Venus

Key points: Conditions on Venus; runaway greenhouse; greenhouse and global warming on Earth

"For a breeze of morning moves,          And the planet of love is on high, Beginning to faint in the light that she loves          On a bed of daffodil sky, To faint in the light of the sun she loves,          To faint in his light, and to die." -- Tennyson, "Maud"   Because it is named after the Greek goddess of love, Venus has received undeserved praise from poets, as we shall see. (to left, courtesy NASA, from Solar Views, http://www.solarviews.com/raw/venus/venusmar.jpg., above, Shukra, the planet Venus, divine sage and tutor of the demons. Here he is riding a rabbit. from The Black Peacock, http://www.goloka.com/index.html)

Superficially very like Earth:

similar mass, diameter, interior structure, not too different in distance from the sun

Rotation synchronized with -- the earth! A "day" is 243 Earth days.

Surface Temp 730K (!!!)

Although now we know better, Venus has not been completely ignored in imagining life elsewhere - and beautiful life at that

Surface Properties of Venus

The Magellan mission mapped the surface of Venus by using radar, which penetrated the Venusian clouds to reveal a variety of surface features including:

Impact Craters

Volcanic domes

Volcanoes and lava flows.(From Magellan Project and D. H. Grinspoon, http://www.funkyscience.net/)

For an imaginary airplane flight over these features, check this animation: (warning: large file (6 MB)) However, the animation shows only the rocky terrain, no lava or clouds because they were not detected by the Magellan radar. The flight might actually look more like this artist's conception that includes volcanic steam, clouds and mist, and lava flows (warning, large file (4MB)) (both animations from Calvin J. Hamilton, http://www.solarviews.com/eng/venus.htm)

This is what the surface would look like if we landed and climbed a hill to look around. (From the Magellan Project, JPL/NASA, http://antwrp.gsfc.nasa.gov/apod/ap960624.html)

In fact, Russian missions have landed on Venus revealing the bare, rocky surface:

Venera 13 spacecraft to right, a picture of the surface above (from Soviet Planetary Exploration Program via Astronomy Picture of the Day, http://antwrp.gsfc.nasa.gov/apod/ap990124.html

On Venus, the lack of water and other conditions have prevented plate building, and the interior heat escapes in other ways .  

The Greenhouse Effect - why the surface is so hot

The surface pressure is about 90 times higher than on Earth. At this pressure, CO₂ is highly absorbing both at 15 microns and near 5 microns = 5000nm. With a surface temperature of about 730K, by Wien's law the emission peak is close to 5000 nm.

where

   is the wavelength of the maximum emission (in nm)

The sunlight is mostly at wavelength of~ 0.6 microns = 600nm, which is not absorbed efficiently in CO2 , so the sunlight scatters its way through the clouds and eventually makes it to the surface. It warms the surface and the surface then radiates in the infrared. However, the CO2 absorbs the infrared radiation and it does not all escape into space, but much of it is trapped between the surface and the CO2 layers in the atmosphere. The end result is that the surface heats to 730K - only at this high temperature is it possible for the energy to escape at the same rate it is absorbed by the surface. By Wien's Law, at this temperature much of the energy is shorted than the CO2 absorption at 4000 to 5000 nm, so it can escape and stop further heating of the surface. (From Germantown Elementary School, http://www.germantown.k12.il.us/html/Venus.html) This process makes the atmosphere incredibly hot and maintains the high surface temperature.

Any water originally on Venus has boiled into the atmosphere and been broken apart by ultraviolet light from the sun. The hydrogen has escaped so water cannot form again. Venus has lost almost all water.

Water is key in controlling the greenhouse effect on Earth: (1) the oceans absorb a lot of CO₂ (2) water and CO₂ react with silicate rock to lock the CO₂ up in carbonaceous rock; and (3) water nourishes plants, which remove CO₂ from the atmosphere. When Venus lost its water, it was no longer able to suppress atmospheric CO₂ and was doomed to develop an extreme greenhouse effect.  

Greenhouse Effect on Earth

Carbon dioxide (and water) have absorptions near 7 and 15 microns that produce a greenhouse effect that warms the earth to a comfortable temperature for life as we know it. The difference from Venus is that carbon dioxide is only 0.03% of the atmosphere of the earth, whereas it is 96.5% of the atmosphere of Venus. This picture shows how it works. Sunlight is either absorbed or reflected by the surface of the earth. The absorbed part heats the surface, and causes it to emit where the 15 micron greenhouse effect operates. Thus, some portion of the energy is trapped and warms the surface above the temperature for a planet without an atmosphere.From UCAR Communications,  http://www.ucar.edu/communications/newsreleases/2001/learnweb.html

Will the greenhouse stay in this "good" range? Or will it increase too much? These records of atmospheric gas concentrations over time are based on studying the gases trapped deep in the Antarctic ice, brought up by drilling "ice cores." The plot goes from about 160,000 years ago (160 Kyr BP - K stands for 1000 and BP for before present) to now. It indicates that carbon dioxide (CO2) in the atmosphere is the largest variable affecting temperature (Ta)over this period . There is also a correlation with methane (CH4), but it is weaker. From NOAA Paleoclimatology Program http://www.ngdc.noaa.gov/paleo/slides/slideset/15/index.html

On top of a large rise starting about 15,000 years ago at the end of the last ice age, CO2 has recently been climbing very rapidly (on a geological time scale) - there is an increase by 20% in the last 40 years! It is now higher than at any time in the past millions of years. We think this rise is due to human activity - fossil fuel burning. (From http://cdiac.esd.ornl.gov/trends/co2/sio-mlo.htm)

We all agree so far -- but what will happen next?  Will the increased CO₂

    1.) Have little effect on our temperature because of effects that compensate by rejecting more heat - like an increase in reflective clouds?

    2.) Cause our climate to get substantially warmer?

    3.) Result in a runaway greenhouse effect like that on Venus? 

Most experts lean toward #2, but it is a very complex problem and there is considerable disagreement. For example, increased temperatures will increase the evaporation of ocean water and hence may increase cloud cover, and clouds can have a large influence on greenhouse trends:(From Science@NASA,   http://science.nasa.gov/headlines/y2002/22apr_ceres.htm?list749499) The net result from these possible counteracting trends is currently beyond our ability to predict In any case, we have to be careful not to blame everything about the climate on human-caused greenhouse gases. Without ignoring our impact, there are many other factors that are also involved. For example, there was a short-term trend toward global cooling from 1940 to 1980, but there has been overall warming from 1880 to the present and the cooling episode now looks insignificant in the big picture.

For more, see http://earthguide.ucsd.edu/earthguide/diagrams/greenhouse/

Odin, associated with Mercury in Northern Europe, http://www.pitt.edu/~dash/odin.html		Horus, Egyptian god of Mars, http://grenier2clio.free.fr/egypte/horus.htm
	Click to return to syllabus
Click to return to Mercury	hypertext G. H. Rieke	Click to go to Mars