Planet in a Test Tube
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about planetary circulation
from Geophysical Fluid Flow Cell experiments
Aug. 16, 1999: What do the racing winds on Jupiter and the snail's pace circulation of molten rock inside the Earth have in common? They're all fluids whose movements were simulated in a "planet in a test tube" flown aboard the Space Shuttle in 1985 and 1995.
Right: Hubble image of Jupiter with possible convection patterns outlined in white. The dotted line represents the approximate location of the hypothesized transition zone between neutral and metallic hydrogen in Jupiter. Links to 800x823-pixel 212K JPG. Credit: NASA/Marshall.
Since the early 1900s scientists have used rotating pans filled with liquids to simulate the flow of Earth's atmosphere. In the last few decades researchers have increasingly used advanced computers to simulate atmospheres and interiors of stars and planets. But even these computer models have been limited by the complexity of the equations that described fluid motions in these objects.
To complement the computer simulations, a team of scientists took a physical laboratory model of stars and planets into space where weightless conditions prevail, and then gave it an artificial radial gravity (meaning it pulled inward towards the center of the spherical test cell) just as found on these objects.
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Enlarged 16mm movie frame from first GFFC flight shows flow patterns as revealed by density changes in the oil. Numbers on each side record experiment conditions. Links to 343x187-pixel, 47K JPG.. Credit: University of Colorado at Boulder.
"An interesting discovery is that you can get multiple flow regimes by starting from the same external conditions like rotation and heating," said Dr. John Hart of the University of Colorado in Boulder. He is the principal investigator for the Geophysical Fluid Flow Cell (GFFC), which NASA called a "planet in a test tube," that flew twice on the Space Shuttle. It was designed to model Earth's climate and interior, the Sun's atmosphere, and the atmospheres of gas giant planets. Results are being published in a NASA Technical Memorandum.
The GFFC was sponsored through NASA's Marshall Space Flight Center as part of the Microgravity Research Program.
The first flight of the GFFC on Spacelab 3 (STS 51-B; April 29-May 6, 1985) was highly successful. More than 100 hours of experiment runs were conducted, and 50,000 16mm film images were recorded. These showed examples of convection structures, instabilities, and turbulence to be expected in a rotating spherical shell of fluid subject to radial gravity and a range of different heating and conditions of rotation.
Building physical models of the atmosphere dates to the early 1900s. Scientists filled pans with water and a visible tracer and then rotated the pans to simulate the flow of air around the Earth. While these models provided insight into basic features of atmospheric flows, the models had the same problems as maps of the Earth: they flatten a three-dimensional, spherical world into a distorted, rectangular view.
Supercomputers and then high-power workstations helped get around the problem in the 1960s and '70s, but even these had limits. This led Hart to propose a scale model, the GFFC, in which they could simulate selected portions of global circulation in space, without gravity making warm fluid settle to the bottom and warm rise.
The heart of the GFFC is a nickel-coated, stainless-steel ball, about the size of a Christmas ornament, under a synthetic sapphire dome. Silicone oil between the two plays the part of the atmosphere of Jupiter or the Sun, or of Earth's molten mantle, all depending on experimental conditions. The rest of the GFFC comprised a temperature-controlled turntable to spin the dome, and a complex optical system to take pictures of the fluid flow patterns.
Finally, the GFFC used an electric charge between the sphere and dome to serve as artificial gravity pulling the oil towards the center of the dome so warm and cold would circulate from the brass sphere to the sapphire dome and back. As the dome rotated, the science team hoped it would set up circulation patterns like those inside Jupiter, the Sun, and Earth.
Two optical techniques let scientists see the flow patterns. Density differences between the warm and cold fluid made the fluid itself act as a lens that would produce areas of shadow or brightness. Alternatively, an ultraviolet flash lamp behind a pinhole mask would darken a light-sensitive dye in the fluid. These spots would go with the flow.
The "planet" inside the GFFC. The white ceramic base shows the depth of the silicone oil "atmosphere" that filled the space between the sphere and the sapphire dome.
Data came from a 16mm film cameras carried on both missions and a solid-state TV camera added for the second mission.
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Before the flight "there was a question of whether you could get convection patterns and wind distributions that resembled those on a gas giant planet," Hart recalled. The question was answered with several new observations, including "banana cells," rapidly rotating columns that formed as differential heating was increased. Subsurface "banana cells" are believed by some scientists to be a key feature in the atmospheric structure of Jupiter. Many of these phenomena were not fully investigated during the Spacelab 3 mission because of time limits on the experiments. Further, scientists could not observe events as they unfolded - the GFFC used 16mm film in its first flight - and thus could not interact with the experiments.
Left: Fred Leslie at work aboard USML-2. Credit: NASA/Marshall.
"The first flight of the GFFC was a little like running an experiment in the lab with the lights off," said Dr. Fred Leslie, a GFFC co-investigator working in the Science and Technology Directorate at NASA/Marshall. "We had no indication how the fluid was responding to the inputs. On the second flight, not only did we have a real-time video camera to observe the flows, but we also had a computer interface through which the crew could interact with the experiment."
Results from the first flight were significant enough to warrant a follow up flight on the U.S. Microgravity Laboratory-2 (STS-73; Oct. 20-Nov. 5 1995). They also appeared in the cover of Science magazine in late 1985.
Now equipped with a TV camera so scientists could observe and modify experiments in real-time, the GFFC carried out 29 separate 6-hour runs. Many of these were conducted by Leslie who flew as a payload specialist.
Video frames from the second GFFC flight on USML-2. At right is a solar model case. The equator is at the top, and the pole is at the bottom. The features propagate prograde at the bottom and top of the frame, while moving retrograde at mid-latitude. The middle image shows a stable polygonal convection pattern. As the rotation rate of the cell is increased from the conditions of the middle image, columnar convection modes appear (right). Credit: NASA/Marshall
|A scientist's experience aboard the Space Shuttle. Dr. Fred Leslie describes running the GFFC and other experiments in space.
GFFC aboard the USML-2 mission.
Modeling of Convection in Deep Atmospheres, studies by Dr. Anil Deane of the University of Maryland using results from the GFFC.
Planet in a test tube fact sheet, written before the USML-2 flight, contains extensive technical information on the GFFC. This is a 288K Acrobat PDF.
"Even under the same external driving, like rotation and heating, small variations in initial conditions at the start of an experiment can lead to different end states," Hart explained. The initial variations are caused by seemingly minor things like starting an experiment with a slightly different protocol than for a previous run.
The finding confirmed a prediction by Dr. Tim Miller, an atmospheric scientist the Global Hydrology and Climate Center in Huntsville and a GFFC co-investigator.
"We designed the experiment for multi-equilibrium states that I found by accident in my computer models," Miller explained. A single-equilibrium experiment ends with similar flow patterns regardless of how you start. A multi-equilibrium experiment ends in different states depending on how you conduct the experiment.
"In our case, we got different flow patterns depending on whether we started with weak 'gravity' [the electrostatic pull on the oil] and ramped up," Miller said, "or starting high and ramping down." And in some cases they wound up with the same flow patterns regardless of where they started.
One post flight computer model based on the GFFC results shows a horseshoe-shaped flow at high latitude. It's actually a break in downwelling at mid-latitudes where upwelling at lower and high latitudes link to each other.
"An extrapolation of the GFFC results is that different initial conditions could have led to a different distribution of continents," Hart continued. "That's speculative, but further investigation of this sensitivity of low rotation convective patterns to changes in initial conditions is warranted."
"One can wonder about the relevance of perturbations like this to the formation of the continents early on planet Earth," Miller said."Initial perturbations get amplified by the natural instability associated with these convecting systems" Hart continued. "These results are interesting, because it means that the distribution of surface features associated with convection may be non-unique." On Earth, the continents are thought to move in response to the steady pull of the viscous mantle beneath the continental plates.
Right: Computer simulation by Anil Deane of the University of Maryland, College Park and Paul Fischer of Argonne National Laboratory, corresponds well to images taken during the second GFFC mission. The top shows a view from the pole, while the bottom shows a view from the equator. Red corresponds to hot fluid rising while blue shows cold fluid falling. Credit: NASA/Goddard. Links to 256-429-pixel, 20K GIF.
Other results from the second mission include:
- Banded structures in planetary atmospheres like Jupiter and Saturn were not seen in the GFFC. "It appears that in order to obtain such multiple jets either experimental conditions far in excess of those possible in the GFFC, or perhaps even different fluid physical properties will be required," Hart suggested.
- Several experiment runs using different rotation and heating rates were used to classify the expected global convection regimes of planets and stars. These runs included the transition between "banana convection" - which is oriented north-south, - to non-aligned convection.
- The experiments showed evidence for baroclinic waves, an instability where cold, dense masses of fluid slide under warm light fluid. The GFFC waves are interesting because they combined attributes of both ordinary thermal convection and rotating slantwise convection. The latter instability is central to the circulation of the Earth's atmosphere. Its occurrence as a combined instability supports recent computational modeling.
- Other experiments with latitudinal heating showed how spiral wave convection breaks down to turbulence by secondary branching.
The experimental side GFFC project has ended as the scientists have moved on to other assignments. But the story might not be over.
"There's a lot more science that can be obtained with the data and the models," Miller said. This includes GEOSIM - the Geophysical Fluid Flow Simulator - which provides the user with three-dimensional, nonlinear simulations of fluid flow in cylindrical and spherical domains. Potential applications include atmospheric dynamics of Earth and other worlds and materials processing.
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