Carving Landscapes - 1 of 3

Carving Landscapes Supplement:

Stories for Pictures on Wall

Impact crater feature / Streamlined island / Permafrost collapse
Triple channel feature / Gusev channel / Chaotic terrain / Shaped by wind, water and meteorite impacts
This casting was made by digitally turning a 3-D picture of a section of Kasai Vallis into a physical model with a 3-dimensional printer that lays down liquid plastic rather than ink. / Detail of lower left corner of model
Teaching Points in brief
  • One current debate about Mars involves the role of liquid water in shaping its landforms. Due to a very low atmospheric pressure on Mars now, liquid water can’t exist for a very long time on the surface of Mars, but perhaps it could have in the past. Since the 2004 rover landings, the warm-wet theory of Mars past got a boost. The question still remaining to be answered is whether Mars ever had clouds, rain and a long-term water cycle. Also, it is not known whether Mars’ water-carved features are due to infrequent, gigantic floods or to slow, long-term erosion by rain runoff as on Earth.
  • Liquid water on Earth carves landscapes in specific and recognizable ways, such as erosion, mass wasting and meandering. For clues about Mars’ history, scientists look for these same water-carved features on the Red Planet.

Interpretive discussion of the pictures

  • Triple channel– Several things stand out about this feature. Notice that there are apparently three runoff channels. These could have been three successive flood events, or they could have been a shift in the channel during one longer event. However, the lack of tributaries seems to weigh against this feature as a long-term drainage basin. The other interesting feature is at the top right. Notice the leaf-like pattern. This can be explained by erosion of permafrost that lies under the ground over most of Mars. When the channel was created, it exposed permafrost in the canyon walls. This permafrost evaporated and reduced the cohesion of the soil. Without a strong binding agent, landslides occurred and exposed more permafrost which then evaporated and led to yet another cave in. (This kind of erosion also widened Candor Chasma and Vallis Marineris from small to gigantic, so they must be very old features.)
  • Channel into Gusev Crater–The round ring (mostly cut off by the very top of the picture) is Gusev Crater, the site where Spirit Rover landed in 2004. The landing site was chosen because it may once have held a large lake. Opportunity (at another site hundreds of miles away) did find water-deposited nodules (“blueberries”) that definitely confirmed the presence of a large lake. This means that the air pressure was higher then, creating an addition mystery: what happened to Mars’ atmosphere? The long channel is over 500 miles long, but has very few tributaries. This weighs against it as a watershed or drainage such as the MississippiValley. Judging from the depth of the channel, the volume of water that formed the canyon must have between immense. There may have been a volcano that erupted, periodically heating the ground, melting the permafrost and releasing huge volumes of water. Try to figure out if the impact crater in the center of the picture was formed before of after the channel.
  • Impact crater feature– The mesa-like hill that runs diagonally across the upper left of the picture is the rim of a crater, probably due to a large meteorite. The power of the impact broke through the underlying rock layers and them lifted up. This local uplift exposed and melted permafrost, and created run-off in small rivulets. However, there is no evidence of recurrent runoff. The wavy feature at the bottom looks like a low dam composed of light soil piled up by the flood of runoff water. But the water in the lake behind the dam either evaporated or drained into the arid Martian soil, for there appears to be no outlet from the lake.
  • Chaotic terrain– Clearly this landscape was not formed in one event. This feature lies at the edge of the Tharis region of Mars where we known there was a giant uplift event that raised the Tharis Plateau high above the rest of the surface of Mars. The two straight parallel canyons connected by a shorter perpendicular canyon are probably grabens. A graben is a long narrow depression formed when a block of land between two parallel faults drops down. The Red Sea on Earth is a very large graben, formed when the African Plate pulled away from the Eurasian Plate. Notice the lack of canyon widening here due to permafrost erosion and landsliding. This may be because either it’s a very young feature or because it lies near the Martian equator, where very little permafrost exists. After the grabens were formed, water flowed through them, perhaps at the same time as the epical flood that created the dagger-shaped island to the center right. Try to determine which direction the sharper end of the dagger points—toward or away from the onrush of flood water.
  • Streamlined “Dagger-shaped” island– Here water in a gigantic flood parted and flowed around a more ancient crater, probably due to meteorite impact present. The canyon walls in some places are higher than the water level, though it looks like water breached the crater wall in the lower end of the picture. It is possible, judging from the shape of the island, to determine which way the water flowed. This dagger-shaped island can be reproduced in the stream table and also seen in the three dimensional model. Try it. Let visitors figure out which way the dagger points.
  • Permafrost collapse– There is a striking parallel in this feature with the impact crater feature discussed above, except that there is no impact crater. So what might have melted the permafrost? Perhaps there is a volcanic hot spring underground and steam from the volcano worked it way to the surface. When the permafrost melted, the surface caved in leaving the depressions that resemble arroyos in the Nevada desert. Try to determine if the water formed a permanent river or lake. Could you make a case that this feature was caused by rain erosion?

Educational Strategy

  • The Stream Table (Carving Landscapes) exhibit provides open-ended, hands-on activity, designed to let visitors experiment on their own, and then compare their results to real-world examples, such as geologic features the visitors might have seen on Earth.
  • The pictures on the wall do capture many features such as the streamlined islands that are created by running water in the stream table.
  • The Cratering and Dustdevil exhibits next to the Stream Table illustrate the other two main ways that the Martian landscape is changed. Guide the visitors to these two exhibits and ask them to observe there too, and come up with reasonable hypotheses.

General Reminders on facilitating

  • Remember that our museum research indicates that visitors want to be placed in the role of co-discoverers, not passive listeners. In facilitating the pictures on the wall, try to get the visitors to think and make intelligent guesses before you give them the answer. In other words wear the NAÏVE INTERLOCUTOR hat. Here are just a few starter questions. Add to this list from your own observations.
  • What might happen to permafrost exposed for the first time?
  • What geologic or cosmic events might heat permafrost and melt.
  • How can you tell which feature was formed first?
  • Which direction did the water flow?
  • Is there evidence of standing water?
  • How does a flood channel differ from a watershed drainage channel?