Astronomy college course/Planetary science

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Planetary Science[edit | edit source]

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Photograph from Apollo 15 orbital unit of the rilles in the vicinity of the crater Aristarchus on the Moon. The arrangement of the two valleys is very similar, although one third the size, to Great Hungarian Plain rivers Danube and Tisza.

Planetary science (rarely planetology) is the scientific study of planets (including Earth), moons, and planetary systems, in particular those of the Solar System and the processes that form them. It studies objects ranging in size from micrometeoroids to gas giants, aiming to determine their composition, dynamics, formation, interrelations and history.

In this sense, the original planetary astronomer would be Galileo, who discovered the four largest moons of Jupiter, the mountains on the Moon, and first observed the rings of Saturn, all objects of intense later study. Galileo's study of the lunar mountains in 1609 also began the study of extraterrestrial landscapes: his observation "that the Moon certainly does not possess a smooth and polished surface" suggested that it and other worlds might appear "just like the face of the Earth itself".

To highlight the interdisciplinary nature of planetary science, note the meandering form of the lunar rille. Let us see what Wikipedia has to say about how water flows on Earth:

Meander[edit | edit source]

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A hypothetical stream bed following a tilted valley. The maximum gradient is along the down-valley axis represented by a hypothetical straight channel. Meanders develop, which lengthen the course of the stream, decreasing the gradient.
Meanders of the Rio Cauto at Guamo Embarcadero, Cuba.

A meander, in general, is a bend in a sinuous watercourse or river. A meander is formed when the moving water in a stream erodes the outer banks and widens its valley and the inner part of the river has less energy and deposits what it is carrying. A stream of any volume may assume a meandering course, alternately eroding sediments from the outside of a bend and depositing them on the inside. The result is a snaking pattern as the stream meanders back and forth across its down-valley axis. When a meander gets cut off from the main stream, an oxbow lake is formed. Over time meanders migrate downstream, sometimes in such a short time as to create civil engineering problems for local municipalities attempting to maintain stable roads and bridges.

Formation[edit | edit source]

Life history of a meander
Spectacular meander scars, oxbow lakes and abandoned meanders in the broad flood plain of the Rio Negro, Argentina. 2010 astronaut photo from ISS.

Meander formation is a result of natural factors and processes. The waveform configuration of a stream is constantly changing. Fluid flows around a bend in a vortex.[1] Once a channel begins to follow a sinusoidal path, the amplitude and concavity of the loops increase dramatically due to the effect of helical flow sweeping dense eroded material towards the inside of the bend, and leaving the outside of the bend unprotected and therefore vulnerable to accelerated erosion, forming a positive feedback loop. In the words of Elizabeth A. Wood:[2]

"... this process of making meanders seems to be a self-intensifying process ... in which greater curvature results in more erosion of the bank, which results in greater curvature ...."

The cross-current along the floor of the channel is part of the secondary flow and sweeps dense eroded material towards the inside of the bend.[3] The cross-current then rises to the surface near the inside and flows towards the outside, forming the helical flow. The greater the curvature of the bend, and the faster the flow, the stronger is the cross-current and the sweeping.[4]

Due to the conservation of angular momentum the speed on the inside of the bend is faster than on the outside.[5]

Since the flow velocity is diminished, so is the centrifugal pressure. However, the pressure of the super-elevated column prevails, developing an unbalanced gradient that moves water back across the bottom from the outside to the inside. The flow is supplied by a counter-flow across the surface from the inside to the outside.[6] This entire situation is very similar to the Tea leaf paradox. This secondary flow carries sediment from the outside of the bend to the inside making the river more meandering.[7]

Other topics[edit | edit source]

The style of this section will differ from those of this and previous sections. Instead of using a single Wikipedia or Wikiveristy page, we shall include only the links to these pages. The quiz will cover not one or two chapters, as has been the case in the past, but will instead cover perhaps four articles. The following is a list of topics that might be suitable for this page. We need to find good articles on these or similar topics, and add quiz questions. Not every topic on this list needs to be included, but the common theme to all these topics should be a connection to planet Earth.

  1. Shield volcanoes: How do volcanoes on Earth compare with those on Mars? I know there are two types of volcanoes, and that the volcanoes on Mars most resemble those of the Hawaiian Islands. They involve a type of lava and the fact that the crust is moving over a "hot spot".
  2. Earth's crust. Why is the earth a sphere? How does the chemical composition change as one goes deeper into the Earth? How do we know the internal structure of Earth? How is the magnetic field produced?
  3. The history of Earth's atmosphere. I understand that there were two or three atmospheres.

Quiz[edit | edit source]

We need EASY questions to be added to this quiz:


References[edit | edit source]

References are hidden
  1. Lewalle, Jacques (2006). "Flow Separation and Secondary Flow: Section 9.1". Lecture Notes in Incompressible Fluid Dynamics: Phenomenology, Concepts and Analytical Tools. Syracuse, NY: Syracuse University. .
  2. Wood, Elizabeth A. (1975). Science from Your Airplane Window: 2nd Revised Edition. New York: Courier Dover Publications. p. 45. ISBN 0-486-23205-0. 
  3. Hickin 2003, p. 432. "One of the important consequences of helical flow in meanders is that sediment eroded from the outside of a meander bend tends to be moved to the inner bank or point bar of the next downstream bend."
  4. Hickin 2003, p. 434.
  5. Hickin 2003, p. 432. "In the absence of secondary flow, bend flow seeks to conserve angular momentum so that it tends to conform to that of a free vortex with high velocity at the smaller radius of the inner bank and lower velocity at the outer bank where radial acceleration is lower."
  6. Hickin 2003, p. 432. "Near the bed, where velocity and thus the centrifugal effects are lowest, the balance of forces is dominated by the inward hydraulic gradient of the super-elevated water surface and secondary flow moves toward the inner bank."
  7. Callander, R.A. "River Meandering," Annual Review of Fluid Mechanics, 1978. 10:129-58