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{\bf General Astronomy, 29:61\\
     Fall, 2005\\
Tenth Homework Set...November 30, 2005\\ Due December 7, 2005}\\
 {\em All problems are worth 5 points except \#7, which is worth 10pts} 
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{\bf (1)}  Callisto is a moon of Jupiter.  We will be learning more about it this week.  What is its expected equilibrium temperature? Think about the physical concepts and data you need, and how to get them.  Explain yourself. \\

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{\bf (2)} This is about Callisto too. As a result of being at a temperature higher than absolute zero, Callisto radiates energy to space.  At what wavelength is this radiation brightest? \\

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{\bf (3)} An oddity in understanding the early history of the Earth is the ``Faint Young Sun Paradox''.  Stellar evolution theory says that the luminosity (i.e. total power output of the Sun) 4.5 Gyr ago should have been $\simeq 75$ \% that which it is today.  There is no evidence that the Earth was correspondingly cooler at that time.  Could a change in the Earth's albedo could have offset the effect of the ``Faint Young Sun''?  Quantitatively describe the change that would have been necessary so that the Earth would not have been cooler. \\

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{\bf (4)} Based on what you have learned in class, discuss (qualitatively this time) another possible way of resolving the Faint Young Sun  paradox.  \\

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{\bf (5)} A fascinating discovery of the past ten years is that the Earth has apparently gone through a number of ``snowball phases'', in which the polar ice caps grew and grey until the whole planet was encased in ice.  Using ideas developed in class, explain why this runaway glaciation would be a ``positive feedback'', in climate terms.  That is, once the polar ice caps began to grow,  they would tend to grow at a faster rate until they covered the whole planet.  You can use words in the answer, but they must utilize concepts introduced in class.   \\

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{\bf (6)} A space ship goes on the ``minimum energy orbit'' from Earth to Jupiter.  This means it travels like any other planet, obeying Kepler's Laws.  How long does the voyage from Earth to Jupiter take?  Explain how you arrive at your answer.  \\

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{\bf (7) \ldots 10pts} {\em Here's an Easter Egg Hunt!!!}  Find an example of a Valley Network channel, or an outflow channel on Mars, and compare its size (length and width) to a large river on Earth.  Rough, approximate numbers are OK.  

This is a way of using resources on the web.  
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\item Use the Mars map in the lab writeup to see the geography of Mars with Martian latitude and latitude.  The map in the lab writeup also identifies surface features. 
\item The following web site shows a map with all of the Valley Network and Outflow channels on Mars: ``barsoom.msss.com/http/ps/channels.channels.html''.  
\item To actual see detailed pictures of the ancient water channels, you need higher resolution images.  Those are available at: ``www.msss.com/moc-gallery''.  Look at the area called MOC Narrow Angle Images.  
\item You can click on any of the time periods, and a map of Mars will come up, divided into quadrants.  When you click on a quadrant of interest, it will come up with a higher resolution map of Mars.  You will see wormy blue lines all over the place.  These are the locations of high resolution pictures.  If there aren't many wormy blue lines in your area of interest, check out the data from a different date.  
\item Click on a wormy blue line in your area of interest.  On the right hand side of the screen there will be ``context image'' showing the general surroundings of the high resolution image (and a very good picture of Mars in its own right), with a white box superposed.  The white box is the outline of the high resolution image, which is show at left and shows much finer detail.  Below the context image is a box with data.  Look for scaled image height and width (in kilometers).  This will give the physical size of the high resolution image and set a scale for both pictures.  
\item Go to work!
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