Download Terrestrial Planets II

Online Astronomy 100!
Lecture 16: Terrestrial Planets II
NASA
1
The Terrestrial Planets, Part 2
Today, we will continue our discussion of the terrestrial planets, looking at the geological histories
ASTR 100
Lecture 16
‣ Topics
for Today:
‣ Histories
of the Terrestrials: Geological activity
2
The Terrestrials (& the Moon)
Mercury
Mars
Venus
Earth
NASA/Wikimedia Commons
Moon
Why have the terrestrial planets turned
out so differently, when they formed at
the same time from the same materials?
3
Earth’s Moon
‣ A small,
barren,
airless world
‣ 25%
Earth’s diameter
‣ 1.2% Earth’s mass
‣ Not geologically active
today
‣ Heavily
cratered
surface
of years old
‣ But, not all parts are
equally cratered
Luc Viatour / www.Lucnix.be
‣ Billions
4
The mass of the Moon is about 1/80th that of the Earth, and its diameter is about 1/4th that of the Earth. The orbit of the Moon
is very nearly circular (eccentricity ~ 0.05) with a mean separation from the Earth of about 384,000 km, which is about 30 Earth
diameters.
Since the moon rotates once per orbit, Lunar day and Lunar night are each about 15 Earth days long. During the Lunar night the
temperature drops to around -113℃, while during the Lunar day the temperature reaches 100℃. The temperature changes are
very rapid since there is no atmosphere or surface water to store heat.
One striking difference between the Lunar surface material and that of Earth concerns the most common kinds of rocks. On the
Earth, the most common rocks are sedimentary, because of atmospheric and water erosion of the surface. On the Moon there is
no atmosphere to speak of and little or no water, and the most common kind of rock is igneous ("fire-formed rocks").
The Moon’s Defining Feature: Impact Craters
‣ Everywhere
on the moon, you will find craters:
giant basins a few hundred km in size...
‣ To microscopic pits in moon rocks!
NASA
NASA
‣ From
Tycho crater,
86 km wide
Tiny microcrater
on a moon rock
5
Because the Moon has no atmosphere, even the smallest impactor can create a crater in the surface rocks.
Lunar regolith
‣ The
Moon is covered
with a gently rolling
layer of powdery soil
with scattered rocks
that is called the
regolith
‣ It is made from debris
blasted out of the
Lunar craters by the
meteor impacts that
created them
Apollo 11 Astronaut
Buzz Aldrin's bootprint
6
No weather or wind, so these footprints will persist for some time.
Lunar Maria & Highlands
‣ Two
types of regions
on the Moon
‣ Highlands
‣ Lighter-colored
hilly
regions
‣ Covered in craters
‣ Maria
smooth, lowlying plains
‣ Few craters
‣ Cover about 17% of the
Moon’s surface
NASA
‣ Darker,
Note the contrast in crater
abundance between the
highlands and maria
7
The Moon’s surface can be divided into light areas called the Lunar Highlands and darker areas called Maria (literally, "seas"; the
singular is Mare). The Maria are lower in altitude than the Highlands, but there are no bodies of water on the Moon so they are
not literally seas.
The maria cover about 17% of the lunar surface, and are found mostly on the near side of the Moon. The dark material filling the
Maria is actually basalt rock - dark, solidified lava - from earlier periods of Lunar volcanism. Both the Maria and the Highlands
exhibit large craters that are the result of meteor impacts. There are many more such impact craters in the Highlands.
The Highlands rocks are largely Anorthosite, which is a kind of igneous rock that forms when lava cools more slowly than in the
case of basalts. This implies that the rocks of the Maria and Highlands cooled at different rates from the molten state and so
were formed under different conditions.
Thought QuesLon
The fact that there are fewer craters on the
lunar maria than on the lunar highlands
indicates that the
A.
B.
C.
D.
highlands are younger than the maria.
craters formed as a result of volcanic eruptions.
maria formed more recently.
maria are of volcanic origin.
8
Answer C
The maria have relatively few craters compared to the highlands, must be younger! (about 3 billion years old)
How did the maria form?
‣ ~4
billion years ago,
large (tens of km
across) objects struck
the lunar surface
‣ Formed giant basins
hundreds of km in
diameter
‣ Up to 10 km deep into Moon’s mantle!
‣ Molten material from
young Moon’s interior
flooded craters
JAXA
Illustration of mare
formation
9
At the end of the heavy bombardment, the largest impacts formed giant basins hundreds of kilometers in diameter. After about
3.8 billion years ago, the cratering rate fell rapidly to the current low rate. The tremendous impacts cracked the crust as deep as
10 kilometers and led to flooding by lava. That formed the maria—from 3.8 to 3.2 billion years ago. Studies of the Apollo
moon rocks show that the basins were flooded by lava flows of dark basalts.
The maria are lava-filled impact basins!
Mercury
is also a
barren airless world
‣ Surface similar to the
Moon’s
NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
‣ Mercury
‣ Heavily
cratered
‣ With regions of smooth
plains
‣ Mercury
is small
‣ Cooled
rapidly
‣ Little geological activity
Image of Mercury taken
by MESSENGER
10
40% Earth’s diameter, 5.5% the mass of the Earth. Orbits the Sun at 0.4 AU, it has the most eccentric orbit of the 8 planets,
e=0.21. Orbits the Sun in 88 Earth days. Each solar day lasts 176 Earth days. Lack of an atmosphere and long days means
daytime temperatures reach a balmy 700 K (430 C), nighttime temperatures plunge to 100 K (-170 C).
Mercury’s “Cracked” Surface
exhibits long cliffs called scarps
‣ Many scarps are over 1 km high!
Scarp
110 km
NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
‣ Mercury
Extending from the upper left edge of this image diagonally
toward the lower right corner is a long scarp (cliff) face.
11
This scarp runs through a large ancient crater right of center in the frame and was seen for the first time during MESSENGER's
second Mercury flyby. Planetary geologists use the Latin term "rupes" for scarps on Mercury. Scarps such as this one have been
identified all over the planet.
Note: The large crater crosscut by the scarp is approximately 110 kilometers (70 miles) in diameter.
How did Mercury’s scarps form?
‣ Mercury
has large
core for a small planet
of its volume!
‣ Compared to 17% for
Earth’s core
Mercury’s
iron core
shrinks
‣ 42%
‣ As
Mercury cooled, its
core shrunk by 20 km
‣ Mercury’s crust
“wrinkled” to
compress down onto
the smaller interior
NASA
Compression of the crust
creates a scarp
NASA/Arizona State University
12
Mercury has a very large iron core for such a small planet, 42% of its internal volume - Earth’s is only 17% of its volume. The
large metal core cooled fast (little insulating rock to hold in the heat). The giant scarps are believed to have formed when
Mercury's interior cooled and the entire planet contracted slightly as a result, causing the surface rocks to fracture and some
blocks of crust to thrust over others along great faults.
What is the geological history of Venus?
‣ Venus
is a large
terrestrial planet
‣ 95%
the diameter of
Earth
‣ We
would expect
similar levels of
geological activity
Craters?
‣ Volcanism?
‣ Tectonics?
‣ Erosion?
NASA
‣ Impact
A radar image of
Venus’ surface from the
Magellan spacecraft
13
Similar in structure and size to Earth, Venus' thick, toxic atmosphere traps heat at the Venusian surface. The scorched world has
temperatures hot enough to melt lead.
Glimpses below the clouds reveal volcanoes and deformed mountains. Venus spins slowly in the opposite direction of most
planets.
Impact craters on Venus
‣ Few
craters
‣ Only
~900 identified
‣ Evenly spread across
surface
‣ No
small craters
(under 2 km across)
atmosphere
burns up smaller
meteoroids
‣ Craters
are “rough”
‣ Rarely
lava-filled
‣ No erosion
NASA
‣ Dense
Danilova, Aglaonice
and Saskja craters
on Venus
14
Low crater count indicates Venus has certainly been geologically active in the recent past.
There are no craters smaller than 2 km, because of the effects of the dense atmosphere on incoming objects.
On Venus, about 85% of craters are in pristine condition - no erosion!
Venus Crater Map
USGS
Even crater distribuLon = global resurfacing event?
Venus' craters are evenly scattered around the
planet, indicating the surface is all about the
same age (~500 million years old)
15
Venus has a surprisingly even crater distribution. No regions are significantly more or less heavily cratered than others.
Compare this to craters on the Moon - the highlands are much more heavily cratered than the maria. We interpret the Moon’s
crater densities to indicate that the highlands are older than the maria. This says, in effect, that the present Venusian surface is
nearly uniformly young geologically.
This rather surprising outcome needs explanation., but that explanation is still being debated. Various models have been
proposed. Most 1) reject any plate tectonics activity, and 2) hold that volcanism has been active during this time period and
hence the surface has been largely repaved.
Venusian Volcanism
1,600 major
volcanoes on Venus
‣ Most are probably long
extinct
‣ 80% of surface is
covered in lava flows
‣ Clouds prevent direct
observation of volcanic
eruptions
‣ There is circumstantial
evidence for ongoing
volcanism
NASA
‣ Over
Venus's highest
volcano Maat Mons
16
The Magellan probe revealed evidence for comparatively recent volcanic activity at Venus's highest volcano Maat Mons (8 km
high), in the form of ash flows near the summit and on the northern flank.
Sulfur dioxide is put into the atmosphere via volcanism on the Earth. It is possible that large observed fluctuations in sulfur
dioxide in Venus’ atmosphere are due to current volcanism on Venus
The Pioneer and Venera probes detected bursts of radio emission in the atmosphere of Venus similar to types seen from
lightning strikes around the plumes of Earth’s volcanoes.
USGS
‣ Distribution
Smithsonian Institution
Venus Volcano Map
of
volcanoes around
Venus’ surface is
random
‣ Earth’s volcanoes
are concentrated
along plate
boundaries
‣ Indicates that
Venus’ crust lacks
plate tectonics
Earth Volcano Map
17
Distribution of volcanoes around Venus’ surface is random, unlike Earth, where volcanic activity is concentrated on plate
boundaries - indicates that Venus’ crust lacks plates.
Venus has tectonics, but not plate tectonics.
Erosion on Venus?
‣ Venus’
surface is too
hot for liquid water or
ice
‣ At the surface, wind is
only a slow breeze no gusts or storms
‣ Photos of rocks taken
by landers sent to
Venus show no
evidence of erosion
Photo taken by the Venera-13
lander on Venus in 1981 and
reprocessed by Don P. Mitchell.
18
Thought QuesLon
‣ The
maps below show representative regions
of three terrestrial worlds. Which world’s
surface is uniformly young, like Venus?
University of Nebraska Lincoln, Astronomy Education Group
19
Answer A
A has the least number of craters.
What are the major geological features of Mars?
Image Credit: National Geographic Society, MOLA Science Team, MSS, JPL, NASA
The Martian terrain includes broad towering volcanoes,
vast windswept plains, and enormous canyons.
20
Mars’ two-­‐faced geology
Southern hemisphere is heavily cratered highlands,
Northern hemisphere is volcanic lowland plains
21
Martian geology: Mars’ surface has two distinct regions. Southern region is mostly heavily cratered highlands. Northern region
is largely volcanic lowland plains, with several large volcanoes. Altitude relief map shows that the southern hemisphere is
generally higher elevation than the north and shows the extensive cratering in the south.
Image Courtesy NASA/JPL-Caltech
Image Credit: National Geographic Society, MOLA Science Team, MSS, JPL, NASA
Olympus Mons is the largest volcano in the Solar System.
As big as Arizona and three times taller than Everest!
22
Olympus Mons - a huge volcano. About 25km high, 624 km in diameter (for comparison, the outline of Arizona is shown). For
comparison: Mauna Loa (largest volcano on Earth) - 10 km high and 120 km across.
The volume of Olympus Mons is about 100 times larger than that of Mauna Loa. But, there are impact craters on the side of
Olympus Mons. Indicates that Olympus Mons is probably extinct.
Image Courtesy NASA/JPL-Caltech
Image Credit: National Geographic Society, MOLA Science Team, MSS, JPL, NASA
Valles Marineris is a vast canyon stretching over about
one-fifth the circumference of Mars!
23
Valles Marineris is a vast canyon stretching over about one-fifth the circumference of Mars! It is 10 times longer, 5 times
deeper, and 20 times wider than the Grand Canyon. Comparing Valles Marineris to North America. it would stretch from New
York City to Los Angeles, and it is nearly as deep as Mount Everest is high!
Valles Marineris is not a river canyon, although it does show some water erosion features (so water likely flowed through it, or
parts of it, at one time). The rising of the Tharsis Bulge, a volcanic rise with three towering volcanoes, at the western end of the
canyon (left in the image) opened a crack in Mars’ crust! Therefore Valles Marineris is a tectonic feature.
Mars shows no evidence of plate tectonics.
Thought QuesLon
Which hemisphere of Mars has an older
surface?
A. The southern hemisphere
B. The northern hemisphere
24
Answer A
How do we know? Higher crater density in the southern hemisphere.
A dried drainage
channel on
Semeykin Crater
NASA/JPL/ASU
NASA/JPL/ASU
Did liquid water once flow on Mars’ surface?
Water erosion
features in Ares Vallis
25
Left images: This impressive drainage system empties into Semeykin Crater on the northern margin of Arabia Terra.
Right image: Impacts, floods, and tectonic forces have all left traces on Ares Vallis, a Martian outflow channel. Outflow channels
formed when massive floods of water poured out of the ground early in Martian history when the planet likely had a thicker
atmosphere and a warmer climate. Scientists estimate the floods had peak volumes many times today's Mississippi River. Teardrop mesas extend like pennants behind impact craters, where the raised rocky rims diverted the floods and protected the
ground from erosion. While Ares Vallis is not the widest or deepest outflow channel, it extends for nearly 1,600 kilometers (1,000
miles).
Mars Global Surveyor found hemaLte on Mars
iron mineral
hematite lies on the
surface of parts of the
Meridiani Planum
region of Mars
‣ Hematite often forms
in the presence of
liquid water
‣ Discovery led to the
Opportunity rover
being sent here
NASA/JPL
‣ The
Hematite abundances
on the Meridiani Planum
26
The iron mineral hematite lies on the surface of parts of Meridiani Planum. Mapped from orbit by the TES instrument on Mars
Global Surveyor, hematite abundances range from 5% (blue) to 20% (red). Because hematite needs liquid water to form, its
presence indicates Meridiani has seen much liquid water in the past. The background image is a THEMIS daytime infrared
mosaic.
Image credit: NASA/JPL/Cornell
Credit: Brenda Beitler, University of Utah
Rovers found hemaLte “blueberries”
"Blueberries": Hematite concretions found on the
Martian surface at Meridiani Planum (left).
Hematite marbles found in Utah (right).
27
Marble-shaped pebbles ("Blueberries", Fig. 1b) were also discovered on Mars by the Mars Exploration Rover Opportunity.
Hematite (Fe2O3) is a mineral that in most cases requires water for its formation. Therefore hematite indicates wetter and warmer
conditions on Mars in time of hematite formation.
Image credit: NASA/JPL/Cornell
Sediments at Erebus Crater
28
A rover view of a stack of fine layers exposed on a ledge in "Erebus Crater" shows a diverse range of primary and secondary
sedimentary textures formed billions of years ago. These structures likely result from an interplay between windblown and
water-involved processes.
Image credit: NASA/JPL/Cornell
Bright Salty Soil
‣ The
rover Spirit's wheels have churned up
light-toned, subsurface soil deposits on Mars
‣ Sulfur-rich salts - possibly left behind by water
evaporation
29
Spirit’s right front wheel became stuck, which slowed its movements, but also churned up the largest amount of bright soil
discovered so far in the mission. Spirit's instruments confirmed that those soils had a salty chemistry dominated by ironbearing sulfates. The salts may record the past presence of water, as they are easily mobilized and concentrated in liquid
solution.
What have we learned?
‣ What
geological processes shaped our Moon?
‣ Early
cratering still present
‣ Maria resulted from volcanism
‣ What
geological processes shaped Mercury?
‣ Cratering
and volcanism similar to Moon
‣ Tectonic features indicate early shrinkage
‣ What
are the major geological features of
Venus?
‣ Venus
has few craters - young surface
‣ Dominated by volcanism
‣ Also has tectonics but little or no erosion
30
What have we learned?
‣ What
are the major geological features of
Mars?
‣ Differences
in cratering across surface
‣ Giant shield volcanoes
‣ Evidence of tectonic activity
‣ What
geological evidence tells us that water
once flowed on Mars?
‣ Features
that look like dry riverbeds
‣ Some craters appear to be eroded
‣ Rovers have found rocks that appear to have
formed in water
31