请详细描述一下火星的特征和环境，并以json格式呈现
user
```json
{
  "planet": "Mars",
  "description": {
    "size": "Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, with a diameter of about 6,779 kilometers.",
    "surface": "It has a rusty, red appearance due to iron oxide (rust) on its surface. The surface is characterized by vast plains, enormous canyons (such as Valles Marineris), and the largest volcanoes in the Solar System, including Olympus Mons.",
    "atmosphere": "The atmosphere of Mars is very thin, composed mostly of carbon dioxide (95.3%), with traces of nitrogen and argon. It has a surface pressure that is only about 1% of Earth's.",
    "temperature": "Temperatures on Mars can vary widely, from a low of about -143°C (-225°F) at the poles in winter to a high of 20°C (68°F) at the equator during summer.",
    "moons": "Mars has two small moons, Phobos and Deimos, which are irregularly shaped and thought to be captured asteroids.",
    "water": "While liquid water does not exist on the surface today, evidence suggests that water once flowed on Mars and that it may still exist in the form of ice beneath the surface or in subsurface reservoirs.",
    "exploration": "Mars has been explored by numerous spacecraft, including orbiters, landers, and rovers. Notable missions include the Mars Pathfinder, the Mars Exploration Rovers (Spirit and Opportunity), the Curiosity rover, and the InSight lander."
  }
}
```
assistant