Martian cratering II: Asteroid impact history     

William K. Hartmann 《Icarus》1971

This paper considers the extent to which Martian craters can be explained by considering asteroidal impact. Sections I, II, and III of this paper derive the diameter distribution of hypothetical asteroidal craters on Mars from recent Palomar-Leiden asteroid statistics and show that the observed Martian craters correspond to a bombardment by roughly 100 times the present number of Mars-crossing asteroids. Section IV discusses the early bombardment history of Mars, based on the capture theory of Öpik and probable orbital parameters of early planetesimals. These results show that the visible craters and surface of Mars should not be identified with the initial, accreted surface. A backward extrapolation of the impact rates based on surviving Mars-crossing asteroids can account for the majority of Mars craters over an interval of several aeons, indicating that we see back in time no further than part-way into a period of intense bombardment. An early period of erosion and deposition is thus suggested. Section V presents a comparison with results and terminology of other authors.  相似文献   

Surface evolution of two-component stone/ice bodies in the Jupiter region     

William K. Hartmann 《Icarus》1980,44(2):441-453

Observational and theoretical data converge on the conclusion that planetesimals in Jupiter's region of the solar nebula were initially composed predominantly of a mixture of roughly 39–70% H₂O ice by volume, and 30–61% dark stony material resembling carbonaceous chondrites. Recent observations emphasize a division of most asteroid and satellite surfaces in this region into two distinct groups: bright icy material and dark stony material. The present model accounts for these by two main processes: an impact-induced buildup of a dark stony regolith in the absence of surface thermal disturbance, and thermal-disturbance-induced eruption of “water magmas” that create icy surfaces. “Thermal disturbances” include tidal and radiative effects caused by nearness of a planet. A correlation of crater density and albedo, Ganymede's dark-ray craters, and other observed phenomena (listed in the summary) appear consistent with the model discussed here.  相似文献   

Martian surface and crust: Review and synthesis     

William K. Hartmann 《Icarus》1973,19(4):550-575

Mariner 9 data and earlier data are combined to investigate the nature of the classical markings on Mars. This leads to a model of crustal evolution and structure. Combination of radar and spectrophotometric data strengthens earlier evidence for petrologic distinctions between surface materials in dark and light regions. The classical surface markings are a complex result of three influences: (1) availability of two types of rock material transportable by wind, (2) topographic control of deposition, and (3) prevailing winds producing quasipermanent preferential deposition patterns. The crust, especially in the Tharsis region, bears strong evidence of mantle-induced uplift of a type recognized on Earth as long ago as 1939. Such uplifts have obliterated ancient craters, caused fractures, and graben systems such as the Coprates canyon, and resulted in intense volcanism. This evidence, combined with an apparent bimodal hypsometric diagram, indicates Mars has current or recent mantle activity sufficient to disturb the crust, aid in petrologic differentiation, and cause development of protocontinental units, but insufficient to cause full-fledged continental drift or fold-causing plate collisions as are common on Earth.  相似文献   

Mass loss from solar-type stars     

L. Hartmann 《Solar physics》1985,100(1-2):587-597

Winds are directly detected from solar-type stars only when they are very young. At ages 10⁶ yr, these stars have mass loss rates 10⁶ times the mass flux of the present solar wind. Although these young T Tauri stars exhibit ultraviolet transition-region and X-ray coronal emission, the large particle densities of the massive winds lead to efficient radiative cooling, and wind temperatures are only 10⁴ K. In these circumstances thermal acceleration is unlikely to play an important role in driving the mass loss. Turbulent energy fluxes may be responsible for the observed mass loss, particularly if substantial magnetic fields are present.The presence of stellar mass loss is indirectly shown by the spindown of low-mass stars as they age. It appears that many solar-mass stars spin up as they contract toward the Main-Sequence, reaching a maximum equatorial velocity of 50 to 100 km s^–1. These stars spin down rapidly upon reaching the Main Sequence. Spindown may be enhanced by a decoupling or lag between convective envelope and radiative core. Because this spindown occurs fairly early in a solar-type star's history, the internal structure of old stars like the Sun may not depend upon initial conditions.  相似文献   

Planet formation: Mechanism of early growth     

William K. Hartmann 《Icarus》1978,33(1):50-61

Experiments in vacuum (approx. 0.5 to 1 mbar) and in air quantify mechanics of collisions, rebound, and fragmentation at low velocities (1–50 m/sec), under the conditions usually postulated for the preplanetary environment in the primitive solar nebula. Such collisions have been little studied experimentally. Contrary to widespread assumptions, accretionary growth of the largest meteoroid- and asteroid-sized bodies in a given swarm results spontaneously from the simple mechanics of these collisions, without other ad hoc sticking mechanisms. The smaller bodies in the swarm are less likely to grow. Granular surfaces form, either by gravitational collapse of dust swarms or by rapid formation of regolith surfaces on solid planetesimals; these surfaces strongly promote further growth by retarding rebound. Growth of large bodies increases modal collision velocities, causing fragmentation of smaller bodies and eventual production of interstellar dust as a by-product planetesimal interactions.  相似文献   

Size distribution of particles in planetary rings     

Richard Greenberg  Donald R. Davis  William K. Hartmann  Clark R. Chapman 《Icarus》1977,30(4):769-779

Harris (Icarus24, 190–192) has suggested that the maximum size of particles in a planetary ring is controlled by collisional fragmentation rather than by tidal stress. While this conclusion is probably true, estimated radius limits must be revised upward from Harris' values of a few kilometers by at least an order of magnitude. Accretion of particles within Roche's limit is also possible. These considerations affect theories concerning the evolution of Saturn's rings, of the Moon, and of possible former satellites of Mercury and Venus. In the case of Saturn's rings, comparison of various theoretical scenarios with available observational evidence suggests that the rings formed from the breakup of larger particles rather than from original condensation as small particles. This process implies a distribution of particle sizes in Saturn's rings possibly ranging up to ～100 km but with most cross-section in cm-scale particles.  相似文献   

Juvenile Saury Pike (Scomberesox Saurus Walb.), an Example of Ichthyoneuston     

Hartmann  Jurgen 《ICES Journal of Marine Science》1970,33(2):245-255

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Circumstellar envelopes and disks of pre-main sequence stars     

L. Hartmann 《Astrophysics and Space Science》1995,224(1-2):3-12

The current state of knowledge about circumstellar matter of young stellar objects is briefly reviewed. It appears that some very young stars yet to accrete substantial amounts of mass may be seen through their dusty infalling envelopes even at optical wavelengths, because of the presence of holes or large departures from spherical symmetry in the envelopes. The evidence for this picture is summarized in the context of one wellstudied young star, HL Tau, indicating that much of the large-scale structure originally identified as a rotating disk is probably a flattened infalling envelope. Departures from spherical symmetry in protostellar clouds are likely to lead to quite flattened structures once collapse gets under way, further suggesting that infall in large-scale toroids may be a general feature of low-mass star formation. The best kinematic evidence for Keplerian disk rotation comes from optical and near-infrared high-resolution spectroscopy of the innermost regions of circumstellar disks. Disk masses are uncertain but are likely to be at least the order of minimum mass solar nebula models, if not much larger.  相似文献   

Preliminary results of optical searches of IPN3 localizations     

C. B. Luginbuhl  F. J. Vrba  R. Hudec  D. Hartmann  K. Hurley  M. Boër  M. Niel  M. Sommer  C. Kouveliotou  C. Meegan  G. Fishman 《Astrophysics and Space Science》1995,231(1-2):289-292

We present preliminary results from deep optical searches of small (5 arcmin²) GRB error boxes determined using the Third Interplanetary Network (IPN³). Two of these fields also have been found to have historical OT events located within the IPN³ error boxes. We compare the preliminary results of these searches to those reported for the larger IPN¹ error boxes. The small size of the IPN³ error boxes should allow a test of the hypothesis suggested by the IPN¹ study that there are excess QSOs associated with the GRB fields.  相似文献   

The leiden/dwingeloo survey of emission from galactic HI     

W. B. Burton  Dap Hartmann 《Astrophysics and Space Science》1994,217(1-2):189-193

The 25-meter radio telescope of the Netherlands Foundation for Research in Astronomy in Dwingeloo has been engaged full-time during the past 5 years in a survey of galactic 21-cm emission from the entire sky accessible from the Netherlands. The new material provides coverage of the sky at -30° on a 0°.5 grid sampled with a 35 main beam, over a velocity range of 1000 km s^–1 at 1 km s^–1 resolution, to a limiting brightness-temperature sensitivity typically better than 0.07 K. The data have been corrected for stray radiation entering both the near and far sidelobes of the antenna.  相似文献