Geotimes Logo NEWS NOTES  August 1999 

Nighttime mysteries on Europa
A martian hunt on Earth
Predicting the South China Sea monsoon
A post-stishovite polymorph
Giant clams of the Triassic

Nighttime mysteries on Europa

 

Voyager 2 image of Europa. Photo courtesy of Calvin J. 
Hamilton. NASA's Jet Propulsion Laboratory.
Galileo observations continue to build an intriguing image of Europa, one of Jupiter’s moons. Previous research generated much debate about the moon’s age and ability to harbor life. One of the latest discoveries, using information collected last year from Galileo’s photopolarimeter-
radiometer (PPR), revealed surprising 

nighttime temperatures on Europa. PPR created maps of the thermal radiation of Europa’s nearly craterless surface, showing expected daytime temperatures but anomalous nighttime temperatures.
   John Spencer, an astronomer at Lowell Observatory in Flagstaff, Ariz., and scientists from the Jet Propulsion Laboratory in Pasadena, Calif., and the Goddard Institute for Space Studies in New York, detail Europa’s thermal emissions in the May 28 issue of Science.
   “The most interesting thing is the strange temperature patterns that we see on the nightside of Europa and, particularly, the strong correlation of temperature with latitude in the regions seen after sunset,” Spencer says.
   Daily temperature fluctuations within the uppermost crust (to a depth of about 4 cm) determine the surface temperatures for Europa, an ice-covered satellite about the size of Earth’s moon. In addition, these surface temperatures are related to surface composition and morphology. Therefore, identifying Europa’s surface tempera-tures can produce a better understanding of the processes that shape the surface, the scientists report.
   Usually, darker surfaces respond to sunlight by increasing in temperature during the day and releasing heat at night, resulting in warm nighttime temperatures. Conversely, brighter surfaces absorb less sunlight and release less heat at night, allowing for cool nighttime temperatures.
   Globally, Europa’s light and dark regions do follow this rule. On a smaller scale, however, some of Europa’s light surfaces emit warmer temperatures at night than do the dark surfaces. For example, the area with the highest nighttime temperature (95 K) includes the bright ejecta blanket of the crater Pwyll.
   The nighttime temperatures of the poles and the equator are also curious. Northern latitude areas are warmer than corresponding southern latitude areas, and equatorial regions are unusually cold at night. Regions have increasingly lighter surface colors away from the relatively darker equator, so the lighter northern and southern latitudes should have cooler nighttime temperatures than the equator. However, the opposite is true. For example, at longitude 55° W, the nighttime temperatures are 94.5 K at 30° N and 90.5 K at 30° S, near the poles, but are 88 K at 2° S, near the equator.
   The scientists have no conclusive reasons for the temperature differences, but they propose several explanations. “It seems that brighter, icier regions are warmer at night, which we might expect because large crystals of ice may be better at storing nighttime heat than the brownish, perhaps salt-rich, regions. But in many other places something else must be going on, particularly, to produce the latitude-dependent post-sunset temperatures,” Spencer says.
   The high nighttime temperatures around the Pwyll crater might be explained by an abundance of rocks or regolith particles that are roughly the same size as the sunlight penetration depth (about 4 cm with a density of 0.5 g/cm3). Because the particles would increase the amount of surface area available for heat flow, these rocks would increase the amount of sunlight the crater regolith would absorb, thereby increasing the nighttime temperatures. The ejecta may also absorb sunlight at depths of 1 to 2 cm.
   Galileo’s near-infrared mapping spectrometer found large photon path lengths in high latitude ice. Larger path lengths result from close contact of grains, which increases sunlight penetration and temperatures. However, a similar theory could also be applied to low altitude areas. In theory, high daytime temperatures will cause grain sintering — bond development through contact between solid grains. These sintered grains will also increase the regolith’s ability to absorb sunlight and raise surface temperatures because greater contact between grains increases conductivity, the scientists write.
   Another theory is that tidal heating — the internal frictional heating of a satellite due to movement caused by the parent planet’s gravitational pull — may be responsible for the unusual nighttime temperatures. Researchers predict that the internal heat generated by tidal heating is two to five times greater in the polar regions than at the equator; this prediction coincides with the observed warmer temperatures near the poles.
   The lithospheric thickness of 0.34 km, which is within the Galileo-based estimated thickness of the crust, is necessary to allow enough endogenic heating to raise polar temperatures above temperatures at the equator. However, the local heat flow required for the temperature increase is beyond estimates of the current model for Europa’s mean tidal heat flow. “We don’t consider an internal heat explanation to be very likely,” Spencer says.
   The ridges and fractures that characterize Europa’s surface suggest previous, and maybe present, geologic activity, but no conclusive evidence for current activity exists. Nighttime temperature maps, however, can be used to locate small areas warmed by internal heat. These “hot spots” could remain decades after geologic activity stops. As yet, scientists have not detected these areas.
   “If our alternative explanation for the nighttime anomalies — that they are due to heat leaking out of the interior — is true, it would imply a very warm, active interior and would increase the chances of an ocean and of life,” Spencer says. Most likely, however, the temperature differences are due to the variability of heat storage on Europa’s surface, such as the Pwyll ejecta blanket and surrounding areas. The implications then will not be known until the scientists have a better understanding of the anomalies.
   The scientists would like to attach a thermal mapping instrument on the future Europa orbiter for a higher resolution map. “This might reveal small, currently active regions where internal heat can be detected at the surface … and should also tell us a lot more about what is really controlling the nighttime temperatures,” Spencer says.

— Julia Cole
Geotimes Intern


A martian hunt on Earth

Timothy Kral can’t go to Mars. But he can simulate Mars-like conditions here on Earth, which he did recently to grow microorganisms that survive in extreme environments.
   Kral, a biology professor at the University of Arkansas, and graduate student Curtis Bekkum presented their research in June at the annual meeting of the American Society for Microbiology.
   Scientists have detected no organic matter on the martian surface, which is extremely cold. Mars also has no liquid water on its surface, but scientists suspect it may harbor some liquid below the surface. To create Mars-like conditions, Kral and Bekkum used ash from Hawaiian volcanoes, known to share characteristics with martian soil. The soil simulant approximates the chemical composition, grain size, density, and magnetic properties of martian soil. To grow microorganisms under inhospitable conditions, they had to find microbes that survive in extreme environments (in this case, a carbon dioxide atmosphere, no organic materials, and very limited water) and that thrive on inorganic matter. They found methanogens.
   Methanogens are anaerobic microorganisms considered to be some of the most primitive life forms on Earth. They exist deep in the ocean or in Earth’s crust — environments, like Mars’ surface, also thought of as harsh or extreme. The methanogens that Kral grew in his Mars soil simulant obtained all the necessary macro and trace minerals they needed to survive, even with very limited access to water. Kral says scientists can use research like this to search for life on Mars.
   “When you’re looking for life there, what do you look for? If you have an idea of what life might look like, you may form better ideas about where to look.” Kral’s biological model of what life on Mars could be like is based on what is already known about the martian landscape. “There’s [no real soil] from Mars for us to work with,” he says. “So you have to play the game from the standpoint of Earth.”
   Other researchers are also using extreme Earth environments — and the organisms that inhabit them — to learn more about the possibilities for life on Mars. Robert W. Sanders, a biology professor at Temple University, spent two months this past winter in Antarctica to study microorganisms and retrieve samples of living protists to analyze in his lab. The National Science Foundation’s Life in Extreme Environments (LExEn) program funded his work.
   By studying organisms that live in harsh environments like Antarctica, scientists can gain greater insight into how the organisms survive in such hostile areas. “The data we gathered, plus the DNA samples and live cultures we brought back, will be the basis of research for several years,” Sanders says. “This project, like others supported by the LExEn program, may have tie-ins with the recent reports of fossilized microbes on meteors from Mars. If there were life on other planets, it would have to be adapted to an extreme environment. Our work will contribute to understanding biology at the limits of life.”
   Scientists at NASA and in Russia are investigating the microbiota found in the permafrost of Siberia, Alaska, and Antarctica for a similar astrobiological study. Richard Hoover of NASA’s Marshall Space Flight Center and Elena Vorobyova of Moscow State University were selected by the Joint U.S./Russian Research in Space Science (JURISS) program to pursue their proposal, “Permafrost as microbial habitat — in situ investigation.”
   NASA’s Space Science News quotes Hoover as saying that the microorganisms found in Earth’s permafrost, glaciers, and polar ice caps have great significance to astrobiology. “Dormant ancient microbes, and even higher plants such as moss, can remain viable by cryo-preservation, resuming metabolic activity upon thawing after being frozen in glacial ice or permafrost for thousands to millions of years,” he said. “The microbial extremophiles in the Arctic and Antarctic glaciers and permafrost represent analogues for cells that might be encountered in the permafrost or ice caps of Mars or other icy bodies of the solar system.”
   Not all of the astrobiology research is taking scientists to the chilly poles. Jack Farmer, a geobiologist at Arizona State University, is studying Mono Lake, Calif., one of the oldest — and saltiest — lakes in North America, to find fossils like those that could someday be found on Mars. The work was reported in NASA’s Space Science News in June.
   “The geology of the Mono Basin reminds me of many old martian lake beds,” Farmer told Space Science News. “Take Gusev Crater for example. It’s a basin on Mars formed by an impact more than 3.5 billion years ago. Water flowed in through channels in a huge canyon called Ma’adim Vallis, but there was no outlet. It was an evaporative lake site.” Mono Lake is also an evaporative basin, and an extreme environment for life.
   But Mono Lake is full of microorganisms. And, even more importantly for Farmer, who is pursuing exopaleontology, it is full of microorganism fossils. “Whenever you have minerals that precipitate rapidly, as they do around the springs in Mono Lake, microorganisms become entombed,” he says. “The fossils of soft-bodied microbes formed by this process could be preserved for billions of years.” Farmer hopes to find microfossils at Mono Lake that could serve as an analog to possible martian carbonate deposits.

— Devra Wexler
Geotimes


Predicting the South China Sea monsoon

 

First visible image of the Chinese geostationary weather
satellite FY-11, showing strong convective activity over 
the South China Sea and vicinity during June 21, 1998. 
More information is available at <http://climate.gsfc.nasa.
gov/~kim/relacs/campaign/scsmex.html>. Courtesy of 
National Satellite Meteorological Center/Chinese Mete- 
orological Administration, People's Republic of China.
With new evidence linking the South China Sea monsoon to cyclone activity over the Indian Ocean, scientists are a step closer to forecasting the monsoon’s arrival each spring. Researchers from the South China Sea Monsoon Experiment (SCSMEX), an international scientific field campaign, presented recent study results at the American Geophysical Union (AGU) meeting in June. SCSMEX is a large-scale experiment to study the water and energy cycles of the Asian monsoon regions. A pilot study in 1996, which ended last year, included climatological data analysis, pilot station and mooring site set up, and the planning of Intensive Operational Periods (IOP) strategies. (IOPs are one-to-two-month periods of intense study.) During a field experiment phase between May 1 and July 31, 1998, scientists made hourly surface observations and dual Doppler radar observations, took aerosondes and

PBL measurements, and made upper air soundings four times a day from stations on the South China Sea. The results of the field experiment, along with data analysis and modeling based on information already collected on rainfall estimates, moisture data, sea surface temperature, and other satellite data, led to the AGU presentations in Boston.

Radar and rainfall
According to William Lau, head of the Climate and Radiation Branch of NASA’s Goddard Space Flight Center and co-chief scientist for SCSMEX, one of the most significant parts of the observational platforms is an intensive flux array, consisting of a dual Doppler radar pair for detailed rainfall measurements over the South China Sea. This array, along with land and shipboard radar and rainfall measurements from the Tropical Rainfall Mapping Mission (TRMM) satellite, provides information on cloud and rain patterns. Detailed analysis of the data helps the researchers pinpoint the start of the summer monsoon.
   The South China Sea region experiences a winter monsoon and a summer monsoon. During the winter, cold, dry air flows into the region from the Arctic. But the flow reverses in the spring, and starts bringing warm, wet air from the south. This transition always takes place at a different time, and the SCSMEX scientists are aiming to time the monsoon’s first appearance. Data gathered during last year’s field phase significantly improved their knowledge of the start dates. For example, they determined that the 1998 summer monsoon settled in over the northern part of the South China Sea on May 15, and over the southern part of the sea on May 20. But is there a way to predict the monsoon’s onset? The researchers hope so.

Clues from cyclones
About 10 years ago, scientists recognized a phenomenon of massive twin cyclones that appeared every spring over the equator. The cyclones, each twice the size of Texas, travel eastward, straddling the equator. They then split up, and the southern cyclone dies out. The northern cyclone moves north from the equator and turns into a monsoon over the Bay of Bengal. It turns out that this happens only a few days before the onset of the South China Sea monsoon. According to Lau, the 1998 SCSMEX operations marked the first time the cyclone system could be linked to the start of the Asian monsoon season.
   Lau plans to watch the twin cyclone system unfold this year to try and understand how its strength and timing can help forecast the monsoon — and perhaps predict flooding in southern China.

The monsoon and flooding
Weather changes in the region seem to be linked to the South China Sea monsoons. Lau reports that he and his colleagues found a connection between the disastrous 1998 Yangtze River flood and the timing and strength of the summer monsoon. “In 1998, the South China Sea monsoon came late and with less than its usual punch, a possible warning of the intense rains over South China and the deadly floods that followed,” Lau says.
   “Analysis of SSM/I (Special Sensor Microwave/Imager) wind and moisture data suggested that the delayed convective activity over the South China Sea may be linked to the weakened northward propagation of [the] monsoon rain band, hence contributing to a persistence of the rain band south of the Yangtze River and [to] the disastrous flood that occurred over this region during mid to late June 1998,” Lau reported at AGU.
   If the monsoon can be predicted, perhaps such effects can be forecasted as well. Clues to the cause, timing, and evolution of the summer monsoon, and a better understanding of how it can vary, will help flood planners, water managers, and farmers reduce losses of life, livestock, agriculture, and property. Data analysis and interpretation and the modeling component of SCSMEX, along with satellite measurements, will continue through 2002.

— Devra Wexler
Geotimes


A post-stishovite polymorph

 

A new, post-stishovite, SiO2 polymorph. Thomas
Sharp, Arizonia State University.

A mineral can only take so much pressure. Increasing the stress on a mineral will either shatter or distort it or cause its crystal structure to adapt by compressing into a new, denser polymorph.
   In the three low pressure polymorphs of SiO2 (quartz, tridymite, and cristobalite), the silicon atoms are bonded, or coordinated, with four surrounding oxygen atoms in a tetrahedral structure. The same tetrahedral coordination also exists in one SiO2 high-pressure polymorph (coesite). However, at higher pressures, the silicon is forced to bond with six oxygen atoms into the denser polymorph known as stishovite.
 
   Stishovite was first synthesized in the laboratory, and later found in the shocked rocks of Meteor Crater, Ariz., and other impact sites. Today, scientists believe that stishovite, and also coesite, are indicators of impact events.
   Thomas Sharp of Arizona State University, Ahmed El Goresy of the Max-Planck-Institut für Kemphyisik in Germany, Brigitte Wopenka of Washington University in St. Louis, and Ming Chen of the Guangzhou Institute of Geochemistry in China announce the discovery of a new, post-stishovite, SiO2 polymorph in the May 28, 1999, Science. The new structure’s existence implies enormous force and may provide insight into the nature of high-pressure environments, from impact target rocks to the mineralogy of Earth’s lower mantle.
   Found within the martian achondritic meteorite Shergotty, the newly discovered mineral falls within the orthorhombic crystal system and possesses a structure similar to that of PbO2. Atoms are closely packed in the post-stishovite SiO2, and the researchers approximate the mineral’s density to be as high as 4.12 g/cm3.
   The shock pressure of an impact is revealed in Shergotty by the post-stishovite SiO2 and the presence of maskelynite, a glass that quenched from the shock-induced melt. “The most interesting thing about the post-stishovite SiO2 in Shergotty is that it suggests a much higher [shock] pressure than previously estimated,” Sharp says. According to Sharp and his collaborators, the presence of the post-stishovite SiO2 in Shergotty suggests that the previously calculated shock pressure of 29 Gigapascals (GPa) is too low a value, and the pressure probably exceeded stishovite’s stability range (45 GPa).
   The researchers also suggest that high differential stress may have been necessary to create the new polymorph, rather than exceedingly high pressures. They base this conclusion on similarities between three mineral phases: the post-stishovite phase, a nearly identical SiO2 phase created in the laboratory (see 1990 work by Tsuchida and Yagi in Nature), and structures exposed to the combined effects of a 50-GPa environment and a differential stress of 10 GPa. The quantitative pressure data is particularly important for determining the martian impact origin of the Shergotty meteorite. “It is hard to explain how material was launched off of Mars without a really big impact,” Sharp says.
   The maximum shock pressure that the new polymorph experienced is not yet clear. Sharp and his collaborators write that in some experiments, extensive melting and extreme shock deformation result from pressures exceeding 80 GPa. However, scientists have compressed chondritic meteorite samples (meteorites that are believed to represent condensed solar nebula materials) under pressures reaching 83 GPa, without seeing evidence that the samples melted significantly. Sharp and his colleagues suggest that they won’t know the true value of the shock pressure until they learn more about the stabilities of the new polymorph’s phases and the role of differential stress on post-stishovite SiO2 phase transitions.
   The discovery of post-stishovite SiO2 could also have implications for hypothesized minerals in Earth’s lower mantle. “Clearly the conditions on the surface of Mars during a large impact event are different from those in Earth’s lower mantle,” Sharp says. “But the possible structures of post-stishovite SiO2 are relevant to the Earth.”
   According to Sharp, most researchers believe that (Mg, Fe) SiO3 in the perovskite structure is the primary mineral component of Earth’s lower mantle. However, recent experi-ments suggest that (Mg,Fe) SiO3-perovskite breaks down at 80 to 100 GPa into SiO2 and magnesiowüstite. “If this is correct, the SiO2 phase would probably be the same or very similar [to the post-stishovite polymorph],” Sharp says. The discovery of a naturally occurring, post- stishovite SiO2 polymorph suggests that, even though we cannot yet observe it directly, this lower-mantle mineral may exist.

— Joshua A. Chamot
Department of Geology,
University of Tennessee, Knoxville


Giant clams of the Triassic
 

Wallowaconchidae
Courtesy of Korla McAlpine, University of Montana.
The discovery of a new family and species of “giant” clams revealed information about Triassic clams and the Triassic Pacific. George Stanley, a University of Montana (UM) geology professor, and Thomas Yancey, a researcher from Texas A&M University, described the “giant” clams, which range up to 3 feet in diameter, in the February 1999 issue of Paleontology
   In the mid-1980s, Stanley and a student discovered the saucer-shaped 

fossils, silhouetted in white in a wall of black limestone, in a quarry of the Wallowa Mountains in Oregon. Unable at the time to separate the fossils from the limestone, they transported the limestone, more than 400 pounds of rock, to UM. The similar densities of the fossils and their matrix rendered X-rays ineffective. The fossils’ identities remained a mystery until two years ago, when Yancey, having the tools and the time, began removing the surrounding rock, and could see the internal morphologies of the giant clams. The researchers assigned the fossils the family name Wallowaconchidae, after the surrounding mountains.
   The fossils had unusual wing-like extensions surrounding their central body cavities.  According to the scientists, the clams used chambers inside their shell extensions to nurture and house algae. Partially translucent shells or natural fiber optics allowed light to pass through the clams’ shells to the symbiotic algae.
   The extinct clams lived in the Triassic period about 200 million years ago. The researchers believe that the clams’ habitat was a shallow lagoon, behind coral reefs, that surrounded isolated volcanic islands like Tahiti or Fiji located in the eastern Pacific. Identical fossils were found in the Yukon and in Sonora, Mexico.
   The different locations of the fossils may help geologists reconstruct the paleogeography of the Pacific during the Triassic. “I believe the clams were living in isolation around an island like the Galapagos and eventually plate tectonics brought them crashing into North America. Later fault systems moved them north and south,” Stanley says.

— Julia Cole
Geotimes Intern


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