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Stop 1 at Hellas

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Terby impact crater with area showing exposed layering
Terby impact crater
Image: Portion of a THEMIS day infrared mosaic (diagonal gray areas are data gaps)
Location: Northern portion of the Hellas rim
Scale: Diameter of the crater is approximately 165 kilometers

Terby is a ~165 kilometer wide impact crater located on the northern rim of the Hellas impact structure. This crater along with Milochau to the north, which we'll visit later, are a few of the many impact craters in the Hellas region that have had material fill in the crater after it was formed and then eroded to expose individual layers.

Why is this important, you may ask? Let's think of the Grand Canyon in Arizona for a moment. What do you see there? Besides a huge canyon, you see layers of different rock types. All these layers are of different age and they record about a billion years of geologic history in this one region. If we think back to Terby now, wouldnt it be exciting to know if these layers record a portion of Martian history? We could learn a lot, couldnt we?

So lets take a closer look at some layers highlighted by the black box in the next image below.

Layers along a mesa-like cliff in Terby crater
Layers along a mesa-like cliff in Terby crater
Image: Portion of THEMIS image V01546003
Location: Southern end of mesa-like cliffs in Terby crater
Scale: Image width is approximately 10.4 kilometers

The image to the left was captured by the Thermal Emission Imaging System (THEMIS) onboard the Mars Odyssey spacecraft which has been orbiting Mars since 2001. THEMIS is a spectrometer, an instrument that is sensitive to light in both the infrared and visible wavelengths. This particular image is in the visible wavelength range and has a resolution of about 17 meters per pixel.

In the image, we can see lots of layers along the large mesa-like cliff. The layers appear to come in "packages" where there is a bench, followed by a steep drop down to the next bench below. Along the face of the steep drop, you can make out several individual layers which make up part of the "package". The top edges of some layers are continuous for many tens to hundreds of meters. Long stretches of unbroken layering without any dramatic changes in thickness is a sign that whatever process deposited the material forming the layers was likely uniform and un-interrupted for some time. Geologic processes on Earth that can produce such layering include advancing sand from eolian (wind) activity, ash fallout from volcanic eruptions, and settling of fine particles in a shallow lake environment. This makes for lots of possibilities for the past environment on Mars!

While scientists can make measurements and use other data to infer the minerals or other physical properties of the layers, we won't truly know all the details unless we go there and see for ourselves. But in the meantime, it is interesting to test and consider different models of how these layers formed, which is exactly what planetary scientists studying Mars are doing today.

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