Download The Sun and Planets Lecture Notes 6.

The Sun and Planets
Lecture Notes 6.
Lecture 6
Venus
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Spring Semester 2017
Prof Dr Ravit Helled
Cover photo: Venus in true color (Courtesy of NASA)
Venus
Properties
Venus is the second brightest natural object in the night sky after the Sun and Moon.
The brightness of Venus is caused by its relatively small distance to the Sun and the
fact that it’s surrounded by thick clouds that reflect ∼75% of the sunlight (i.e., it has a
high Albedo).
The semi-major axis of Venus (i.e., its distance from the Sun) is 108’208’000 km (0.723
AU). It is the second closest planet to the Sun and the closest planet to Earth.
One year on Venus (its orbital period), the time it takes to complete one orbit around
the Sun, is 243 days. A day on Venus (its rotation period), or the time it takes to
complete one rotation about its axis, is 225 days (almost the same as its year).
Venus is very similar to Earth in terms of size (only 5% smaller), mass, density, and
composition.
The surface gravity on Venus is almost the same as on Earth (0.9g).
Internal Structure
The internal structure of Venus is remarkably similar to Earth. Venus has a core, mantle,
and crust. Venus’ core is at least partially liquid. Venus’ smaller size relative to the Earth
means that the pressure is ∼24% lower in its deep interior.
Figure 1: Interal structure of Venus compared to that of Earth.
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Surface
Venus has 167 large volcanoes that are over 100 km across. The only volcanic complex
of this size on Earth is the “Big Island” of Hawaii. This is not because Venus is more
volcanically active than Earth, but rather because its crust is older.
Earth’s oceanic crust is continentally recycled by subduction at the tectonic plate boundaries and has an average age of ∼100 million years. Venus’ surface is a few hundred million
years old.
Venus’ surface consists of impact craters, mountains, and valleys. Special geological features on Venus include:
• Flat-topped volcanic features called “farra”, which resemble pancakes
with diameters of 20–50 km and are 100–1000 meters high.
• Radial, star-like fracture systems called “novae”.
• Spider web-like features, known as “arachnoids”.
• Circular rings of fractures, sometimes surrounded by a depression,
called “coronae”.
All of the above features are volcanic in origin.
Volcanism
Are Venus’ volcanoes still active? Some volcanoes should still be active on Venus, although
we have not observed any eruptions directly. However, the presence of sulfur dioxide (SO2 )
in Venus’ atmosphere suggest that volcanic outgassing is still occurring on Venus—at least
on geological timescales (i.e., within the past 100 million years). SO2 is gradually removed
from the atmosphere by chemical reactions with surface rocks.
In 2008/2009, ESA’s Venus Express spacecraft detected hotspots (at infrared wavelengths)
that are associated with lava flows resulting from volcanic eruptions.
Observations of the Venusian Surface
Venus’ thick clouds prevents us from seeing its surface with visible light. Therefore,
geological features on Venus’ surface are discovered using radar, since radio waves are
able to pass through the clouds.
Between 1990 and 1993, the Magellan spacecraft used radar to map the surface of Venus.
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Figure 2: Topographic map of the Venus as mapped by the Magellan spacecraft (Mercator
projection).
Atmosphere
Composition
The atmosphere of Venus is 96.5% carbon dioxide (CO2 ), 3.5% nitrogen, and traces of
several other elements and molecules (see Figure 3). Sulfuric acid is present in the atmosphere as the result of volcanic outgassing in the absence of rainfall.
Temperature & Pressure
The mean surface temperature on Venus is an unpleasant 735 K (462◦ C). At the surface,
the pressure is a crushing 90 bars (roughly equivalent to an ocean depth of 886 meters).
However, at an altitude of ∼50 km, the temperature and pressure are Earth-like.
Weather
The weather on Venus is nearly unchanging. The slow rotation of the planet leads to
little wind, with maximum wind speeds only reaching ∼6 km/h. Venus has no seasons
because the planet’s rotational axis is not tiled relative to the Sun (ψ = 0). Therefore,
temperatures remain the same all year.
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Figure 3: Elemental composition of Venus’ atmosphere measured in PPM (Parts Per
Million).
An atmosphere is a layer of gases surrounding a planet or other planetary object.
This gas is held in place by the gravity of the object. Since a collection of molecules
may be moving at wide range of velocities, there will always be some molecules travelling fast enough to escape the object (vmolecule > vescape ). This produces a slow
leakage of gas into space. An atmosphere is more likely to be retained if its gravity is
high and the atmospheres temperature is low.
Sources of atmospheric gas:
1. Outgassing: Volcanoes (and possibly accretion of volatile planetesimals)
2. Evaporation/sublimation: evaporation of surface liquids and ices
3. Surface ejection: tiny impacts of micrometeorites
Losses of atmospheric gas:
1. Condensation: e.g., rain and snow
2. Chemical reactions: iron rust—removing oxygen from the atmosphere
(a) Solar wind stripping: Without a magnetosphere, particles from the solar
wind will gradually carry away gas particles from the upper atmosphere.
(b) Thermal escape: If a gas atom/molecule reaches a high enough velocity
(vmolecule > vescape — i.e., it becomes hot enough), it can escape the planet.
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How do atmosphere’s affect planets?
• Atmospheres create pressure that determines whether or not liquid water (H2 O) can
exist on a planet’s surface.
• Atmospheres absorb and scatter light. Scattering makes daytime skies bright on
planets with atmospheres, and absorption prevents (some) radiation from reaching
the surface.
• Atmospheres can create wind and weather.
• Interactions between atmospheric gases and the solar wind creates a protective magnetosphere around planets with strong magnetic fields.
• Atmospheres can make planetary surfaces warmer than they would be otherwise via
the greenhouse effect.
The Greenhouse Effect
The main heat source for the Venusian atmosphere is radiation from the Sun (this is true
for all planets in the Solar System). The radiation from the Sun is primarily at visible
wavelengths, where the Sun is the brightest (recall blackbody radiation and Wien’s law).
Cooling of the atmosphere occurs via radiation to space (radiative cooling). This
outgoing radiation is primarily at infrared wavelengths.
The final temperature is determined by the equilibrium point, whereby the heating
rate is balanced by the cooling rate.
Certain gases act like a planet-sized blanket, in that they act to keep the planet warm by
partially preventing radiative cooling (at infrared wavelengths). These are known as the
greenhouse gases—e.g. carbon dioxide (CO2 ), water vapor (H2 O), and methane (CH4 ).
The magnitude of warming due to these greenhouse gases is called the greenhouse effect.
On Earth, the greenhouse effect is ∼ 30◦ C, whereas on Venus it is much stronger at
∼ 400◦ C.
The surface temperature on Venus is 467◦ C. This makes Venus’ surface temperature the
highest in the Solar System—even hotter than Mercury despite being more than twice the
distance from the Sun (and therefore receiving only ∼25% of Mercury’s solar irradiance).
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The Greenhouse Effect on Earth
The water vapor (H2 O) and carbon dioxide (CO2 ) that is found naturally in Earth’s
atmosphere keeps Earth warmer than it would otherwise be. Our relatively clear
atmosphere allows incoming sunlight (at visible wavelengths) to penetrate the atmosphere and warm Earth’s surface. At the same time, the surface radiates energy as
infrared radiation, which is absorbed by the water vapor and CO2 in the atmosphere.
Figure 4: Illustration of the Greenhouse Effect on Earth.
Why did Venus get hot?
• Is it hot because of its small distance to the Sun? No. Despite its small distance
to the Sun, its high Albedo (due to its clouds) would make Venus cold.
• Venus became too hot to develop liquid oceans. Without oceans to dissolve outgassed CO2 —and subsequently lock it into carbonate rocks—all of the CO2 remained
in the atmosphere, which resulted in its intense greenhouse effect.
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The Evolution of Venus’ Atmosphere
Billions of years ago, liquid water could have been existed on Venus’ surface as oceans.
During this time, CO2 would have able to dissolve in the oceans (removing it from the
atmosphere). N2 would have been in the atmosphere.
However, an increase in the Sun’s radiation led to an increase in surface temperature,
which in turn led to the evaporation of the oceans. The result was that water vapor
(H2 O) and carbon dioxide (CO2 ) moved into the atmosphere. Remember that both of
these are greenhouse gases.
Photodissociation in the upper atmosphere by solar radiation leads the thermal
escape of atomic hydrogen, e.g.
H2 O + hγ (sunlight) → 2H + O
The water (H2 O) molecule is split into two hydrogen atoms (H) and an oxygen atom
by sunlight. Whereas a H2 O molecule is too heavy to escape the planet, the two individual hydrogen atoms can easily escape. Therefore, when water is in the atmosphere,
some of it will be destroyed (photodissociated) and the resulting hydrogen will escape
to space.
Deuterium (2 H) is a stable isotope of hydrogen, but is heavier than atomic hydrogen. Therefore, the lighter atomic hydrogen will escape more easily to space than
deuterium. Therefore, we use the deuterium-to-hydrogen ratio as a measure of how
much hydrogen has been lost to space as a result of photodissociation and thermal
escape.
This has led to a D/H (Deuterium-to-Hydrogen) ratio on Venus that is 100 higher than
that on Earth. This means that more than 99% of Venus’ water has been lost! Therefore,
Venus is no longer habitable despite its similarity to Earth.
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