The Mars Orbiter Camera (MOC) onboard the Mars Global Surveyor (MGS) spacecraft continued to obtain high resolution images of the red planet into August 1998. At this time, each ground track (the portion of Mars available for MOC imaging on a given orbit) covers areas from about 40N on the late afternoon side of the planet, up over the sunlit north polar cap, and down the early morning side of Mars to about 20N latitude. Early morning and late afternoon views provide good shadowing to reveal subtle details on the martian surface. Views of Mars with such excellent lighting conditions will not be seen by MOC once MGS's Science Phasing Orbits end in mid-September 1998.The image shown here, MOC image 47903, was targeted on Friday afternoon (PDT), August 7, 1998. This picture of ejecta from a nameless 9.1 kilometer (5.7 mile)-diameter crater was designed to take full advantage of the present lighting conditions. When the image was taken (around 5:38 p.m. (PDT) on Saturday, August 8, 1998), the Sun had just risen and was only about 6 above the eastern horizon. With the Sun so low in the local sky, the contrast between sunlit and shadowed surfaces allowed new, subtle details to be revealed on the surface of the crater ejecta deposit.The crater shown here has ejecta of a type that was first identified in Mariner 9 and Viking Orbiter images as \"fluidized\" ejecta. Ejecta is the material that is thrown out from the crater during the explosion that results when a meteor--piece of a comet or asteroid--collides with the planet. Fluidized ejecta is characterized by its lobate appearance, and sometimes by the presence of a ridge along the margin of the ejecta deposit. In the case of the crater shown here, there are two ridges that encircle the crater ejecta--this type of ejecta deposit is sometimes called a double-lobe rampart deposit. The MOC image shows that this particular crater also has \"normal\" ejecta that occurs out on the plains, beyond the outermost ridge of the main, fluidized ejecta deposit.Fluidized or \"rampart\" ejecta deposits have long been thought by many Mars scientists to result from an impact into a surface that contains water. The water would have been underground, and could have been frozen or liquid. According to the prevailing model, when the meteor hit, this water was released--along with tons of rock and debris--and the ejecta flowed like mud. Images with resolutions higher than those presently attainable from the 11.6 hr elliptical orbit are needed to see the specific features (such as large boulders \"rafted\" by the dense mud) that would confirm or refute this model. Such images may be acquired once MGS is in its mapping orbit.MOC image 47903 was received and processed by the MOC team at Malin Space Science Systems on Monday afternoon (PDT), August 10, 1998. The image center is located at 27.92N latitude and 184.66W longitude, in the northern Tartarus Montes region.
Lawrence Livermore National Laboratory researchers Fady Najjar, left, and Garry Maskaly led the work described in the Journal of Applied Physics paper identifying a previously unknown ejecta production mechanism called Shallow Bubble Collapse.
New research led by Lawrence Livermore National Laboratory (LLNL) provides a better understanding of ejecta production, which has been the subject of broad interest for more than 60 years throughout the scientific community.
Garry Maskaly, lead author of a paper featured in the Journal of Applied Physics, said the research team identified a previously unknown ejecta production mechanism called Shallow Bubble Collapse (SBC) that is not based on Richtmyer-Meshkov instabilities (RMI), when shock waves interact with and separate two fluids of different density. RMI ejecta previously have been believed to be the main source of shock-driven metal ejecta and have been the subject of decades of research.
The work solidifies the tight collaboration amongst LLNL, LANL, and NNSS-STL, pushing forward a new area of ejecta physics. Shock-driven ejecta production from metal surfaces often occurs in high-explosive-driven experiments with metal free surfaces.
The key highlights from this research summarized how the SBC mechanism can produce substantially more ejecta (10 times) with a much higher temperature (two times) than RMI ejecta produced under similar shock strengths.
Maskaly said that SBC explains a regime of multiple shock-driven ejecta behaviors that were previously either unexplained or unexplored. With SBC, the team demonstrates that enough momentum is ejected that the bulk hydrodynamics can be impacted.
Sample curtain reflecting the trajectories of ejecta particles. Once an impact is detected, the grid is refined to better reflect the actual detection probability. Color images available online at www.liebertonline.com/ast
The probability of collision per revolution of the ejecta particle with the planet/moon is calculated from the orbital elements of both orbits, a, e, and i; the relative velocity, Vr; the orbital period, P; and the collision cross-sectional area, σ, which is a function of the radii and the masses of the two bodies. The equation for the probability of collision is given by
where A and B are constant coefficients for a given crater determined from the best-fit computation with the observation data. However, the coefficients for the power laws are specific to a given crater, and the crater diameters in Vickery's studies varied over a large range. In this analysis, a more global approach is adopted that consists of defining a sequence of craters in terms of diameter. Thus, to examine the size distribution of ejecta particles that impact Phobos, the size-velocity relations as defined in Eq. 10 must include the diameter of the crater. Relationships of the form
Ejecta from Dimorphos 1.7 days after impact taken on September 28, 2022. The base of the ejecta cone started to be twisted by the gravity of Didymos, rotating in clockwise direction, forming two curved streams.
Shinmoedake volcano is one of the active andesitic volcanoes in the Kirishima mountain range, Kyushu, Japan (Fig. 1). A series of explosion events, accompanying subplinian, vulcanian and dome-building stages, occurred at the volcano between January and May in 2011 (Nakada et al., 2013). In this paper, the initial condition of ballistic ejecta expelled from the vulcanian explosion at a growing andesitic lava dome on Shinmoedake volcano on 1 February, 2011, is estimated, based on the sizes of impact craters created by ballistic ejecta. In this estimation, we use a ballistic trajectory model (Mastin, 2008) and a scaling law for impact crater formation (Housen et al., 1983; Holsapple, 1993). The source condition is constrained using a vulcanian explosion model by Fagents and Wilson (1993). This combined approach, and the results, will be useful for understanding the process of vulcanian explosions and related hazards.
Numerous ballistic ejecta were expelled from the summit crater through the expanding explosion cloud. The explosion cloud reached the rim of the summit crater (300 m) in a few seconds. Finer materials were entrained into the rising plume. The plume height reached 2 km above a vent based on visual observation by JMA, while Shimbori and Fukui (2012) reported that the plume height reached 7 km above sea level based on Doppler radar observation. The plume then moved toward the southeast. Pyroclastic flows were not observed. The maximum amplitude of seismicity (3767 µm/s) was recorded at the Shinmoedake Southwest station, 1.7 km from the summit crater, and maximum air pressure (458.4 Pa) was measured at the Yunono station, 3 km southwest of the crater. The amplitudes of seismicity were the largest in the historical record of Shinmoedake eruptions (Japan Meteorological Agency, 2011). In the city of Kobayashi, over 10 km from the crater, roofs and windows of buildings were partially destroyed by small fragments of lava transported by the plume. In areas at the southern foot of the volcano, windows were broken by the shock wave. In some places to the southwest where ballistic ejecta and their fragments were landed, fires were started. Tephra distribution was unknown; hence tephra volume is not constrained, but probably is on the order of 104 m3 based on a comparison to tephra data our group obtained for similar-scale vulcanian explosions during February to March 2011 (Unpublished data of Earthquake Research Institute).
In the same morning after the vulcanian explosion, the lava dome diameter was measured at about 500 m by a JMA airborne survey. This size was nearly the same as that on 31 January. The dome height was slightly reduced because the explosion destroyed its surface. Although the change of dome height was not precisely measured, it seemed very small compared to the thickness of the lava dome. The dome center was covered by blocky lava, but lava wrinkles at the outer part of the dome were still observed. The observation indicates that the explosion site was confined to the dome center, approximately within 250 m in diameter (half the size of the dome). The Geospatial Information Authority of Japan (2011) estimated the altitude of the top of the lava dome to be 1360 m, based on Daichi Phased Array type L-band Synthetic Aperture Radar (PALSAR) intensity images on the same day after the explosion. The dome growth then slowed and almost ceased the next day. Based on these observations, we assumed that ballistic ejecta were launched from the lava dome center at the altitude of 1360 m. The thickness of the lava dome at the explosion site was estimated to be about 130 m, based on topographic data of pre- and post-explosion.
Ballistic blocks were dense but have a few bubble textures, and there was no evidence of delayed vesiculation such as bread crust surfaces (Fig. 5(a)). This suggests that ballistic ejecta found in this area originated from solid lava that would seal pressurized gas beneath the dome but were still at a high temperature, so they caused fires and burned grass when they emplaced. 59ce067264