https://www.cnet.com/news/nasa-telescopes-snap-christmas-comet-the-brightest-of-2018/

The image above was taken by Hubble's Wide Field Camera 3 (WFC3) on Dec. 13 when the comet was 7.4 million miles (12 million kilometers) from Earth. The solid core of the comet, the nucleus, hides within the blue aura and is composed of gas and rock. The glow is created by the comet heating up as it passes through the solar system, releasing the trapped gases into the surrounding space. Although 46P/Wirtanen has been described as a "ghostly green", post-processing has painted the image blue.

Because 46P/Wirtanen came so close to the Earth, astronomers have been able to investigate what the comet is made of and how the gases within may be changed as they zip through space exposed to sunlight and radiation.