Part 6—Create Comparative Planetology Images

Step 1 –
Comparing Features from Different Planets

Voyager has image data for 20 unique planets and moons of our solar system. You can create images to compare features among these planets and moons. For example, you may want to compare a canyon on Earth to one on Mars.

Grand Canyon of Earth
Morphology of Grand Canyon, Earth
Valles Marineris of Mars
Morphology of Valles Marineris, Mars


Comparing their shapes (morphology), we can see that both planets have generally linear canyons. Are we missing something by only comparing the canyons' shapes though?

How much of each planet is actually displayed in these images?

Index image of Earth's topography shown at correct scale relative to Mars. Magenta outline shows area displayed in Grand Canyon image below.



Index image of Mars topography shown at correct scale relative to Earth. Magenta outline shows area displayed in Valles Marineris image below.



The example above shows that Earth's Grand Canyon is actually not very grand at all compared to Mars' Valles Marineris. In this final part, you'll learn how to create images of different worlds at the same relative scale. You'll use the URL shortcuts described in Part 3 to create the images.

Step 2 –
Create Correctly Scaled Maps to Compare Volcanoes on 3 Planets

Your challenge is to create correctly scaled images to compare the following volcanoes from Earth, Venus, and Mars.
In addition to the local images, you should also create an Index (view-from-space) image of each planet at the correct scale.

http://jules.unavco.org/Voyager/Earth?n=21.5&e=-160.4
http://jules.unavco.org/Voyager/Venus?n=65.0&e=4.6
http://jules.unavco.org/Voyager/Mars?n=17.91&e=226.83

Step 3 –
Creating Regional Maps that Cover the Same Extent

The ability to control the areal extent of maps makes it easy to create map images from different planets that show the same amount of area. For instance, to show a 500 km by 500 km square on any world, add ?kmn=250&kme=250 to the end its URL (see Part 3 for an explanation of this URL shortcut). You can choose whatever dimensions you like for the area; simply keep it consistent from world to world.

The following two URLs will produce images that show 500 km x 500 km areas so you can compare features on Ganymede and Enceladus.
  • http://jules.unavco.org/Voyager/Jupiter/Ganymede?kmn=250&kme=250
  • http://jules.unavco.org/Voyager/Saturn/Enceladus?kmn=250&kme=250

Experiment with different sizes to find dimensions that will show the three volcanoes at the same size.

Step 4 –
Create Index images at the Same Relative Scale

To create index images of each world at the same scale relative to each other, you need to start with the largest of the worlds you are comparing.
  1. Consult the table of planetary radii below to identify the largest world you want to compare.
  2. Values for planetary radii
  3. Set up Voyager for the largest planet you'll be comparing, zoomed in to the area you've chosen, and click the Index button to create the planet-from-space view.
  4. To create Index maps of other worlds at the correct scale, you'll set up Voyager to display the appropriate area of each world, then reduce the width of the local image, and create the index map from the reduced local image.
    1. Calculate the reduction factor by dividing the radius of each smaller world by the radius of the largest world.
    2. Multiply each result by 720 pixels (Voyager's default window size) to come up with the appropriate width (in pixels) for the smaller world's regional image.
    3. If the largest world radius = A
      and a smaller world's radius = B
      Then the width (w) to set for the smaller world's local image = B/A x 720 pixels.
    4. Set the local images' width by adding w=[result in pixels] to the URL
    5. Click the Index button to generate the Index map.
    Use the information above to create correctly scaled index images from each of your regional maps. After you've created each index image, check yourself by examining the example below.
    From the table of planetary radii, Earth is the largest of the three planets.
    So (Venus' radius/Earth's radius) x 720 pixels = Reduced width (in pixels)
    (6051.85/6378.15) x 720 pixels = (0.95) x 720 pixels = 684 pixels

    Therefore the URL for reduced regional map of Venus is
    http://jules.unavco.org/Voyager/Venus?e=4.6&n=65.0&kmn=400&kme=800&w=684

    Once this image loads, click the Index button to create an Index image of Venus at the correct scale relative to Earth.
    From the table of planetary radii, Earth is the largest of the three planets.
    So (Mars' radius/Earth's radius) x 720 pixels = Reduced width (in pixels)
    (3396/6378.15) x 720 pixels = (.53) x 720 pixels = 381 pixels

    Therefore the URL for the reduced regional map of Mars is
    http://jules.unavco.org/Voyager/Mars?e=-111.4&n=-4.0&kme=800&kmn=400&w=381

    Once this image loads, click the Index button to create an Index image of Mars at the correct scale relative to Earth.

    When Voyager creates an Index image (orthographic projection), the default window width of 720 pixels is reduced to a width of about 583 pixels. This is 81% as large as the original. When starting with a smaller window however, the reduction factor changes slightly (for instance, a 205 pixel-wide regional map will create an index image that is 171 pixels wide, a reduction factor of 83%). Because of this range of reduction factors, the width of index images may be off by a few pixels for smaller worlds.


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