Ejecta Cloud Research Articles

Visible and near-infrared spectrophotometry of the Deep Impact ejecta of Comet 9P/Tempel 1

We have obtained optical spectrophotometry of the evolution of Comet 9P/Tempel 1 after the impact of the Deep Impact probe, using the Supernova Integral Field Spectrograph (SNIFS) at the UH 2.2-m telescope, as well as simultaneous optical and infrared spectra using the Lick Visible-to-Near-Infrared Imaging Spectrograph (VNIRIS). The spatial distribution and temporal evolution of the “violet band” CN (0–0) emission and of the 630 nm [OI] emission was studied. We found that CN emission centered on the nucleus increased in the 2 h after impact, but that this CN emission was delayed compared to the light curve of dust-scattered sunlight. The CN emission also expanded faster than the cloud of scattering dust. The emission of [OI] at 630 nm rose similarly to the scattered light, but then remained nearly constant for several hours after impact. On the day following the impact, both CN and [OI] emission concentrated on the comet nucleus had returned nearly to pre-impact levels. We have also searched for differences in the scattering properties of the dust ejected by the impact compared to the dust released under normal conditions. Compared to the pre-impact state of the comet, we find evidence that the color of the comet was slightly bluer during the post-impact rise in brightness. Long after the impact, in the following nights, the comet colors returned to their pre-impact values. This can be explained by postulating a change to a smaller particle size distribution in the ejecta cloud, in agreement with the findings from mid-infrared observations, or by postulating a large fraction of clean ice particles, or by a combination of these two.

Hubble Space Telescope observations of Comet 9P/Tempel 1 during the Deep Impact encounter

We report on the Hubble Space Telescope program to observe periodic Comet 9P/Tempel 1 in conjunction with NASA's Deep Impact Mission. Our objectives were to study the generation and evolution of the coma resulting from the impact and to obtain wide-band images of the visual outburst generated by the impact. Two observing campaigns utilizing a total of 17 HST orbits were carried out: the first occurred on 2005 June 13–14 and fortuitously recorded the appearance of a new, short-lived fan in the sunward direction on June 14. The principal campaign began two days before impact and was followed by contiguous orbits through impact plus several hours and then snapshots one, seven, and twelve days later. All of the observations were made using the Advanced Camera for Surveys (ACS). For imaging, the ACS High Resolution Channel (HRC) provides a spatial resolution of 36 km (16 km pixel −1) at the comet at the time of impact. Baseline images of the comet, made prior to impact, photometrically resolved the comet's nucleus. The derived diameter, 6.1 km, is in excellent agreement with the 6.0 ± 0.2 km diameter derived from the spacecraft imagers. Following the impact, the HRC images illustrate the temporal and spatial evolution of the ejecta cloud and allow for a determination of its expansion velocity distribution. One day after impact the ejecta cloud had passed out of the field-of-view of the HRC.

Searching for the Deep Impact crater on Comet 9P/Tempel 1 using image processing techniques

In this work we attempt to obtain direct images of the crater associated with the impact of the Deep Impact impactor spacecraft on the nucleus of Comet 9P/Tempel 1 on July 4, 2005. The impact generated a large and bright ejecta cloud that hampers the clear view of the post-impact nucleus surface. We used image restoration techniques to enhance spatial resolution and contrast on a subset of selected post-impact high resolution images. No unambiguous evidence for the crater can be found; however, indirect evidence is consistent with a crater size in the 150–200 m range.

Photometry of Comet 9P/Tempel 1 during the 2004/2005 approach and the Deep Impact module impact

The results of the 9P/Tempel 1 CARA (Cometary Archive for Amateur Astronomers) observing campaign is presented. The main goal was to perform an extended survey of the comet as a support to the Deep Impact (DI) Mission. CCD R, I and narrowband aperture photometries were used to monitor the Afρ quantity. The observed behavior showed a peak of 310 cm 83 days before perihelion, but we argue that it can be distorted by the phase effect, too. The phase effect is roughly estimated around 0.0275 mag/degree, but we had no chance for direct determination because of the very similar geometry of the observed apparitions. The log-slope of Afρ was around −0.5 between about 180–100 days before the impact but evolved near the steady-state like 0 value by the impact time. The DI module impact caused about a 60% increase in the value of Afρ and a cloud feature in the coma profile which was observed just after the event. The expansion of the ejecta cloud was consistent with a fountain model with initial projected velocity of 0.2 km/s and β = 0.73 . Referring to a 25,000 km radius area centered on the nucleus, the total cross section of the ejected dust was 8.2 / A km 2 0.06 days after the impact, and 1.2 / A km 2 1.93 days after the impact ( A is the dust albedo). Five days after the event no signs of the impact were detected, nor deviations from the expected activity referring both to the average pre-impact behavior and to the previous apparitions.

Near-infrared light curve of Comet 9P/Tempel 1 during Deep Impact

On UT 2005 July 4 we observed Comet 9P/Tempel 1 during its encounter with the Deep Impact flyby spacecraft and impactor. Using the SpeX near-infrared spectrograph mounted on NASA's Infrared Telescope Facility, we obtained 0.8-to-2.5 μm flux-calibrated spectral light curves of the comet for 12 min before and 14 min after impact. Our cadence was just 1.1 s. The light curve shows constant flux before the impact and an overall brightening trend after the impact, but not at a constant rate. Within a 0.8-arcsec-radius circular aperture, the comet rapidly-brightened by 0.63 mag at 1.2 μm in the first minute. Thereafter, brightening was more modest, averaging about 0.091 mag/min at 1.2 μm, although apparently not quite constant. In addition we see a bluing in the spectrum over the post-impact period of about 0.07 mag in J–H and 0.35 mag in J–K. The majority of this bluing happened in the first minute, and the dust only marginally blued after that, in stark contrast to the continued brightening. The photometric behavior in the light curve is due to a combination of crater formation effects, expansion of the ejecta cloud, and evolution of liberated dust grains. The bluing is likely due to an icy component on those grains, and the icy grains would have had to have a devolatilization timescale longer than 14 min (unless they were shielded by the optical depth of the cloud). The bluing could also have been caused by the decrease in the “typical” size of the dust grains after impact. Ejecta dominated by submicron grains, as inferred from other observations, would have stronger scattering at shorter wavelengths than the much larger grains observed before impact.

Physical and chemical evidence of the 1850 Ma Sudbury impact event in the Baraga Group, Michigan

Research Article| September 01, 2007 Physical and chemical evidence of the 1850 Ma Sudbury impact event in the Baraga Group, Michigan Peir K. Pufahl; Peir K. Pufahl 1Department of Earth and Environmental Science, Acadia University, Wolfville, Nova Scotia B4P 2R6, Canada Search for other works by this author on: GSW Google Scholar Eric E. Hiatt; Eric E. Hiatt 2Department of Geology, University of Wisconsin, Oshkosh, Wisconsin 54901, USA Search for other works by this author on: GSW Google Scholar Clifford R. Stanley; Clifford R. Stanley 3Department of Earth and Environmental Science, Acadia University, Wolfville, Nova Scotia B4P 2R6, Canada Search for other works by this author on: GSW Google Scholar Jared R. Morrow; Jared R. Morrow 4Department of Geological Sciences, San Diego State University, San Diego, California 92182-1020, USA Search for other works by this author on: GSW Google Scholar Gabriel J. Nelson; Gabriel J. Nelson 5Department of Earth and Environmental Science, Acadia University, Wolfville, Nova Scotia B4P 2R6, Canada Search for other works by this author on: GSW Google Scholar Cole T. Edwards Cole T. Edwards 6Department of Geology, University of Wisconsin, Oshkosh, Wisconsin 54901, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Peir K. Pufahl 1Department of Earth and Environmental Science, Acadia University, Wolfville, Nova Scotia B4P 2R6, Canada Eric E. Hiatt 2Department of Geology, University of Wisconsin, Oshkosh, Wisconsin 54901, USA Clifford R. Stanley 3Department of Earth and Environmental Science, Acadia University, Wolfville, Nova Scotia B4P 2R6, Canada Jared R. Morrow 4Department of Geological Sciences, San Diego State University, San Diego, California 92182-1020, USA Gabriel J. Nelson 5Department of Earth and Environmental Science, Acadia University, Wolfville, Nova Scotia B4P 2R6, Canada Cole T. Edwards 6Department of Geology, University of Wisconsin, Oshkosh, Wisconsin 54901, USA Publisher: Geological Society of America Received: 28 Jan 2007 Revision Received: 02 May 2007 Accepted: 02 May 2007 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 The Geological Society of America, Inc. Geology (2007) 35 (9): 827–830. https://doi.org/10.1130/G23751A.1 Article history Received: 28 Jan 2007 Revision Received: 02 May 2007 Accepted: 02 May 2007 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Peir K. Pufahl, Eric E. Hiatt, Clifford R. Stanley, Jared R. Morrow, Gabriel J. Nelson, Cole T. Edwards; Physical and chemical evidence of the 1850 Ma Sudbury impact event in the Baraga Group, Michigan. Geology 2007;; 35 (9): 827–830. doi: https://doi.org/10.1130/G23751A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract An ejecta layer produced by the Sudbury impact event ca. 1850 Ma occurs within the Baraga Group of northern Michigan and provides an excellent record of impact-related depositional processes. This newly discovered, ∼2–4-m-thick horizon accumulated in a peritidal environment during a minor sea-level lowstand that punctuated a period of marine transgression. Common ejecta clasts include shock-metamorphosed quartz grains, splash-form melt spherules and tektites, accretionary lapilli, and glassy shards, suggesting sedimentation near the terminus of the continuous ejecta blanket. Sedimentologic and geochemical data indicate that primary fallout from a turbulent ejecta cloud was reworked to varying degrees by an impact-generated tsunami wave train. Observed platinum group element anomalies (Ir, Rh, and Ru) within the Sudbury ejecta horizon are sufficient to suggest that the impactor was a meteorite. Documenting and interpreting the detailed characteristics of the Sudbury ejecta horizon in Michigan have yielded a fingerprint to identify this chronostratigraphic marker in other Paleoproterozoic basins. For the first time a foundation exists to assess the consequences of the Sudbury impact on Precambrian ocean chemistry and early life. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Global scale lunar sample return using projectiles launched from a low-flying spacecraft

Since the Moon has no atmosphere it is possible to fly over the surface at very low altitudes without experiencing drag forces. If a spacecraft flying at a low altitude were to fire a projectile into the lunar surface, a second trailing spacecraft could capture material from the resulting cloud of ejecta. This procedure could be repeated over many sites on the Moon with a fresh collector for each location. Eventually, the collector spacecraft would seal its cargo in a reentry vehicle and return to Earth with the samples. Compared with a robotic lander, the advantage of this architecture is the ability to sample locations over the entire Moon, wherever the topography will permit such maneuvers. Our crater ejecta models show that 1–10 g of material can be collected from the ejecta curtain of a 2 m radius crater at an altitude of 150 m, assuming a collector surface area of 1 square meter. We studied numerous means of creating these craters and developed two scenarios: a reduced velocity explosive excavator (EE), and a higher velocity impact excavator (IE).

Photometry of Comet 9P/Tempel 1 during the 2004/2005 approach and the Deep Impact module impact

The results of the 9P/Tempel 1 CARA (Cometary Archive for Amateur Astronomers) observing campaign is presented. The main goal was to perform an extended survey of the comet as a support to the Deep Impact (DI) Mission. CCD R, I and narrowband aperture photometries were used to monitor the Afρ quantity. The observed behavior showed a peak of 310 cm 83 days before perihelion, but we argue that it can be distorted by the phase effect, too. The phase effect is roughly estimated around 0.0275 mag/degree, but we had no chance for direct determination because of the very similar geometry of the observed apparitions. The log-slope of Afρ was around −0.5 between about 180–100 days before the impact but evolved near the steady-state like 0 value by the impact time. The DI module impact caused about a 60% increase in the value of Afρ and a cloud feature in the coma profile which was observed just after the event. The expansion of the ejecta cloud was consistent with a fountain model with initial projected velocity of 0.2 km/s and β = 0.73 . Referring to a 25,000 km radius area centered on the nucleus, the total cross section of the ejected dust was 8.2 / A km 2 0.06 days after the impact, and 1.2 / A km 2 1.93 days after the impact ( A is the dust albedo). Five days after the event no signs of the impact were detected, nor deviations from the expected activity referring both to the average pre-impact behavior and to the previous apparitions.

Near-infrared light curve of Comet 9P/Tempel 1 during Deep Impact

On UT 2005 July 4 we observed Comet 9P/Tempel 1 during its encounter with the Deep Impact flyby spacecraft and impactor. Using the SpeX near-infrared spectrograph mounted on NASA's Infrared Telescope Facility, we obtained 0.8-to-2.5 μm flux-calibrated spectral light curves of the comet for 12 min before and 14 min after impact. Our cadence was just 1.1 s. The light curve shows constant flux before the impact and an overall brightening trend after the impact, but not at a constant rate. Within a 0.8-arcsec-radius circular aperture, the comet rapidly-brightened by 0.63 mag at 1.2 μm in the first minute. Thereafter, brightening was more modest, averaging about 0.091 mag/min at 1.2 μm, although apparently not quite constant. In addition we see a bluing in the spectrum over the post-impact period of about 0.07 mag in J–H and 0.35 mag in J–K. The majority of this bluing happened in the first minute, and the dust only marginally blued after that, in stark contrast to the continued brightening. The photometric behavior in the light curve is due to a combination of crater formation effects, expansion of the ejecta cloud, and evolution of liberated dust grains. The bluing is likely due to an icy component on those grains, and the icy grains would have had to have a devolatilization timescale longer than 14 min (unless they were shielded by the optical depth of the cloud). The bluing could also have been caused by the decrease in the “typical” size of the dust grains after impact. Ejecta dominated by submicron grains, as inferred from other observations, would have stronger scattering at shorter wavelengths than the much larger grains observed before impact.

Visible and near-infrared spectrophotometry of the Deep Impact ejecta of Comet 9P/Tempel 1

We have obtained optical spectrophotometry of the evolution of Comet 9P/Tempel 1 after the impact of the Deep Impact probe, using the Supernova Integral Field Spectrograph (SNIFS) at the UH 2.2-m telescope, as well as simultaneous optical and infrared spectra using the Lick Visible-to-Near-Infrared Imaging Spectrograph (VNIRIS). The spatial distribution and temporal evolution of the “violet band” CN (0–0) emission and of the 630 nm [OI] emission was studied. We found that CN emission centered on the nucleus increased in the 2 h after impact, but that this CN emission was delayed compared to the light curve of dust-scattered sunlight. The CN emission also expanded faster than the cloud of scattering dust. The emission of [OI] at 630 nm rose similarly to the scattered light, but then remained nearly constant for several hours after impact. On the day following the impact, both CN and [OI] emission concentrated on the comet nucleus had returned nearly to pre-impact levels. We have also searched for differences in the scattering properties of the dust ejected by the impact compared to the dust released under normal conditions. Compared to the pre-impact state of the comet, we find evidence that the color of the comet was slightly bluer during the post-impact rise in brightness. Long after the impact, in the following nights, the comet colors returned to their pre-impact values. This can be explained by postulating a change to a smaller particle size distribution in the ejecta cloud, in agreement with the findings from mid-infrared observations, or by postulating a large fraction of clean ice particles, or by a combination of these two.

Hubble Space Telescope observations of Comet 9P/Tempel 1 during the Deep Impact encounter

We report on the Hubble Space Telescope program to observe periodic Comet 9P/Tempel 1 in conjunction with NASA's Deep Impact Mission. Our objectives were to study the generation and evolution of the coma resulting from the impact and to obtain wide-band images of the visual outburst generated by the impact. Two observing campaigns utilizing a total of 17 HST orbits were carried out: the first occurred on 2005 June 13–14 and fortuitously recorded the appearance of a new, short-lived fan in the sunward direction on June 14. The principal campaign began two days before impact and was followed by contiguous orbits through impact plus several hours and then snapshots one, seven, and twelve days later. All of the observations were made using the Advanced Camera for Surveys (ACS). For imaging, the ACS High Resolution Channel (HRC) provides a spatial resolution of 36 km (16 km pixel −1) at the comet at the time of impact. Baseline images of the comet, made prior to impact, photometrically resolved the comet's nucleus. The derived diameter, 6.1 km, is in excellent agreement with the 6.0 ± 0.2 km diameter derived from the spacecraft imagers. Following the impact, the HRC images illustrate the temporal and spatial evolution of the ejecta cloud and allow for a determination of its expansion velocity distribution. One day after impact the ejecta cloud had passed out of the field-of-view of the HRC.

Observations of CN and dust activity of comet 9P/Tempel 1 around Deep Impact

Aims. We present observations of CN emission and the scattered solar light on cometary dust particles around the impact time of the Deep Space spacecraft (NASA) into the nucleus of comet 9P/Tempel 1. The purpose of the observations was to compare post-impact activity to the conditions pre-impact to search for new spectral emission lines after impact, to quantify the increase in gas activity due to the impact and to study the long-term activity changes. Methods. We performed long-slit spectroscopy observations of comet 9P/Tempel 1 at the VLT, ESO, using the FORS instruments from July 2 to July 12, 2005. A wavelengths range of 370–920 nm was covered using two grisms. Four different position angle settings of the slit were applied each night with the projected Sun-comet line as standard setting, for which we report results here. Results. The optical spectra of comet 9P/Tempel 1 showed the usual emission bands in the optical wavelengths range of the radicals: CN, C3 ,C 2 and NH2. No new emission bands were detected after impact. The ejecta cloud of gas and dust caused by the impacting spacecraft into the cometary nucleus could be followed over the observing period. The projected expansion velocities have been determined. The night after impact we observed about (3.9 ± 1.2) × 10 29 molecules of the CN parent in the ejected cloud. However, after five days the appearance of the gas and dust coma was back to pre-impact conditions.

Use of flash X-radiography to observe ejecta from shaped charge penetrators in concrete

This paper describes the use of Flash X-Radiography for imaging the ejecta cloud from a small-scale shaped charge jet impacting a semi-infinite concrete target. The aim is to understand the timescale and extent of the ejecta as well as measuring the general velocity field. This provides useful additional information for validating material and fracture models used in simulations. The technique required the novel use of the X-ray equipment, since it was necessary to image the ejecta cloud in a region just above the surface of the concrete target and to take two images very close together, to allow a reasonably accurate velocity measurement. Also the extent of the ejecta cloud was important in terms of determining whether it originated from the borehole or as a result of the surface spallation leading to crater formation. Various techniques of target doping were used and these had varying degrees of success since the doping method could also affect the borehole formation. These were compounded by limits in the resolution of the equipment. General results are presented and recommendations made for developing this promising technique further.

Characterization of Jovian plasma-embedded dust particles

As the data from space missions and laboratories improve, a research domain combining plasmas and charged dust is gaining in prominence. Our solar system provides many natural laboratories such as planetary rings, comet comae and tails, ejecta clouds around moons and asteroids, and Earth's noctilucent clouds for which to closely study plasma-embedded cosmic dust. One natural laboratory to study electromagnetically controlled cosmic dust has been provided by the Jovian dust streams and the data from the instruments which were on board the Galileo spacecraft. Given the prodigious quantity of dust poured into the Jovian magnetosphere by Io and its volcanoes resulting in the dust streams, the possibility of dusty plasma conditions exist. This paper characterizes the main parameters for those interested in studying dust embedded in a plasma with a focus on the Jupiter environment. I show how to distinguish between dust-in-plasma and dusty-plasma and how the Havnes parameter P can be used to support or negate the possibility of collective behavior of the dusty plasma. The result of applying these tools to the Jovian dust streams reveals mostly dust-in-plasma behavior. In the orbits displaying the highest dust stream fluxes, portions of orbits E4, G7, G8, C21 satisfy the minimum requirements for a dusty plasma. However, the P parameter demonstrates that these mild dusty plasma conditions do not lead to collective behavior of the dust stream particles.

Magnetic clouds, cosmic ray decreases, and geomagnetic storms

Abstract The relationship between magnetic clouds, cosmic ray decreases and geomagnetic storms has been investigated by using some cosmic ray hourly intensities recorded with ground-based monitors at Alert, Deep River and Mount Washington, as well as the geomagnetic activity Dst index, and the interplanetary magnetic field (IMF) and the solar wind plasma (SWP) bulk-speed, density and temperature in the near-Earth space, on 28–30 September 1978, 24–26 April 1979, 13–15 January 1967, 3–5 January 1978, and 27–29 November 1989. Due to the interplanetary coronal mass ejection (ICME) impacting on slow solar wind, there is a sheath upstream of the ICME led by a fast forward shock. And the large IMF variations in this sheath, which sustain the depressions in the cosmic ray intensity during Forbush decreases (FDs), were found not to influence the main phase storm, but rather the southward IMF in the said sheath and magnetic cloud was the major source in triggering geomagnetic storms, by allowing a strong coupling between the solar wind and the magnetosphere. It was also observed that the initial set of the main phase storm always began in the sheath where, and when, the sustained southward-oriented IMF first occurred, but ceased when the IMF was rotated to a strong northward-orientation, only to resume at subsequent sustained southward-oriented IMF within the sheath and the leading (i.e., front) region of the magnetic cloud. The front boundary of the magnetic cloud was found to be well defined by the relatively high (≳10 nT) rms of the IMF components, which prominently separates both the Lull region of the sheath and the onset of the second decrease of the two-step FD, from the magnetic cloud. There were some instances where a two-step main phase storm, caused by the combination of a sheath and cloud structure, occurred, the two steps sometimes both starting in the sheath itself. Also, in some cases, the sheath and the leading region of the magnetic cloud together produced a single-step storm. In addition, enhanced IMF south latitude and IMF intensity in the sheath and magnetic cloud during the IMF sustained southern orientation, were each observed to produce enhanced geomagnetic activity, even for intense storms. And high SWP bulk speed was found to reduce the depth of the Dst index. Therefore, it appears that when the magnetosphere is exposed to a sustained southward-oriented IMF in the magnetic cloud and the sheath preceding it, a valve (i.e., valve-like IMF direction) opens and allows direct transfer of energy between the solar wind and the magnetosphere to trigger the geomagnetic storms, such that the stronger the sustained IMF south-ward orientation, the wider the valve opens, the higher the SWP bulk speed, the narrower the opening in the valve becomes. And the more the IMF strength during the IMF southern orientation, the larger is the solar wind energy density that is available for transfer through the valve. The valve closes when the IMF is rotated to a strong northward-orientation, and the geomagnetic storms cease. Index terms: 2104 Interplanetary Physics: Cosmic rays; 2111 Interplanetary Physics: Ejecta, driver gases, and magnetic clouds; 2139 Interplanetary Physics: Interplanetary shocks; 7513 Solar Physics, Astrophysics, and Astronomy: Coronal mass ejections.

Composition and magnetic structure of interplanetary coronal mass ejections at 1 AU

We study the magnetic structure and charge state ratio of interplanetary coronal mass ejections (ICMEs) observed by ACE and Wind spacecraft. Measurements of abundances and charge state ratio of heavy ions (e.g. O 7+/O 6+, C 6+/C 5+, and Mg 10+/O 6+) in the plasma as well as magnetic field structure are important tracers for physical conditions and processes in the source regions of ICMEs. We used ion composition (from ACE), plasma (from Wind) and magnetic field (from Wind and ACE) data from 1998 to 2002. Using the low proton temperature criterion, a common plasma signature of ICMEs, we identified 154 events which include magnetic clouds, non-cloud ejecta and complex ICMEs. The latter one refers to compound events resulting from the overtaking of successive ICMEs which can include both magnetic clouds and non-cloud ejecta. We find that there is a close relationship between the increase in the charge state ionization factor and the magnetic structure of ICMEs. Events with magnetic cloud topology show higher Q O 7 + / O 6 + and Q Mg 10 + / O 6 + charge state ratios than those with non-magnetic cloud structure. However, both magnetic cloud and non-cloud events show an increase in these ratios when compared with the ambient solar wind. In contrast, perhaps due to instrumental effects, the charge state ratio Q C 6 + / C 5 + for all events does not show a real enhancement when compared with the ambient solar wind. The difference in ionization states between non-cloud ejecta and magnetic clouds is more pronounced in fast solar wind than when events are embedded in slow wind.

Catastrophic emplacement of the Heart Mountain block slide, Wyoming and Montana, USA

The mechanism that allowed many tens of km of movement of the enormous block slide floored by the rootless Heart Mountain detachment fault in NW Wyoming has long been a puzzle. Carbonat-rich microbreccia that is widespread along the fault and in dikes in the upper plate contains accreted grains indistinguishable from those observed as fallout from volcanic eruption clouds (accretionary lapilli) and impact ejecta clouds and in intrusive diatremes. In these settings and also in industrial processing, accreted grains form when particles in a turbulent gaseous suspension containing limited water adhere to a nucleating grain or to each other. Elongate grains in thick microbreccia have strong but diverse shap-preferred orientations unlike those reported from other fault rocks but instead suggestive of turbulent flow, and the microbreccia contains layering and other features of sedimentary character that appear to record deposition from suspension rather than frictional processes along a fault. We suggest that frictional heating led to dissociation of carbonate rock along the fault, producing supercritical CO 2 as the suspending medium. High CO 2 pressure drastically reduced friction along the fault and allowed continuation of catastrophic movement, probably initiated by a volcanic or phreatomagmatic explosion, resulting in very large displacement on a low-dipping surface. Earlier slower sliding may have occurred but final emplacement was rapid (minutes) and spectacular.

What have we learned with SOHO?

The Solar and Heliospheric Observatory (SOHO), a space mission of international collaboration between ESA and NASA, has been operating almost continuosly since early 1996. The Sun and the heliosphere went through both: the minimum and maxumum of solar activity in 1996 and 2000, respectively. The perfectly working set of modern solar telescopes and insitu instrumentation has been producing an unprecedented set of most valuable observational data that are almost immediately available to the public via the Internet. A wealth of new results has been published in innumerable papers. For CME research in particular, SOHO has started a new era. CME evolution can now be studied from their initiation up to the arrival of the ejecta clouds at 1 AU. For the first time, helioseismological observations reveal flow vortices underneath sunspots, i.e., activity centers that are involved in subsequebt eruptions. Combined EUV disk observations and coronagraph images allow to differentiate between CMEs pointed towards to or away from the Earth. Thus, space weather predictions have achieved a new quality. The occurrence of “EIT waves” at CME onset was discovered, the internal structure of CMEs (including “disconnection”, magnetic topology and helicity, etc.) was made visible, statitics about CME properties and their change with solar activity were refined. Spectacular CME images and animations have been attracting the public to an unexpected extent, to the benefit of solar research in general.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

“Comet‐tail” ejecta streaks: A predicted cratering landform unique to Titan

Abstract— A model for an impact ejecta landform peculiar to Saturn's moon Titan is presented. Expansion of the ejecta plume from moderate‐sized craters is constrained by Titan's thick atmosphere. Much of the plume is collimated along the incoming bolide's trajectory, as was observed for plumes from impacts on Jupiter of P/Shoemaker‐Levy‐9, but is retained as a linear, diagonal ejecta cloud, unlike on Venus where the plume “blows out.” On Titan, the blowout is suppressed because the vertically‐extended atmosphere requires a long wake to reach the vacuum of space, and the modest impact velocities mean plume expansion along the wake is slow enough to allow the wake to close off. Beyond the immediate ejecta blanket around the crater, distal ejecta is released into the atmosphere from an oblique line source: this material is winnowed by the zonal wind field to form streaks, with coarse radar‐bright particles transported less far than fine radar‐dark material. Thus, the ejecta form two distinct streaks faintly reminiscent of dual comet tails, a sharply W‐E radar‐dark one, and a less swept and sometimes comma‐shaped radar‐bright one.

Impact-generated dust clouds around planetary satellites: spherically symmetric case

An analytic model of an impact-generated, steady-state, spherically symmetric dust cloud around an atmosphereless planetary satellite (or planet—Mercury, Pluto) is constructed. The projectiles are assumed to be interplanetary micrometeoroids. The model provides the expected mass, density, and velocity distributions of dust in the vicinities of parent bodies. Applications are made to Jupiter's moon Ganymede and six outer satellites of Saturn. In the former case, the model is shown to be consistent with the measurements of the dust detector system onboard the Galileo spacecraft. In the latter case, estimates are given and recommendations are made for the planned experiment with the Cassini cosmic dust analyzer (CDA) during targeted flybys of the spacecraft with the moons. The best CDA pointing to maximize the number of detections is in the ram direction. With this pointing, measurements are possible within a few to about 20 min from the closest approach, with maximum minute impact rates ranging from about 1 for Phoebe and Hyperion to thousands for Enceladus. Detections of the ejecta clouds will still be likely if CDA's angular offset from the ram direction does not exceed 45°. The same model can be applied to dust measurements by other space missions, like New Horizons to Pluto or BepiColombo to Mercury.