Sunsets are gorgeous no matter what planet you’re standing on. NASA’s Mars Curiosity rover pointed its high resolution mast camera at the setting Sun to capture this 4-image sequence shown below on April 15, the mission’s 956th Martian day. Don’t you just love the contrast between the pink solar disk and hazy blue sky?
Back on Earth, dust and other fine particles in the atmosphere scatter the blues and greens from the setting or rising Sun to color it yellow, orange and red. At the Red Planet, the ever-present, fine dust in the Martian atmosphere absorbs blue light and scatters the warmer colors, coloring the sky well away from the Sun a familiar red. At the same time, dust particles in the Sun’s direction scatter blue light forward to create a cool, blue, glowing disk near the setting Sun. Standing on Mars, you’d only notice the blue glow when the Sun was near the horizon, the time when its light passes through the greatest depth of atmosphere and dust.
Mars has plenty of light to see around, but because it’s half again as far from the Sun as Earth, the light is weaker there. More like a lightly overcast afternoon or morning here on Earth. If you placed a safe solar filter over your eyes, you’d notice another odd thing about the Sun. It’s smaller. Only two-thirds as big as normal.
Because Mars’ atmosphere is so thin, just 1% as thick as Earth’s, we see another surprising phenomenon. Even when close to the horizon, it remains circular instead of shaped like a grape.
When the Sun sets or rises on Earth, our much thicker atmosphere, especially near the horizon, bends or refracts the Sun’s light upward, “lifting” the bottom of the solar disk into the top half and warping it into an oval.
Once the Sun’s about a fist high, we see it through much less atmosphere. Refraction effects diminish and it becomes a circle again. On Mars, the air’s so thin that there’s little noticeable bending of light (warping of the Sun) even when it hovers just above the horizon.
What a difference an atmosphere makes. The air on Venus is 92 times denser than Earth’s. Can you imagine how intense refraction would be? Would the Sun look like a straight line when it rose? Too bad we’ll never see the sight through its perpetual clouds.