Spring Colloquium 98: Mars Exploration—Past, Present, and Future

PASADENA—The San Gabriel Valley Section of the American Institute of Aeronautics and Astronautics will present "Spring Colloquium 98: Mars Exploration—Past, Present, and Future" on Tuesday, June 9, from 6–9:30 p.m. in von Karman Auditorium at the Jet Propulsion Laboratory, located at 4800 Oak Grove Drive in Pasadena. This year's program will provide a comprehensive overview of America's past, present, and potential future Mars exploration missions. The public is invited and encouraged to attend. Light refreshments will be served.

The program speakers, each an expert in a particular aspect of Mars exploration, together will provide a unique opportunity to learn about the nation's Mars exploration missions and how the missions work together to create a systematic approach to understanding and exploring the Red Planet.

Dr. Arden Albee will first present an overview of past and present Mars missions from a space science viewpoint. Next, Robert Manning will present an overview of NASA's ongoing Mars Exploration Program, a series of robotic missions that will greatly expand our knowledge of Mars, and also provide the knowledge needed to mount a future human mission to Mars. Finally, William Siegfried will present an overview of NASA's planning for a future human mission to Mars.

Please join us for a fascinating and informative evening dedicated to Mars Exploration.

Dr. Arden Albee is dean of graduate studies at the California Institute of Technology in Pasadena. Dr. Albee was a science investigator on a number of past Mars missions, and is currently the principal investigator for NASA's Mars Global Surveyor mission.

Mr. Robert Manning, of the Jet Propulsion Laboratory in Pasadena, is the chief engineer for NASA's Mars Exploration Program. Mr. Manning was previously the chief engineer for the Mars Pathfinder spacecraft.

Mr. William Siegfried, of the Boeing Company in Huntington Beach, California, has worked on a number of crewed space programs including Skylab, Space Transportation System, and the International Space Station. He has also served on several key space advisory committees, and is currently co-chair of the IAA Lunar-Mars Committee.

The San Gabriel Valley Section of the American Institute of Aeronautics and Astronautics sponsors the annual event as a forum for aerospace-related topics of interest to its membership and the public.

For more information, contact the AIAA Western Office at (800) 683-AIAA.

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Posthumous Paper by Gene Shoemaker Details Evidence of Comet Shower That Pummeled Earth 36 Million Years Ago

PASADENA—Geochemical evidence from a rock quarry in northern Italy indicates that a shower of comets hit Earth about 36 million years ago.

The findings not only account for the huge craters at Popagai in Siberia and at Chesapeake Bay in Maryland, but posit that they were but a tiny fraction of the comets active during a period of two or three million years during the late Eocene period. The work provides indirect evidence that a gravitational perturbation of the Oort comet cloud outside the orbit of Pluto was responsible for sending a wave of comets swarming toward the center of the solar system.

In a paper published today in the journal Science, a group from the California Institute of Technology, the U.S. Geological Survey Flagstaff office, and the Coldigioco Geological Observatory in Italy, report their evidence of a very large increase in the amount of extraterrestrial dust hitting Earth in the late Eocene period. The writers include the husband-and-wife team of Gene and Carolyn Shoemaker. Gene Shoemaker died in a car crash last year while the research was in progress.

According to lead author Ken Farley, a geochemist at Caltech, the contribution of Shoemaker was especially crucial in the breakthrough.

"Basically, Gene saw my earlier work and recognized it as a new way to test an important question: are large impact craters on Earth produced by collisions with comets or asteroids," Farley says.

"He suggested we study a quarry near Massignano, Italy, where seafloor deposits record debris related to the large impact events 36 million years ago. He said that if there had been a comet shower, the technique I've been working on might show it clearly in these sediments."

Carolyn Shoemaker said that she and her husband went to Italy last year to perform field work in support of the paper.

"Gene was pretty excited about the work Ken was doing," she said. "He was glad Ken was taking it on. It's exciting work, and it's a rather new type of work."

The matter involved detecting the helium isotope known as 3He, which is rare on Earth but common in extraterrestrial materials. 3He is very abundant in the sun, and some of it is ejected from the sun as solar wind throughout the solar system. The helium is easily picked up and carried along by extraterrestrial objects such as asteroids and comets and their associated dust particles.

Thus, arrival of extraterrestrial matter on Earth's surface can be detected by measuring its associated 3He. And even this material is unlikely to include large objects like asteroids and comets. Because these heavy, solid objects fall into the atmosphere with a high velocity, they melt or vaporize, giving their helium up to the atmosphere. This 3He never falls below very high altitudes, and soon reenters space.

But tiny particles entering the atmosphere are another story. These particles can pass through the atmosphere at low temperatures, and so retain helium. These particles accumulate on the seafloor, and seafloor sediments provide an archive of these particles going back hundreds of millions of years.

Elevated levels of 3He would suggest an unusually dusty inner solar system, possibly because of enhanced abundances of active comets. Such an elevated abundance of comets might arise when a passing star or other gravity anomaly kicks a huge number of comets from the Oort cloud into elliptical, sun-approaching orbits.

When Farley took Shoemaker's suggestion and traveled to the Italian quarry, he discovered that there was indeed an elevated flux of 3He-laced materials in a sedimentary layer some 50 feet beneath the surface. Because this region of Italy was submerged in water until about 10 million years ago, the comet impacts and microscopic debris had accumulated on the ocean bed, and this debris was preserved because dying organisms had cooperatively covered the debris over the eons.

The depth of the sedimentary layer suggested to the researchers that the 3He had been deposited about 36 million years ago. This corresponds to the dating of the craters at Popagai and Chesapeake Bay.

More precisely, the 3He measurements show enhanced solar system dustiness associated with the impacts 36 million years ago, but with the dustiness beginning 0.5 million years before the impacts and continuing for about 1.5 million years after. The conclusion is that there were a large number of Earth-crossing comets and much dust from their tails for a period of about 2.5 million years.

In addition to Gene and Carolyn Shoemaker and Ken Farley, the paper was cowritten by Alessandro Montanari, who holds joint appointments at the Coldigioco Geological Observatory in Apiro, Italy, and the School of Mines in Paris.

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Robert Tindol
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Geophysicists model the Cretaceous motions of Australia

PASADENA--The theory of plate tectonics says that Earth's crust has moved horizontally by thousands of miles over millions of years. For visual evidence, one need look no further than a map showing how nicely South America and Africa fit together.

But plate tectonics also literally has its ups and downs. In addition to the horizontal motions that long ago tore South America and Africa asunder, plate tectonics can also cause entire land masses to steadily rise and lower by thousands of feet. These vertical motions are driven by the internal planetary heat engine that makes the plates slide around horizontally.

One of the best examples of this vertical motion is Australia, and researchers have now completed a three-dimensional model of Earth's internal heat engine that provides the best-ever description of the process. The research is published in the current issue of Science.

According to Caltech geophysicist Michael Gurnis, the lead author of the paper, the research is aimed at improving the understanding of how heat convection in the mantle relates to the motions of the crustal plates. More specifically, the work answers nagging questions about oddities in the Australian plate.

"Normally, the ridges you see in the ocean floor are associated with upwellings of hot material from the mantle," Gurnis says. But in the ocean between Australia and Antarctica, researchers have long noted a downwelling at this mid-ocean ridge.

According to Gurnis, "We don't know why there's a downwelling, but it's at least 20 million years old and probably a few hundred million years old."

Also, Gurnis explains, there is longstanding evidence that Australia's seacoast significantly retreated during the Cretaceous period (about 60 to 125 million years ago). This was caused by the aforementioned vertical tectonic motions, which bowed the entire continent upward.

The result was a continent with terrain that at times was well above sea level—and thus dry—but at other times relatively low and thus inundated with water.

For example, the maximum flooding of Australia occurred about 120 million years ago and covered more than half the present land mass with water. But what has long perplexed geologists was that 70 million years ago, while nearly half the surface area of other continents (including the Americas, Europe, and Russia) was covered by shallow inland seas, Australia was high and dry!

The achievement of Gurnis and his colleagues is to model these risings and fallings of continents very precisely, so that a vertical level can be "predicted" for a certain time period.

Also, their dynamic models show that the entire effect was caused by a plate that subducted (or passed under) Australia, later stagnated in the mantle hundreds of miles below Earth's surface, and is now being drawn back up by the South East Indian Ridge.

"This is one of the best examples of this process, which is called 'continental epeirogeny,'" Gurnis says. "As such, it is an ideal place for tying vertical motion to how Earth's heat engine works."

The dynamic models published by Gurnis and his colleagues predict a downwelling between Australia and Antarctica in precisely the position it is observed. "The correspondence between sea-floor topography, chemistry, and crustal thickness is impressive," says Gurnis.

The work represents a breakthrough because of the ability to predict the timing and geography of geologic events separated widely in space and time. Gurnis suggests that this work opens up a huge new frontier in which the motions of plates can be predicted from computer models of the Earth's internal heat engine, in much the same way that scientists use sophisticated global circulation models to study climate change.

Says Gurnis, "Such models can be tested against the vast library of Earth's history locked in the geologic record within continents and on the sea floor."

The other authors of the paper are R. Dietmar Mueller of the University of Sydney in Australia and Louis Moresi of the Commonwealth Scientific and Industrial Research Organization in Perth, Australia.

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Robert Tindol
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Don Anderson Will Be Awarded the 1998 Crafoord Prize

PASADENA—The Royal Swedish Academy of Sciences is to award the 1998 Crafoord Prize in geosciences, with special emphasis upon "the dynamics of the deeper parts of the Earth," to Don L. Anderson of the California Institute of Technology and to Adam M. Dziewonski from Harvard University for their fundamental contributions to our knowledge of the structures and processes in Earth's interior. The 1998 Crafoord Prize is valued at $500,000 dollars, and will be presented to the prizewinners at a ceremony on September 16 in Sweden.

On hearing the news that he had been awarded the 1998 Crafoord Prize, Caltech's Eleanor and John R. McMillan Professor of Geophysics Don Anderson said, "I think it's very significant that deep-Earth geophysics is being honored by this award. It is rare for our field to be acknowledged in this way. I am really delighted that Adam Dziewonski, a close colleague of mine, is also being honored for his work. Most people, when they think of geophysics, think of earthquakes, but seismologists do other things, such as x-raying Earth using seismic tomography to see what is going on in the deep Earth."

Caltech president David Baltimore congratulated Professor Anderson and noted that "the Institute is very proud and pleased that Don will be receiving the Crafoord. It is exciting news. Don's work is truly deserving of this great prize. He is one of the world's most prominent scientists in the area."

According to the Royal Academy, Anderson and Dziewonski have together developed a generally accepted standard model of how Earth is organized and of the dynamics of the processes at its core and in its mantle that govern continental drift, volcanism, and earthquakes.

Anderson and his team have researched changes arising from the pressure deep down in Earth's mantle. Sudden changes in the rock types at depths of 400 kilometers and 660 kilometers are explained by conversions undergone by the rock types, so that they contain minerals entirely unknown at Earth's surface. At 400 kilometers, the mineral olivine, common in lava, changes to spinel, a high-pressure mineral. At 660 kilometers, the mineral perovskite is formed, a mineral otherwise only produced in the laboratory at very high pressures and temperatures. Anderson's research has shown that such changes in composition of the mantle may explain the occurrence of tensions in Earth's crust that can lead to earthquakes. Anderson and his research team have also used seismic data to study convection currents in the mantle, important for understanding continental drift and volcanism. Recently, Anderson has also used geochemical and chemical-isotope methods not only for mapping Earth's development, but also for understanding the development of the moon and the planets Mars and Venus.

Anderson was born in 1933 in Maryland and received his doctorate in geophysics from Caltech in 1962. He has been a leading figure in "deep Earth" research since the 1960s. He was director of the Seismological Laboratory at Caltech from 1967 to 1989. In 1989 he published his "Theory of the Earth," a remarkable synthesis of his broad and provocative research and a guide for geo-researchers from different fields for future exploration of the dynamics of the deep parts of Earth.

The Crafoord Prize is awarded at a ceremony held on September 16, Crafoord Day. On this occasion, the prizewinner gives a public lecture and the Royal Academy organizes an international scientific symposium on a subject from the chosen discipline of the year.

The Anna-Greta and Holger Crafoord's Fund was established in 1980 to promote basic research in mathematics, astronomy, the biosciences (particularly ecology), the geosciences, and polyarthritis. Both an international prize and research grants to Swedish scientists are awarded among the scientific fields mentioned above.

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Sue Pitts McHugh
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Geologists find more evidence for an active fault beneath downtown and east Los Angeles

LONG BEACH--Geologists report new evidence for a fault beneath Los Angeles that could cause damaging earthquakes in the future.

According to Michael Oskin, a graduate student at the California Institute of Technology (Caltech), the new study concerns an 11-mile-long, previously known geologic fold that runs through the hills north and east of Downtown Los Angeles. This fold provides indirect evidence for an underlying fault.

"Our evidence from the surface is that the fold is still growing," says Oskin. "This indicates that the fault that lies beneath it must also be active."

The fold, first associated with earthquakes at the time of the Whittier Narrows Earthquake, in 1987, is formally known as the Elysian Park Anticlinorium and runs northwest-southeast from Hollywood to Whittier Narrows. Three smaller "wrinkles" formed upon the southwest-facing flank of this fold have been investigated in detail by the Caltech scientists.

Their studies of sediment deposited by the Los Angeles River and its tributaries indicate that these small folds have been active during the past 60,000 years. During that time, the area has been contracting north-south at a rate of at least a half-millimeter per year.

"Our evidence that this structure is active does not increase the overall hazard in the metropolitan region," says coauthor Kerry Sieh, a professor of geology at Caltech. "Rather, it allows us to be more specific about how, where, and how fast deformation is occurring in the area.

The length of the surface features suggests that the underlying fault is about 11 miles in length and may extend 10 or so miles into the earth. Such a fault, if it ruptured all at once, could produce a 6.5- to 6.8-magnitude earthquake.

The rate of deformation suggests that such events might occur, on average, about once every one to three thousand years.

Also contributing data and resources to the study were the Southern California Earthquake Center, Metropolitan Transit Authority (MTA), Engineering Management Consultant, and Earth Technology Corporation.

Oskin and Sieh reported on their work at the Geological Society of America meeting in Long Beach April 7, 1998.

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Robert Tindol
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Yucca Mountain Is Possibly More Seismically Active Than Once Believed, Geologists Discover

PASADENA—Recent geodetic measurements using Global Positioning System (GPS) satellites show that the Yucca Mountain area in southern Nevada is straining roughly 10 to 100 times faster than expected on the basis of the geologic history of the area. And for the moment at least, geologists are at a loss to explain the anomaly.

In the March 28 issue of the journal Science, Brian Wernicke of the California Institute of Technology (Caltech) and his colleagues at the [Smithsonian Astrophysical Observatory] in Cambridge, Massachusetts, report on Global Positioning System surveys they conducted from 1991 to 1997. Those surveys show that the Yucca Mountain area is stretching apart at about one millimeter per year east-southeastward.

"The question is, why are the predicted geological rates of stretching so much lower than what we are measuring?" asks Wernicke. "That's something we need to think through and understand."

The answer is likely to be of interest to quite a few people, because Yucca Mountain has been proposed as a site for the permanent disposal of high-level radioactive waste. Experts believe that the waste-disposal site can accommodate a certain amount of seismic activity, but they nonetheless would like for any site to have a certain amount of stability over the next 10,000 to 100,000 years.

Yucca Mountain was already known to have both seismic and volcanic activity, Wernicke says. An example of the former is the 5.4-magnitude "Little Skull Mountain" earthquake that occurred in 1992. And an example of the latter is the 80,000-year-old volcano to the south of the mountain. The volcano is inactive, but still must be studied according to Department of Energy regulations.

The problem the new study poses is that the strain is building up in the crust at a rate about one-fourth that of the most rapidly straining areas of the earth's crust, such as near the San Andreas fault, Wernicke says. But there could be other factors at work.

"There are three possibilities that we outline in the paper as to why the satellite data doesn't agree with the average predicted by the geological record," he says. "Either the average is wrong, or we are wrong, or there's some kind of pulse of activity going on and we just happened to take our data during the pulse."

The latter scenario, Wernicke believes, could turn out to be the case. But if Yucca Mountain is really as seismically active as the current data indicate at face value, the likelihood of magmatic and tectonic events could be 10 times higher than once believed.

 

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Robert Tindol
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Mars Global Surveyor already bringing in scientific payoff

PASADENA—Despite a 12-month delay in aerobraking into a circular orbit, the Mars Global Surveyor is already returning a wealth of data about the atmosphere and surface of the Red Planet.

According to mission scientist Arden Albee of the California Institute of Technology, all scientific instruments on the Mars probe are fully functioning and providing good data. Early results from data collected during the 18 elliptical orbits in October and November are being reported in this week's issue of Science.

"For the first time, a spacecraft has captured the start of a major dust storm on Mars and has followed it through its development and demise," Albee says. "Also, we've received a number of narrow-angle high-resolution images that are enough to put any planetary geologist into a state of ecstasy."

These accomplishments are especially noteworthy when considering that the probe developed a glitch when it first began tightening up its orbit last September. For various reasons having to do with design and cost, Global Surveyor was set on a course that took it initially into a huge sweeping elliptical orbit of Mars.

On its near approach in each orbit, the probe was to dip into the upper atmosphere of Mars in a maneuver known as "aerobraking," which would effectively slow the probe down and eventually place it into a near-circular orbit.

But a solar–panel damper failed early in the mission, and damage to the solar panel forced the team to slow down the aerobraking. At the current rate of aerobraking, Mars Global Surveyor will enter its circular mapping orbit in March 1999.

This has delayed the systematic mapping of Mars, but Albee says that the new mission plan nonetheless permits the collection of science data in a 12-hour elliptical orbit, from March to September of this year.

"Another exciting discovery is that the Martian crust exhibits layering to much greater depths than would have been expected," Albee says. "Steep walls of canyons, valleys, and craters show the Martian crust to be stratified at scales of a few tens of meters.

"At this point, it is simply not known whether these layers represent piles of volcanic flows or sedimentary rocks that might have formed in a standing body of water," he adds.

The Mars Global Surveyor team has previously announced that the on-board magnetometer shows Mars to have a more complex magnetic field than once thought. Of particular interest is the fact that the magnetic field was apparently once about the same strength as that of present-day Earth.

Many experts think that a strong magnetic field may be crucial for the evolution of life on a planet. Without a magnetic field, a planet tends to have any existing atmospheric particles blasted away by cosmic rays in a process known as "sputtering."

And finally, the Mars Orbiting Laser Altimeter (MOLA) has already sent back 18 very good topographic profiles of the northern hemisphere. "Characterization of these features is leading to a new understanding of the surface processes of Mars," Albee says.

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Robert Tindol
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Biological Activity the Likely Culprit of Atmospheric Nitrous Oxide Increases

PASADENA–Nitrous oxide (N2O) is an atmospheric gas known to contribute both to global warming and ozone depletion. New research suggests that its changing concentration in the atmosphere is largely a result of biological activity.

In the December 5 issue of the journal Science, Yuk Yung of the California Institute of Technology and Charles Miller of Caltech's Jet Propulsion Lab (now at the Haverford College Department of Chemistry) describe their work on isotopes of N2O, a greenhouse gas that is of increasing concern because its concentration in the atmosphere has been rising for several decades.

N2O is a molecule with two atoms of nitrogen and a single atom of oxygen. It is created in the decay of organic material, principally plants, but is also generated in the manufacture of nylon.

Scientists have known for years that N2O enters the nitrogen cycle, but the ultimate sources and sinks of the gas have been unclear. By contrast, carbon dioxide, another greenhouse gas, is known to be a direct consequence of industrial activity.

"Nitrous oxide is less important as a greenhouse molecule than carbon dioxide, and slightly less important than methane," says Yung, a professor of planetary science at Caltech. "But the concentrations have been increasing since good measurements began 20 years ago, and ice core samples in Greenland and Antarctica suggest that it has been increasing since the Industrial Revolution began."

Yung and Miller specifically looked at isotopes of nitrous oxide once it enters the stratosphere. Isotopes are variations of a chemical element that have the same number of protons (and thus the same atomic number), but a different number of neutrons. Thus, a 15N atom of nitrogen has its regular seven protons and seven neutrons, but an additional neutron as well.

A careful analysis of isotopic variations is an effective way of tracing substances to their sources. If the nitrogen-based fertilizer in agriculture has a known isotopic makeup and that same percentage is found in the stratosphere, for example, then it can be concluded that agricultural fertilization is a contributor.

Yung and Miller examined theoretically how isotopes of nitrous oxide interact with ultraviolet light energy. They predict that, as N2O is destroyed by light, heavier isotopes survive preferentially because molecules comprising slightly heavier isotopes require a bit more energy for the atoms to separate.

From their theory and related atmospheric measurements presented in the same issue by researchers at the Scripps Institution of Oceanography and the University of California at San Diego, Yung and Miller conclude that new chemical sources do not need to be introduced to account for the isotopic concentrations that are indeed observed in the stratosphere.

Thus, sources such as the decay of plant life and the burning of rainforests and other biomass burning can account for the signatures that are seen. Experimental verification of the predictions is now under way in the laser spectroscopy lab of Caltech cosmochemist Geoff Blake.

Understanding the sources can give society a better grip on the possibilities of dealing with the problem, Yung says.

"I think the most reasonable explanation for the increase is that we are accelerating biological activity globally," he says. "Because of global warming, the use of agricultural fertilizers, and nitrogen made from pollution that acts just like a fertilizer, the biosphere has been stimulated. This fosters the growth/decay cycle which leads to N2O release."

The next step for the researchers is to pin down the precise isotopic signatures of various biological and atmospheric processes. But there may be little that can realistically or politically be done if biology on a planetary scale is responsible, Geoff Blake says.

"We may just have to live with it.

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Robert Tindol
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Geophysicists Develop Model to Describe Huge Gravity Anomaly of Hudson Bay Region

PASADENA—While the gravity field of Earth is commonly thought of as constant, in reality there are small variations in the gravitational field as one moves around the surface of the planet.

These variations have typical magnitudes of about one–ten thousandth of the average gravitational attraction, which is approximately 9.8 meters per second per second. A global map of these variations shows large undulations at a variety of length scales. These undulations are known as gravity anomalies.

There are many such anomalies in Earth's gravity field, but one of the largest negative gravity anomalies (implying the attractions of gravity being a little less than average, or in other words, a mass deficit) centered over Hudson Bay, Canada. Using a new approach to analyzing planetary gravity fields, two geophysicists, Mark Simons at the California Institute of Technology and Bradford Hager at M.I.T., have shown that incomplete glacial rebound can account for a substantial portion of the Hudson Bay gravity anomaly.

With this new information, Simons and Hager were able to place new constraints on the variations in strength of the materials that constitute the outer layers of Earth's interior (the crust and mantle). Their work appears in the December 4 issue of the journal Nature.

About 18,000 years ago, Hudson Bay was at the center of a continental–sized glacier. Known as the Laurentide ice sheet, this glacier had a thickness of several kilometers. The weight of the ice bowed the surface of Earth down. The vast majority of the ice eventually melted at the end the Ice Age, leaving a depression in its wake.

While this depression has endured for thousands of years, it has been gradually recovering or "flattening itself out." The term "glacial rebound" refers to this exact behavior, whereby the land in formerly glaciated areas rises after the ice load has disappeared.

Evidence of this is seen in coastlines located near the center of the former ice sheet. These coastlines have already risen several hundred meters and will continue to rebound.

"The rate at which the area rebounds is a function of the viscosity of Earth," says Simons. "By looking at the rate of rebound going on, it's possible to learn about the planet's viscosity."

Simons says that geophysicists have known for some time about the Hudson Bay gravity anomaly, but have hitherto been uncertain how much of the gravity anomaly is a result of glacial rebound and how much is due to mantle convection or other processes.

The gravity anomaly is measured from both the ground and from space. Simons and Hager use a gravity data set developed by researchers at the Goddard Space Flight Center.

However, knowing how much of an anomaly exists at a certain site on Earth is not sufficient to determine the pliability of the materials beneath it. For this, Simons and his former M.I.T. colleague Hager have developed a new mathematical tool that looks at the spatial variations of the spectrum of the gravity field.

In many instances, this approach allows one to separate the signatures of geologic processes that occur at different locations on Earth. In particular, Simons and Hager were able to isolate the glacial rebound signature from signatures of other processes, such as manifestations of plate tectonics, that dominate that gravity field but are concentrated at other geographic locations.

Having an estimate of incomplete postglacial rebound allowed Simons and Hager to derive a model of how the viscosity of the mantle changes with depth. Simons and Hager propose one such model that explains both the gravity anomaly as well as the uplift rates estimated from the coastlines.

Their favored model suggests that underneath the oldest parts of continents (some of which are over 4 billion years old) the viscosity of the outer 400 kilometers of Earth is much stiffer than under the oceans. Therefore, these continental keels can resist the erosion by the convective flow that drives plate tectonics.

 

 

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Robert Tindol
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Caltech Scientists Find Evidence For Massive Ice Age When Earth Was 2.4 billion Years Old

PASADENA— Those who think the winter of '97 was rough should be relieved that they weren't around 2.2 billion years ago. Scientists have discovered evidence for an ice age at the time that was severe enough to partially freeze over the equator. In today's new issue of Nature, California Institute of Technology geologists Dave Evans and Joseph Kirschvink report evidence that glaciers came within a few degrees of the equator's latitude when the planet was about 2.4 billion years old. They base their conclusion on glacial deposits discovered in present-day South Africa, plus magnetic evidence showing where South Africa's crustal plate was located at that time.

Based on that evidence, the Caltech researchers think they have documented the extremely rare "Snowball Earth" phenomenon, in which virtually the entire planet may have been covered in ice and snow. According to Kirschvink, who originally proposed the Snowball Earth theory, there have probably been only two episodes in which glaciation of the planet reached such an extent — one less than a billion years ago during the Neoproterozoic Era, and the one that has now been discovered from the Paleoproterozoic Era 2.2 billion years ago.

"The young Earth didn't catch a cold very often," says Evans, a graduate student in Kirschvink's lab. "But when it did, it seems to have been pretty severe."

The researchers collected their data by drilling rock specimens in South Africa and carefully recording the magnetic directions of the samples. From this information, the researchers then computed the direction and distance to the ancient north and south poles.

The conclusion was that the place in which they were drilling was 11 degrees (plus or minus five degrees) from the equator when Earth was 2.4 billion years old. Plate tectonic motions since that time have caused South Africa to drift all over the planet, to its current position at about 30 degrees south latitude. Additional tests showed that the samples were from glacial deposits, and further, were characteristic of a widespread region.

Kirschvink and Evans say that the preliminary implications are that Earth can somehow manage to pull itself out of a period of severe glaciation. Because ice and snow tend to reflect sunlight much better than land and water, Earth would normally be expect to have a hard time reheating itself in order to leave an ice age. Thus, one would expect a Snowball Earth to remain forever.

Yet, the planet obviously recovered both times from the severe glaciation. "We think it is likely that the intricacies of global climate feedback are not yet completely understood, especially concerning major departures from today's climate," says Evans. "If the Snowball Earth model is correct, then our planet has a remarkable resilience to abrupt shifts in climate.

"Somehow, the planet recovered from these ice ages, probably as a result of increased carbon dioxide — the main greenhouse gas."

Evans says that an asteroid or comet impact could have caused carbon dioxide to pour into the atmosphere, allowing Earth to trap solar energy and reheat itself. But evidence of an impact during this age, such as a remote crater, is lacking.

Large volcanic outpourings could also have released a lot of carbon dioxide, as well as other factors, such as sedimentary processes and biological factors.

At any rate, the evidence for the robustness of the planet and the life that inhabits it is encouraging, the researchers say. Not only did Earth pull itself out of both periods of severe glaciation, but many of the single-celled organisms that existed at the time managed to persevere.

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Robert Tindol
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