Peter H. Schultz

Highly oblique impacts represent a rare and special class of impact events manifested as elliptical craters preserved on present‐day planetary surfaces. Crater excavation in highly oblique impacts differs from that of standard vertical impacts due to early time asymmetries imparted by the downrange‐directed coupling of energy between the projectile and target. Such asymmetries are preserved as scouring and mobilized surface materials produced by high‐velocity, low‐angle ejecta and blast winds along the initial impact trajectory axis downrange. Subsequent deposition of resulting melt‐rich products occurs in the form of concentrated strewnfields located downrange from the impact. The distinct nature of these distal strewnfields is separate from later time, more proximal ejecta accumulations and generally allows connecting these specific deposits with their parent crater. The present study involves a three‐dimensional computational impact simulation (modeled using the CTH shock physics hydrocode) of an event similar to the impact that produced the elliptical Hale impact crater on Mars (125 km × 150 km diameter crater located at 36°S, 36°W). Results generally capture the predicted patterns indicative of highly oblique impacts. Evidence is presented for correlated distal strewnfields located downrange from Hale that coincide with prominent low‐albedo surface deposits and wind streaks on the present‐day Martian surface, thereby providing implications for possible identification for other dark glassy strewnfields as well as impact‐generated winds on Mars.

Watering the Moon: About a year ago, a spent upper stage of an Atlas rocket was deliberately crashed into a crater at the south pole of the Moon, ejecting a plume of debris, dust, and vapor. The goal of this event, the Lunar Crater Observation and Sensing Satellite (LCROSS) experiment, was to search for water and other volatiles in the soil of one of the coldest places on the Moon: the permanently shadowed region within the Cabeus crater. Using ultraviolet, visible, and near-infrared spectroscopy data from accompanying craft, Colaprete et al. (p. 463 ; see the news story by Kerr ; see the cover) found evidence for the presence of water and other volatiles within the ejecta cloud. Schultz et al. (p. 468 ) monitored the different stages of the impact and the resulting plume. Gladstone et al. (p. 472 ), using an ultraviolet spectrograph onboard the Lunar Reconnaissance Orbiter (LRO), detected H ₂ , CO, Ca, Hg, and Mg in the impact plume, and Hayne et al. (p. 477 ) measured the thermal signature of the impact and discovered that it had heated a 30 to 200 square-meter region from ∼40 kelvin to at least 950 kelvin. Paige et al. (p. 479) mapped cryogenic zones predictive of volatile entrapment, and Mitrofanov et al. (p. 483 ) used LRO instruments to confirm that surface temperatures in the south polar region persist even in sunlight. In all, about 155 kilograms of water vapor was emitted during the impact; meanwhile, the LRO continues to orbit the Moon, sending back a stream of data to help us understand the evolution of its complex surface structures.

Satellite radar signals suggest that something unusual is buried beneath one of the most enigmatic regions of Mars.

Samples of material returned from the Moon have established that widespread lunar volcanism ceased about 3.2 Gyr ago. Crater statistics and degradation models indicate that last-gasp eruptions of thin basalt flows continued until less than 1.0 Gyr ago¹, but the Moon is now considered to be unaffected by internal processes today, other than weak tidally driven moonquakes² and young fault systems³. It is therefore widely assumed that only impact craters have reshaped the lunar landscape over the past billion years. Here we report that patches of the lunar regolith in the Ina structure^2,3,4,5 were recently removed. The preservation state of relief, the number of superimposed small craters, and the ‘freshness’ (spectral maturity) of the regolith together indicate that features within this structure must be as young as 10 Myr, and perhaps are still forming today. We propose that these features result from recent, episodic out-gassing from deep within the Moon. Such out-gassing probably contributed to the radiogenic gases detected during past lunar missions. Future monitoring (including Earth-based observations) should reveal the composition of the gas, yielding important clues to volatiles archived at great depth over the past 4–4.5 Gyr.