In both cases, high concentrations of the gas are dark, while low concentrations are light.

在两幅图中，气体密度越高的区域颜色也就越深，低密度区则更浅。

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They are very light, that is to say, low mass.

很轻，也就是说，质量很小。

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They are very light, that is to say, low mass.

很轻，也就是说，质量很小。

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