Number Of Large Craters Research Articles

Hypervelocity Impact Experiments in Iron‐Nickel Ingots and Iron Meteorites: Implications for the NASA Psyche Mission

AbstractThe National Aeronautics and Space Administration (NASA) Psyche mission will visit the 226‐km diameter main belt asteroid (16) Psyche, our first opportunity to visit a metal‐rich object at close range. The unique and poorly understood nature of Psyche offers a challenge to the mission as we have little understanding of the surface morphology and composition. It is commonly accepted that the main evolutionary process for asteroid surfaces is impact cratering. While a considerable body of literature is available on collisions on rocky/icy objects, less work is available for metallic targets with compositions relevant to Psyche. Here we present a suite of impact experiments performed at the NASA Ames Vertical Gun Range facility on several types of iron meteorites and foundry‐cast ingots that have similar Fe‐Ni compositions as the iron meteorites. Our experiments were designed to better understand crater formation (e.g., size, depth), over a range of impact conditions, including target temperature and composition. We find that the target strength, as inferred from crater sizes, ranges from 700 to 1,300 MPa. Target temperature has measurable effects on strength, with cooled targets typically 10–20% stronger. Crater morphologies are characterized by sharp, raised rims and deep cavities. Further, we derive broad implications for Psyche's collisional evolution, in light of available low resolution shape models. We find that the number of large craters (>50 km) is particularly diagnostic for the overall bulk strength of Psyche. If confirmed, the number of putative large craters may indicate that Psyche's bulk strength is significantly reduced compared to that of intact iron meteorites.

The shape of (7) Iris as evidence of an ancient large impact?

Context. Asteroid (7) Iris is an ideal target for disk-resolved imaging owing to its brightness (V ~ 7–8) and large angular size of 0.33′′ during its apparitions. Iris is believed to belong to the category of large unfragmented asteroids that avoided internal differentiation, implying that its current shape and topography may record the first few 100 Myr of the solar system’s collisional evolution. Aims. We recovered information about the shape and surface topography of Iris from disk-resolved VLT/SPHERE/ZIMPOL images acquired in the frame of our ESO large program. Methods. We used the All-Data Asteroid Modeling (ADAM) shape reconstruction algorithm to model the 3D shape of Iris, using optical disk-integrated data and disk-resolved images from SPHERE and earlier AO systems as inputs. We analyzed the SPHERE images and our model to infer the asteroid’s global shape and the morphology of its main craters. Results. We present the 3D shape, volume-equivalent diameter Deq = 214 ± 5 km, and bulk density ρ = 2.7 ± 0.3 g cm−3 of Iris. Its shape appears to be consistent with that of an oblate spheroid with a large equatorial excavation. We identified eight putative surface features 20–40 km in diameter detected at several epochs, which we interpret as impact craters, and several additional crater candidates. Craters on Iris have depth-to-diameter ratios that are similar to those of analogous 10 km craters on Vesta. Conclusions. The bulk density of Iris is consistent with that of its meteoritic analog based on spectroscopic observations, namely LL ordinary chondrites. Considering the absence of a collisional family related to Iris and the number of large craters on its surface, we suggest that its equatorial depression may be the remnant of an ancient (at least 3 Gyr) impact. Iris’s shape further opens the possibility that large planetesimals formed as almost perfect oblate spheroids. Finally, we attribute the difference in crater morphology between Iris and Vesta to their different surface gravities, and the absence of a substantial impact-induced regolith on Iris.

Double-layer high-porous copper/duralumin bumper

Normal impacts of a spherical steel projectile 2-mm in diameter on double-layer bumpers are investigated experimentally in a velocity range from 2 to 7.3 km/s. One layer of the bumpers is made of fine copper powder, and the other is made of duralumin. The high-porosity copper/duralumin double-layer bumper of the same thickness and areal density performed better than a single-layer duralumin bumper for impact velocities ranging from 3 to 5 km/s. The maximum depth of penetration of projectile fragments decreased and a smaller number of large craters were observed in the witness plates for the tests with the double-layer bumpers. The order of the layers in the bumper did not have a significant effect on the projectile's failure. As the impact velocity increased to 7 km/s, the difference between the protective properties of the single-layer and double-layer bumpers disappeared.

Close Encounters and Collisions of Short-Period Comets with Jupiter and Its Satellites

The collisional probability of short-period (SP) comets with Jupiter was estimated from numerical integrations of the orbits of real 165 SP comets over 4400 years. The cumulative close-encounter distance vs frequency plot was extrapolated to the radius of Jupiter taking account of the gravitational focusing effects in order to obtain the mean collisional probability. The obtained value gave a fairly good agreement with that calculated by D. Olsson-Steel (1987, Mon. Not. R. Astron. Soc. 227, 501-524) based on a form of Öpik's method. Incorporating the estimated flux of invisible SP comets near the Jovian orbit in the observed range of H 10-magnitude, the intrinsic collision rate with Jupiter was found to be once per 950 years, which is nearly 15 times more frequent than the collision rate for long-period comets calculated by Olsson-Steel (1987). The total impact numbers of comets with each jovian satellite over the Solar System age were calculated for the steady-state flux model of comets. It is shown that the surface densities of ray-craters (youngest) on Ganymede and Callisto ( D gt; 30 km) are in agreement, within a factor of two, with our calculated intrinsic flux of SP comets. From the fact that the number of large craters on Amalthea (JV) is nearly equal to the total impact number predicted from the steady-state flux of comets, it is proposed that Amalthea may be the remnant core of a larger parent body whose original surface was spalled by many catastrophic impacts, or it may be a new object recently captured into the present orbit. We also suggest that the maximum size of craters on the retrograde outer satellites, which could be measured by the Galileo spacecraft, be a good tester for the knowledge of the cometary flux integrated over 4.6 × 10 9 years near the orbit of Jupiter. It is discussed that current survival of some inner small satellites might give a clue to the upper limit of the integrated cometary flux there.

The terrestrial cratering record: II. The crater production rate

The terrestrial cratering record for the Phanerozoic has a size-frequency distribution of NαD −2.05 for D > 22.6 km and NαD −0.24 for D < 11.3 km. This shallowing of the distribution slope at D > 22.6 km reflects the removal of small terrestrial craters by erosion. The number of large craters on the North American and East European cratons provide estimated terrestrial crater production rates for D > 20 km of 0.36 ± 0.1 and 0.33 ± 0.2 × 10 −14 km −2 year −1, respectively. These rates are in good agreement with previous estimates and astronomical observations on Apollo bodies. Comparisons with the lunar rate, taking account of the effects of variations in impact velocity, surface gravity, and gravitational cross section, indicate that the lunar and terrestrial rates overlap, if the cratering flux has been constant during the last 3.4 by. If the early (pre 4.0 by) high-flux rate did not decay to a constant value until 3.0 to 2.5 by then the rates differ by a factor of 2 and the Phanerozoic can be interpreted as a period of higher than normal cratering.