Elevation = Colorized MOLA (Mars Orbiter Laser Altimeter) – I find this the most useful map to orient myself on the planet when I zoom pretty far out. The map isn’t very high resolution, but large global-scale features are easily identifiable.
	Visible = MOC (Mars Orbital Camera) wide-angle map. In this example, the visible map is clouded over by bright haze. That’s actually typical for this region – because it has such a low elevation, clouds form there during most of the year.
	Infrared = THEMIS (THermal EMission Imaging System) daytime IR – these maps are high-resolution, so they’re good for close-in context. They’re harder to interpret, though, because most people aren’t used to looking at infrared (IR) images. IR observations measure the temperature of the surface, not albedo (brightness/darkness) like a regular visible-light image. You do see shapes in daytime IR, like you would see in a visible image; shapes are detected because shadows are darker (and thus cooler) than sunlit areas. In addition, though, you can also get an idea of the type of material in an IR image. For example, dusty areas will be brighter in daytime IR images because they heat up faster during the day. Rocky areas will be darker, because it takes them longer to warm up from the cold night. (This article has a good explanation of this, using White Rock as an example.)

Using these maps, I was able to figure out that the “enigmatic terrain” in the above picture (PSP_009548_1420) is in the western part of Hellas Basin, which is a large, deep depression in the southern hemisphere of Mars. I could also tell it’s part of a larger isolated patch of this type of stuff, which seems to run concentrically along the inside of the basin rim. In this case I could have figured some of that out from the caption and the coordinates, but this is more fun.

The magic happens when we succeed at coregistering the IR and BG to the RED parts of the image to produce the center strip, false color images. More about this in an upcoming post. The maximum width of a color image is 4048 pixels. Some HiRISE images are 100,000 pixels long, which makes for a very long skinny image. These are affectionately dubbed “color noodles” by the HiPI (PI=Principle Investigator).

The image below illustrates where the color portion of the image is located. The zoomed in part of the same image just shows more clearly how the colors can offer more detailed geologic information than is available in the RED (black and white) image. For detailed information about the use of the color products and how they can be interpreted for scientific purposes, please refer to “Information for Scientific Users of HiRISE Color Products”

Is this what Mars really looks like? The images are not true color. The three color images taken by HiRISE are coregistered and stacked on top of each other. Then each color layer is assigned to red, blue or green, because those are the colors that are projected on your screen. So you can see how the word “color” becomes quite confusing. First, red is black and white. Then, we have all those I’s R’s and G’s and B’s! The color in HiRISE color products is really false color, because we are assigning a visible color to one that is invisible to human eyes. Also, there are only three wavelengths of light, not the full visible spectrum we are used to seeing. The RGB products are more similar to “natural” color. Even with HiRISE’s limited color capability, there is still an incredible amount of information gained by having the two extra wavelengths.

Why is there a garish green strip along the right side of the color image (left side in the nomap products)? You will notice this in some of the HiRISE color products. It will be apparent in the IRB, but not the RGB products. This is due to one half of the IR10 CCD having electronics issues during the earlier part of the mission. This problem was resolved for most cases, so that later images have both channels of IR10 — no green strip. Some of the earlier images were also able to be reprocessed to restore the missing IR information.

What is the difference between “RGB” and “IRB”? The RGB products are different than the IRB products in that the IR channel has been replaced by a “synthetic blue” layer that creates an image that is somewhat closer to natural color. In many of the images, the infrared band does not contribute a lot of information. The bands in this product have also been stretched to provide better contrast. In other words, the RGB images are more aesthetic. The IRB product is a science product. It contains the IR, RED and BG layers.

In the IAS viewer, you can turn the bands on and off to see what information each one contributes to a particular image. Use this button to switch from color to grayscale. This dialogue will also allow you to switch the color assigned to each band. The way the images are stacked in the HiRISE images goes like this:

Changing two bands to display the same color will show what kind of information is contributed by each band.

Below is a detail from PSP_004052_2045 showing the IRB color overlaid on the RED image. It is a beautiful example of how the color available in HiRISE images gives us new information that aids in interpreting the images. They are also just plain beautiful.