Neutral particle release from Europa’s surface     

C. Plainaki  A. Milillo  S. Orsini 《Icarus》2010,210(1):385-395

In this paper, we look at space weathering processes on the icy surface of Jupiter’s moon Europa. The heavy energetic ions of the jovian plasma (H⁺, O⁺, S⁺, C⁺) can erode the surface of Europa via ion sputtering (IS), ejecting up to 1000 H₂O molecules per ion. UV photons impinging the Europa’s surface can also result in neutral atom release via photon-stimulated desorption (PSD) and chemical change (photolysis). In this work, we study the efficiency of the IS and PSD processes for ejecting water molecules, simulating the resulting neutral H₂O density. We also estimate the contribution to the total neutral atom release by the Ion Backscattering (IBS) process. Moreover, we estimate the possibility of detecting the sputtered high energy atoms, in order to distinguish the action of the IS process from other surface release mechanisms. Our main results are: (1) The most significant sputtered-particle flux and the largest contribution to the neutral H₂O density come from the incident S⁺ ions; (2) the H₂O density produced via PSD is lower than that due to sputtering by ∼1.5 orders of magnitude; (3) in the energy range below 1 keV, the IBS can be considered negligible for the production of neutrals, whereas in the higher energy range it becomes the dominant neutral emission mechanism; (4) the total sputtering rate for Europa is 2.0 × 10²⁷ H₂O s⁻¹; and (5) the fraction of escaping H₂O via IS is 22% of the total sputtered population, while the escape fraction for H₂O produced by PSD is 30% of the total PSD population. Since the PSD exosphere is lower than the IS one, the major agent for Europa’s surface total net erosion is IS on both the non-illuminated and illuminated side. Lastly, the exospheric neutral density, estimated from the Galileo electron density measurements appears to be higher than that calculated for H₂O alone; this favors the scenario of the presence of O₂ produced by radiolysis and photolysis.  相似文献   

In-line digital holographic microscopy for terrestrial and exobiological research     

S.K. Jericho  J. Nadeau  M.H. Jericho  H.J. Kreuzer 《Planetary and Space Science》2010,58(4):701-1752

We describe here a simple digital in-line holographic microscope (DIHM) that was used to investigate the microbial life forms that exist in perennial springs and glacial melt-water pools on Axel Heiberg Island at near 80°N latitude in the Canadian High Arctic. The instrument determined an upper limit of the density of microbial organisms in the springs and also found an abundance of algae and bacteria in the pools formed from glacial run off. The discovery of life in extra-terrestrial regions of our solar system has been the aim of several space missions. DIHM can capture the dynamics of objects throughout an imaging volume with wavelength limited resolution. The simplicity of DIHM technology furthermore allows the construction of very light-weight and rugged instruments that we believe can be easily adapted for space missions and exobiological studies.  相似文献   

Galileo PPR observations of Europa: Hotspot detection limits and surface thermal properties     

Julie A. Rathbun  Nathaniel J. Rodriguez  John R. Spencer 《Icarus》2010,210(2):763-769

The Galileo photopolarimeter–radiometer (PPR) made over 100 observations of Europa’s surface temperature. We have used these data to constrain a diurnal thermal model and, thus, map the thermal inertia and bolometric albedo over 20% of the surface. We find an increased thermal inertia at mid-latitudes that is widespread in longitude and does not appear to correlate with geology, albedo, or other observables. Our derived thermophysical properties can be used to predict volatile stability across the surface over the course of a day and in planning of infrared instruments on future missions. Furthermore, while observations in the thermal infrared can and have been used to find endogenic activity, no such activity was detected at Europa. We have calculated the detection limits of these PPR observations and find that 100 km² hotspots with temperatures of 116–1200 K could exist undetected on the surface, depending on the location.  相似文献   

Constraints on Europa’s rotational dynamics from modeling of tidally-driven fractures     

Alyssa Rose Rhoden  Burkhard Militzer  Eric M. Huff  Terry A. Hurford  Michael Manga  Mark A. Richards 《Icarus》2010,210(2):770-784

Cycloids, arcuate features observed on Europa’s surface, have been interpreted as tensile cracks that form in response to diurnal tidal stress caused by Europa’s orbital eccentricity. Stress from non-synchronous rotation may also contribute to tidal stress, and its influence on cycloid shapes has been investigated as well. Obliquity, fast precession, and physical libration would contribute to tidal stress but have often been neglected because they were expected to be negligibly small. However, more sophisticated analyses that include the influence of Jupiter’s other large satellites and the state of Europa’s interior indicate that perhaps these rotational parameters are large enough to alter the tidal stress field and the formation of tidally-driven fractures. We test tidal models that include obliquity, fast precession, stress due to non-synchronous rotation, and physical libration by comparing how well each model reproduces observed cycloids. To do this, we have designed and implemented an automated parameter-searching algorithm that relies on a quantitative measure of fit quality, which we use to identify the best fits to observed cycloids. We then apply statistical techniques to determine the tidal model best supported by the data. By incorporating obliquity, fits to observed southern hemisphere cycloids improve, and we can reproduce equatorial and equator-crossing cycloids. Furthermore, we find that obliquity plus physical libration is the tidal model best supported by the data. With this model, the obliquities range from 0.32° to 1.35°. The libration amplitudes are 0.72–2.44°, and the libration phases are −6.04° to 17.72° with one outlier at 84.5°. The variability in obliquity is expected if Europa’s ice shell is mechanically decoupled from the interior, and the libration amplitudes are plausible in the presence of a subsurface ocean. Indeed, the presence of a decoupling ocean may result in feedbacks that cause all of these rotational parameters to become time-variable.  相似文献   

Growth of spotted ragged-tooth sharks Carcharias taurus (Rafinesque) in captivity     

《African Journal of Marine Science》2013,35(1):15-19

Eight spotted ragged-tooth sharks Carcharias taurus (95–248,4 cm total length TL), ranging in known age from 0 to 16,6 years, were used to investigate the growth of the species in captivity. Total length was measured by a standard technique in some cases and estimated from photographs in others. The accuracy of the photographic technique was evaluated and found adequate for captive growth studies in sharks. Parameters of various growth models were estimated by non-linear regression and the special Von Bertalanffy growth model provided the best fit to the age-length data of the combined sexes. The parameters of the special Von Bertalanffy model are L _∞ = 249,8 cm TL, K = 0,233·year^?1, t ₀ = ?2,238 years. Longevity of captive C. taurus is greater than 16,6 years.  相似文献   

Rockin' and Rappin' in American Music: Themes and Resources     

《The Journal of geography》2012,111(6):261-270

Abstract

As a relevant aspect of contemporary culture, music is a significant part of the lifestyles of students at all levels of education. Thus, use of various genres of American music is framed in the context of the National Standards five themes of geography—location, movement, human-environment interaction, region, and place. Several classroom projects appropriate for grades 7–12 are identified, including data collection, map preparation, and development of a research paper. Teaching resources consist of references used in text, music geography bibliography in addition to references, data sources, useful music-related websites, and list of research paper topics.  相似文献   

Looking Toward the Future: A Concepts,Themes, and Standards Approach to Preservice Teacher Education     

《The Journal of geography》2012,111(5):513-518

Abstract

Preservice teacher education based on content, skills, and perspectives specified in Geography for Life: National Geography Standards 1994 is essential for dissemination of the Standards to classrooms all across the country. Curricular materials are often structured around the five fundamental themes of geography; however, it is essential that teacher preparation courses integrate the content of the Standards into the themes of location, place, human—environment interaction, and region. One model of this approach to preservice education, as summarized here, provides future teachers with a conceptual basis for learning and teaching geography, enhancing opportunities for disseminating solid geography content to all grade levels nationwide.  相似文献   

The National Geography Standards and Your Undergraduate Curriculum: The Opportunity That Knocks More Than Once     

《The Journal of geography》2012,111(5):534-537

Abstract

Geography For Life: National Geography Standards 1994 offers university geographers opportunities in the areas of program assessment and development. This article describes an assessment process that is applicable to both individual courses and geography programs. The process can strengthen programs by making them more consistent. As a result, geography may have a more powerful presence in teacher certification programs. Additional techniques for enhancing geography's role in preservice education curricula are also outlined. Finally, the article expresses concern that geographers at the university level will not take the recently published National Geography Standards seriously, thinking they apply only to K-12 education. They are challenged to extend the Standards' scope and content to the baccalaureate degree.  相似文献   

Future Teachers of Geography: Whose Opportunity?     

《The Journal of geography》2012,111(5):487-494

Abstract

Although geography is being revived in the K-12 curriculum, preservice teachers—candidates for the teaching profession—now in our universities are not universally reaping the benefits of cooperative efforts of geography and college of education faculty. Reform is appropriate in both camps. Colleges of education, influenced by many agents, including Certificate/licensing regulations, accrediting agencies, professional organizations, and a stream of reform movements, are bringing education to the level of a true profession. The dimensions of the reform underway and several key players offer great opportunities for geography education and for improving the general climate for teaching excellence in institutions of higher learning. Candidates for the new teaching profession and their education faculty are teaching and learning collaboratively, and conducting research in diverse school districts. Geographers have new reasons and new opportunities to participate in the education of the geography teachers of tomorrow.  相似文献   

Spatial distribution patterns of wetland plants in relation to environmental gradient in the Honghe National Nature Reserve, Northeast China     

ZHOU Demin  LUAN Zhaoqing  GUO Xiaoyu  LOU Yanjing 《地理学报》2012,22(1):57-70

Quantifying correlation between the spatial patterns of natural wetland plants and environmental gradient gives better understanding of wetland habitats, which is the fundamental for the strategy making on the protection and restoration of natural wetlands. In this study, the spatial patterns of wetland plants and the environmental gradient of wetland habitats were assessed in the Honghe National Nature Reserve (HNNR) in Northeast China, a wetland of international importance on the Ramsar list. Biophysical parameters' values of wetland plants were obtained by field sampling methods, and wetland mapping at the community scale was completed using remote sensing techniques. Digital delineation of the surface water system, hydrological zoning and wetness index were produced by spatial analysis methods in Geographic Information System. An ecological ordination method and two clustering methods were used to quantify the relationship between the spatial distribution patterns of wetland plants and the corresponding environmental gradients. Such quantitative analyses also present the specific diversity of different types of wetland plants based on the environmental attributes of their habitats. With the support from modern geo-information techniques, the experimental results indicate how four ecotypes of wetland plants spatially transit from forest swamp, shrub wetland and meadow into marsh wetland with increasing wetness index and water table. And they also show how wetland spatial distribution patterns are controlled by an environmental gradient of wetness. Another key finding of this research work is that our results present the exact fundamental differences between marsh and non-marsh plants of 11 wetland plant communities within the core study area. Hence, this case study gives a good sample for better understanding of the complex correlation between the spatial patterns of wetland plants and their environmental attributes using advanced digital analysis methods. It is also useful to show how to integrate geoinformatic techniques with statistical analysis methods based on the field data base.  相似文献