Comet Impact Research Articles

A time-dependent photochemical model for Titan’s atmosphere and the origin of H2O

Context. Titan’s stratosphere contains oxygen compounds (CO, CO2 , and H2 O), implying an external source of oxygen whose nature is still uncertain. Recent observations from the Herschel Space Observatory using the HIFI and PACS instruments and the Cassini /CIRS, as well as steady-state photochemical modeling indicate that the amounts of CO2 and H2 O in Titan’s stratosphere may imply inconsistent values of the OH/H2 O input flux, and that the oxygen source is time-variable.Aims. We attempt to reconcile the H2 O and CO2 observed profiles in Titan’s atmosphere by using an updated photochemical scheme and developing several time-dependent scenarios for the influx/evolution of oxygen species.Methods. We use a time-dependent photochemical model of Titan’s atmosphere to calculate effective lifetimes and the response of Titan’s oxygen compounds to changes in the oxygen input flux. Two variants for the C-H-O chemical network are considered. We investigate a time-variable Enceladus source and the evolution of material delivered by a cometary impact.Results. We find that the effective lifetime of H2 O in Titan’s atmosphere is only a factor of six shorter than that of CO2 and exceeds 10 yr below 200 km. A time-variable Enceladus source, involving a decrease by a factor of 5–20 in the OH/H2 O flux over the last few centuries, shows promise in explaining the relative CO2 /H2 O profiles. However, if the previous measurements from the Herschel Space Observatory are representative of Titan’s atmospheric water, an additional H2 O loss to the haze term is needed to bring the model in full agreement with the data. In an alternate situation, CO2 production following a cometary impact that occurred at least 220–300 yr ago can in principle explain the CO2 “excess” in Titan’s stratosphere, but this scenario is highly unlikely, given the estimates of the impact rate at Titan.

The carbon-14 spike in the 8th century was not caused by a cometary impact on Earth

A mysterious increase of radiocarbon 14C ca. 775 AD in the Earth’s atmosphere has been recently found by Miyake et al. (Miyake, F., Nagaya, K., Masuda, K., Nakamura, T. [2012]. Nature, 486, 240). A possible source of this event has been discussed widely, the most likely being an extreme solar energetic particle event. A new exotic hypothesis has been presented recently by Liu et al. (Liu, Y. [2014]. Sci. Rep., 4, 3728) who proposed that the event was caused by a cometary impact on Earth bringing additional 14C to the atmosphere. Here we calculated a realistic mass and size of such a comet to show that it would have been huge (≈100km across and 1017–1020g of mass) and would have produced a disastrous geological/biological impact on Earth. The absence of an evidence for such a dramatic event makes this hypothesis invalid.

Dark matter as a trigger for periodic comet impacts.

Although statistical evidence is not overwhelming, possible support for an approximately 35×106 yr periodicity in the crater record on Earth could indicate a nonrandom underlying enhancement of meteorite impacts at regular intervals. A proposed explanation in terms of tidal effects on Oort cloud comet perturbations as the Solar System passes through the galactic midplane is hampered by lack of an underlying cause for sufficiently enhanced gravitational effects over a sufficiently short time interval and by the time frame between such possible enhancements. We show that a smooth dark disk in the galactic midplane would address both these issues and create a periodic enhancement of the sort that has potentially been observed. Such a disk is motivated by a novel dark matter component with dissipative cooling that we considered in earlier work. We show how to evaluate the statistical evidence for periodicity by input of appropriate measured priors from the galactic model, justifying or ruling out periodic cratering with more confidence than by evaluating the data without an underlying model. We find that, marginalizing over astrophysical uncertainties, the likelihood ratio for such a model relative to one with a constant cratering rate is 3.0, which moderately favors the dark disk model. Our analysis furthermore yields a posterior distribution that, based on current crater data, singles out a dark matter disk surface density of approximately 10M⊙/pc2. The geological record thereby motivates a particular model of dark matter that will be probed in the near future.

Layering in the Paleocene/Eocene boundary of the Millville core is drilling disturbance

In a study of a sediment core from the US Atlantic Coastal Plain (Ocean Drilling Program Site 174X-Millville), Wright and Schaller (1) claim to have resolved the onset of the Paleocene/Eocene carbon isotope excursion (CIE) across 13 y. Such a rapid change would require an enormous and instantaneous release of isotopically light carbon into the ocean/atmosphere, implicating comet impact. The claim rests on the interpretation of rhythmic layering in the sediment core as annual couplets but here it is proposed that they are an artifact of drilling disturbance.

Chelyabinsk event as an astronomical phenomenon

The fall of a meteorite near the town of Chelyabinsk is considered from the viewpoint of astronomy, and the major witness facts and entry characteristics (including the measured entry velocity and the height of the explosion) are analyzed. The aerodynamic phenomena that accompanied the entry of the meteorite in the atmosphere at an ultrasonic velocity and the origin of a shock wave that induced damage on the Earth’s surface are analyzed. The paper also reports the estimated frequency of the falls of celestial bodies depending on their size, and consequences of collisions of these bodies with the Earth. It is emphasized that studies of small bodies in the Solar System can provide insight into the origin of the protoplanetary disk and the processes that produced the planets. The studies of small bodies, such as the Chelyabinsk meteorite, are directly related to the problem of asteroid and comet impact hazard (ACIH). The paper reports the sizes of potentially hazardous celestial bodies whose monitoring requires the deployment of a network of specialized telescopes on the Earth to mitigate ACH and a system of space-based systems for the identification and monitoring of such bodies in near space.

ASSESSING THE INFLUENCE OF THE SOLAR ORBIT ON TERRESTRIAL BIODIVERSITY

The terrestrial fossil record shows a significant variation in the extinction and origination rates of species during the past half billion years. Numerous studies have claimed an association between this variation and the motion of the Sun around the Galaxy, invoking the modulation of cosmic rays, gamma rays and comet impact frequency as a cause of this biodiversity variation. However, some of these studies exhibit methodological problems, or were based on coarse assumptions (such as a strict periodicity of the solar orbit). Here we investigate this link in more detail, using a model of the Galaxy to reconstruct the solar orbit and thus a predictive model of the temporal variation of the extinction rate due to astronomical mechanisms. We compare these predictions as well as those of various reference models with paleontological data. Our approach involves Bayesian model comparison, which takes into account the uncertainties in the paleontological data as well as the distribution of solar orbits consistent with the uncertainties in the astronomical data. We find that various versions of the orbital model are not favored beyond simpler reference models. In particular, the distribution of mass extinction events can be explained just as well by a uniform random distribution as by any other model tested. Although our negative results on the orbital model are robust to changes in the Galaxy model, the Sun's coordinates and the errors in the data, we also find that it would be very difficult to positively identify the orbital model even if it were the true one. (In contrast, we do find evidence against simpler periodic models.) Thus while we cannot rule out there being some connection between solar motion and biodiversity variations on the Earth, we conclude that it is difficult to give convincing positive conclusions of such a connection using current data.

Constraining the origins of Neptune’s carbon monoxide abundance with CARMA millimeter-wave observations

We present observations of Neptune’s 1- and 3-mm spectrum from the Combined Array for Research in Millimeter-wave Astronomy (CARMA). Radiative transfer analysis of the CO (2–1) and (1–0) rotation lines was performed to constrain the CO vertical abundance profile. We find that the data are well matched by a CO mole fraction of 0.1-0.1+0.2 parts per million (ppm) in the troposphere, and 1.1-0.3+0.2 ppm in the stratosphere. A flux of 0.5–20×108 CO molecules cm−2s−1 to the upper stratosphere is implied. Using the Zahnle et al. (Zahnle, K., Schenk, P., Levison, H., Dones, L. [2003]. Icarus 163, 263–289) estimate for cometary impact rates at Neptune, we calculate the CO flux that could be formed from (sub)kilometer-sized comets; we find that if the diffusion rate near the tropopause is small (200cm2s−1), these impacts could produce a flux as high as 0.5-0.4+0.8×108 CO molecules cm−2s−1. We also revisit the calculation of Neptune’s internal CO contribution using revised calculations for the CO→CH4 conversion timescale in the deep atmosphere (Visscher, C., Moses, J.I. [2011]. Astrophys. J. 738, 72). We find that an upwelled CO mole fraction of 0.1ppm implies a global O/H enrichment of at least 400, and likely more than 650, times the protosolar value.

Gas-hydrodynamic analysis of the genesis of green and orange glasses in lunar rocks

In order to elucidate the genesis of green and orange glasses in Apollo 15 and Apollo 17 samples of lunar rocks, two alternative hypotheses are analyzed, according to which the glasses are produced either (1) by a comet or meteorite impact (impact model) or (2) by volcanic activity (volcanic model). The green and orange glasses are clearly genetically and petrochemically autonomous, i.e., the composition of the glasses themselves differs from those of the major types of the primary rocks. The mechanisms responsible for the origin of the glasses are analyzed along with mathematical models of their genesis. The theoretically calculated size distribution of glass particles is in good agreement with those measured in Apollo 15 and Apollo 17 samples. Simulation results and the analysis of the composition and structure of the green and orange glasses lead to the conclusion that the impact hypothesis of their genesis is the most realistic.

Comparing the Earth impact flux from comets and near-Earth asteroids

When compared to the discovered set of near-Earth asteroids, the Earth impact flux of all known active comets, including Jupiter family comets, Halley-type and long period comets, is below 1%. However, the comet impact fluxes may be higher for those objects large enough to cause extinction level events.

IMPACT! - BOLIDES, CRATERS, AND CATASTROPHES

It is now universally accepted that the impact of planetesimals, asteroids, and comets has been a fundamental process throughout the Solar System. Catastrophic impact events have been instrumental in developing the early history of the planets and have caused environmental disasters throughout Earth history. A major mass extinction at the Cretaceous–Paleogene boundary has been confidently related to an impact event (Chicxulub, Mexico). While the study of impact cratering is a multidisciplinary field, mineralogical and geochemical investigations have been central since the beginning, focusing on the nature of impact-generated rocks and of the extraterrestrial projectiles as well as their interaction with geological materials. Chemical and isotopic techniques have allowed the dating of impact events and the identification of traces of meteoritic projectiles in impact-formed rocks on Earth and the Moon.

Synergistic approach of asteroid exploitation and planetary protection

The asteroid and cometary impact hazard has long been recognised as an important issue requiring risk assessment and contingency planning. At the same time asteroids have also been acknowledged as possible sources of raw materials for future large-scale space engineering ventures. This paper explores possible synergies between these two apparently opposed views; planetary protection and space resource exploitation. In particular, the paper assumes a 5 tonne low-thrust spacecraft as a baseline for asteroid deflection and capture (or resource transport) missions. The system is assumed to land on the asteroid and provide a continuous thrust able to modify the orbit of the asteroid according to the mission objective. The paper analyses the capability of such a near-term system to provide both planetary protection and asteroid resources to Earth. Results show that a 5 tonne spacecraft could provide a high level of protection for modest impact hazards: airburst and local damage events (caused by 15–170 m diameter objects). At the same time, the same spacecraft could also be used to transport to bound Earth orbits significant quantities of material through judicious use of orbital dynamics and passively safe aero-capture manoeuvres or low energy ballistic capture. As will be shown, a 5 tonne low-thrust spacecraft could potentially transport between 12 and 350 times its own mass of asteroid resources by means of ballistic capture or aero-capture trajectories that pose very low dynamical pressures on the object.

Enhanced coastal paleoproductivity and nutrient supply in Upper Egypt during the Paleocene/Eocene Thermal Maximum (PETM): Mineralogical and geochemical evidence

We have analyzed the geochemistry and mineralogy of the five characteristic beds that constitute the Dababiya Quarry Member (DQM) recovered from the Dababiya Quarry Core located near the Global Stratotype Section and Point (GSSP) for the base of the Eocene. Well developed in Upper Egypt, these beds are known to record the isotopic and biotic signatures of the Paleocene/Eocene thermal maximum (PETM). We have extracted the mineral phases from these beds (representing a total thickness of 2.35 m) and compared them with those of the encasing shales through qualitative and semi-quantitative SEM analysis using EDX technology. Total organic carbon was determined using the titrimetric method. Major and trace elements were analyzed using an inductively coupled plasma-optical emission spectrometer, and enrichment factors were established relative to Al content. We discuss the significance of our data with regard to basaltic volcanism and cometary impact, for neither of which we find support. In agreement with other studies, our data indicate very high biological productivity during the deposition of the DQM. This may be related to upwelling and/or increased nutrient supply, the latter because of an enhanced hydrologic cycle.

The mysterious onset of the Younger Dryas

The 1300-year-long Younger Dryas cold reversal (12,900–11,600 cal BP) is seen most clearly in Greenland ice cores. Contemporaneous changes of climate and environment in more southerly regions were variably expressed. The precise timing of the abrupt Younger Dryas onset is still uncertain. It was linked to a sudden increase in 14C manifest as a “cliff” where 14C dates drop from 11,000 to 10,600 radiocarbon years BP within a century of real time, followed by a long plateau. Changes in ocean and/or atmospheric circulation or solar radiation, and even a comet impact have been proposed as triggers, but the abruptness and severity of YD onset still elude explanation.

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.

PLUME DEVELOPMENT OF THE SHOEMAKER-LEVY 9 COMET IMPACT

We have studied plume formation after a Jovian comet impact using the ZEUS-MP 2 hydrodynamics code. The three-dimensional models followed objects with 500, 750, and 1000 meter diameters. Our simulations show the development of a fast, upward-moving component of the plume in the wake of the impacting comet that "pinches off" from the bulk of the cometary material ~50 km below the 1 bar pressure level, ~100 km above the depth of greatest mass and energy deposition. The fast-moving component contains about twice the mass of the initial comet, but consists almost entirely (>99.9%) of Jovian atmosphere rather than cometary material. The ejecta rise mainly along the impact trajectory, but an additional vertical velocity component due to buoyancy establishes itself within seconds of impact, leading to an asymmetry in the ejecta with respect to the entry trajectory. The mass of the upward-moving component follows a velocity distribution M(>v) approximately proportional to v^-1.4 (v^-1.6 for the 750 m and 500 m cases) in the velocity range 0.1 < v < 10 km/s.

Lunar swirls: Examining crustal magnetic anomalies and space weathering trends

[1] We have used multispectral images from Clementine and data from Lunar Prospector's magnetometer to conduct a survey of lunar crustal magnetic anomalies, prominent lunar swirls, and lesser known swirl markings to provide new information on the nature of swirls and their association with magnetic anomalies. We find that all swirls and swirl-like albedo patterns are associated with areas of magnetized crust, but not all areas of magnetized crust are colocated with swirl-like albedo anomalies. All observed swirls exhibit spectral characteristics similar to immature material and generally have slightly lower FeO values compared with their surroundings as determined with a multispectral iron-mapping method. We discuss these results in relation to the various hypotheses for swirl formation. The comet impact hypothesis for lunar swirls would not predict a difference in the spectrally determined FeO content between swirls and nearby ordinary surfaces. The compositional difference could be explained as a consequence of (1) magnetic shielding of the surface from the solar wind, which could produce anomalous space weathering (little darkening with limited reddening) and potentially alter the predictions of the multispectral iron-mapping algorithm while the compositional contrast could be enhanced by delivery of lower-FeO ejecta from outside the swirl; and (2) accumulation of fine plagioclase-rich dust moving under the influence of electric fields induced by solar wind interactions with a magnetic anomaly. Therefore, we cannot at present clearly distinguish between the solar wind shielding and electrostatic dust accumulation models for swirl formation. We describe future measurements that could contribute to solution of the puzzle of swirl origin.

δ18O and chemical composition of Libyan Desert Glass, country rocks, and sands: New considerations on target material

– Oxygen isotope and chemical measurements were carried out on 25 samples of Libyan Desert Glass (LDG), 21 samples of sandstone, and 3 of sand from the same area. The δ18O of LDG samples range from 9.0‰ to 11.9‰ (Vienna Standard Mean Ocean Water [VSMOW]); some correlations between isotope data and typological features of the LDG samples are pointed out. The initial δ18O of a bulk parent material may be slightly increased by fusion due to the loss of isotopically light pore water with no isotope exchange with oxygen containing minerals. Accordingly, the δ18O of the bulk parent material of LDG may have been about 9.0 ± 1‰ (VSMOW). The measured bulk sandstone and sand samples have δ18O values ranging from 12.6‰ to 19.5‰ and are consequently ruled out as parent materials, matching the results of previous studies. However, separated quartz fractions have δ18O values compatible with the LDG values suggesting that the modern surface sand inherited quartz from the target material. This hypothesis fits previous findings of lechatelierite and baddeleyite in these materials. As the age of the parent material reported in previous studies is Pan-African, we measured the δ18O values of bulk rock and quartz from intrusives of Pan-African age and the results obtained were compatible with the LDG values. The main element abundances (Fe, Mg, Ca, K, Na) in our LDG samples conform to previous estimates; Fe, Mg, and K tend to be higher in heterogeneous samples with dark layers. The hypothesis of a low-altitude airburst involving silica-rich surface materials deriving from weathered intrusives of Pan-African age, partially melted and blown over a huge surface by supersonic winds matches the results obtained.

Seismic signatures of a proterozoic thermal plume below southwestern part of the Cuddapah Basin, Dharwar craton

Origin and evolutionary history of the Cuddapah Basin in SE India has remained a subject of considerable speculation whether it was evolved through vertical tectonic movements, extentional stretching or even cometary impact. Based on detailed seismic and other geophysical studies (Gravity, magnetotelluric and heat flow), we have delineated signatures of a possible deep seated mantle plume below southwestern part of the Cuddapah Basin, which may have been responsible for the 1.1 Ga kimberlitic magmatism in the eastern part of the Dharwar craton (EDC). The thermal anomaly associated with this mantle plume appears to have resulted into 15–20 km thick magmatic underplating (Vp: 7.10–7.30 km/s; density 3.07–3.16 g/cm3) below the Parnapalle region of the southwestern Cuddapah Basin, which also coincides with the high gravity and high conductivity anomaly. The massive underplating led to thickening of the crust to about 40–44 km below southwestern part of the Cuddapah Basin, compared to about 34± 2 km in the surrounding regions of EDC, indicating thermal restructuring of the crust / mantle boundary. This plume, which was apparently active in an area of about 500 km radius, may have also affected the Closepet granitic region, which is ∼100 km west of Cuddapah Basin.

Jupiter’s stratospheric hydrocarbons and temperatures after the July 2009 impact from VLT infrared spectroscopy

Aims. Thermal infrared imaging and spectroscopy of the July 19, 2009 Jupiter impact site has been used to identify unique features of the physical and chemical atmospheric response to this unexpected collision. Methods. Images and high-resolution spectra of methane, ethane and acetylene emission (7−13 μm) from the 2009 impact site were obtained by the Very Large Telescope (VLT) mid-infrared camera/spectrometer instrument, VISIR. An optimal estimation retrieval algorithm was used to determine the atmospheric temperatures and hydrocarbon distribution in the month following the impact. Results. Ethane spectra at 12.25 μm could not be explained by a rise in temperature alone. Ethane was enhanced by 1.7−3.2 times the background abundance on July 26, implying production as the result of shock chemistry in a high C/O ratio environment, favouring an asteroidal origin for the 2009 impactor. Small enhancements in acetylene emission were also observed over the impact site. However, no excess methane emission was found over the impact longitude, either with broadband 7.9-μm imaging 21 h after the impact, or with center-to-limb scans of strong and weak methane lines between 7.9 and 8.1 μm in the ensuing days, indicating either extremely rapid cooling in the initial stages, or an absence of heating in the upper stratosphere (p 10 mbar), though this solution is highly dependent on the spectral properties of stratospheric debris. The enhanced ethane emission was localised over the impact streak, and was diluted in the ensuing weeks by redistribution of heated gases by zonal flow and mixing with the unperturbed jovian air. Conclusions. The different thermal energy deposition profiles, in addition to the highly reducing (C/O > 1) environment and shallow impactor angle, suggest that (a) the 2009 plume and shock-fronts did not reach the sub-microbar altitudes of the Shoemaker-Levy 9 plumes; and (b) models of a cometary impact are not directly applicable to the unique impact circumstances of July 2009.

Mammoth-Killer Impact Flunks Out

After a new study failed to find nanodiamonds, impact experts are flatly rejecting outsiders' claims that an impact 12,900 years ago devastated the megafauna.