Real Science

Wednesday, 4 July 2007

Martian wandering

University of California, Berkeley: 13-Jun-2007 13:00 Eastern US Time

There were once great oceans on Mars, say a team of scientists at the University of California, Berkeley.

A large plain at the planet's north pole looks very like an ocean basin, even from Earth. Images taken by the Viking spacecraft in the 1980s showed two possible ancient shorelines. Each of these was thousands of kilometres long, with features like those found in Earth's coastal regions.

These dried-up shorelines, as they seemed to be, were name Arabia and Deuteronilus. Scientists estimated the dates when they were filled with water at between 2 and 4 billion years ago.

But then in the 1990s Mars Global Surveyor measured the surface of Mars to a resolution of 300 metres. They found that the shorelines vary in height by several kilometres (more than a mile).

They rise and fall like waves, with several thousand kilometres from one peak to the next.

Here on Earth the height of any shoreline is pretty much the same everywhere. So experts began to reject the notion that Mars once had oceans.

But the UC Berkeley scientists have now found an explanation for the undulating Martian shorelines. The north and south poles of Mars have moved by nearly 3,000 kilometres along its surface. This happened within the past 2 or 3 billion years.

Spinning objects bulge at their equator. So this "true polar wander" could have caused the change in height of the shorelines that we now see on Mars, say the scientists.

"When the spin axis moves relative to the surface, the surface deforms," says study co-author Michael Manga. He is UC Berkeley professor of earth and planetary science. "That is recorded in the shoreline."

The paper will appear in this week's issue of Nature. The lead author is Taylor Perron, a former UC Berkeley graduate student, now a postdoctoral fellow at Harvard University.

Perron's calculations show that the response of Mars' elastic crust could create very large elevation differences for features like a shoreline. This is exactly what we see. The Arabia shoreline varies in elevation by about 2.5 kilometres. The Deuteronilus shoreline varies by about 0.7 kilometres.

"This is a beautiful result that Taylor got," says co-author Mark Richards. He is professor of earth and planetary science and dean of mathematical and physical sciences at UC Berkeley.

"The mere fact that you can explain a good fraction of the information about the shorelines with such a simple model is just amazing. It's something I never would have guessed at the outset.

"This really confirms that there was an ocean on Mars."

So now the question is: What caused Mars' spin axis to move?

A spinning planet is most stable when its mass is farthest from its spin axis. So any shift of mass on the planet could cause the spin axis to move. This might be a shift of mass within the mantle. It could be a mass shift between the mantle and the crust to form a volcano. It could even be an addition of mass caused by a meteorite hitting the planet.

Richards has modelled polar wander in Earth's past. This was generated by the upwelling of hot mantle. Some scientists believe this shifted Earth's spin axis 800 million years ago, by 90 degrees, tipping the planet on its side.

The UC team calculates that on Mars an initial shift of 50 degrees from today's pole would have been enough to disrupt the Arabia shoreline. This is equal to about 3,000 kilometres on the surface.

A shift of 20 degrees from today's pole, or 700 kilometres, would have changed the Deuteronilus shoreline.

Interestingly, today's pole and the two ancient poles lie in a line that is a constant distance from the planet's largest feature. This is the Tharsis rise, a huge bulge near the equator which contains Mars' most recent volcanic vent, Olympus Mons.

Tharsis is the largest volcano in the solar system. It formed about 4 billion years ago, not long after Mars solidified.

The positions in relation to each other of Tharsis and the path of the poles is exactly what scientists would expect if a mass had shifted that was smaller than the Tharsis rise. This is because the planet would then rotate so that the large mass of Tharsis stayed on the equator - as far away from the axis as possible.

"This alignment is unlikely to occur by coincidence," the team writes.

Manga has a hunch about the mass shift that led to the tilt of Mars' spin axis. If a flood of water had filled the Arabia ocean 3 billion years ago, to a depth of several kilometres, that might have been enough to shift it 50 degrees to the south.

When the water disappeared, the pole could have shifted back again. Then it could have shifted again by 20 degrees during the flood that created the Deuteronilus shoreline.

The unknown source of water must have produced a flood greater than any seen on Earth, Manga says. Huge canyons have been cut in the flanks of the Tharsis rise. Where has the water gone?

Well it might have evaporated. But there is another, more intriguing possibility. All that water might have sunk into underground dikes. These would be frozen near the surface.

But they could be liquid below.

More help with words













What's it all about?

  1. What is the main conclusion of this latest research?
  2. Whereabouts on Mars is the large plain that looks like an ocean basin?
  3. Roughly how long are the two possible shorelines photographed by the Viking spacecraft in the 1980s
  4. What names were they given?
  5. How old were they estimated to be?
  6. For a while the plains were thought to be the remains of ancient oceans. Then new images started coming in during the 1990s. Which spacecraft took these images?
  7. What kind of information about the surface of Mars did these new images show in much more detail than the Viking images?
  8. What did this new information about the surface of Mars show about the edges of the large plains?
  9. Shorelines on Earth are pretty much at the same height. So there was no obvious reason for shorelines on Mars to vary in height by as much as several ----------.
  10. Scientists therefore began to think these were not ancient ----------.
  11. But now the UC scientists have found a way to explain how a shoreline could end up with some parts much ------- than others.
  12. It is all about what happens when things spin. What happens to the surface of spinning objects near their equators?
  13. But the large plain that looks like an ocean basin is nowhere near the equator nowadays. Where is it?
  14. For the scientists' new idea to work the large plain must have been much closer to the equator in the past than it is now. In just a few words what makes the equator move? (The article actually talks about the spin axis moving. But if the spin axis moves the equator must move. This is because the equator is an imaginary line around the planet half-way between the ends of the spin axis.)
  15. If you stick a lump of wet putty or plasticine onto a spinning ball its spin axis will shift. Exactly the same thing happens if a big rock hits a planet from space. A big rock from space is called a ---------.
  16. The spin axis will also shift if big masses of stuff move around the planet – from the inside to the surface or from the surface to its insides. Give one example, which we also see on Earth, of material from inside a planet getting to its surface.
  17. So the scientists' idea is that a big mass of something moved on Mars. This made the spin axis shift so that what looks like shoreline was closer to the ------- than it is now.
  18. Remember that the surface of a planet ------ near the equator. So parts of the shoreline closer to the equator would have ended up higher than other parts.
  19. This is the scientists' idea for explaining why the shoreline is nothing like as level as shorelines here on -----.
  20. Manga thinks the moving mass that made the spin axis shift was a flood of water into the ocean. Does he have any evidence for this?
  21. Why do you think the possibility that there is liquid water under the surface of Mars is described as intriguing?
  22. How sure do you think we can be that things happened just as these scientists say?
  23. Make a list of the actual observations that are mentioned in the story, as distinct from ideas, suggestions and the results of calculations.
  24. Do you think there might be another explanation for these observations?
  25. If you were these scientists what would you like to do next?
  26. What would be the aim of that research?

What kind of story is this?
Learning to do science is about learning to think. Experiments, direct teaching, group activities and discussions all have a part to play. So do science news stories.

Like other non-fiction texts, science stories contain different kinds of statements. To get at the science behind the words - and to make reading them an active experience - students should pull a text apart and explore the kinds of statement it contains.

We've met some of these in the later questions of the previous activity. Science news stories usually include the aims of the research or reasons for doing it. They often contain a hypothesis. Sometimes evidence for a hypothesis is given, or a hypothesis is used to make a prediction. Towards the end of a story the direction of future research the scientists are planning is often discussed, as well as outstanding questions the research will be designed to answer.

All these types of statement occur in some science stories. Virtually all science stories, however, will contain statements of the following four types:

  • new findings or developments;
  • the technology and methods the scientists used;
  • previous or accepted knowledge, which may or may not be supported by the new findings;
  • issues, implications and applications of the research.

So the next activity is designed to engage students with the latest science news by exploring the meaning and structure of a story as revealed by the content and balance of these four statement types:

Pulling it apart
In groups students should read through the story looking for
new findings or developments. Once they have reached agreement, or at least consensus, and have underlined all the statements about what the scientists have just discovered or achieved, they can compare and discuss.

In groups they should go through the story again looking for
the technology and methods the scientists used in their research. Once they have reached agreement or consensus, and have underlined the statements that talk about the methods and equipment the scientists used, they can compare and discuss.

They should repeat the activity for
existing knowledge.

Any areas of disagreement in these activities - whether among the students or between teacher and students - should be regarded as opportunities for discussion rather than errors to be corrected.

Having fully engaged with the latest science news through the above activities, students will be far better able to talk and think about the science and its implications than someone who has simply read about it in a newspaper or watched a brief item on television.

Now it's time for them to get to grips with the issues raised by the research.

Young people have opinions. But school science traditionally allowed little scope for forming and expressing these - which is why it turned many of them off the subject for life.

Putting it together again

In groups, students should read through the latest story looking for issues, implications and applications. Once they have reached agreement, or at least consensus, and have underlined all the relevant statements in the story, they can compare and discuss.

Having done all this the students are well armed to explore the issues raised by the story. A suggested discussion topic specific to this new story is provided below.

Topic for discussion, research or pupil presentations

A simple demonstration of a spin axis shifting can be done with a football and a lump of something sticky, like wet mud or honey. If the football is set spinning on the floor with its axis straight up, and the sticky stuff dropped in the region of its "north pole" what happens next simulates how the spin axis of Mars has shifted, according to the UC scientists.

Two things to notice, the second not obvious from the story:

1) The sticky stuff ends up going around the equator.

2) It is not the spin axis that shifts, but the planet with respect to the axis. The orientation of the spin axis in space is fixed (apart from the effect of the small torque of the incoming mass), but the positions of the poles on the planet move.

Working in groups pupils should predict what will happen, do the experiment, then compare their predictions with what actually happened. They should then try to explain what they saw.

It takes a bit of teacher practice to get the spin rate and stickiness just right, so that big blobs of honey don't go flying around the room - although this is quite entertaining and isntructive.

A less messy approach would be to use the teaching aid movie on polar wander set up by Dave Stegman at Monash University

Teachers can find more activities, resources and lessons about Mars here

Tips for science class discussions and groupwork

No 52

"The 5-E model is designed to help kids construct their own understanding and then form questions to complete that understanding. Some teachers may find that some items do not lend themselves to the 5-E model, but I have found in my biology class that any subject can be taught with the 5-E model. Where most people turn from the 5-E model is because there is less planning involved and less "classroom confusion" (students may at times be actively moving around the room and creating noise in class). They also feel more in control by being the leader of the classroom instead of being an active part of the learning experience.

For myself, I find it exciting to develop lessons where I get to be creative and I can keep the creativity in my classroom. Personal opinion, but I think that as we move students through to graduation, we are being forced to take the creativity out of their learning experience. The 5-E lets me keep that creativity and excitement level in my class because I am actively involved in the learning. It also lets me use Multiple Intelligences to make learning the information relevant to each student. Sometimes, depending on how the students create their meaning, we are led down different paths than another group of students.

For those that like every class at the same point every day, this can be a hard item to deal with."

NSTA forum entry by teacher Bob Penrose (May 2007)

For more see 5E model and constructivism.


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