Download Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
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Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Lecture 15:
Venus
Picture: from The Nine Planets, Bill Arnett
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Twin Planets?
• Venus and Earth are very
nearly the same size and
density.
Diameter (km)
Venus
Earth
12,104
12,756
• Venus’s shorter orbital
period is in line with its
smaller orbit about the Sun.
Density (g/cm 3 )
5.5
5.3
Semi-major axis (AU)
0.72
1.00
• However, Venus’s rotation
period on its axis is 243 Earth
days: longer than its year of
225 Earth days!
Orbital Period (days)
224.70
365.26
Rotation Period (days)
243r
1
Albedo
0.75
0.3
• Also, Venus rotates in a retrograde direction, opposite to its orbital motion.
• We will encounter many other differences: temperature, volcanism,
atmospheric composition.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Albedo and Atmosphere
• Venus is perpetually shrouded in a thick blanket of white clouds,
obscuring the surface, and confounding early efforts to determine its
rate of rotation.
• The clouds reflect 75% of incident solar radiation, much more than the
Earth’s 30% or the Moon’s 11% albedo.
• The temperature of the cloud tops was measured in the 1930s to be
230 K, similar to the stratosphere of the Earth.
• These were assumed to be water clouds, like the Earth.
• Carbon dioxide was the only gas detected until the 1960s, and it
seemed plausible that a lot of nitrogen was also present but
undetected.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Taking The Temperature
• Venus is closer to the Sun, but its clouds also reflect more than twice
as much radiation as those of the Earth. Heat-balance calculations
indicated that Venus should be at about 280 K.
• However, radio astronomers in
1958 found that Venus was
radiating radio waves as
strongly as a much hotter body:
600 K ! Many astronomers
thought there must be some
other explanation, until…
• In 1962 Mariner 2 flew by Venus
and confirmed that the radiation
really did come from the
surface. (Mariner 1 failed shortly
after launch).
Figure credit: NASA/OSS
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Surprising Radar Data
• At about the same time as the Mariner 2 mission, radar
astronomers had succeeded in bouncing waves off the
surface to measure the rotation rate: which was found to
an extremely slow backward motion.
• Venus takes 243 days to rotate (‘backwards’) on its axis,
but only 225 days to orbit the Sun. This results in a solar
day (sunrise to sunrise) of 117 days, but note the Sun rises
in the west and sets in the east!
• The radar data also showed how extended the atmosphere
was: with a surface pressure at least 50 times that of the
Earth.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Touchdown on the surface
• A series of Soviet robot probes were sent to Venus with the intention of
landing on the surface, to settle once and for all what the conditions
were like.
• Venera 4 (1967) was the first to enter
the atmosphere, but was either
crushed or melted 23 km above the
surface, at T=500 K and p=20 bars.
• Venera 7 (1970, right) was the first
spacecraft to successfully return data
from the surface another planet.
• As was by then expected, the surface
temperature was found to be 750 K,
and the pressure 90 bars, the same as
under 900 meters of ocean!
Figure credit: USSR, from NASA NSSDC
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Why is Venus so hot?
• We now know that Venus is the hottest planet in the solar system,
despite being further from the Sun than Mercury. Any ideas why?
• The answer is the greenhouse effect, which we just discussed in the
context of the Earth. However, clearly the size of the effect is much
larger for Venus.
• Although the cloud tops of Venus are around 230 K, the temperature
increases rapidly toward the surface. Remember that the troposphere
(lower atmosphere) has a temperature gradient maintained by
convection.
• The cause of the greenhouse effect, worked out by Carl Sagan and
James Pollack, was the massive atmosphere, which is mainly CO2, a
very effective greenhouse gas.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Venus Greenhouse Effect
• Scientists at first could not
believe that any sunlight
could reach the ground
through Venus’s massive
atmosphere.
• In fact, the greenhouse
gases are transparent in
the visible, and even the
clouds only reflect 75% of
sunlight, so plenty still
reaches the surface.
• The huge blanketing effect
causes Venus’s surface
temperatures to be almost
constant across the planet.
Figure credit: Greg Holmes, Embry-Riddle Univ.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Greenhouse warming on various planets
•
Different planets have differing amounts of natural greenhouse
warming: Venus is the most extreme example, followed by the Earth.
• Mercury has no
atmosphere, and hence
no greenhouse effect.
• For the outer planets, we
will see that there is a
different reason why the
temperature may be
higher than expected.
•
Figure credit: Univ. Michigan, Global Change
Could the Earth become
like Venus if humanity
releases enough CO2?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Atmospheric Composition
• Apart from CO2, the next
most abundant gas is N2.
• All other gases are trace
constituents (<1%). ‘ppm’
means ‘parts per million’
in the atmosphere.
• Scientists were surprised
at first not to find more N2
and O2, but especially the
low amounts of H2O were
perplexing. The answer
concerns differential
escape of hydrogen- see
Chap. 12).
Table after Morrison and Owen
Gas
Formula
Abundance
Main Constituents
Carbon Dioxide
CO2
Nitrogen
N2
Trace Constituents
Sulfur dioxide
Argon (40)
Argon (36)
Oxygen
SO2
Ar-40
Ar-36
O2
130 ppm
33 ppm
30 ppm
30 ppm
Water Vapour
H2O
30 ppm
Carbon monoxide
Carbonyl Sulfide
Neon
Hydrochloric acid
Hydrofluoric
CO2
OCS
Ne
HCl
HF
0 ppm
10 ppm
9 ppm
0.6 ppm
0.005 ppm
96.50%
3.50%
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Venusian Clouds
• Elucidating the composition of Venus’s clouds posed a particular
problem. They could not be water droplets: there was insufficient water
vapor present for that.
• Spectroscopy does not help much with solids and liquids: they do not
show the sharp absorption and emission features as do gases
(although they do display harder-to-see broad spectral absorptions).
• Finally, NASA’s airborne telescope in the 1970s was able to detect
spectral features of H2SO4 – concentrated sulfuric acid. Other evidence
indicated the same conclusion.
• Sulfuric acid is produced on Venus by reactions between H2O and SO2,
driven by the action of solar UV light. The SO2 is probably released by
periodic volcanic eruptions, as on Earth.
• Other cloud layers apart from the main H2SO4 clouds have been
detected at 30-60 km altitude, but their composition remains unknown.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Runaway Greenhouse Effect
• Let’s imagine that we could move the Earth closer to the Sun, at the
distance of Venus. What would be the effect?
• First of all, there would be twice as much sunlight hitting the Earth. So,
more water would evaporate from the oceans.
• Water vapor is a greenhouse gas, and so the atmosphere would warm
up. Therefore, there would be more evaporation… and so on in a cycle.
• This is called a positive feedback loop: a change occurs which feeds
on itself, increasing the magnitude of the change.
• In this case, the heating might continue and runaway until the oceans
boiled off: this is called a runaway greenhouse effect.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
The mystery of Venus’s water
• On the Earth, water vapor cannot get past the tropopause, which acts
as a cold trap: condensing water back to liquid.
• But in the runaway greenhouse situation, the atmosphere can heat up
sufficiently so that water vapor can attain greater height: high enough
to be affected by solar UV radiation.
• Just as in comets, UV radiation breaks the molecule apart, into O and
H atoms. Free hydrogen atoms could then escape to space, while the
oxygen combines with rocks on the surface.
• We can envisage this occurring in the past on Venus. The net effect
was that water was permanently destroyed and hydrogen lost. This
could explain why Venus has so little water today.
• We might also ask: did Venus have oceans in the past, for long enough
so that life could begin?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Upper Atmosphere
• Venus shows no features in visible light, but seen through an ultraviolet
filter, curious markings appear.
• The picture (right) was taken by
the Pioneer Venus orbiter in UV.
• The features can be tracked as
they fly around the planet in a
retrograde direction, like the
surface rotation.
• However, the clouds only require 4
Earth days to make one complete
circuit, traveling at 360 km/hour:
much faster than the solid planet. A
very different circulation to the
Earth.
Figure credit: NASA/OSS
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Temperature Profile
• The atmosphere of
Venus was extensively
investigated by probes
from a number of
countries from 1968 on,
culminating in a SovietFrench joint balloon
flight in 1985.
• Venus is actually colder
than the Earth at high
altitudes, although
much warmer near the
surface.
• Wind speeds are low near the surface,
where the atmosphere is very thick.
Figure credit: from J Scott Shaw, U Ga.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Atmospheric Circulation
• Venus’s lower atmosphere exhibits the simplest type of global
circulation possible: an equator to pole movement in each hemisphere,
called a Hadley cell.
• In each cell, the atmosphere is
heated more at the equator, where
is rises and then begins to move
pole-ward.
• At the poles, it cools and sinks,
moving equator-ward again to
complete the cycle.
• This was first proposed for the
Earth, but in fact, the Earth’s
circulation turned out to be more
complex.
Figure credit: Max Planck Inst: Enid
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Earth-Venus Circulation Differences
•
The key difference between the Earth and Venus which causes the
circulations to differ is rotation speed. Venus has a much longer day,
leading to a stronger diurnal heating effect.
• On the Earth, faster rotation
means a stronger Coriolis
effect (sideways deflection of
air into cyclones).
• The two large Hadley cells
each fracture into three
smaller ones: the true cell
near the equator, plus a midlatitude cell and a polar cell.
The Earth also has a much
more complex stratospheric
wind pattern.
Figure credit: Lutgens and Tarbuck: The Atmosphere, 8th Edition
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Topography of Venus
• The Pioneer Venus spacecraft was the first to map the surface of
Venus in detail, using a technique called radar altimetry: essentially,
measuring the distance from the ground to the spacecraft by bouncing
radar waves.
• The spacecraft orbited N-S
and mapped the planet as it
turned W-E underneath,
over a 2 yr period (‘78-’80).
• The spatial resolution was
just 50 km: not good
enough to map craters, and
about the same as the
image of the Moon with the
unaided eye.
Figure credit: NASA/NSSDC/Don Sawyer
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Pioneer Venus Surface Topography Map
Figure credit: NASA/NSSDC
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Venus Topography from Pioneer
• The topographical map of Venus shows big differences from a similar
map of the Earth (c.f. fig. 10.10 of M&O):
• The contrast between ‘continents’ and ‘oceans’, or highlands and
lowlands, is much less than the Earth.
• Venus has about 10% ‘continents’ compared with 45% of the Earth.
• On the Earth, the uplifted continents and low-lying oceanic crust reflect
the underlying plate tectonics, with deep rifts at the boundaries.
• On Venus, plate tectonics must be much less active, as the boundaries
between ‘continents’ and ‘oceans’ are so much less well defined.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Named Features Of Venus
• The largest continent, in the mid-southern latitudes is called Aphrodite,
using the Greek word for Venus.
• Ishtar is the prominent highland in the north, named after the
Babylonian incarnation of Venus. The highest elevations on Venus, the
Maxwell mountains are found there.
• Other features found later,
are named after other
famous females, such as the
craters: Ariadne, Callas,
Cleopatra, Dickinson, JoliotCurie, Mead, Meitner, Stuart;
and the coronae: Artemis,
Gaia, Nefertiti, Nightingale,
Sacajawea and Sappho.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Improvements In Mapping
• In order to really assess the geology of Venus, including the cratering
record and volcanic activity, higher resolution images were needed.
• The Soviet Venera 15 and 16 spacecraft in 1983 made the first
improvements, imaging the northern hemisphere at 2 km resolution.
• However the best data we have comes from
the Magellan spacecraft (artist’s impression,
right), which used an advanced form of radar
called ‘synthetic aperture radar’ (SAR) to
map the entire planet down to 100 m
resolution.
• The task took 2 years, from 1990-1992, and
Magellan finally returned more data than all
previous missions combined!
Picture credit: NASA/JPL
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Magellan View of Venus
• This Magellan image is
centered on the Ovda
Regio region in the
western arm of the
Aphrodite highland.
• In this map brightness is
an indication of
roughness, and hence the
dark rings are smoother
material.
• Magellan found over 1000
impact craters, from 280
km diameter to 2 km size.
Figure credit: NASA/NSSDC
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Cratering
• The absence of larger craters (e.g. impact basins) immediately tells us
that Venus has been tectonically active since the end of the late heavy
bombardment.
• The absence of smaller craters tells us that Venus’s atmosphere has
effectively shielded the planet from objects as large as 500 m (the
Earth’s atmosphere filters only 50 m or less).
• The crater Dickenson (69 km across) shows a
mixture of radar-bright and radar-dark terrain.
• The bright terrain here is the ejecta: the uneven
distribution may show that impact was oblique
from the west (left).
• This is a complex crater, with a partial central
ring, and flooded floor.
Figure credit: NASA/NSSDC
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Cratering (contd)
• Why are the crater floors smooth? We think it may be due to impact
melting of crustal material: Venus is already so hot, that it is easier to
melt rock on Venus than on the Earth.
• We also see fluid outflows around some craters: evidence scientists
believe may be lava flows of melted material.
• The craters of Venus appear
relatively new, compared to those
on the Earth. This must be an
artifact of less erosion, not less
age.
• The image (left) shows a 3-D
view of three craters.
Figure credit: NASA/JPL
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Cratering and Aging
• We normally associate crater densities with age of the material. Can
this tell us anything about different aged terrains on Venus?
• The continents on Venus appear to have less craters than the
lowlands: the opposite to the situation on the Moon.
• The average density of 10-km craters is only 15% that of the lunar
maria, implying a youthful age of 500 million years for the terrain
(compare to ocean basins on the Earth: average 100 million years).
• The fact we must explain is that the craters remain ‘fresh’ in
appearance, seemingly until they are wiped out, rather than being
gradually eroded as on the Earth. What process could be responsible?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Tectonic Activity: Tension and Compression
•
Tectonic activity can take the form of tension or compression; both of
which tend to produce parallel cracks or folds.
•
The regularly-spaced grid of cracks (bright) and ridges (dark) in the
image of the Lakshmi plains (below center) results from tension in one
direction and compression in a perpendicular direction.
• The far right image
shows the Lakshmi
plains running into the
Maxwell Montes, with
the Cleopatra crater
(105 km) center. The
dark wrinkle ridges in
Lakshmi are 10-20 km
apart, due to
compression.
Figure credit: NASA/NSSDC
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Fold Mountains
• The most elevated region of Venus occurs in the northern Ishtar
continent (previous slide), around the Lakshmi plateau.
• In many respects this area rivals and exceeds the Himalayas on Earth:
• The Lakshmi plateau is 6 km elevation, with the highest peaks in
the nearby Maxwell Mountains around 11 km.
• The Himalayan plateau is 4-5 km above sea level, with the highest
peaks 8 km high.
• Were the Lakshmi mountains created by plate tectonics, as on the
Earth?
• In fact, we do not believe that Venus has well-defined plates like the
Earth: rather the compression is distributed evenly over the crust.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Volcanism On Venus
• Does Venus undergo volcanic activity, as we have seen on other
worlds?
• Yes! Venus does have lava plains, volcanoes similar to the Earth, and
at least one new form of volcanism not seen elsewhere.
• One outcome of the Magellan
radar mapping of the surface was
3-D visualizations of the surface.
• The 3-D reconstruction right is of
the volcano Sif Mons. Beware that
the color is not real, and the height
has been greatly enhanced. The
volcano is really 2 km high by 300
km across.
Figure credit: Univ. Tenn. Knoxville
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Lava Plains
•
Venus is about 80% covered in lava plains, similar to the lunar maria,
or the ocean crust of the Earth, but even more extensive.
• These plains have been dated to 500 Myrs old by crater counts.
•
If we estimate the amount of outflow required to cover such an area to
a depth of 2 km (to cover all original craters) we find a mean outflow of
1.6 km3/year (compare to the Earth: 10 km3/year).
•
If this was a continuous process, we would expect to see some 20% of
Venus’s craters being partly covered today, but in fact, we see only 5%
affected. Why?
•
We hypothesize that the outflow has not been continuous: in fact, most
of the outflow occurred in an episode about 500 Myr ago, with a much
reduced level of activity since then.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Shield Volcanoes
•
Venus exhibits shield volcanoes, like Mauna Loa on the Earth. These
show the same shallow slopes, and summit craters (calderas).
•
Sapas Mons (right) is a good
example. The volcano is 400
km across but only 1.5 km
high.
• The bright areas are rougher:
the darker areas smoother, in
this radar image.
• A number of different
overlapping flows can be
seen.
Figure credit: NASA/NSSDC
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Pancake Domes
•
Venus also has curious volcanic formations nick-named ‘pancake
domes’ (see below) which may be 65 km across and 2-3 km high.
• These appear to be caused by a single, viscous eruption, rather than a
build-up of successive layers as in most volcanoes.
• The result is an
extremely symmetric,
circular shape.
• These domes were
imaged by Magellan in
the Eistla region.
Figure credit: NASA/NSSDC
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Coronae
• The coronae of Venus (not to be confused with the corona of the Sun)
have no counterpart on Earth.
• Aine Corona (right) is about 200 km in diameter, with associated
pancake domes.
• Coronae are circular or oval
features, 100s to 1000s of
km across. They are
characterized by:
(i) a low central dome,
(ii) a trough around the dome
(iii) concentric tectonic cracks.
• We believe these are
caused by magma plumes
which have failed to break
the surface.
Image: NASA/NSSDC
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Blob Tectonics?
•
We now return to the questions of tectonics and re-surfacing.
•
Venus clearly has a different form of tectonics from the Earth, in which
perhaps the lithosphere cannot slide easily over the mantle, as on
Earth.
•
The major remaining question is the issue of the sudden resurfacing
event 500 Myr ago. Was Venus catastrophically re-surfaced, creating
the massive lava plains we see today?
•
This seems to be in contrast to the Earth, which has slow plate
movements (few meters per century) at a fairly constant rate over time.
• Venus hence has the youngest surface of any of the terrestrial planets,
and it is unlikely we will find any rocks as old as terrestrial continents.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Surviving Venus
• Exploring the surface of
Venus was a great
challenge.
• The surface (860°F) is
substantially hotter than a
domestic oven, with crushing
pressure also to contend
with.
• The Soviet Venera series of
landers were eventually
successful however.
• These spacecraft, such as Venera 13 (above right) were armored and
equipped with cooling. The longest survived nearly 2 hours.
Image: NASA/NSSDC
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Pictures from the Surface
•
Veneras 9 & 10 (1975) transmitted the first surface close-ups to Earth.
• The more advanced Veneras 13, 14 (1982) and 18 sent back panoramic pictures of the soil and surface, but were also equipped with
gamma ray detectors and X-ray sources, to enable composition to be
found from natural radioactivity and stimulated emission respectively.
• The ladder-like boom in the Venera 13 image (below) is a device to
measure surface hardness: the Venera 14 device suffered an
unfortunate encounter with the ejected camera lens cover!
Image: NASA/NSSDC
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Venus Express
• Venus Express is an ESA mission to
Venus, launched in Nov 2005, and
reached Venus orbit in May 2006.
• The spacecraft was adapted from
Mars Express at low development
cost.
• Venus Express (or VEX) is now in
polar orbit, and will map the planet
over a 243-day period.
• Instruments include cameras,
spectrometers, and magenetic fields
and particle experiments.
Images: ESA
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Venus Express Results
• The false-color movie (right) was made by
the VMC during approach in May 2006, in
ultraviolet light at 20,000-40,000 km
distance.
• The complex cloud structure shows up in
detail.
• This VIRTIS movie (left) was made in
infrared light, and shows a doubleeyed vortex at the south pole.
• The altitude seen is about 59 km, at
the height of the main clouds. Brighter
areas are deeper (warmer) in the
atmosphere.
Images: ESA
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Pair Discussion
• Comparative
planetology is sometimes used as a
justification for expenditure on planetary science:
• Is this a sufficient reason alone for expenditure
on interplanetary robotic space missions?
• What other reasons are there for planetary
science missions, and do these collectively justify
the expense?
• Should the US undertake a new Venus mission,
and with what objective(s)?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Quiz-Summary
1. Describe the rotation and orbit of Venus. Which is longer, the year, or
the solar day (sunrise to sunrise)?
2. Which poses the greater challenge in your opinion: visiting the bottom
of the oceans (on Earth) or the surface of Venus?
3. Why is the surface of Venus hotter than the surface of Mercury?
4. What is responsible for the greenhouse effect of Venus?
5. What are Venus’s clouds made of, and how did we find out?
6. Did Venus have more water in the past, and if so, what happened to it?
7. What do we know about winds in the upper atmosphere of Venus?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Quiz-Summary
8. Describe the atmospheric circulation of Venus’s troposphere. Is it the
same, or different to the Earth?
9. How do the basaltic lowlands of Venus compare to the oceanic crust of
the Earth, and the lunar maria?
10. What do the craters of Venus tell us about age?
11.Does Venus have plate tectonics, like the Earth?
12. What types of volcanism do we see on Venus?
13. What is the implication of the freshness of craters on the lava plains?
14. What produces a ‘pancake dome’?
Dr Conor Nixon Fall 2006