Look up to the evening sky and you'll often spot Venus shining brightly — but what would you see on Mars? This picture from the Curiosity rover's Mastcam just after the Martian sunset shows that the brightest thing in the sky is Earth itself.
It makes sense: Just as Venus, the next planet in from Earth, is often the brightest point in the sky to us, so Earth, the next planet in from Mars, is the brightest point from there.
The caption for the photo goes on to explain that a human observer on Mars (hopefully wearing a spacesuit) would see both Earth and the moon (just discernable in the photo) as bright and distinct lights.
Mars and Earth were 99 million miles away from each other when this photo was taken; Venus, at its brightest and closest, comes about 25 million miles of Earth. The thin atmosphere on Mars likely adds to our planet's visibility.
You can see the picture without annotation here, and a closeup of the Earth and our moon is here.