National Center for Earth and Space Science Education, Capitol Heights, Maryland, USA.
Astrobiology. 2011 Nov;11(9):907-30. doi: 10.1089/ast.2011.0614. Epub 2011 Nov 11.
NASA's EPOXI mission observed the disc-integrated Earth and Moon to test techniques for reconnoitering extrasolar terrestrial planets, using the Deep Impact flyby spacecraft to observe Earth at the beginning and end of Northern Hemisphere spring, 2008, from a range of ∼1/6 to 1/3 AU. These observations furnish high-precision and high-cadence empirical photometry and spectroscopy of Earth, suitable as "ground truth" for numerically simulating realistic observational scenarios for an Earth-like exoplanet with finite signal-to-noise ratio. Earth was observed at near-equatorial sub-spacecraft latitude on 18-19 March, 28-29 May, and 4-5 June (UT), in the range of 372-4540 nm wavelength with low visible resolving power (λ/Δλ=5-13) and moderate IR resolving power (λ/Δλ=215-730). Spectrophotometry in seven filters yields light curves at ∼372-948 nm filter-averaged wavelength, modulated by Earth's rotation with peak-to-peak amplitude of ≤20%. The spatially resolved Sun glint is a minor contributor to disc-integrated reflectance. Spectroscopy at 1100-4540 nm reveals gaseous water and carbon dioxide, with minor features of molecular oxygen, methane, and nitrous oxide. One-day changes in global cloud cover resulted in differences between the light curve beginning and end of ≤5%. The light curve of a lunar transit of Earth on 29 May is color-dependent due to the Moon's red spectrum partially occulting Earth's relatively blue spectrum. The "vegetation red edge" spectral contrast observed between two long-wavelength visible/near-IR bands is ambiguous, not clearly distinguishing between the verdant Earth diluted by cloud cover versus the desolate mineral regolith of the Moon. Spectrophotometry in at least one other comparison band at short wavelength is required to distinguish between Earth-like and Moon-like surfaces in reconnaissance observations. However, measurements at 850 nm alone, the high-reflectance side of the red edge, could be sufficient to establish periodicity in the light curve and deduce Earth's diurnal period and the existence of fixed surface units.
美国宇航局的 EPOXI 任务观察了整合在盘中的地球和月球，以测试用于侦察系外类地行星的技术，利用深度撞击飞越飞船在 2008 年北半球春季开始和结束时从 ∼1/6 到 1/3 AU 的范围观察地球。这些观测提供了高精度和高节拍的地球实证光度和光谱学，适合作为数值模拟具有有限信噪比的类地系外行星实际观测场景的“地面实况”。地球于 2008 年 3 月 18-19 日、5 月 28-29 日和 6 月 4-5 日（UT）在近赤道亚空间飞船纬度进行了观测，波长范围为 372-4540nm，可见光分辨率较低（λ/Δλ=5-13），红外分辨率适中（λ/Δλ=215-730）。在七个滤光片中的分光光度法在 ∼372-948nm 滤光片平均波长处产生光曲线，其被地球自转调制，峰峰值幅度为≤20%。空间分辨的太阳耀斑是盘状反射率的次要贡献者。在 1100-4540nm 处的光谱学揭示了水蒸气和二氧化碳，以及分子氧、甲烷和氧化亚氮的较小特征。全球云量的一天变化导致光曲线开始和结束之间的差异≤5%。5 月 29 日月球凌日期间的光曲线由于月球的红色光谱部分遮挡了地球相对较蓝的光谱而呈现出颜色依赖性。在两个长波长可见光/近红外带之间观察到的“植被红边”光谱对比度存在歧义，无法清楚地区分被云层稀释的绿色地球与月球荒凉的矿物质风化层。在侦察观测中，需要至少在另一个短波长比较带中进行分光光度法测量，以区分类地和类月表面。然而，仅在 850nm 处的测量，即红边的高反射侧，可能足以确定光曲线的周期性，并推断出地球的昼夜周期和固定表面单元的存在。
