astr 330: the solar system

ASTR 330: The Solar System

Announcements

Dr Conor Nixon Fall 2006

• Mid-term exam #1 results at last.

Class average was 168 out of 200 (84%)! Well done to everyone. Overall course class average is now 78%.

• New materials on-line later today.

• Congratulations - as of today you are halfway through the course!


Lecture 15:

Venus

Dr Conor Nixon Fall 2006Picture: from The Nine Planets, Bill Arnett


Twin Planets?


• Venus and Earth are very nearly the same size and density.

• Venus’s shorter orbital period is in line with its smaller orbit about the Sun.

• However, Venus’s rotation period on its axis is 243 Earth days: longer than its year of 225 Earth days!

Venus Earth

Diameter (km) 12,104 12,756

Density (g/cm 3 ) 5.5 5.3

Semi-major axis (AU) 0.72 1.00

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.


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.


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


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.


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.

Figure credit: USSR, from NASA NSSDC

• 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!


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.


Venus Greenhouse Effect

Dr Conor Nixon Fall 2006Figure credit: Greg Holmes, Embry-Riddle Univ.

• 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.


Greenhouse warming on various planets


• Different planets have differing amounts of natural greenhouse warming: Venus is the most extreme example, followed by the Earth.

Figure credit: Univ. Michigan, Global Change

• 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.

• Could the Earth become like Venus if humanity releases enough CO2?


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 96.50%

Nitrogen N2 3.50%

Trace Constituents

Sulfur dioxide SO2 130 ppm

Argon (40) Ar-40 33 ppmArgon (36) Ar-36 30 ppm

Oxygen O2 30 ppm

Water Vapour H2O 30 ppm

Carbon monoxide CO2 0 ppm

Carbonyl Sulfide OCS 10 ppmNeon Ne 9 ppmHydrochloric acid HCl 0.6 ppmHydrofluoric HF 0.005 ppm


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.


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.


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?


Upper Atmosphere


• Venus shows no features in visible light, but seen through an ultraviolet filter, curious markings appear.

Figure credit: NASA/OSS

• 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.


Temperature Profile


• The atmosphere of Venus was extensively investigated by probes from a number of countries from 1968 on, culminating in a Soviet-French joint balloon flight in 1985.

• Venus is actually colder than the Earth at high altitudes, although much warmer near the surface.

Figure credit: from J Scott Shaw, U Ga.

• Wind speeds are low near the surface, where the atmosphere is very thick.


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


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.

Figure credit: Lutgens and Tarbuck: The Atmosphere, 8th Edition

• 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 mid-latitude cell and a polar cell. The Earth also has a much more complex stratospheric wind pattern.


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.

Figure credit: NASA/NSSDC/Don Sawyer

• 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.


Pioneer Venus Surface Topography Map

Dr Conor Nixon Fall 2006Figure credit: NASA/NSSDC


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.


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, Joliot-Curie, Mead, Meitner, Stuart; and the coronae: Artemis, Gaia, Nefertiti, Nightingale, Sacajawea and Sappho.


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


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


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.


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.

Figure credit: NASA/JPL

• 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.


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?


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.


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.


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.

Figure credit: Univ. Tenn. Knoxville

• 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.


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.


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.


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.


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.

Image: NASA/NSSDC

• 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.


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.


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.

Image: NASA/NSSDC

• These spacecraft, such as Venera 13 (above right) were armored and equipped with cooling. The longest survived nearly 2 hours.


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 pan-oramic 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


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.

Images: ESA

• 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.


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.

Images: ESA

• This VIRTIS movie (left) was made in infrared light, and shows a double-eyed 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.


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)?


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?


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’?

astr 330: the solar system

Documents

earth days

venusdr conor nixon

surfacedr conor nixon

atmospheredr conor nixon

temperaturedr conor

solar systemalbedo

solar systemlecture

solar systemtouchdown