Derived Production Rates Research Articles

Determination of regolith production rates from 238U-234U-230Th disequilibrium in deep weathering profiles (Longnan, SE China)

The present study seeks to evaluate the application of the 238U-234U-230Th radioactive disequilibrium methodology for the determination of the regolith production rates in thick weathering profiles marked by long histories, encountered under various climate regimes, but still very little studied by these techniques. For this purpose, 238U-234U-230Th disequilibria have been analyzed in the top 11 m of a lateritic profile developed on a granitic bedrock in south China (Longnan, Jiangxi Province) under a subtropical climate. The results demonstrate that in such a weathering profile the determination of weathering rates from the analysis of U-series nuclides in bulk rock samples cannot be recovered by applying in one step to the entire alteration profile the modeling approach classically used to interpret the U-series nuclides, i.e. the “gain and loss” model. The modeling has to be made on subsections of relatively small size (<1 or 2 m of thickness), so that the model assumptions can be met, especially the constancy of the mobility parameters along the weathering zone. The results also confirm that the upper part of the weathering profiles marked by the vegetation/biological influences and responding to the short-term climate variations is not well adapted for applying the U-series nuclides methodology for recovering regolith production rates. Based on the data, regolith production rates were estimated independently on four different deeper zones of the profile. Similar values of ~2 m/Ma have been obtained whatever the level, suggesting that such a profile of more than 5 million years would be formed at a relatively stable long-term production rate (averaged over several thousand years). This production rate is slow compared to the production rate deduced from previous in situ 10Be data, which can be explained by assuming non steady-state erosion of the upper part of the profile. Slow denudation rates similar to the U-series derived production rates of 2 m/Ma can thus be obtained with a minimum exposure time of 60 ky, and an inherited component of 20,000 atoms/g originating from the exhumed deeper part of the profile. Altogether the data demonstrate that the combined analysis of U-series and cosmogenic nuclides, which has the potential to become a relevant approach to constrain the dynamics of continental surfaces, requires (a) dense and deep sampling for both nuclides studies, and (b) also to consider more systematically the polyphased and variable history of erosion of the continental surface during the Quaternary.

The surface distributions of the production of the major volatile species, H2O, CO2, CO and O2, from the nucleus of comet 67P/Churyumov-Gerasimenko throughout the Rosetta Mission as measured by the ROSINA double focusing mass spectrometer

The Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) suite of instruments operated throughout the over two years of the Rosetta mission operations in the vicinity of comet 67P/Churyumov-Gerasimenko. It measured gas densities and composition throughout the comet's atmosphere, or coma. Here we present two-years' worth of measurements of the relative densities of the four major volatile species in the coma of the comet, H2O, CO2, CO and O2, by one of the ROSINA sub-systems called the Double Focusing Mass Spectrometer (DFMS). The absolute total gas densities were provided by the Comet Pressure Sensor (COPS), another ROSINA sub-system. DFMS is a very high mass resolution and high sensitivity mass spectrometer able to resolve at a tiny fraction of an atomic mass unit. We have analyzed the combined DFMS and COPS measurements using an inversion scheme based on spherical harmonics that solves for the distribution of potential surface activity of each species as the comet rotates, changing solar illumination, over short time intervals and as the comet changes distance from the sun and orientation of its spin axis over long time intervals. We also use the surface boundary conditions derived from the inversion scheme to simulate the whole coma with our fully kinetic Direct Simulation Monte Carlo model and calculate the production rates of the four major species throughout the mission. We compare the derived production rates with revised remote sensing observations by the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) as well as with published observations from the Microwave Instrument for the Rosetta Orbiter (MIRO). Finally we use the variation of the surface production of the major species to calculate the total mass loss over the mission and, for different estimates of the dust/gas ratio, calculate the variation of surface loss all over the nucleus.

Spatially resolved evolution of the local H2O production rates of comet 67P/Churyumov-Gerasimenko from the MIRO instrument on Rosetta

Aims. Using spectroscopic and continuum data measured by the MIRO instrument on board Rosetta of comet 67P/Churyumov-Gerasimenko, it is possible to derive and track the change in the water production rate, to learn how the outgassing evolves with heliocentric distance. The MIRO data are well suited to investigate the evolution of 67P, in unprecedented spatial and temporal detail.Methods. To obtain estimates of the local effective Haser production rates we developed an efficient and reliable retrieval approach with precalculated lookup tables. We employed line area ratios (H2 16 O/H2 18 O) from pure nadir observations as the key variable, along with the Doppler shift velocity, and continuum temperature. This method was applied to the MIRO data from August 2014 until April 2016. Perihelion occurred on August 13, 2015 when the comet was 1.24 AU from the Sun.Results. During the perihelion approach, the water production rates increased by an order of magnitude, and from the observations, the derived maximum for a single observation on August 29, 2015 is (1.42 ± 0.51) ×1028 . Modeling the data indicates that there is an offset in the peak outgassing, occurring 34 ± 10 days after perihelion. During the pre-perihelion phase, the production rate changes with heliocentric distance as r h −3.8±0.2 ; during post-perihelion, the dependence is r h −4.3±0.2 . The comet is calculated to have lost 0.12 ± 0.06 % of its mass during the perihelion passage, considering only water ice sublimation. Additionally, this method provides well sampled data to determine the spatial distribution of outgassing versus heliocentric distance. The time evolution is definitely not uniform across the surface. Pre- and post-perihelion, the surface temperature on the southern hemisphere changes rapidly, as does the sublimation rate with an exponent of ~−6. There is a strong latitudinal dependence on the r h exponent with significant variation between northern and southern hemispheres, and so the average over the comet surface may only be of limited importance. We present more detailed regional variation in the outgassing, demonstrating that the highest derived production rates originate from the Wosret, Neith and Bes regions during perihelion.

SIMULTANEOUS OBSERVATIONS OF COMET C/2002 T7 (LINEAR) WITH THE BERKELEY-ILLINOIS-MARYLAND ASSOCIATION AND OWENS VALLEY RADIO OBSERVATORY INTERFEROMETERS: HCN AND CH3OH

We present observations of HCN J = 1-0 and CH_3OH J(K_a, K_c) = 3(1, 3)-4(0, 4) A+ emission from comet C/2002 T7 (LINEAR) obtained simultaneously with the Owens Valley Radio Observatory (OVRO) and Berkeley-Illinois-Maryland Association (BIMA) millimeter interferometers. We combined the data from both arrays to increase the (u, v) sampling and signal to noise of the detected line emission. We also report the detection of CH_3OH J(K_a, K_c) = 8(0, 8)-7(1, 7) A^+ with OVRO data alone. Using a molecular excitation code that includes the effects of collisions with water and electrons, as well as pumping by the Solar infrared photons (for HCN alone), we find a production rate of HCN of 2.9 × 10^(26) s^(–1) and for CH_3OH of 2.2 × 10^(27) s^(–1). Compared to the adopted water production rate of 3 × 10^(29) s^(–1), this corresponds to an HCN/H_2O ratio of 0.1% and a CH_3OH/H_2O ratio of 0.7%. We critically assess the uncertainty of these values due to the noise (~10%), the uncertainties in the adopted comet model (~50%), and the uncertainties in the adopted collisional excitation rates (up to a factor of 2). Pumping by Solar infrared photons is found to be a minor effect for HCN, because our 15 synthesized beam is dominated by the region in the coma where collisions dominate. Since the uncertainties in the derived production rates are at least as large as one-third of the differences found between comets, we conclude that reliable collision rates and an accurate comet model are essential. Because the collisionally dominated region critically depends on the water production rate, using the same approximate method for different comets may introduce biases in the derived production rates. Multiline observations that directly constrain the molecular excitation provide much more reliable production rates.

In situ cosmogenic 10Be and 21Ne in sanidine and in situ cosmogenic 3He in Fe–Ti-oxide minerals

We report concentrations of in situ cosmogenic 10Be and 21Ne from coexisting quartz and sanidine separates and of cosmogenic 3He in coexisting Fe–Ti-oxide minerals from ignimbritic successions of northern Chile (Oxaya and Lauca ignimbrites). New mineral-isotope pairs such as sanidine and Fe–Ti-oxide minerals are helpful in quantitative geomorphology for geological settings where the lithology lacks the commonly used minerals quartz, pyroxene or olivine. Production rates in sanidine and Fe–Ti-oxide minerals were determined by normalizing nuclide concentrations to established production rates in quartz. The experimentally determined production rates are compared to model production rates calculated with new cross-sections for 3He, 21Ne, and 10Be production from the individual target elements. The mean experimental 21Ne production rate for five sanidine samples is 30.4 ± 3.7 atoms g − 1 yr − 1 (30.4 ± 5.4 atoms g − 1 yr − 1 including the uncertainty of the 21Ne production rate in quartz, P 21Ne qtz). This is in excellent agreement with the modelled value of 28.3 atoms g − 1 yr − 1 , which has an estimated uncertainty of 20%. The 21Ne production rate in sanidine is thus about 50% higher than that in quartz. The cosmogenic neon in sanidine is entirely released in low temperature steps (400–600 °C) and no signs of an interfering nucleogenic neon component were observed. This is in stark contrast to quartz and makes sanidine an attractive mineral for terrestrial cosmogenic neon studies. 3He diffuses out of the sanidine structure. Preliminary results also indicate that sanidine is well suited for 10Be studies. The mean experimental 10Be production rate from two sanidine samples is 4.45 ± 0.38 atoms g − 1 yr − 1 (4.45 ± 0.42 atoms g − 1 yr − 1 including uncertainty of P 10Be qtz), very close to the modelled value of 4.55 atoms g − 1 yr − 1 , which has an estimated uncertainty of 20%. We emphasize that 10Be and 21Ne production in sanidine is composition dependent. Therefore, major element analyses should be carried out and production rates calculated on a sample by sample basis. Fe–Ti-oxide minerals retain 3He quantitatively. Experimentally derived production rates are in excellent agreement with new values derived from physical modelling. The mean experimental 3He production rate for five Fe–Ti-oxide minerals samples is 120 ± 11 atoms g − 1 yr − 1 (120 ± 12 atoms g − 1 yr − 1 including the uncertainties of P 21Ne qtz), compared to a mean modelled value of 124 atoms g − 1 yr − 1 , which has an estimated uncertainty of 20%. The variable chemical and structural composition of the solid solution lines of Fe–Ti-oxide minerals has little effect on the total 3He production rate. Cosmogenic 21Ne is not produced in significant quantities in Fe–Ti-oxide minerals due to the absence of suitable target elements.

Ethane Production and Release in Comet C/1995 O1 Hale–Bopp

Ethane (C 2H 6) was detected in Comet C/1995 O1 Hale–Bopp on 13 dates between UT 1996 September 20.3 ( R h=3.01 AU pre-perihelion) and 1997 September 25.7 ( R h=2.83 AU post-perihelion) using high-resolution infrared spectroscopy. Production rates and rotational temperatures were measured, and the derived heliocentric dependence for ethane production was Q=(5.52±0.20)×10 28 [ R h (−2.43±0.13)] molecules s −1. The spatial distribution of C 2H 6 molecules in the coma was consistent with all ethane being released directly from the nucleus, although the possibility that a small fraction was released as a distributed source cannot be excluded. When our derived production rates for ethane, water, and acetylene (C 2H 2) are compared, we obtain an average relative abundance of C 2H 6/H 2O=(6.23±0.42)×10 −3, and C 2H 6/C 2H 2=2.4±0.7. The high ethane abundance relative to acetylene in Hale–Bopp suggests its ices were altered by radiation processing and/or hydrogen-atom addition reactions on the surfaces of ice-mantled grains in the natal cloud. These results are not consistent with ices in Hale–Bopp originating in a thermally or chemically equilibrated region of the solar nebula.

Carbonyl Sulfide in Comets C/1996 B2 (Hyakutake) and C/1995 O1 (Hale–Bopp): Evidence for an Extended Source in Hale–Bopp

Carbonyl sulfide was detected in Comet Hale–Bopp (C/1995 O1) using high dispersion infrared spectroscopy at the NASA Infrared Telescope Facility on Mauna Kea, Hawaii. Six lines (P10, P11, P12, P14, P15, and P17) of the ν3fundamental band near 4.85 μm were detected on UT April 16.0, April 30.0, and May 1.2, 1997, and two additional lines (P18 and P22) were detected on April 30.0. Water was detected directly through emission lines of its ν1–ν2hot band on UT April 16.0, April 30.0, and May 1.2, with a rotational temperature of 93 ± 16 K on May 1.2. Adopting a rotational temperature of 100 K for OCS, our derived production rates are (3.80 ± 0.67) and (1.77 ± 0.85) × 1028molecules s−1on April 16.0 and May 1.2, respectively. The measured production rates imply mixing ratios (QOCS/QH2O) of (4.58 ± 0.77) and (2.76 ± 1.34) × 10−3on April 16.0 and May 1.2, respectively. Comparison of spatial profiles for OCS, H2O, and the dust continuum derived from spectra taken less than 1 h apart suggests that on April 16.0 a significant amount of OCS (∼70% of the total) was released as an extended source. On April 30.0 and May 1.2 the quality of the data prevent a firm conclusion regarding an extended source contribution. If the nature of the OCS source did not change between April 16.0 and May 1.2, the weighted mean mixing ratio is (4.13 ± 0.77) × 10−3. We searched for lines P1 through P8 of the ν3band of OCS in Comet Hyakutake (C/1996 B2) on UT March 24.5, 1996. Our upper limit (3σ) for the OCS production rate in Hyakutake is 1.0 × 1027molecules s−1(QOCS/QH2O< 5.3 × 10−3).

Comet Lightcurves: Effects of Active Regions and Topography

The lightcurves and derived production rates of volatiles such as water for short-period comets show asymmetries around perihelion. These asymmetries do not follow a specific trend, and short-period comets are as likely to be brighter preperihelion as postperihelion. This and images of the nucleus of Comet P/Halley suggest that the activity of comet nuclei is driven by a small number of active regions. Lightcurve asymmetries may then be understood by changing geometry of the active regions with respect to the Sun. Solutions of the heat diffusion equation for a short-period comet with active regions covering the full range of spin axis orientations show that active regions can produce asymmetries of several orders of magnitude in the sublimation rate of water from the nucleus. These large and sudden swings in activity are reduced if the active zone lies within a crater or topographical depression on the nucleus. Simulation of the sublimation rate of H2O from craters on a comet nucleus show that topography can qualitatively change the dependence of water production on heliocentric distance. The effect of negative relief topography is to slow the loss of heat from the nucleus and maintain higher sublimation rates than from a flat surface active region experiencing winter. In the absence of topography, a single active region can lead to sublimation rates that vary over several orders of magnitude on short time scales. Negative relief topography reduces the magnitude and speed of these seasonal changes in comet activity, with rougher surfaces having smaller seasonal variations. The magnitude of seasonal changes in comet activity can be used with the results of this model to place upper limits on the amount of surface roughness at known active regions.

IUE observations of the evolution of Comet Wilson (1986/): Comparison with P/Halley

The ultraviolet spectrum of comet Wilson (1986 l) was observed with the International Ultraviolet Explorer ( IUE) satellite observatory between September 1986 and November 1987. Observations were made at weekly and biweekly intervals near perihelion, making comet Wilson the first dynamically “new” comet to be observed with the IUE through perihelion. This provided the opportunity to compare a “new” comet, Wilson, with a highly evolved comet, P/Halley (1986 III), which was observed both preperihelion and postperihelion at about the same heliocentric distances. The derived production rates of several gases, as well as the visual brightness of comet Wilson, were found to decrease monotonically near perihelion, even though the comet remained at approximately the same heliocentric distance. No short term variability of the type seen in P/Halley was observed in comet Wilson. The water production rates, derived from the observed OH emission, were roughly the same for both comets near 1.2 AU, although earlier (near 2.5 AU inbound), comet Wilson showed significantly greater water production than comet Halley. The column densities of several gas species, relative to OH, were about half as high in comet Wilson as in P/Halley preperihelion at comparable heliocentric and geocentric distances. Previously unidentified spectral features were detected near 1474 and 1425 Å; these were identified as Si emissions, with significant fluorescence efficiencies only when the heliocentric velocity of the comet is near zero. However, the intensity of the λ1474 feature, relative to Si λ1814, is inconsistent with atomic data in the literature.