| Lecture |
|
Venus |
[Lecture based on Ch. 8 of The New Solar System, 1999, Beatty, Collins Petersen, Chaikin (eds.)]
After hearing the lecture and reviewing these online notes, you should be able to:
Explain why we expect Venus' geologic history to be very similar to Earth's.
Explain the process whereby Venus departed from being Earthlike. Give the
overall properties of the Venusian atmosphere and temperature at the surface.
Describe how the Magellan orbiter mapped the surface of Venus.
Why was it able to see through the clouds while we, from Earth, cannot?
Outline the process of getting information from the Magellan orbiter-from the
sending of radar waves to the surface to the printing of an image on Earth.
Tectonics seem to be as important on Venus (at least in the past) as on Earth.
Explain how the tectonics are similar and how they are different.
Crater counts on the surfaces of the terrestrial planets give good
relative ages of the surfaces when compared to each other. Relate the use of crater
counts on other planets to the use on Venus. Why would one be
cautious using relative dating based on Venus's crater counts?
Present the evidence that leads us to think that Venus experienced
a global resurfacing event around 500 million years ago.
Volcanism on Venus is also comparable to that on Earth in many ways.
List two similarities (saying "they both have volcanoes won't work).
List two differences.
Discuss what is meant by primary, secondary, and tertiary atmospheres of
Venus, Earth, and Mars.
Explain how the runaway greenhouse effect works.
Assess whether or not we earthlings should be concerned about our
greenhouse gas emission.
Venus is only 30 per cent closer to the Sun than Earth. It is about 75 per cent the size of
Earth. Everything we know about Venus suggests that it is made of the same stuff as
Earth and in about the same proportions. One key ingredient is missing: water.
Venus is, in fact, very different from Earth. Always has been, and (we hope, since Venus is not
about to make any changes) always will be.
We will discuss the atmosphere of Venus more fully below. The inner planets probably all had
temporary atmospheres of hydrogen and helium just after forming--what we call their primary
atmospheres. As the infant Sun went through its violent birth, the hydrogen and helium escaped,
these planets were too warm and had too little mass to retain it. Then, Venus, Earth and
Mars developed their secondary atmospheres. As much
carbon dioxide came out of the Earth's interior, mostly in volcanic eruptions, as is in
Venus' atmosphere now. The difference? Our oceans remove carbon dioxide from the
atmosphere and put it into limestone and other carbon-bearing rocks, leaving an atmosphere made
mostly of nitrogen. The appearance of life on Earth gave us our third, or tertiary atmosphere,
one containing around 21% oxygen. Venus and Mars are left with their secondary atmospheres.
Its thick atmosphere
of carbon dioxide is the reason why we have no direct images of the surface of Venus,
other than the few snapshots sent back by the Russion Venera space probes before they experienced
meltdown.
One important clue to the age of a planetary surface is the numbers, size, and condition of
the impact craters it bears. Impact events are more or less random in space and time
when viewed over the lifetime of a body. The older the surface, the more craters it
should have. Crater counts, however, give us only relative ages (see discussion on
impact cratering). We cannot put an absolute number on an age until we actually date
the rock. How? By going to the planet and getting a sample.
The impact craters on Venus were surprising to scientists.
Additional characteristics of the craters
Much of the surface of Venus resembles the basalt flows in Hawaii and on the Snake River
Plains of Idaho. These basalts generally form from the melting of mantle materials, and
they are abundant on each of the terrestrial planets and the Moon.
As if the hot temperature of Venus due to the greenhouse effect wasn't enough, Venus is
also covered by evidence of global volcanism. Flat plains, lava rivers, (sinuous rilles),
small shield volcanoes, and flat-topped steep-sided domes are abundant. The steep sided
domes are evidence for high viscosity lava. Remember that the viscosity of a substance
is increased if it contains abundant gas bubbles. Imagine releasing carbon dioxide under
Earth's atmospheric pressure and then imagine releasing carbon dioxide under an atmosphere
with 90 times the pressure. Gases remain trapped longer in the magma, and then are released
explosively.
Coronae form a distinctive class of large volanoes on Venus. They are characterized by
large, concentric rings of fractures, ridges, and troughs. Evidence of upwelling of
hot material, fracturing of the crust, and then collapse. Pancake domes are thought
of form in a similar way.
Volcanoes on Venus are not concentrated in regions such as the Ring of Fire on Earth. The
Venusian volcanoes are all over the planet. Whether or not Venus still has active
volcanism is not known. Hopefully we will find out in the near future.
Venus is thought to have just one gigantic plate resting on a warm mantle. Rather than
subduction or rift zones, material on Venus gets stretched or shoved together. Venus's surface
is covered with large faults, wrinkles, striations, mountains, all thought to be the result
of tectonics.
Erosion on Venus is extremely slow compared to Earth. The thick atmosphere is very
sluggish at the surface. (Why would there not be violent winds on the surface? Think of
what causes violent winds on Earth.) Venus has evidence of landslides. Venus has no
plate subduction. Surface recycling occurs vertically, with mantle upwellings triggering
volcanism and mantle downwellings resulting in compression.
Above the surface, within the thick clouds, on Venus, it rains hydrosulphuric acid. The
ultimate acid rain evaporates before actually reaching the surface. The atmosphere of Venus
is made up of 96% carbon dioxide, and 4% nitrogen. With the excellent greenhouse qualities of
carbon dioxide, the surface of Venus remains at a toasty 700+ Kelvin (around 860 degrees
Fahrenheit). The melting temperature of lead is around 500 Kelvin.
Earth's atmosphere acts like a greenhouse, warming our planet in much the same way that
an ordinary greenhouse warms the air inside its glass walls. Like glass, the gases in
the atmosphere let in light yet prevent heat from escaping. This natural warming of
the planet is called the greenhouse effect.
Greenhouse gases -- carbon dioxide, methane, nitrous oxide, and others -- are transparent
to certain wavelengths of the Sun's radiant energy, allowing them to penetrate deep into
the atmosphere or all the way to Earth's surface. Clouds, ice caps, and particles in
the air reflect about 30 percent of this radiation, but oceans and land masses absorb
the rest, then release it back toward space as infrared radiation. The greenhouse gases
and clouds effectively prevent some of the infrared radiation from escaping; they trap
the heat near Earth's surface where it warms the lower atmosphere. If this natural barrier
of atmospheric gases were not present, the heat would escape into space, and Earth's mean
global temperatures could be as much as 33 degrees Celsius cooler [about -18 degrees Celsius
as opposed to 15 degrees Celsius].
The greenhouse effect is important to life on Earth, without it the Earth would be far too
cold for us. But some scientists are concerned that humans are producing too many greenhouse
gases, and that we may be warming the Earth too much. The first step to a safe future is to
be aware of the possible problems. Next we must study climate and atmospheric trends, which
will take many years of observation. NASA is very involved in these studies, measuring
greenhouse gases in the atmosphere from satellites in space. As we develop a deeper
understanding of climate trends, we can better predict what affects we have on our environment,
and how to ensure a healthy future.
From NASA
Liftoff to Space Exploration
For Venus, its being just a little bit closer
to the Sun made all the difference. It may have once had liquid water on its surface, but
the oceans started to evaporate due to the higher heat. Water vapor is a greenhouse gas,
just like carbon dioxide. While in the atmosphere, the water molecules were dissociated by
the Sun's radiation. The hydrogen escaped, and the oxygen combined with other atoms (oxygen
is a reactive element). As more of the Sun's radiation entered the atmosphere and heated the
surface of Venus, the visible radiation was changed to infrared radiation, heating the planet
even more. Eventually all of the water evaporated and disappeared, leaving a surface devoid
of hydration--there was no water left to dissolve the carbon dioxide, and thus nothing to
reduce the heating. Venus continued to heat up until equilibrium was met. And that is where
it is today--hot like Hell.
At the risk of overdoing it, here it is summarized in yet another way:
Visible (and some ultraviolet) radiation comes to us from the Sun and is transmitted
through our atmosphere. [Fortunately for us, the atmosphere blocks all of the
rest of the harmful radiation from ever reaching Earth--as long as we have our
ozone layer.] The visible light hits the surface of the Earth and heats the rocks.
This heat is reflected as infrared radiation. Now, the greenhouse gases in the
atmosphere are those that just happen to "like" infrared radiation: carbon dioxide,
water, methane being the main culprits. These molecules absorb the infrared radiation.
Since energy is conserved, the radiation goes into the kinetic energy of these
molecules raising the temperature of the atmosphere. When the molecules get rid
of the energy, the radiation is released in random directions, to be absorbed by
yet another molecule.
What happens as we raise the carbon dioxide levels of our atmosphere? The atmosphere
heats up (since we seem to also be chopping down the trees that would normally be
using that carbon dioxide for life--and what's worse, burning them!). As the
temperature rises, more water evaporates from our oceans, building up a greater
abundance of this greenhouse gas, which traps the infrared radiation, heating
up the atmosphere even more. This scenario is why it is described as "Runaway"!
The slides shown in class (and included here) are those from the Magellan mission. The images
can be found at
NASA's Magellan Images web site.
Look for the following images and read about the features by clicking on the images. There
is some important information there that will help you learn more.
Volcanic "pancake" domes in Tinatin Planitia, Venus; Wheatley Crater, Venus;
Lava Channels, Lo Shen Valles, Venus from Magellan cycle 2; Akna Montes mountain belt, Venus;
Addams Crater, Venus and outflow; Corona chain near Themis Regio,Venus; Press release image of wind streaks on Venus;
For more global views, visit
NSSDC Photo Gallery
of Venus.
For Venus in 3D, go to
the Lunar and Planetary Institute 3D Tour of the Solar System.
Note: Regions that are bright in the reconstructed radar images have higher radar reflectivity.
These areas are often rougher than their surroundings on the scale of the wavelength of the
radar (cm to m) Terrain may look especially bright if it faces towards the spacecraft.
Concepts Covered
Formation and Evolution of Venus
The Magellan Mission
We have "seen through" to the surface via radar imaging. The Magellan orbitor, which
systematically mapped Venus from September 1992 to October 1994 really reveal Venus' true nature.
Click on the image at the right and read more, a whole lot more, about this mission.
Information from Magellan Fact
Sheet
Geological Processes
Impact Cratering
Two hypotheses have been proposed:
Planetwide Volcanism
Venus has a unique volcanic feature called "pancake" domes. These domes have flattened tops and
steep edges, looking much like their namesakes. It is thought that these domes were formed by
the extrusion of high viscosity lava.
Tectonics
Erosion
The Greenhouse Effect
Speaking of "self-destruction"
Global Warming: Is There a Venus-World in Our Future?
The Greenhouse Effect
How the Greenhouse Effect Works (image from
Environment Canada)
Surface Features
Take the Quiz!
Last updated on: