Detection of sporadic impact flashes on the Moon: Implications for the luminous efficiency of hypervelocity impacts and derived terrestrial impact rates

We present the first redundant detection of sporadic impact flashes on the Moon from a systematic survey performed between 2001 and 2004. Our wide-field lunar monitoring allows us to estimate the impact rate of large meteoroids on the Moon as a function of the luminous energy received on Earth. It also shows that some historical well-documented mysterious lunar events fit in a clear impact context. Using these data and traditional values of the luminous efficiency for this kind of event we obtain that the impact rate on Earth of large meteoroids (0.1-10 m) would be at least one order of magnitude larger than currently thought. This discrepancy indicates that the luminous efficiency of the hypervelocity impacts is higher than 10⁻², much larger than the common belief, or the latest impact fluxes are somewhat too low, or, most likely, a combination of both. Our nominal analysis implies that on Earth, collisions of bodies with masses larger than 1 kg can be as frequent as 80,000 per year and blasts larger than 15-kton could be as frequent as one per year, but this is highly dependent on the exact choice of the luminous efficiency value. As a direct application of our results, we expect that the impact flash of the SMART-1 spacecraft should be detectable from Earth with medium-sized telescopes.     

Microwave remote sensing of Jupiter's atmosphere from an orbiting spacecraft

Microwave remote sounding from a spacecraft flying by or in orbit around Jupiter offers new possibilities for retrieving important and presently poorly understood properties of its atmosphere. In particular, we show that precise measurements of relative brightness temperature as a function of off-nadir emission angles, combined with absolute brightness temperature measurements, can allow us to determine the global abundances of water and ammonia and study the dynamics and deep circulations of the atmosphere in the altitude range from the ammonia cloud region to depths greater than 30 bars in a manner which would not be achievable with ground-based telescopes.     

Cometary impact and the magnetization of the Moon

Collisions of comets with planetary bodies are capable of impressing patterns of magnetization onto them that match those observed for the Moon and possibly for Mercury. The ambient solar wind magnetic field is briefly but strongly enhanced as the large partially ionized cometary atmosphere is compressed against the planetary surface. Just at the time of peak field enhancement, the solid part of the comet collides with the surface and the compressed fields are permanently imprinted by shock magnetization.     

A Moon orbiting observatory for the measurement of the elemental and isotopic composition in the primary cosmic radiation

We describe a project for the measurement of elemental composition of the primary cosmic radiation to be performed by a space observatory orbiting around the Moon. The absence of atmosphere and the low intrinsic magnetic field of the Moon give access to the very low energy component of the cosmic radiation, allowing the search for rare events.The quest for antinuclei, the determination of the lunar lepton albedo and the abundance measurement of galactic radioactive clocks (Be¹⁰, C¹⁴, Al²⁶) are the major tasks of the ANTARES apparatus (ANTimatter Assessment RESearch).We report details of the instrument design, the expected performance for single detectors, their capability to accomplish the proposed measurements and the characteristics of the space mission.     

Transitional impact craters on the Moon: Insight into the effect of target lithology on the impact cratering process

We studied a data set of 28 well‐preserved lunar craters in the transitional (simple‐to‐complex) regime with the aim of investigating the underlying cause(s) for morphological differences of these craters in mare versus highland terrains. These transitional craters range from 15 to 42 km in diameter, demonstrating that the transition from simple to complex craters is not abrupt and occurs over a broad diameter range. We examined and measured the following crater attributes: depth (d), diameter (D), floor diameter (D_f), rim height (h), and wall width (w), as well as the number and onset of terraces and rock slides. The number of terraces increases with increasing crater size and, in general, mare craters possess more terraces than highland craters of the same diameter. There are also clear differences in the d/D ratio of mare versus highland craters, with transitional craters in mare targets being noticeably shallower than similarly sized highland craters. We propose that layering in mare targets is a major driver for these differences. Layering provides pre‐existing planes of weakness that facilitate crater collapse, thus explaining the overall shallower depths of mare craters and the onset of crater collapse (i.e., the transition from simple to complex crater morphology) at smaller diameters as compared to highland craters. This suggests that layering and its interplay with target strength and porosity may play a more significant role than previously considered.     

Sounding of Titan’s atmosphere at submillimeter wavelengths from an orbiting spacecraft

An investigation of the capabilities and science goals of a submillimeter-wave heterodyne sounder onboard a Titan orbiter is presented. Based on a model of Titan’s submillimeter spectrum, and including realistic instrumental performances, we show that passive limb observations of Titan’s submillimeter radiation would bring novel and unique information on the dynamical and chemical state of Titan’s atmosphere, particularly in the so far poorly probed 500-900 km region. The 300-360, 540-660 and 1080-1280 GHz spectral ranges appear especially promising, and could be explored with an instrument equipped with a tunable local oscillator system. Vertical temperature profiles can be determined up to ∼1200 km using rotational lines of CH₄, CO, and HCN. Winds can be measured over the 200-1200 km altitude range with an accuracy of 3-5 m/s from Doppler shift measurements of any strong optically thin line. Numerous molecular species are accessible, including H₂O, NH₃, CH₃C₂H, CH₂NH, and several nitriles (HC₃N, HC₅N, CH₃CN, and C₂H₃CN). Many of them are expected to be detectable in a large fraction of the atmosphere and in some cases at all levels, providing an observational link between stratospheric and thermospheric chemistry. Isotopic variants of some of these species can also be measured, providing new measurements of H, C, N, and O isotopic ratios. Mapping of the thermal, wind, and composition fields, best achieved from a polar orbit and with an articulated antenna, would provide a new view of the couplings between chemistry and dynamics over an extended altitude range of Titan’s atmosphere. Additional science goals at Saturn and Enceladus are briefly discussed.     

The measurement of the gravitational constant in an orbiting laboratory

We propose to measure the gravitational constantG by putting in an orbiting laboratory a known mass of very high density and by tracking the motion of a small test mass under the gravitational influence of the primary mass. We analyse the different sources of perturbation; the consideration of the Earth's gravity gradient leads us to conclude that, if the laboratory is in a low Earth orbit, we cannot get stable satellite-like orbits of the test mass, but we must study only a process of gravitational scattering. In order to maximize the time of interaction it is proposed to use the practical stability of a collinear equilibrium point of the system Earth-primary mass, by putting the test mass as close as possible to the stable manifold of an equilibrium point. This method will allow the determination of the value ofG within a few parts over 10⁵, as shown by some computer simulations of the experiment taking into account also some unknown perturbation and random noise.Two main problems are involved in this experiment: (a) refined numerical methods are needed to take into account all significant perturbations and to extract the result aboutG from the experimental data; (b) during the motion of the test mass, the primary mass must always be free-falling inside the laboratory, so that this experiment needs a drag-free satellite technique of the same type which is necessary for high-precision gravimetric measurements.     

Remote sensing of a comet at millimeter and submillimeter wavelengths from an orbiting spacecraft

The ESA Rosetta Spacecraft, launched on March 2, 2004 with the ultimate destination being Comet 67P/Churyumov–Gerasimenko, carries a relatively small and lightweight millimeter–submillimeter spectrometer instrument, the first of its kind launched into deep space. The instrument, named Microwave Instrument for the Rosetta Orbiter (MIRO), consists of a 30-cm diameter, offset parabolic reflector telescope, which couples energy in the millimeter and submillimeter bands to two heterodyne receivers. Center-band operating frequencies are near 190 GHz (1.6 mm) and 562 GHz (0.5 mm). Broadband, total power continuum measurements can be made in both bands. A 4096-channel spectrometer with 44 kHz resolution is connected to the submillimeter receiver. The spectral resolution is sufficient to observe individual, thermally broadened spectral lines (T⩾10 K). The submillimeter radiometer/spectrometer is fixed tuned to measure four volatile species—CO, CH₃OH, NH₃ and three isotopes of water, H₂¹⁶O, H₂¹⁷O and H₂¹⁸O. The MIRO experiment will use these species as probes of the physical conditions within the nucleus and coma. The basic quantities measured by MIRO are surface temperature, gas production rates and relative abundances, and velocity and excitation temperature of each species, along with their spatial and temporal variability. This information will be used to infer coma structure and outgassing processes, including the nature of the nucleus/coma interface.     

Theory of an experiment in an orbiting space laboratory to determine the gravitational constant

The paper analyzes an experiment in an orbiting laboratory to determine the gravitational constantG. A massive sphere, according to a suggestion of L. S. Wilk, is to have three tunnels drilled through it along mutually perpendicular diameters. The sphere either floats in the orbiting laboratory, with its center held fixed by means of external jets issuing from the spacecraft, or is tethered to the spacecraft. In either case it is free to rotate; in the second case this freedom would be achieved by a system of gimbals.Each tunnel contains a small test object, which is held on the tunnel's axis by means of a suspension system, perhaps electrostatic, and held at rest relative to the sphere by slowly rotating the latter by means of inertia reaction wheels, governed by a servomechanism. Fundamentally, one balances the gravitational forces on the test objects by centrifugal force, determines the latter by measuring the components of angular velocity, and calculatesG from the resulting balance. It is better to use three tunnels than one because their use minimizes the effects of the Earth's gravity-gradient.Many other measurements and corrections are required. The latter arise from Earth gravity-gradient, aerodynamic drag (with the tethered sphere), gravitational forces produced by the spacecraft itself, and the force reductions produced by the empty space in all three tunnels. After the consideration of these effects there is a presentation and discussion of the equations required to reduce the observations to obtainG. There then follow the extra equations, not needed in the reduction, that are required for a computer simulation to investigate the possible extraction of a test object and to aid in designing the servomechanisms.In Appendix B, I have devised another version of the experiment, in which the sphere is kept intact, but has short thin hollow vestigial tunnels attached to the outside of the sphere, along perpendicular diameters. These external tunnels would contain the test objects and the suspension systems. The servomechanisms would then have to prevent collision of a test object with the sphere, as well as extraction. This second method could allow for some inhomogeneities in the sphere, would require no accurate drilling, and would make the suspension systems more accessible for construction and adjustment.This paper was prepared under the sponsorship of the National Aeronautics and Space Administration through NASA Contract NAS 9-8328.     

Volatile retention from cometary impacts on the Moon

Impacts of comets and asteroids play an important role in volatile delivery on the Moon. We use a novel method for tracking vapor masses that reach escape velocity in hydrocode simulations of cometary impacts to explore the effects of volatile retention. We model impacts on the Moon to find the mass of vapor plume gravitationally trapped on the Moon as a function of impact velocity. We apply this result to the impactor velocity distribution and find that the total impactor mass retained on the Moon is approximately 6.5% of the impactor mass flux. Making reasonable assumptions about water content of comets and the comet size-frequency distribution, we derive a water flux for the Moon. After accounting for migration and stability of water ice at the poles, we estimate a total 1.3×10⁸-4.3×10⁹ metric tons of water is delivered to the Moon and remains stable at the poles over 1 Ga. A factor of 30 uncertainty in the estimated cometary impact flux is primarily responsible for this large range of values. The calculated mass of water is sufficient to account for the neutron fluxes poleward of 75° observed by Lunar Prospector. A similar analysis for water delivery to the Moon via asteroid impacts shows that asteroids provide six times more water mass via impacts than comets.     

Statistics of faint gamma-ray bursts from the GRIF experiment on the Mir orbiting station

During the GRIF experiment onboard the Mir orbiting station, cosmic gamma-ray bursts (GRBs) were observed in the photon energy range 10–300 keV. We developed a technique for selecting events, cosmic GRB candidates, based on output readings from the PX-2 scintillation spectrometer, the main astrophysical instrument. Six events interpreted as cosmic GRBs were identified at a threshold sensitivity level of ≥10^?7 erg cm^?2. The GRIF burst detection rate recalculated to all the sky is ～10³ yr^?1 (fluence ≥10^?7 erg cm^?2). This rate matches the BATSE/CGRO estimate and significantly differs from the value predicted by the S^?3/2 dependence, which holds for a spatially uniform source distribution. The GRB detection rate at low peak fluxes is compared with the results of analysis for BATSE/CGRO “nontriggered” events and with predictions of major cosmological models. We conclude that the PX-2 observational data on faint cosmic GRBs are consistent with predictions of models with the highest frequency of GRB occurrence at z ≥1.5–2.     

X‐ray spectra and polarization from accreting black holes: polarization from an orbiting spot

The polarization from a spot orbiting around Schwarzschild and extreme Kerr black holes is studied. We assume different models of local polarization. Firstly, as a toy model we set the local polarization vector either normal to the disc plane, or perpendicular to the toroidal magnetic field. Then we examine the more realistic situation with a spot arising due to the emission from the primary source above the disc. We employ either Rayleigh single scattering or Compton multiple scattering approximations. The time dependence of the degree and angle of polarization during the spot revolution is examined as a function of the observer's inclination angle and black hole angular momentum. The gravitational and Doppler shifts, lensing effect as well as time delays are taken into account. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The shape and appearance of craters formed by oblique impact on the Moon and Venus

Abstract— We surveyed the impact crater populations of Venus and the Moon, dry targets with and without an atmosphere, to characterize how the 3‐dimensional shape of a crater and the appearance of the ejecta blanket varies with impact angle. An empirical estimate of the impact angle below which particular phenomena occur was inferred from the cumulative percentage of impact craters exhibiting different traits. The results of the surveys were mostly consistent with predictions from experimental work. Assuming a sin²θ dependence for the cumulative fraction of craters forming below angle θ, on the Moon, the following transitions occur: >?45 degrees, the ejecta blanket becomes asymmetric; >?25 degrees, a forbidden zone develops in the uprange portion of the ejecta blanket, and the crater rim is depressed in that direction; >?15 degrees, the rim becomes saddle‐shaped; >?10 degrees, the rim becomes elongated in the direction of impact and the ejecta forms a “butterfly” pattern. On Venus, the atmosphere causes asymmetries in the ejecta blanket to occur at higher impact angles. The transitions on Venus are: >?55 degrees, the ejecta becomes heavily concentrated downrange; >?40 degrees, a notch in the ejecta that extends to the rim appears, and as impact angle decreases, the notch develops into a larger forbidden zone; >?10 degrees, a fly‐wing pattern develops, where material is ejected in the crossrange direction but gets swept downrange. No relationship between location or shape of the central structure and impact angle was observed on either planet. No uprange steepening and no variation in internal slope or crater depth could be associated with impact angle on the Moon. For both planets, as the impact angle decreases from vertical, first the uprange and then the downrange rim decreases in elevation, while the remainder of the rim stays at a constant elevation. For craters on Venus >?15 km in diameter, a variety of crater shapes are observed because meteoroid fragment dispersal is a significant fraction of crater diameter. The longer path length for oblique impacts causes a correlation of clustered impact effects with oblique impact effects. One consequence of this correlation is a shallowing of the crater with decreasing impact angle for small craters.     

Photometry of Venus below 2200 Å from the orbiting astronomical observatory-2

Spectrophotometry of Venus from 2170 to about 1950 Å has been obtained by OAO-2 at 10 Å resolution. The new data confirm and extend previous indications that the geometric albedo decreases continuously below 2500 Å. Secular changes in either the amount or distribution, or both, of absorbing constituents in the upper atmosphere are strongly suggested. A narrow absorption feature is found near 2145 Å, confirming an earlier report by Anderson et al. [J. Atmos. Sci.26 (1969), 874–888]. Absorption by trace amounts of nitrogen-bearing molecules, including N₂O, HNO₃ in aqeous solution, and possibly also NO, together with Rayleigh scattering from CO₂, can account for the variation in albedo below 3200 Å, but other explanations are not excluded. For example, H₂S may contribute to or be responsible for the decrease in albedo below 2500 Å.     

Prospect for UV observations from the Moon

Space astronomy in the last 40 years has largely been done from spacecraft in low Earth orbit (LEO) for which the technology is proven and delivery mechanisms are readily available. However, new opportunities are arising with the surge in commercial aerospace missions. We describe here one such possibility: deploying a small instrument on the Moon. This can be accomplished by flying onboard the Indian entry to the Google Lunar X PRIZE competition, Team Indus mission, which is expected to deliver a nearly 30 kgs of payloads to the Moon, with a rover as its primary payload. We propose to mount a wide-field far-UV (130–180 nm) imaging telescope as a payload on the Team Indus lander. Our baseline operation is a fixed zenith pointing but with the option of a mechanism to allow observations of different attitudes. Pointing towards intermediate ecliptic latitude (50^° or above) ensures that the Sun is at least 40^° off the line of sight at all times. In this position, the telescope can cover higher galactic latitudes as well as parts of Galactic plane. The scientific objectives of such a prospective are delineated and discussed.     

Simulations of a comet impact on the Moon and associated ice deposition in polar cold traps

Modeling results of the water vapor plume produced by a comet impact on the Moon and of the resulting water ice deposits in the lunar cold traps are presented. The water vapor plume is simulated near the point of impact by the SOVA hydrocode and in the far field by the Direct Simulation Monte Carlo (DSMC) method using as input the SOVA hydrocode solution at a fixed hemispherical interface. The SOVA hydrocode models the physics of the impact event such as the surface deformation and material phase changes during the impact. The further transport and retention processes, including gravity, photodestruction processes, and variable surface temperature with local polar cold traps, are modeled by the DSMC method for months after impact. In order to follow the water from the near field of the impact to the full planetary induced atmosphere, the 3D parallel DSMC code used a collision limiting scheme and an unsteady multi-domain approach. 3D results for the 45° oblique impact of a 2 km in diameter comet on the surface of the Moon at 30 km/s are presented. Most of the cometary water is lost due to escape just after impact and only ∼3% of the cometary water is initially retained on the Moon. Early downrange focusing of the water vapor plume is observed but the later material that is moving more slowly takes on a more symmetric shape with time. Several locations for the point of impact were investigated and final retention rates of ∼0.1% of the comet mass were observed. Based on the surface area of the cold traps used in the present simulations, ∼1 mm of ice would have accumulated in the cold traps after such an impact. Estimates for the total mass of water accumulated in the polar cold traps over 1 byr are consistent with recent observations.     

Field geology on the Moon: Some lessons learned from the exploration of the Haughton impact structure, Devon Island, Canadian High Arctic

With the prospect of humans returning to Moon by the end of the next decade, considerable attention is being paid to technologies required to transport astronauts to the lunar surface and then to be able to carry out surface science. Recent and ongoing initiatives have focused on scientific questions to be asked. In contrast, few studies have addressed how these scientific priorities will be achieved. In this contribution, we provide some of the lessons learned from the exploration of the Haughton impact structure, an ideal lunar analogue site in the Canadian Arctic. Essentially, by studying how geologists carry out field science, we can provide guidelines for lunar surface operations. Our goal in this contribution is to inform the engineers and managers involved in mission planning, rather than the field geology community. Our results show that the exploration of the Haughton impact structure can be broken down into 3 distinct phases: (1) reconnaissance; (2) systematic regional-scale mapping and sampling; and (3) detailed local-scale mapping and sampling. This break down is similar to the classic scientific method practiced by field geologists of regional exploratory mapping followed by directed mapping at a local scale, except that we distinguish between two different phases of exploratory mapping. Our data show that the number of stops versus the number of samples collected versus the amount of data collected varied depending on the mission phase, as does the total distance covered per EVA. Thus, operational scenarios could take these differences into account, depending on the goals and duration of the mission. Important lessons learned include the need for flexibility in mission planning in order to account for serendipitous discoveries, the highlighting of key “science supersites” that may require return visits, the need for a rugged but simple human-operated rover, laboratory space in the habitat, and adequate room for returned samples, both in the habitat and in the return vehicle. The proposed set of recommendations ideally should be tried and tested in future analogue missions at terrestrial impact sites prior to planetary missions.     

Radial mixing in the outer Milky Way disc caused by an orbiting satellite

The turbulent diffusion tensor describing the evolution of the mean concentration of a passive scalar is investigated for non-helically forced turbulence in the presence of rotation or a magnetic field. With rotation, the Coriolis force causes a sideways deflection of the flux of mean concentration. Within the magnetohydrodynamics approximation there is no analogous effect from the magnetic field because the effects on the flow do not depend on the sign of the field. Both rotation and magnetic fields tend to suppress turbulent transport, but this suppression is weaker in the direction along the magnetic field. Turbulent transport along the rotation axis is not strongly affected by rotation, except on shorter length-scales, i.e. when the scale of the variation of the mean field becomes comparable with the scale of the energy-carrying eddies. These results are discussed in the context of anisotropic convective energy transport in the Sun.