seen on the steep sides of crater walls, and are 10s to
100s of meters wide and 10s of km long.
• The runoff channels are old, as old as the cratered highlands, putting
their age around 4 Gyr. This is long before the formation of the Tharsis
bulge or northern plains.
• Clearly, the conditions for liquid water to exist on the surface have long
since passed. Mars must have had a thicker, warmer atmosphere in the
past.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Outflow and Floods
• The outflow channels are much larger and less common than the runoff
channels, and are found in the equatorial regions. Such channels are at
least 10 km wide and 100s of km long.
• We believe these channels to have been caused by intermittent or
periodic flooding. A mechanism could be the breaking of an ice dam,
holding back a lake, with catastrophic results.
• Characteristic features of the outflow terrain
includes teardrop islands, seen in the Viking
image (right), terraced walls, and sandbars.
• These islands were carved by the flood of
water rushing over original plateau terrain,
descending from the uplands into the Chryse
basin.
Image credit: NASA/JPL
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Chaos Terrain
• Chaos is the name given to the jumbled mixture of hills and valleys we
see in certain parts of the cratered uplands.
• This image of the
Iani Chaos comes
from the HRSC on
ESA Mars Express.
• It lies east of the
Valles Marineris,
and is composed of
mesas 1 to 8 km
across and up to a
km high.
Image credit: ESA/DLR/FU Berlin
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Chaos and Floods
• We believe the chaos terrains are associated with the major
floods and outflow channels, about 3.5 Gyr ago, after the runoff
channels had been created, and about the same time as the
Tharsis uplift.
•
Underground water was probably the source of the floods, but
what was the exact mechanism? Three have been proposed:
1. Melting of sub-surface ice by volcanic activity.
2. Chemical release of water bound to the Martian soil.
3. Movement of liquid water, due to the uplifting of Tharsis.
• We do not currently know which was the culprit: perhaps all three
were involved.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Gullies
• Martian gullies are the
best evidence we have for
liquid water on Mars today.
• These fresh-cut features
are found on the inner walls
of some craters and the old
runoff channels.
• For water to produce
these features, it must have
been released in a torrent,
for a slow trickle will not
suffice.
• This MGS/MOC image
shows recent gullies in a
crater wall.
Image credit: NASA/JPL/MSSS
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Gullies: explanation
• The gullies are found only at high latitudes, in the shaded walls of
craters and channels, some of the coldest places on Mars. Why?
• Our theory starts with the idea that there is permafrost under the
surface at the high latitudes.
• Now imagine that a hot magma plume approaches from below. The
permafrost is melted, perhaps even causing steam.
• If the water cannot reach the surface, due to over-lying rock layers, it
will try to escape sideways.
• Some will probably re-freeze as it nears the channel or crater wall, and
will slowly build up into an ice plug.
• Eventually, the ice plug could be released explosively as the pressure
gets too much, and the liberated water will gush out.
Dr Conor Nixon Fall 2004
ASTR 330: The Solar System
Quiz-Summary
1. Are there canals on Mars? How did the idea start, and how was the
issue resolved?
2. What features were discovered by Mariner 9?
3. What was the greatest accomplishment of the Viking missions?
4. Does Mars have any large impact basins?
5. Describe the main geologic features of the Tharsis uplift. How was
Tharsis produced?
6. Compare Martian volcanoes to terrestrial and Venusian mountains.
7. Are the Valles Marineris on Mars bigger versions of the Earth’s Grand
Canyon?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Quiz-Summary
8. Are craters on Mars the same as those on the Moon and Mercury? If
not, what differences are there?
9. What differences are there between the northern and southern
hemispheres on Mars?
10. What types of rocks were found on Mars. Is this what we expected?
11. What three types of channels are found on Mars, and what caused
them?
12. Is there liquid water on the surface of Mars today?
13.Compare the Martian atmosphere to that of (i) Venus (ii) the Earth.
14. What missions are currently underway to explore Mars?
Dr Conor Nixon Fall 2006