Yes, the concept of terraforming Mars using comets has been proposed by some scientists and science fiction authors. The idea involves redirecting comets from the outer regions of the solar system and deliberately crashing them into Mars. The comets would bring substantial amounts of water, which could help create a thicker atmosphere and potentially contribute to the development of liquid water on the planet's surface.
The process of crashing comets into Mars would release large amounts of water vapor and carbon dioxide into the atmosphere, which could help to warm the planet and thicken the atmosphere through the greenhouse effect. This, in turn, could potentially lead to the melting of subsurface ice and the formation of liquid water bodies.
However, it's important to note that terraforming Mars is an immensely complex and challenging task. The proposal to use comets as a means of terraforming Mars is just one of many ideas that have been suggested, and each idea has its own advantages, drawbacks, and feasibility concerns. The process of transforming Mars into a habitable planet would require a deep understanding of planetary science, extensive technological capabilities, and likely take centuries or even millennia to achieve, if it's achievable at all.
Currently, the focus is primarily on Mars exploration and understanding the planet better, rather than actively pursuing terraforming as a short-term goal.