Impact Flash Spectroscopy
Impact Flash Spectroscopy
复制标题
冲击闪光光谱
DOI：
10.1016/j.icarus.2005.06.010
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发表时间：
1996
期刊：
影响因子：
3.2
通讯作者：
S. Sugita
中科院分区：
文献类型：
--
作者：
P. Schultz;M. A. Adams;J. Perry;J. Goguen;S. Sugita
对富含挥发物的目标进行超高速撞击会产生自发光云，该云会在存在大气层的情况下按照理论预期膨胀和减速 (1, 2)。此类云的光谱揭示了由蒸发的目标成分以及与撞击器的反应产生的显着发射线 (1, 3)。由于汽化程度和云光度随撞击速度、角度和撞击目标组合而变化，因此使用 NASA 艾姆斯垂直炮靶场设计了实验来评估不断变化的光谱内容。我们的结果表明，对于各种碳酸盐和硅酸盐目标，可以从相对适中的撞击速度（5-6 W s ）的发射光谱中获得大量的成分信息。早期研究将这种现象视为几乎瞬时的闪光 (4, 5)，与此相反，我们的结果记录了在时间和空间上演化的复杂蒸气云。因此，光谱学提供了一种强大的新工具，不仅可以探测撞击过程，还可以利用这些信息来确定传统遥感技术可能难以测量或解释的行星表面成分。方法：使用了三种光谱系统。第一个是高分辨率（0.2 nm）照相衍射光谱仪（PDS），覆盖范围从 400 nm 到 630 nm。第二个是 CCD 个人光谱仪 I1 (PS-11)，覆盖范围从 380 nm 到 880 nm，低分辨率 (5 nm)。这两种仪器相对于撞击事件的时间分辨率较差：本质上是开放式曝光。然而，蒸气云运动越过视场导致有效曝光时间小于 1 毫秒。这种方法揭示了各种目标和撞击条件的综合光谱内容。第三个系统是 Oriel 公司的门控增强 CCD 光谱仪，可以根据需要调整光谱范围、分辨率和曝光时间。对于这些实验，我们选择了相对较窄的光谱范围（360 nm 至 580 nm），光谱分辨率为 1 nm。光谱仪的时间选通允许在撞击后的前 2.3 毫秒和 2.3 毫秒至 50 毫秒内进行测量。这种更高的时间和波长分辨率对于从光谱中获取定量信息至关重要。这三个系统不是同时使用的，而是在不同的时间用于不同的目的。它们共同提供了对复杂现象和过程的有趣概述，这将有助于限制未来的研究。美国宇航局艾姆斯垂直射击场非常适合这项工作。可变的发射角度允许使用松散和液体目标。同样重要的是，大的腔室尺寸允许蒸气云在近真空 (3 m) 中自由膨胀，而不会受到腔室壁的干扰。此外，多个观察口允许替代的观察几何形状。照相摄谱仪位于侧端口，提供了当蒸汽云经过成像狭缝时不断演变的蒸汽云的高分辨率二维视图。目标包括干冰 (CO2) 和 CaCO3 粉末，在距水平面 15" 处受到 5 kds-5.5 k d s 铝球 (0.635 cm) 的撞击。低分辨率 PS-II 也位于侧端口，并在蒸汽云内的特定位置上使用窄视角 (5")，以实现不同的撞击角度（15"、30°、90"）和撞击器类型（铝和耐热玻璃）。这些实验可以对云内的撞击角度效应和成分异质性进行初步评估。最后，使用 Oriel 系统根据观察到的光谱确定三种不同天然目标材料（白云石粉末、方解石晶体和辉长岩 [辉绿岩] 块）的成分。铝弹丸（0.3 × 175 cm 球体）在 15"（距水平方向）处撞击这些目标。结果：PDS 的高光谱分辨率揭示了撞击体（铝）和碳酸盐目标离解成分的清晰特征 (1)。470,480 和 510 nm 附近的独特 A10+ 分子谱带表明目标和撞击体之间发生快速化学反应。已识别的目标排放物包括 NaI （5891590 nm）、CaO（554、604 和 622 nm）、CaI（423,527,560、586,610 和 612 nm）以及部分掩蔽的 CO（561 和 609 nm）。此类冲击产物与预期一致 (6,7)。 (A10+) 扩散和 NdK 分馏。图 1 显示了随着 15"、30° 和 90"（水平方向）冲击角的增加，Na 和 K 的排放强度逐渐降低。然而，随着冲击角从 15" 处的 1.2 增加到 30" 处的 1.3 和 90" 处的 1.5，NdK 比率系统性地增加。正如先前推断的撞击碳酸盐目标的情况一样，在较低的撞击角度下 (2, 3)，图 1 还表明，随着撞击角度的增加，撞击组件的强度会随着时间的推移而变化，这是由于蒸汽云内的条件变化（反应、混合、冷却、轨迹）和暴露（穿过探测器）而产生的，因此门控 Oriel 光谱仪对于得出有意义的元素丰度至关重要，图 2 对比了早期（0 2.3 ms）和后期捕获的光谱。 (2.3 25 ms) 次，以 15 英寸（0.318 厘米铝）冲击方解石。与侧视视图（图 1）相反，图 2 显示了从上方聚焦在距离撞击点 5 厘米处的直径 10 厘米的点上采集的数据。它不仅揭示了当气体云向下移动到观察区域之外时强度的快速衰减，而且还揭示了由于辐射、反应和传输过程而演化的物种。早期曝光捕获了撞击物（AlI、A10+），而后期曝光则更清楚地揭示了目标成分（CO、CaO、CaI），尽管强度大大降低。使用方解石、白云石和辉长岩进行的其他实验可以根据其发射线的相对强度（图 3）确定 Mg 和 Ca 的相对丰度，这与其已知的成分一致。
Hypervelocity impacts into volatile-rich targets generate a self-luminous cloud that expands and decelerates in the presence of an atmosphere following theoretical expectations (1, 2). Spectra of such clouds reveal prominent emission lines resulting from vaporized target components and reactions with the impactor (1, 3). Because the degree of vaporization and cloud luminosity is found to vary with impact velocity, angle, and impactorltarget combinations, experiments were designed using the NASA Ames Vertical Gun Range to assess the evolving spectral content. Our results establish that a wealth of compositional information can be derived from emission spectra at relatively modest impact velocities (5-6 W s ) for a wide range of carbonate and silicate targets. In contrast with much earlier studies that viewed the phenomena as a nearly instantaneous flash (4, 5), our results document a complex vapor cloud that evolves in both time and space. Consequently, spectroscopy provides a powerful new tool not only for probing the impact process but also for exploiting this information to determine planetary surface compositions that might be difficult to measure or interpret with conventional remote-sensing techniques. Approach: Three spectrographic systems have been used. The first is a high-resolution (0.2 nm) photographic diffraction spectrograph (PDS) covering the range from 400 nm to 630 nm. The second is a CCD Personal Spectrometer I1 (PS-11) covering the range from 380 nm to 880 nm at low-resolution (5 nm). These two instruments provide poor time resolution relative to the impact event: essentially open exposures. The motion of the vapor cloud past the field of view, however, resulted in an effective time exposure of less than 1 ms. This approach revealed the integrated spectral content for a variety of targets and impact conditions. The third system is a gated-intensified CCD spectrograph by the Oriel Corporation allowing the spectral range, resolution, and exposure time to be tuned according to need. For these experiments, we chose a relatively narrow spectral range (360 nm to 580 nm) with a spectral resolution of 1 nm. Time gating of the spectrograph allowed measurements over the first 2.3 ms and from 2.3 ms-50 ms after the impact. This higher resolution in both time and wavelength is critical for deriving quantitative information from the spectra. The three systems were not used simultaneously but at different times for different purposes. Collectively they provide an intriguing overview of the complex phenomena and processes that will help to constrain future studies. The NASA Ames Vertical Gun Range is uniquely suited for this effort. The variable launch angle allows use of unconsolidated and liquid targets. Equally important, the large chamber size permits free expansion of the vapor cloud in a near-vacuum (3 m) without interference from the chamber walls. Moreover, multiple viewing ports allow alternative viewing geometries. The photographic spectrograph was positioned at the side port and provided a highresolution, two-dimensional view of the evolving vapor cloud as it passed by the imaged slit. Targets included dry ice (C02) and CaC03 powder impacted by 5 kds-5.5 k d s aluminum spheres (0.635 cm) at 15" from the horizontal. The low-resolution PS-II was also positioned at the side port and used a narrow viewing angle (5") on specific sites within the vapor cloud for different impact angles (15", 30°, 90") and impactor types (aluminum and pyrex). These experiments allowed a preliminary assessment of impact angle effects and compositional heterogeneity within the cloud. Lastly, the Oriel system was used to determine composition from observed spectra for three distinct natural target materials (dolomite powder, calcite crystal, and gabbroic [diabase] block). Aluminum projectiles (0.3 175 cm spheres) impacted these targets at 15" (from the horizontal). Results: The high spectral resolution from the PDS reveals clear signatures from both the impactor (aluminum) and dissociated components of carbonate targets (1). The distinctive A10+ molecular bands near 470,480, and 510 nm indicate rapid chemical reactions between target and impactor. Identified emissions from the target include NaI (5891590 nm), CaO (554, 604, and 622 nm), CaI (423,527,560, and 586,610, and 612 nm) and partly masked CO (561 and 609 nm). Such impact products are consistent with expectations (6,7). Impacts into non-volatile silicate powders allow tracing the effect of impact angle on the impactor (A10+) dispersal and NdK fractionation. Figure 1 shows the decreasing intensities of emissions from Na and K with increasing impact angles of 15", 30°, and 90" (from the horizontal). The NdK ratio, however, systematically increases with increasing impact angle from 1.2 at 15", to 1.3 at 30°, and to 1.5 at 90". These trends may indicate enhanced vaporized mass with lower temperatures at lower impact angles as previously inferred for impacts into carbonate targets (2, 3). Figure 1 also reveals that the intensity of the impactor component decreases as impact angle increases. Because the observed spectral emissions change with time due to evolving conditions within the vapor cloud (reactions, mixing, cooling, trajectories) and exposure (motion across the detector), the gated Oriel spectrometer is necessary for deriving meaningful elemental abundances. Figure 2 contrasts spectra captured during early (0 2.3 ms) and late (2.3 25 ms) times following 15" impacts (0.318 cm aluminum) into calcite. In contrast with the sidelooking views (Fig. l), Fig. 2 shows data acquired focused from above on a 10 cm-diameter spot positioned 5 cm downrange from the point of impact. It reveals not only the rapid decay in the intensity as the gas cloud moves downrange beyond the viewing area but also the evolving species due to radiative, reactive, and transport processes. While the early-time exposure captures the impactor (AlI, A10+), the late-time exposure reveals more clearly the target components (CO, CaO, CaI) although considerably reduced in intensity. Additional experiments using calcite, dolomite, and gabbro allowed determining the relative abundances of Mg and Ca from the relative intensities of their emission lines (Fig. 3) that were consistent with their known compositions.