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“Very early in the mission, we’ve found out Bennu is going to provide the type of material we want to return,在任务的早期，我们发现Bennu将提供我们想要返回的材料类型” said principal investigator Dante Lauretta of the University of Arizona in Tucson. “It definitely looks like we’ve gone to the right place.” OSIRIS-REx’s onboard spectrometers measure the chemical signatures of various minerals based on the wavelengths of light they emit and absorb.基于矿物发射和吸收的波长，OSIRIS-REx的机载光谱仪可以测量各种矿物的化学特征 The instruments were able to see signs of hydrated minerals on Bennu’s surface about a month before the spacecraft arrived at the asteroid, and the signal has remained strong all over the asteroid’s surface as the spacecraft approached, 大约在探测器抵达小行星前个月，这些仪器就能在本努的表面看到含水矿物的迹象。探测器接近小行星时，小行星表面的信号一直很强烈，Simon said. Those minerals can form only in the presence of liquid water, and suggest that Bennu had a hydrothermal system in its past.这些矿物只有在液态水的存在下才能形成，这表明Bennu在过去有一个水热系统。 Bennu’s surface is also covered in more boulders and craters than the team had expected based on observations of the asteroid taken from Earth.本努的表面还覆盖着更多的巨石和陨石坑，这比研究小组根据从地球上获取的小行星观测到的结果所预期的要多。 Remote observations led the team to expect a few large boulders, about 10 meters wide. Instead they see hundreds, some of them up to 50 meters wide. “It’s a little more rugged of an environment,” Lauretta said. But that rough surface can reveal details of Bennu’s internal structure and history.洛蕾塔说:“这里的环境比较恶劣。但这个粗糙的表面可以揭示本努内部结构和历史的细节。 If Bennu were one solid mass, for instance, a major impact could crack or shatter its entire surface. 例如，如果Bennu是一个固体，一个主要的冲击可能会使其整个表面开裂或碎裂。The fact that it has large craters means it has survived impacts intact. 它有巨大的陨石坑，这意味着它完好无损地经受住了撞击。It may be more of a rubble pile loosely held together by its own gravity.它可能更多的是由自身重力松散地粘合在一起的碎石堆。 Telling scars Crater pockmarks on Bennu, like this 20-meter-wide depression photographed on December 2, may turn out to be the safest places for the spacecraft to pick up a sample of the asteroid. Bennu，就像12月2日拍摄的这张20米宽的低气压照片一样，可能是航天器采集小行星样本最安全的地方。Craters also tell scientists that the asteroid is not one solid mass but instead different materials clumped together. 陨石坑还告诉科学家，这颗小行星不是一个固体，而是不同的物质聚集在一起。If Bennu had been a solid rock, large impacts could crack or shatter its surface. UNIV. OF ARIZ. GODDARD/NASA The asteroid’s density supports the rubble pile idea. OSIRIS-REx’s first estimate of Bennu’s density shows it is about 1,200 kilograms per cubic centimeter, Lauretta said.罗斯-雷克斯对本努密度的首次估计显示，它的密度约为每立方厘米1200公斤，洛蕾塔说 The average rock is about 3,000 kilograms per cubic centimeter. The hydrated minerals go some way towards lowering the asteroid’s density, since water is less dense than rock. 由于水的密度小于岩石，含水矿物在一定程度上降低了小行星的密度。But up to 40 percent of the asteroid may be full of caves and voids as well, Lauretta said. Some of the rocks on the surface appear to be fractured in a spindly pattern.表面上的一些岩石似乎是呈纺锤形断裂的 “If you drop a dinner plate on the ground, you get a spider web of fractures,” says team member Kevin Walsh of the Southwest Research Institute in Boulder, Colo. “We’re seeing this in some boulders.” The boulders may have cracked in response to the drastic change in temperatures they experience as the asteroid spins这些巨石可能是由于小行星自转时所经历的剧烈温度变化而破裂的. Studying those fracture patterns in more detail will reveal the properties of the rocks. The OSIRIS-REx team also needs to know how many boulders of various sizes are strewn across the asteroid’s surface. Any rock larger than about 20 centimeters across would pose a hazard to the spacecraft’s sampling arm, says Keara Burke of the University of Arizona.亚利桑那大学(University of Arizona)的凯拉伯克(Keara Burke)说，任何直径超过20厘米的岩石都会对航天器的取样臂构成威胁 Burke, an undergraduate engineering student, is heading up a boulder mapping project.伯克是工程系的本科生，他正在领导一个描述大石头分布地图项目。 Cracked surface Some of the rocks strewn across Bennu, like this 35-meter-wide boulder photographed on December 2, are fractured in a spindly pattern. The fractures may be the result of sudden, drastic temperature change. Studying the fracture patterns could help scientists learn more about the properties of the rocks. UNIV. OF ARIZ./GODDARD/NASA “My primary goal is safety,” she says. “If it looks like a boulder to me, within reasonable guidelines, then I mark it as a boulder. We can’t sample anything if we’re going to crash.” The team also needs to know where the smallest grains of rock and dust are, as OSIRIS-REx’s sampling arm can pick up grains only about 2 centimeters across. One way to find the small rocks is to measure how well the asteroid’s surface retains heat. Bigger rocks are slower to heat up and slower to cool down, so they’ll radiate heat out into space even on the asteroid’s night side.较大的岩石升温较慢，降温较慢，因此即使在小行星的夜晚，它们也会向太空辐射热量。 Smaller grains of dust heat up and cool down much more quickly. “It’s exactly like a beach,” Walsh says. “During the day it’s scalding hot, but then it’s instantly cold when the sun sets.”白天热得像滚烫，但太阳下山的时候马上就冷了。” Measurements of the asteroid’s heat storage so far suggest that there are regions with grains as small as 1 or 2 centimeters across, Lauretta said, though it is still too early to be certain. “I am confident that we’ll find some fine-grained regions,” Lauretta said. Some may be located inside craters. The challenge will be finding an area wide enough that the spacecraft’s navigation system can steer to it accurately.挑战将是找到一个足够宽的区域，使航天器的导航系统能够准确地导航到它。 Citations Welcome to Bennu: first asteroid science data obtained by the OSIRIS-REx mission. American Geophysical Union meeting, Washington, D.C., December 10, 2018. D. Lauretta. OSIRIS-REx encounters Bennu: initial assessment from the approach phase. American Geophysical Union meeting, Washington, D.C., December 11, 2018. D. DellaGiustina. First resolved images of Asteroid (101955) Bennu. American Geophysical Union meeting, Washington, D.C., December 11, 2018. K. Walsh et al. Initial perspectives on surface geology of (101955) Bennu including comparisons to (162173) Ryugu. American Geophysical Union meeting, Washington, D.C., December 11, 2018. K. Burke et al. Boulder size frequency distribution
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