As light from a quasar travels through space, it passes through clouds of gas that imprint absorptionlines onto its spectrum.
当光从一个类星体划过太空,便会穿过将吸收线特征印在光谱上的气体云。
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The three sets of observations that purported to see methane on Mars all relied on the absorptionlines which the gas imposes on light at specific wavelengths.