Paleomagnetism and petrophysics of the Jänisjärvi impact structure,Russian Karelia

Abstract— Paleomagnetic, rock magnetic, and petrophysical results are presented for impactites and target rocks from the Lake Jänisjärvi impact structure, Russian Karelia. The impactites (tagamites, suevites, and lithic breccias) are characterized by increased porosity and magnetization, which is in agreement with observations performed at other impact structures. Thermomagnetic, hysteresis, and scanning electron microscope (SEM) analysis document the presence of primary multidomain titanomagnetite with additional secondary titanomaghemite and ilmenohematite. The characteristic impact‐related remanent magnetization (ChRM) direction (D = 101.5°, I = 73.1°, α₉₅ = 6.2°) yields a pole (Lat. = 45.0°N, Long. = 76.9°E, dp = 9.9°, dm = 11.0°). Additionally, the same component is observed as an overprint on some rocks located in the vicinity of the structure, which provides proofs of its primary origin. An attempt was made to determine the ancient geomagnetic field intensity. Seven reliable results were obtained, yielding an ancient intensity of 68.7 ± 7.6 μT (corresponding to VDM of 10.3 ± 1.1 times 10²² Am²). The intensity, however, appears to be biased toward high values mainly because of the concave shape of the Arai diagrams. The new paleomagnetic data and published isotopic ages for the structure are in disagreement. According to well‐defined paleomagnetic data, two possible ages for magnetization of Jänisjärvi rocks exist: 1) Late Sveconorwegian age (900–850 Myr) or 2) Late Cambrian age (?500 Myr). However, published isotopic ages are 718 ± 5 Myr (K‐Ar) and 698 ± 22 Myr (³⁹Ar‐⁴⁰Ar), but such isotopic dating methods are often ambiguous for the impactites.     

The Sun's Distance from the Galactic Plane

Use is made of 93,106 parallaxes from the Hipparcos catalog, with a mixture of spectrum-luminosity classes, to derive the position of the Galactic plane. The reduction technique, mixed total least squares-least squares, takes into account the errors in the parallaxes, and the condition that the direction cosines of the Galactic pole have unit Euclidean norm is rigorously enforced. To obtain an acceptable solution it is necessary to eliminate the stars of classes O and B that belong to the Gould belt. The Sun is found to lie 34.56±0.56 pc above the plane. The coordinates of the Galactic pole, l _g , b _g, are found to be: l _g =0.°004±0.°039; b _g =89.°427±0.°035.This agrees well with what radio observations find and demonstrates that the IAU's recommendation in 1960 to use only radio observations to determine the Galactic pole, although correct at the time because of the paucity of optical observations, can no longer be justified given the plethora of observations contained in the Hipparcos catalog and an adequate reduction technique, unavailable in 1960. The reduction technique is also demonstrably superior to others because it involves fewer assumptions and calculates smaller mean errors. This revised version was published online in July 2006 with corrections to the Cover Date.     

Spectrophotometry of Saturn and its rings from 60 to 180 microns

We observed Saturn at far-infrared and submillimeter wavelengths during the Earth's March 1980 passage through the plane of Saturn's rings. Comparison with earlier spectroscopic observations by D. B. Ward [Icarus32, 437–442 (1977)], obtained at a time when the tilt angle of the rings was 21.8°, permits separation of the disk and ring contributions to the flux observed in this wavelength range. We present two main results: (1) The observed emission of the disk between 60 and 180 μm corresponds to a brightness temperature of 104 ± 2°K; (2) the brightness temperature of the rings drops approximately 20°K between 60 and 80 μm. Our data, in conjunction with the data obtained by other observers between 1 μm and 1 mm, permit us to derive an improved estimate for the total Saturnian surface brightness of (4.84 ± 0.32) × 10^?4W cm^?2 corresponding to an effective temperature of 96.1 ± 1.6°K. The ratio of radiated to incident power, P_R/P_I, is (1.46 ± 0.08)/(1 - A), where A is the Bond albedo. For A = 0.337 ± 0.029, P_R/P_I = 2.20 ± 0.15 and Saturn's intrinsic luminosity is L_S = (2.9 ± 0.5) × 10^?10L_⊙.     

Pole coordinates of the asteroid 511 Davida as determined via the Amplitude-Magnitude method

The Amplitude-Magnitude (AM) method is used for the pole determination of the asteroid 511 Davida, using observations from six oppositions. The possible North poles are found to be λ₁ = 92° ± 7°; β₁ = 33° ± 6°, and λ₂ = 303° ± 4°; β₂ = 34° ± 5°, when scattering effect is not taken into account. When scattering is accounted for, solutions not significantly different from (λ₁, β₁) and (λ₂, β₂) are obtained. The moderately eccentric and inclined orbit of 511 Davida does not allow us to distinguish between the two pole solutions. A comparison with other methods is necessary in order to make a definitive choice.     

A two-color CCD survey of the North Celestial Cap: I. The method

We describe technical aspects of an astrometric and photometric survey of the North Celestial Cap (NCC), from the Pole (δ=90°) to δ=80°, in support of the TAUVEX mission. This region, at galactic latitudes from ∼17° to ∼37°, has poor coverage in modern CCD-based surveys. The observations are performed with the Wise Observatory one-meter reflector and with a new mosaic CCD camera (LAIWO) that images in the Johnson–Cousins R and I bands a one-square-degree field with sub-arcsec pixels. The images are treated using IRAF and SExtractor to produce a final catalogue of sources. The astrometry, based on the USNO-A2.0 catalogue, is good to ∼1 arcsec and the photometry is good to ∼0.1 mag for point sources brighter than R=20.0 or I=19.1 mag. The limiting magnitudes of the survey, defined at photometric errors smaller than 0.15 mag, are 20.6 mag (R) and 19.6 (I). We separate stars from non-stellar objects based on the object shapes in the R and I bands, attempting to reproduce the SDSS star/galaxy dichotomy. The completeness test indicates that the catalogue is complete to the limiting magnitudes.     

Shapes and Pole Orientations of Asteroids (360) Carlova and (209) Dido

Several light-curves of asteroid (360) Carlova and (209) Dido in different epochs were analyzed to determine shapes and pole orientations by means of AM-method and least squares method. New values of a/b, b/c, λ _p and β _p for asteroid (360) Carlova were obtained, which are 1.52°, 1.5°, 120 ± 6° and 66 ± 7°, respectively. We report a first determination of the parameters of (209) Dido which are 1.3°, 1.1°, 221 ± 6° and 37 ± 3°, respectively.     

A Late Mesoproterozoic 40Ar/39Ar age for a melt breccia from the Keurusselkä impact structure,Finland

Field investigations in the eroded central uplift of the ≤30 km Keurusselkä impact structure, Finland, revealed a thin, dark melt vein that intersects the autochthonous shatter cone‐bearing target rocks near the homestead of Kirkkoranta, close to the center of the impact structure. The petrographic analysis of quartz in this melt breccia and the wall rock granite indicate weak shock metamorphic overprint not exceeding ~8–10 GPa. The mode of occurrence and composition of the melt breccia suggest its formation as some kind of pseudotachylitic breccia. ⁴⁰Ar/³⁹Ar dating of dark and clast‐poor whole‐rock chips yielded five concordant Late Mesoproterozoic miniplateau ages and one plateau age of 1151 ± 10 Ma [± 11 Ma] (2σ; MSWD = 0.11; P = 0.98), considered here as the statistically most robust age for the rock. The new ⁴⁰Ar/³⁹Ar age is incompatible with ~1.88 Ga Svecofennian tectonism and magmatism in south‐central Finland and probably reflects the Keurusselkä impact, followed by impact‐induced hydrothermal chloritization of the crater basement. In keeping with the crosscutting relationships in the outcrop and the possible influence of postimpact alteration, the Late Mesoproterozoic ⁴⁰Ar/³⁹Ar age of ~1150 Ma should be treated as a minimum age for the impact. The new ⁴⁰Ar/³⁹Ar results are consistent with paleomagnetic results that suggested a similar age for Keurusselkä, which is shown to be one of the oldest impact structures currently known in Europe and worldwide.     

Intensity and polarization line profiles in a semi-infinite Rayleigh-scattering planetary atmosphere. II. Variations of equivalent width over the disk

The study of the variation of equivalent width in a Rayleighscattering planetary atmosphere along the intensity equator and along the mirror meridian on whichμ =μ ₀ shows that the equivalent widths decrease monotonically towards the poles, the limb and the terminator with the following characteristics: (i) the weakest lines exhibit the maximum change; (ii) theI ^e _r component shows more change than theI ^e _r component; (iii) the decrease towards the limb or the terminator is not as sharp as that towards the poles; (iv)I ^e _r component shows more decrease towards the limb whileI ^e _r component shows more decrease towards the terminator; and (v) the relationW (μ, φ;μ ₀,φ ₀)= W (μ ₀,φ ₀;μ, φ) holds for the total intensity. These results are qualitatively in agreement with the observations of absorption bands in the spectra of Venus, Jupiter and Saturn.     

Pole coordinates of the asteroids 9 metis, 22 kalliope,and 44 Nysa

By means of new photoelectric observations made in 1974 an attempt to determine the poles of asteroids 9 and 44 was made. Following a method based upon the magnitude-aspect and amplitude-aspect relations, the coordinates of the poles for 9 and 44 were found to be, respectively, λ₀ = 191° ± 5°, β₀ = 56° ± 6° and λ₀ = 100° ± 10°, β₀ = 50° ± 10°. The previously published pole for asteroid 22, λ₀ = 215° ± 10°, β₀ = 45° ± 15°, was confirmed. From its phase relation we determined the phase coefficient of 44 Nysa, a very high albedo object (p_v = 0.377). The very low phase coefficient obtained (β_v = 0.018 mag/deg) agrees very well with an inverse relation between geometrical albedo and phase coefficient. The results are summarized in a table.     

Photoelectric lightcurves of the asteroid 1862 Apollo

Photoelectric lightcurves of the asteroid 1862 Apollo were obtained in November–December 1980 and in April–May 1982. The period of rotation is unambiguously determined to be 3.0655 ± 0.0008 hr. The 1980 observations span a range of solar phase angle from 30° to 90°, and the 1982 observations, 0.°2 to 90°. The Lumme-Bowell-Harris phase relation can be fit to the absolute magnitudes at maximum light with an RMS scatter of 0.06 magnitude over the entire range of phase angle. The constants of the solution are absolute V magnitude at zero phase angle and at maximum light, 16.23 ± 0.02; slope parameter, 0.23 ± 0.01. These constant corresponds to values in the linear phase coefficient system of V(1, 0) = 16.50 ± 0.02 and a phase coefficient of β_v = 0.0305 ± 0.0012 mag/degree in the phase range 10°–20°. The slope of the phase curve is typical for a moderate albedo asteroid. The absolute magnitudes observed in 1980 and 1982 fall along a common phase curve. That is, Apollo was not intrinsically brighter at one apparition than the other. This is not surprising, since the two apparitions were almost exactly opposite one another in the sky. A pole position was calculated from the observed deviation of the lightcurve from constant periodicity (synodic-sidereal difference) during both apparitions. The computed 1950 ecliptic coordinates of the pole are: longitude = 56°, latitude = −26°. This is the “north” pole with respect to right-handed (counter-clockwise) rotation. The formal uncertainty of the solution for the pole position is less than 10°, but realistically may be several times that, or even completely wrong. The sidereal period of rotation asscociated with this pole solution is 3.065436 ± 0.000012 hr.     

Shadow mechanism and the opposition effect of brightness of atmosphereless celestial bodies

We consider the Irvine-Yanovistkii modification of the shadow model developed by Hapke for the opposition effect of brightness. The relation between the single scattering albedo ω and the transparency coefficient of particles κ is suggested to be used in the form κ = (1 ? ω) ⁿ , which allows the number of unknowns in the model to be reduced to two parameters (the packing density of particles g and ω) and the single-scattering phase function χ(α). The analysis of spectrophotometric measurements of the moon and Mars showed that the data on the observed opposition effect and the changes in the color index with the phase angle α well agree if the values of n = 0.25 and g = 0.4 (the moon) and 0.6 (Mars) are assumed in calculations. When being applied to asteroids of several types, this method also yielded a satisfactory agreement. For the E-type asteroids, the sets of parameters are [g = 0.6, ω = 0.6, A _g = 0.21, and q = 0.83] or [g = 0.3, ω = 0.4, A _g = 0.15, and q = 0.71] under the Martian single-scattering phase function; for the M-type asteroids, it is [g = 0.4, ω ≤ 0.1, A _g ≤ 0.075, and q ≤ 0.42] under the lunar single-scattering phase function; for the S-type asteroids, it is [g = 0.4, ω = 0.4, A _g = 0.28, and q = 0.49] under the lunar single-scattering phase function; and for the C-type asteroids, it is [g = 0.6, ω ≤ 0.1, A _g ≤ 0.075, and q = 0.43] under the modified lunar single-scattering phase function. The polarization measurements fulfilled by Gehrels et al. (1964) for the bright feature on the lunar surface, Copernicus (L = -20°08′, φ = +10°11′), at a phase angle α = 1.6° revealed the deviations in the position of the polarization plane from that typical for the negative branch. They were 22° and 12° in the G and I filters, respectively. At the same time, the deviation was within the error (±3°) in the U filter and for the dark feature Plato (L = -10°32′, φ = +51°25′), which can be caused by the coherent mechanism of the formation of the polarization peak.     

Longitudinal variation of equatorial electrojet parameters derived from POGO satellite observations

A method of analysis has been developed making it possible to obtain electrojet parameters from vertical profiles of the POGO electrojet data. This has provided an unprecedented quantity of electrojet parameters—about 500 each of its peak eastward current intensity J₀, its total eastward current I₊, and its half width w—spanning through 360° longitude round the Earth, 5 h of daytime and September equinoctial months of the years 1967, 1968 and 1969. The daytime all-longitude averages of the parameters for the 3 years are 235 ± 14 km for w, 232 ± 63A km^?1 for J₀ and (54 ± 11) × 10³A for I₊. This first coverage round the globe shows that J₀ and I₊ vary considerably with longitude unlike w. While confirming the expected maximum of J₀ and I₊ at about 280°E longitude, we find that each of them also has an unexpected major maximum at about 100°E and a subsidiary maximum at 190°E longitude. The major maxima are found to be prominent only at hours near local noon. It is argued that the surprising longitudinal variations of J₀ and I₊ cannot be explained satisfactorily by variations in the upper mantle conductivity along the dip equator but possibly by longitudinal variations of wind systems and gradients at electrojet altitudes.     

Pole orientation of asteroid 44 Nysa via photometric astrometry,including a discussion of the method's application and its limitations

The results of photometric astrometry, a method of determining the orientation of a rotation axis, as applied to asteroid 44 Nysa are presented. The pole orientation of Nysa was found to be λ₀ = 100°, β₀ = +60° with an uncertainty of 10°. The sidereal period is 0^d.26755902 ± 0.00000006, and the rotation prograde. Refinements to, and limitations of, the application of the method of photometric astrometry are discussed. In light of the results presented herein, we believe that all photometric astrometry pole determinations of the past should be redone.     

Orbital parameters and variability of the emission spectrum for the massive binary system 103 Tau

Based on high-resolution spectra taken near the He I 6678 Å line for the massive binary system 103 Tau, we have detected a weak absorption component belonging to the binary’s secondary component. We have measured the radial velocities of both components, improved the previously known orbital parameters, and determined the new ones. The binary has an orbital period P _orb = 58.305^d, an orbital eccentricity e = 0.277, a radial velocity semi-amplitude of the bright component K _A = 44.8 km s^?1, and a component mass ratio M _A /M _B = 1.77. The absence of photometric variability and the estimates of physical parameters for the primary component suggest that the binary most likely has a considerable inclination of the orbital plane to the observer, i ≈ 50°?60°. In this case, the secondary component is probably a normal dwarf of spectral type B5–B8. Based on the spectra taken near the H _α line, we have studied the variability of the emission profile. It is shown to be formed in the Roche lobe of the secondary component, but no traces of active mass exchange in the binary have been detected.     

Two Regularities in the Coronal Green-Line Brightness – Magnetic Field Coupling and the Heating of the Corona

To study the quantitative relationship between the brightness of the coronal green line 530.5 nm Fe xiv and the strength of the magnetic field in the corona, we have calculated the cross-correlation of the corresponding synoptic maps for the period 1977 – 2001. The maps of distribution of the green-line brightness I were plotted using every-day monitoring data. The maps of the magnetic field strength B and the tangential B _t and radial B _r field components at the distance 1.1 R _⊙ were calculated under potential approximation from the Wilcox Solar Observatory (WSO) photospheric data. It is shown that the correlation I with the field and its components calculated separately for the sunspot formation zone ±30° and the zone 40 – 70° has a cyclic character, the corresponding correlation coefficients in these zones changing in anti-phase. In the sunspot formation zone, all three coefficients are positive and have the greatest values near the cycle minimum decreasing significantly by the maximum. Above 40°, the coefficients are alternating in sign and reach the greatest positive values at the maximum and the greatest negative values, at the minimum of the cycle. It is inferred that the green-line emission in the zone ±30° is mainly controlled by B _t, probably due to the existence of low arch systems. In the high-latitude zone, particularly at the minimum of the cycle, an essential influence is exerted by B _r, which may be a manifestation of the dominant role of large-scale magnetic fields. Near the activity minimum, when the magnetic field organization is relatively simple, the relation between I and B for the two latitudinal zones under consideration can be represented as a power-law function of the type I ∝ B ^q. In the sunspot formation zone, the power index q is positive and varies from 0.75 to 1.00. In the zone 40 – 70°, it is negative and varies from −0.6 to −0.8. It is found that there is a short time interval approximately at the middle of the ascending branch of the cycle, when the relationship between I and B vanishes. The results obtained are considered in relation to various mechanisms of the corona heating.     

GJ 900: A new hierarchical system with low-mass components

Speckle interferometric observations made with the 6 m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences in 2000 revealed the triple nature of the nearby (π _Hip = 51.80 ± 1.74 mas) low-mass young (≈ 200 Myr) star GJ 900. The configuration of the triple system allowed it to be dynamically unstable. Differential photometry performed from 2000 through 2004 yielded I- and K-band absolute magnitudes and spectral types for the components to be I _A =6.66±0.08, I _B =9.15±0.11, I _C =10.08±0.26, K _A =4.84±0.08, K _B =6.76±0.20, K _C =7.39±0.31, Sp _A ≈K5?K7, Sp _B ≈M3?M4, Sp _C ≈M5?M6. The “mass-luminosity” relation is used to estimate the individual masses of the components: M _A ≈0.64M _⊙, M _B ≈0.21M _⊙, M _C ≈0.13M _⊙. From the observations of the components’ relative motion in the period 2000–2006, we conclude that GJ 900 is a hierarchical triple star with the possible orbital periods P_A-BC≈80 yrs and P_BC≈20 yrs. An analysis of the 2MASS images of the region around GJ 900 leads us to suggest that the system can include other very-low-mass components.     

Photometric observations and modelling of the asteroid 85 Io in conjunction with data from an occultation event during the 1995–96 apparition

The asteroid 85 Io has been observed using CCD and photoelectric photometry on 18 nights during its 1995–96 and 1997 apparitions. We present the observed lightcurves, determined colour indices and modelling of the asteroid spin vector and shape. The colour indices (U-B = 0.35±0.02, B-V = 0.66±0.02, V-R = 0.34±0.02, R-I = 0.36±0.02) are as expected for a C-type asteroid. The allowed spin vector solutions have the pole co-ordinates λ₀ = 285±4°, β₀ = −52±9° or λ₀ = 108±10°, β₀ = −46±10° and λ₀ = 290±10°, β₀ = −16±10° with a retrograde sense of rotation and a sidereal period P_sid = 0^d.286463±0^d.000001. During the 1995–96 apparition the International Occultation Time Association (IOTA) observed an occultation event by 85 Io. The observations and modelling presented here were analysed together with the occultation data to develop improved constraints on the size of the asteroid. The derived value of 164 km is about 5% larger than the IRAS diameter. © 1999 Elsevier Science Ltd. All rights reserved.     

Pole orientation of 16 psyche by two independent methods

Nineteen new lightcurves of 16 Psyche are presented along with a pole orientation derived using two independent methods, namely, photometric astrometry (PA) and magnitude-amplitude-shape-aspect (MASA). The pole orientations found using these two methods agree to within 4°. The results from applying photometric astrometry were prograde rotation, a sidereal period of $\text{0}\text{d}\text{dot}\text{1748143 ± 0}\text{d}\text{dot}\text{0000003}$, and a pole at longitude 223° and latitude +37°, with an uncertainty of 10°; and, from applying magnitude-amplitude-shape-aspect a pole at 220 ± 1°, +40 ± 4°, and a modeled triaxial ellipsoid shape (a > b > c) with a/b = 1.33 ± 0.02 and b/c = 1.33 ± 0.07. The discrepancy between the high pole latitude found here and the low latitudes reported by others is discussed.     

Analytical Reference Functions <Emphasis Type="Italic">F</Emphasis>(λ) for the Sun's Limb Darkening and Its Absolute Continuum Intensities (λλ 300 to 1100 m)

It is shown (1) that the coefficients A_i of the limb darkening functions I(μ)/I_center = P₅ (μ) = ∑A_i μⁱ (i = 0... 5; μ = cos ϑ), which had been published by Neckel and Labs (Solar Phys. 153, 91, 1994), can well be approximated by analytical functions of wavelength λ, and (2) that at first sight purely formal extrapolation of the functions P₅(μ) to the very limb (μ = 0.0) is not meaningless: in combination with absolute intensities for the disk center these functions yield ‘limb intensities’ which all correspond to almost the same ‘limb temperature’, T_limb≈4746 K. Together these results lead to ‘reference functions’ which can quickly yield rather reliable values of the Sun's continuum intensities, for any values of μ and λ.     

Reclassification and thermal history of Trenzano chondrite

Abstract— We present a new single‐crystal X‐ray diffraction (XRD) study performed on a suite of six orthopyroxene grains from the low‐shocked H6 Trenzano meteorite. The quenched intracrystalline Fe²⁺‐Mg ordering state in orthopyroxene preserves the memory of the cooling rate near closure temperature T_c, thus yielding useful constraints on the last thermal event undergone by the host rock. The orthopyroxene T_c of 522 ± 13 °C, calculated using a new calibration equation obtained by Stimpfl (2005b), is higher than in previously published H chondrite data. The orthopyroxene cooling rate at this T_c is about 100 °C/kyr. This fast rate is inconsistent with the much slower cooling rate expected for H6 in the onion shell structural and thermal model of chondrite parent bodies. A petrographic study carried out at the same time indicated that the Trenzano meteorite is an H5 chondrite and not an H6 chondrite, as it is officially classified. Furthermore, the two‐pyroxene equilibrium temperature of Trenzano (824 ± 24 °C), calculated with QUILF95, is similar to the two‐pyroxene temperature of 750–840 °C obtained for the Carcote (H5) chondrite (Kleinschrot and Okrusch 1999).