Measurement of Impact Ejecta from Regolith Targets in Oblique Impacts

This paper reports the results of experiments on projectile impact into regolith targets at various impact angles. Copper projectiles of 240 mg are accelerated to 197 to 272 m s⁻¹ using an electromagnetic gun. The ejecta are detected by thin Al foil targets as secondary targets, and the resulting holes on the foil are measured to derive the spatial distribution of the ejecta. The ejecta that penetrated the foil are concentrated toward the downrange azimuths of impacting projectiles in oblique impacts. In order to investigate the ejecta velocity distribution, the nondimensional volume of ejecta with velocities higher than a given value is calculated from the spatial distribution. In the case of the vertical impact of the projectile, most ejecta have velocities lower than 24% of the projectile speed (∼50 m s⁻¹), and there are only several ejecta with velocities higher than 72 m s⁻¹. This result confirms the existence of an upper limit to the ejection velocity in the ejecta velocity distribution (Hartmann cutoff velocity) (W. K. Hartmann, 1985, Icarus63, 69-98). On the other hand, it is found that, in the oblique impacts, there are a large number of ejecta with velocities higher than the Hartmann cutoff velocity. The relative quantity of ejecta above the Hartmann cutoff velocity increases as the projectile impact angle decreases. Taking these results with the results of S. Yamamoto and A. M. Nakamura (1997, Icarus128, 160-170) from impact experiments using an impact angle of 30°, it can be concluded that the ejecta from these regolith targets exhibit a bimodal velocity distribution. Below a few tens of m s⁻¹, we see the expected velocity distribution of ejecta, but above this velocity we see a separate group of high-velocity ejecta.     

The NASA Lunar Impact Monitoring Program

NASA’s Meteoroid Environment Office has implemented a program to monitor the Moon for meteoroid impacts from the Marshall Space Flight Center. Using off-the-shelf telescopes and video equipment, the Moon is monitored for as many as 10 nights per month, depending on weather. Custom software automatically detects flashes which are confirmed by a second telescope, photometrically calibrated using background stars, and published on a website for correlation with other observations. Hypervelocity impact tests at the Ames Vertical Gun Range facility have begun to determine the luminous efficiency and ejecta characteristics. The purpose of this research is to define the impact ejecta environment for use by lunar spacecraft designers of the Constellation manned lunar program. The observational techniques and preliminary results will be discussed. The U.S. Government's right to retain a non-exclusive, royalty-free license in and to any copyright is acknowledged.     

Ejecta size-velocity relation derived from the distribution of the secondary craters of kilometer-sized craters on Mars

The relation between the size and velocity of impact crater ejecta has been studied by both laboratory experiments and numerical modeling. An alternative method, used here, is to analyze the record of past impact events, such as the distribution of secondary craters on planetary surfaces, as described by Vickery (Icarus 67 (1986) 224; Geophys. Res. Lett. 14 (1987) 726). We first applied the method to lunar images taken by the CLEMENTINE mission, which revealed that the size-velocity relations of ejecta from craters 32 and 40 km in diameter were similar to those derived by Vickery for a crater 39 km in diameter. Next, we studied the distribution of small craters in the vicinity of kilometer-sized craters on three images from the Mars Orbiter Camera (MOC) on board the Mars Global Surveyor (MGS). If these small craters are assumed to be secondaries ejected from the kilometer-sized crater in each image, the ejection velocities are of hundreds of meters per second. These data fill a gap between the previous results of Vickery and those of laboratory studies.     

The Observation and Characterization of Lunar Meteoroid Impact Phenomena

Confirmed observations of meteoroids from the Leonid stream impacting the Moon in 1999 and 2001 have opened up new opportunities in observational and theoretical astronomy. These opportunities could help bridge the gap between the ground-based (atmospheric) sampling of the smallest meteoroids and the larger objects observable with ground-based telescopes. The Moon provides a laboratory for the study of hypervelocity impacts, with collision velocities not yet possible in ground-based laboratories. Development of automatic detection software removes the time-intensive activity of laboriously reviewing data for impact event signatures, freeing the observer to engage in other activities. The dynamics of professional-amateur astronomer collaboration have the promise of advancing the study of lunar meteoritic phenomenon considerably. These three factors will assist greatly in the development of a systematic, comprehensive program for monitoring the Moon for meteoroid impacts and determining the physical nature of these impacts.     

Impact cratering on Titan II. Global melt, escaping ejecta, and aqueous alteration of surface organics

New three-dimensional hydrodynamic simulations of hypervelocity impacts into the crust of Titan were undertaken to determine the fraction of liquid water generated on the surface of Saturn's largest moon over its history and, hence, the potential for surface—modification of hydrocarbons and nitriles by exposure to liquid water. We model in detail an individual impact event in terms of ejecta produced and melt generated, and use this to estimate melt production over Titan's history, taking into account the total flux of the impactors and its decay over time. Our estimates show that a global melt layer at any time after the very beginning of Titan's history is improbable; but transient melting local to newly formed craters has occurred over large parts of the surface. Local maxima of the melt are connected with the largest impact events. We also calculate the amount of volatiles delivered at the impact with various impact velocities (from 3 km/s for possible Hyperion fragments to 11 km/s for Jupiter family comets) and their retention as a possible source of Titan's atmosphere. We find the probability of impact ejecta escaping Titan with its modern dense and thick atmosphere is rather low, and dispersal of Titan organics throughout the rest of the Solar System requires impactors tens of kilometers in diameter. Water ice melting and exposure of organics to liquid water has been widespread because of impacts, but burial or obscuration of craters by organic deposits or cryovolcanism is aided by viscous relaxation. The largest impactors may breach an ammonia-water mantle layer, creating a circular albedo contrast rather than a crater.     

The sunlit lunar atmosphere: A comprehensive study by CHACE on the Moon Impact Probe of Chandrayaan-1

The altitudinal/latitudinal profile of the lunar atmospheric composition on the sunlit side was unraveled for the first time by the Chandra’s Altitudinal Composition Explorer (CHACE) on the Moon Impact Probe, a standalone micro-satellite that impacted at the lunar south pole, as a part of the first Indian mission to Moon, Chandrayaan-1. Systematic measurements were carried out during the descent phase of the impactor with an altitude resolution of ∼250 m and a latitudinal resolution of ∼0.1°. The overall pressure on the dayside and the neutral composition in the mass range 1-100 amu have been measured by identifying 44 and 18 amu as the dominant constituents. Significant amounts of heavier (>50 amu) species also have been detected, the details of which are presented and discussed.     

‘Direct’ evidence for water (H2O) in the sunlit lunar ambience from CHACE on MIP of Chandrayaan I

Direct detection of water in its vapour phase in the tenuous lunar environment through in situ measurements carried out by the Chandra’s Altitudinal Composition Explorer (CHACE) payload, onboard the Moon Impact Probe (MIP) of Chandrayaan I mission vindicates the presence of water on the surface of the moon in form of ice at higher lunar latitudes inferred from IR absorption spectroscopy, (especially that of OH), by the Moon Mineralogy Mapper (M³) of Chandrayaan I. The quadrupole mass spectrometer based payload, CHACE, sampled the lunar neutral atmosphere every 4 s with a broad latitudinal (∼40°N to 90°S, with a resolution of ∼0.1°) and altitudinal (from 98 km up to impact on the lunar surface with a resolution of ∼0.25 km) coverage in the sunlit side of the moon for the first time. These two (CHACE and M³) complementary experiments are shown to collectively provide unambiguous signatures for the distribution of water in solid and gaseous phases in Earth’s moon.