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1.
Abstract— A significant fraction of the Earth's prebiotic volatile inventory may have been delivered by asteroidal and cometary impacts during the period of heavy bombardment. The realization that comets are particularly rich in organic material seemed to strengthen this suggestion. Previous modeling studies, however, indicated that most organics would be entirely destroyed in large comet and asteroid impacts. The availability of new kinetic parameters for the thermal degradation of amino acids in the solid phase made it possible to readdress this question. We present the results of new high-resolution hydrocode simulations of asteroid and comet impact coupled with recent experimental data for amino acid pyrolysis in the solid phase. Differences due to impact velocity as well as projectile material have been investigated. Effects of angle of impacts were also addressed. The results suggest that some amino acids would survive the shock heating of large (kilometer-radius) cometary impacts. At the time of the origins of life on Earth, the steady-state oceanic concentration of certain amino acids (like aspartic and glutamic acid) delivered by comets could have equaled or substantially exceeded concentrations due to Miller-Urey synthesis in a CO2-rich atmosphere. Furthermore, in the unlikely case of a grazing impact (impact angle ～5° from the horizontal), an amount of some amino acids comparable to that due to the background steady-state production or delivery would be delivered to the early Earth.  相似文献   

2.
The possibility of impacts between comets belonging to the Jupiter Family and other small bodies orbiting in the main asteroid belt, and the consequences in relation to cometary activity are discussed. The probability of such events and the jumps in cometary brightness caused by impacts are examined. The results are compared with the results of the Deep Impact mission to Comet 9P/Tempel 1. The main conclusion of this paper is in agreement with previous findings, namely that an impact mechanism cannot be the main cause of the outburst activity of comets.  相似文献   

3.
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.  相似文献   

4.
Comet 9P/Tempel 1, the target of the Deep Impact mission, has been intensively observed for a long time period before the encounter. Pre-impact ground based monitoring of the comet was an important prerequisite for the success of the first space experiment in which a comet is treated by an artificial impact. It provided the background data needed to disentangle the features caused by the impact from variations caused by the natural activity of the comet. In this paper we present results from the ESO-monitoring of the comet, conducted in the thermal infrared and optical spectral ranges during several months before the Deep Impact encounter with the comet.  相似文献   

5.
Effects of collisions with interplanetary particles are investigated. To this purpose, collision probabilities for comets with different orbital elements are computed. It is found that collisions may have a non-negligible effect on the physical evolution of comets. In this connection, it is shown that under certain conditions collisional lifetimes may be shorter than dynamical or vaporization lifetimes. In particular, collisional lifetimes are on average shorter for comets in retrograde orbits than those for direct ones. It is further suggested that catastrophic collisions may contribute to prevent long-period comets in retrograde orbits from reaching short-period orbits by orbital diffusion. Collisions may also produce irregularities of the nucleus brightness by leaving exposed regions of fresh volatile material and may in this way lead to a rejuvenation of old dusty short-period comets. Catastrophic collision probabilities are too low to account for the observed comet splittings, so other trigger mechanisms should be at work. However, it is shown that collisional mini-bursts (increases in brightness of one magnitude or so) caused by decimeter-sized bodies may occur rather frequently on short-period comets when they pass through the asteroid belt. The burst observed in comet Tempel-2 at 3 AU in December, 1978 could be an example of such collisional mini-bursts. The systematic observation of periodic comets when they pass through the asteroid belt could give valuable information about the spatial density of decimeter and meter-sized bodies. In particular, collisional effects for comet Halley, for which a continuous surveillance is planned, are evaluated.  相似文献   

6.
The orbit of Comet C/2002C1 (Ikeya–Zhang) has a similarity to that of Comet C/1661C1 (Hevelius), and the numerical integration of the motion of C/2002C1 backward shows a possible linkage of those two comets. Thus, 153P/Ikeya–Zhang was designated a periodic comet. Historical records of comets in 877 and 1273 are also identified with Comet 153P/Ikeya–Zhang. The integrated orbital elements during 77 and 2362, and historical records of the comet are also presented and discussed.  相似文献   

7.
Time variation in impact probability is studied by assuming that the periodic flux of the Oort Cloud comets within 15 au arises from the motion of the Sun with respect to the Galactic mid-plane. The periodic flux clearly shows up in the impact rate of the captured Oort Cloud cometary population, with a phase shift caused by the orbital evolution. Depending on the assumed flux of comets and the size distribution of comets, the impact rate of the Oort Cloud comets of 1 km in diameter or greater is from 5 to 700 impacts Myr−1 on the Earth and from 0.5 to 70 impacts per 1000 yr on Jupiter. The relative fractions of impacts are 0.09, 0.11, 0.26 and 0.54 for long-period comets, Halley type comets, Jupiter family comets and near-Earth objects, respectively. For Jupiter, the corresponding fractions in the first three categories are 0.18, 0.31 and 0.51. If we consider physical fading of comet activity that is compatible with the observations, then the impact rates of active comets are two orders of magnitude smaller than the total impact rates by all kinds of comets and cometary asteroids of size 1 km or greater.  相似文献   

8.
Zdenek Sekanina 《Icarus》1978,33(1):173-185
A multiparameter model has been devised to discriminate the effects of the velocity of separation of two fragments of a split comet on their observed motions from the effects of the postsplit differential nongravitational forces. The iterative differential-correction procedure is applied to five extensively observed split comets: West 1975n, Ikeya-Seki 1965 VIII, Wirtanen 1957 VI, the Southern Comet 1947 XII, and Periodic Brooks 2 1889 V. The present model confirms that the major contribution to the observed rate of separation of the fragments comes from the postsplit forces. The model removes or substantially reduces the systematic trends in the residuals of comets West, Wirtanen, and P/Brooks 2, which could not be eliminated by a simple model that ignored the effects of the separation velocity. In particular, the results for Comet Wirtanen now indicate that the comet split at a heliocentric distance of more than 9 AU and that the initial rate of separation of the two fragments did not amount to more than 26 cm/sec. The separation velocities for the other comets range from 1 to 2 m/sec. Difficulties of the interpretation of the observed separation velocities are briefly discussed.  相似文献   

9.
The orbit of asteroid 2003 EH1 is very similar to the mean orbit of the Quadrantid meteoroid stream so that a close relationship between the two is very likely. It has already been suggested that Comet C/1490 Y1 could be the parent of the Quadrantids. If this is the case, then some relationship between the comet and the asteroid might be expected. The orbit of C/1490 Y1 is based on a short observing arc of about 6 weeks and all the observations were with the naked eye, so that its elements are very poorly determined. Hence, forward integration to determine whether asteroid 2003 EH1 represents the re-discovery of the dormant nucleus of C/1490 Y1 is not feasible. Instead we choose to integrate back in time the orbit of 2003 EH1, which is far better determined, and a family of 3500 clones, all of which are moving on an orbit that is consistent with the present known orbit of 2003EH1. We compare the results primarily with the recorded observations of the comet rather than the orbit of the comet derived by Hasegawa. We find that one clone is consistent with these observations.  相似文献   

10.
An Öpik-based geometric algorithm is used to compute impact probabilities and velocity distributions for various near-Earth object (NEO) populations. The resulting crater size distributions for the Earth and Moon are calculated by combining these distributions with assumed NEO size distributions and a selection of crater scaling laws. This crater probability distribution indicates that the largest craters on both the Earth and the Moon are dominated by comets. However, from a calculation of the fractional probabilities of iridium deposition, and the velocity distributions at impact of each NEO population, the only realistic possibilities for the Chicxulub impactor are a short-period comet (possibly inactive) or a near-Earth asteroid. For these classes of object, sufficiently large impacts have mean intervals of 100 and 300 Myr respectively, slightly favouring the cometary hypothesis.  相似文献   

11.
《Planetary and Space Science》1999,47(3-4):301-304
The recent developments in cometary studies suggest rather low mean densities and weak structures for the nuclei. They appear to be accumulations of fairly discrete units loosely bound together, as deduced from the observations of Comet Shoemaker–Levy 9 during its encounter with Jupiter. The compressive strengths deduced from comet splitting by Öpik and Sekanina are extremely low. These values are confirmed by theory developed here, assuming that Comet P/Holmes had a companion that collided with it in 1892. There follows a short discussion that suggests that the mean densities of comets should increase with comet dimensions. The place of origin of short-period comets may relate to these properties.  相似文献   

12.
The Canada-France-Hawaii Telescope Legacy Survey, specifically the Very Wide segment of data, is used to search for possible main-belt comets. In the first data set, 952 separate objects with asteroidal orbits within the main-belt are examined using a three-level technique. First, the full-width-half-maximum of each object is compared to stars of similar magnitude, to look for evidence of a coma. Second, the brightness profiles of each object are compared with three stars of the same magnitude, which are nearby on the image to ensure any extended profile is not due to imaging variations. Finally, the star profiles are subtracted from the asteroid profile and the residuals are compared with the background using an unpaired T-test. No objects in this survey show evidence of cometary activity. The second survey includes 11438 objects in the main-belt, which are examined visually. One object, an unknown comet, is found to show cometary activity. Its motion is consistent with being a main-belt asteroid, but the observed arc is too short for a definitive orbit calculation. No other body in this survey shows evidence of cometary activity. Upper limits of the number of weakly and strongly active main-belt comets are derived to be 630±77 and 87±28, respectively. These limits are consistent with those expected from asteroid collisions. In addition, data extracted from the Canada-France-Hawaii Telescope image archive of main-belt Comet 176P/LINEAR is presented.  相似文献   

13.
I. Toth  C.M. Lisse 《Icarus》2006,181(1):162-177
Here we estimate the regions of stability, fragmentation, and destruction for cometary bodies versus rotational breakup in the radius-rotational period plane. By testing different plausible physical models of the cometary nucleus equation of state, we show that the plane is divided into 3 segments: the allowed, damaged, and forbidden regions. We then compare the location of well-observed comets with respect to the separation lines. The range of constituent material parameters from the literature for cometary nuclei are used to show that all the observed comets lie in the allowed region, except for Comet C/1995 O1 (Hale-Bopp), which resides in the damaged region (where the body is fractured and only held together gravitationally). We speculate that the extremely high activity demonstrated by Comet Hale-Bopp during the 1997 apparition may have been due to its highly fractured state. Comet Hyakutake, observed to emit fragments at perigee in 1996, may be near the boundary of the damaged region. Comet C/1999 S4 (LINEAR) was solidly in the rotationally allowed region, making its disintegration in July 2000 due to centrifugal forces unlikely. In contrast to the comets, the centaurs do not cluster in the allowed region, with the majority falling instead into the rotationally damaged and forbidden regions. The centaurs are only stable against breakup assuming much stronger solid water ice properties, strongly suggesting that on the whole, these bodies have different bulk physical properties than cometary nuclei.  相似文献   

14.
The interaction between the Comet P/Grigg-Skjellerup ionosphere and the oncoming solar wind is numerically simulated on the basis of the well-known gasdynamic single-fluid Biermann model. Numerical solutions are obtained using a discontinuity-fitting technique based on the implicit second-order Babenko-Rusanov method. The salient features of the interaction pattern for GS comet are nearly the same as in the previously considered case of the Comet P/Halley, although the gas productions of the two comets near the perihelion differ by an order of magnitude. The results of the numerical simulation are correlated with the data of onboard measurements performed during the `Giotto' encounter with GS comet in July 1992. The interaction region geometry, as well as the profiles of the plasma velocity and the disturbed magnetic field strength along the `Giotto' path, are compared. Theoretical and measured data are in a fairly good agreement, except for a near-nucleus region, where the MHD effects may become predominant. In general, the MHD effects appear to be more pronounced in the GS case, as compared with that of Halley comet, since during the `Giotto' – GS encounter the comet found itself in the `magnetic cloud' conditions. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

15.
Oort cloud comets occasionally obtain orbits which take them through the planetary region. The perturbations by the planets are likely to change the orbit of the comet. We model this process by using a Monte Carlo method and cross sections for orbital changes, i.e. changes in energy, inclination and perihelion distance, in a single planet-comet encounter. The influence of all major planets is considered. We study the distributions of orbital parameters of observable comets, i.e. those which have perihelion distance smaller than a given value. We find that enough comets are captured from the Oort cloud in order to explain the present populations of short period comets. The median value of cos i for the Jupiter family is 0.985 while it is 0.27 for the Halley types. The results may explain the orbital features of short period comets, assuming that the active lifetime of a comet is not much greater than 400 orbital revolutions.  相似文献   

16.
Oort cloud comets occasionally obtain orbits which take them through the planetary region. The perturbations by the planets are likely to change the orbit of the comet. We model this process by using a Monte Carlo method and cross sections for orbital changes, i.e. changes in energy, inclination and perihelion distance, in a single planet-comet encounter. The influence of all major planets is considered. We study the distributions of orbital parameters of observable comets, i.e. those which have perihelion distance smaller than a given value. We find that enough comets are captured from the Oort cloud in order to explain the present populations of short period comets. The median value of cos i for the Jupiter family is 0.985 while it is 0.27 for the Halley types. The results may explain the orbital features of short period comets, assuming that the active lifetime of a comet is not much greater than 400 orbital revolutions.  相似文献   

17.
Comet C/1860 M1 (Great Comet of 1860) is one of a large number of comets with parabolic orbits. Given that there are sufficient observations of the comet, 261 in right ascension and 251 in declination, it proves possible to calculate a better orbit. The comet's orbit is hyperbolic, and statistically different from a parabola. The comet, therefore, cannot be considered to be a Near Earth Oject. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

18.
Jack D. Drummond 《Icarus》1981,45(3):545-553
A reevaluation of the comet/meteor shower and shower/shower associations suggested by Cook (1973, in Evolutionary and Physical Properties of Meteoroids, U.S. Govt. Printing Office, Washington, D.C., NASA SP-319) is made using two orbital discriminant techniques. Twenty-six of his pairings are confirmed, five are rejected, and one new match is found; Comet Ikeya (1964 VIII) is asserted to be the source of the ? Geminids, bringing to sixteen the number of comets which produce meteor showers in Cook's list. No known asteroid shows a convincing relationship to any of the showers.  相似文献   

19.
E. PierazzoC.F. Chyba 《Icarus》2002,157(1):120-127
Jupiter's moon Europa may harbor an ocean beneath its ice cover, but the composition of that ocean and the overlying ice is nearly entirely unknown. Regardless of uncertainties in models for Europa's formation, we estimate lower limits for Europa's inventory of biogenic elements (such as C, N, O, and P) by investigating the contribution to the inventory of impact events over Europa's geologic history. A series of high-resolution hydrocode simulations were carried out over a range of comet densities (1.1, 0.8, and 0.6 g/cm3, corresponding to porosities between 0 and 45%) and impact velocities (16, 21.5, 26.5, and 30.5 km/s). We found that at typical impact velocities on Europa most impactor material reaches escape velocity, and it is assumed to be lost from Europa. For a nonporous comet, some fraction (20% or higher) of the projectile is retained by Europa even at the highest impact velocity modeled, 30.5 km/s. For porous comets, however, a significant fraction of the projectile (above 25%) is retained only for the lowest impact velocity modeled, 16 km/s. Integrated over solar system history, this suggests that 1 to 10 Gt of carbon could have been successfully delivered to Europa's surface by impacts of large comets (around 1 km in diameter). This is a few times more carbon than is contained in the procaryotic biomass of the upper 200 meters of the Earth's oceans, but about 2 orders of magnitude less if the whole depth of the oceans is considered. Therefore, regardless of its initial formation conditions, Europa should have a substantial inventory of “biogenic” elements, with implications for the chemistry of its oceans, ice cover, and the possibility of life.  相似文献   

20.
《Icarus》1987,69(1):185-188
If other stars possess Oort-like comet clouds, then some such clouds will pass sufficiently close to our Sun to induce an influx of Extra-Solar comets through the planetary region. We investigate this source, and find that while the expected number of planetary impacts due to Extra-Solar Oort comets will dominate impacts caused by free interstellar comets, only a few such comets have impacted the terrestial planets over time.  相似文献   

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