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Public Outreach Astrophysics & |
Planets and Comets of our Solar SystemCometary StudiesKrasnopolsky, Mumma, and Abbott continued their study of soft X-rays in comets with the EUVE by observing post perihelion Hale-Bopp and analyzing observations of comets Encke, Mueller, and Borrelly from the EUVE archive. Five of eight observations in their EUVE database show the detectable X-ray emissions. Correlation of the X-ray luminosities with the gas production rates favors charge transfer from the solar-wind heavy ions as an excitation mechanism. However, the EUVE observations of Hale-Bopp, its nondetection with the ROSAT, and its high X-ray luminosity observed with the BeppoSAX require a substantial contribution from the attogram dust scattering of the solar X-rays.Planetary ScienceTo study thermal structure and hydrodynamic escape in Pluto's upper atmosphere, Krasnopolsky suggested two limiting models of Pluto's atmosphere. He found that heating by the UV absorption of CH$_4$ (previously neglected) is even stronger than the EUV heating by N$_2$. He proved the existence of hydrodynamic flow of N$_2$ and developed a method of analytical solution for the equation of hydrodynamic flow. Structure of Pluto's upper atmosphere was calculated by this method at various solar activity. The models show that escape on Pluto disagrees with the classic theory and is case of a slow hydrodynamic escape. Pluto's atmosphere is restricted to 4000 km, which makes possible a close flyby of future spacecraft.Krasnopolsky and Cruikshank developed a
photochemical model of Pluto's
atmosphere and ionosphere. This model is based on a set of the continuity
equations for 44 neutral and 23 ion species and includes 191 chemical
reactions. Altitude profiles, column abundances, escape and precipitation rates
were calculated for each species. The model results are addressed to the basic
aspects of Pluto's atmospheric composition, chemistry, and evolution. NEARClark and Starr are associate team members on the Near Earth Asteroid Rendezvous (NEAR) mission X-ray/Gamma-Ray Spectrometer (XGRS) experiment. They support operations during the cruise and orbital phases of the mission and will have lead roles in data analysis. NEAR will arrive at Eros 433 in January 2000, and will remain in orbit around the asteroid for 1 year. The rendezvous was delayed by about one year because of a failed rendezvous burn maneuver on December 20, 1998. A successful rendezvous burn maneuver was accomplished on January 3, 1999. The XGRS will collect thousands of X-ray and gamma-ray spectra while in orbit around Eros. These observations will be used to obtain elemental composition maps of the surface of Eros. These mapped compositions will be compared to known meteorite compositions in order to determine whether Eros has the characteristics of any of these meteorite types. The maps will also be of importance in determining the nature of formation and evolution of the Eros. The XGRS science team leader is Jacob Trombka at Goddard Space Flight Center.Observations of solar spectra by the XGRS solar monitors on board the NEAR spacecraft during high activity periods have been obtained. These spectra show both continuum and discrete line calcium and iron emissions. Information on gamma-ray detector activation due to cosmic rays has been obtained. The NEAR XGRS has been included in the Inter-Planetary Network (IPN) for the detection of Gamma-Ray Bursts (GRB). The IPN now incorporates GRB information from the NEAR, Ulysses, Compton-GRO, GGS-Wind, and Konus spacecraft. The precision of the timing of the NEAR XGRS GRB detection system has been determined by using the detection of the GRB-associated optical transient GRB 990510. The delay between the arrival time at NEAR and at Konus was both measured and then calculated. The one-sigma difference in arrival time between measurement and calculation was found to be about 100 milliseconds. This difference then corresponds to a possible difference in location of the GRB of about one half arc-minute when 1.5 AU separates the spacecraft. This is well within the two arc-minute requirements for locating position of GRB's in the IPN network. Effects of long-term radiation exposure have been observed in the gamma-ray/X-ray detectors and the mechanism producing this degradation is understood. The results of these have been published. A number of students from both universities and high schools have participated in developing significant parts of the NEAR XGRS data management acquisition analysis processing system. Clark and Starr are currently in the process of creating a master meteoritical information database from a variety of sources, including Jarosewich (the Smithsonian), Mittlefelhdt (JSC), Kallemeyn (from Wasson's group, chondrites), and the Japanese (Antarctic meteorites), which includes elemental abundances (for major, minor, and radioactive elements including Mg, Al, Si, Ca, Ti, Fe, S, K, Th, U, P, Cr, Mn, Zn, Cu, and Ni), ratios, and useful ancillary information, including methodology, laboratory, class, nature of sample, weathering information, and reference for each analysis. They are developing a range of IDL and EXCEL based tools to allow them to determine intrinsic variations in geologically significant elemental abundances and ratios associated with major meteorite classes and components. Products they can generate include spreadsheets, mean and standard deviation, histograms, density plots, and statistically significant populations associated with each class using cluster analysis. They also have interactive capability to pull up all the information for any point on the plot. They will use the database to exclude variations which result from instrumental or laboratory biases and to identify the most useful ratios which can be derived from their spectral XGRS data and used to determine the nearest meteorite analogue and differentiation history of Eros. Thus, they are focusing on data from meteorites which are the most likely analogues, including chondrites, stony irons, and achondrites. Clark and Starr
have established a systematic relationship between their observed
spectra from the solar monitor and the level of solar activity as
determined by the GOES solar X-ray instrument in orbit around the
Earth. They have verified that their solar model data closely matches their
observations. They have also determined that increase in the level of
solar activity is not correlated with increased in high energy charged
particle induced background. Increases in background are associated
with solar mass ejections that are generally associated with, although
they do not occur in every case for very energetic solar flares. They
are now establishing a record of changes in background for the
detectors during cruise, and will verify causes for each shift in
background. On the basis they have generated inputs for the NEAR
analytical software (quicklook) for treating and interpreting their
measurements according to level of solar activity. Information on
typical means and standard deviations for each element for major
meteorite classes and components generated from their interactive
meteorite database are now being used to generate model spectra under a
range of solar conditions anticipated during the mission. Plots of XRF
intensity ratio versus concentration ratio, with realistic error bars
from their interactive database, will be used to determine which ratios
will be most useful in interpreting their results. The predicted results
will be used as tools for planning their observations and developing data
treatment software. Lunar Data Analysis (Lunar Prospector Mission)Starr is a Principal Investigator and Clark a Co-Investigator in the Lunar Data Analysis Program. Clark has used quantitative (sensor fusion) techniques to combined data from Clementine spectral reflectance and Apollo gamma ray spectra derived iron and titanium variation on the Moon. By using this approach, they have optimized the utility of measurements from each data set. In their approach, gamma-ray measurements are used to provide bulk abundances for these two elements (based on calibration to bulk soil abundances), in the case of spectral reflectance measurements, recalibrating published Ti and Fe data (on the basis of soil bulk abundances and mineralogy) to represent Fe and Ti in minerals which absorb most efficiently at the selected wavelengths, namely Fe in pyroxene and Ti in ilmenite grains. They are developing this approach, and a classification matrix based on parameters derivable from near infrared, X-ray, and Gamma-ray spectral measurements, for use in both the analysis of recently obtained gamma-ray observations of the Moon (Lunar Prospector) and for other planetary surfaces with more limited ground truth, in particular for the asteroid Eros which we will encounter on the NEAR mission. Establishing this relationship between near infrared observations, which have been used to classify asteroids on the basis of mineralogy, and high energy spectral data, from which bulk elemental abundances can be derived, can also be useful in the interpretation of ground-based near infrared spectral measurements in terms of their geochemical, as well as mineralogical, significance.MarsMichael Smith is an associate member of the Mars Global Surveyor Thermal Emission Spectrometer (TES) science team. The Mars Global Surveyor has been in orbit around Mars since September 1997 and has been in its primary mapping mission since April 1999. Smith has led an investigation of the ubiquitous dust aerosols that are in the Mars atmosphere. TES has observed several large regional dust storms (including the large November 1997 Noachis Dust Storm), numerous local dust storms, and has monitored the seasonal and geographical variations of the background dust activity. Smith has also led the development of retrieval algorithms to separate the contributions of the atmosphere and the surface from TES spectra. These algorithms are being used to identify and to map mineralogical units on the surface of Mars.
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