Blue sunset

  •   Ricky
  •   July 31, 2017
  •   104
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Even robots can’t tear their eyes from a beautiful sunset. NASA’s Mars Curiosity rover pointed its high resolution mast camera at the setting Sun to capture this 4-image sequence on April 15 at the conclusion of the mission’s 956th Martian day. While it resembles an earthly sunset, closer inspection reveals alien oddities.

A day on Mars lasts 24 hours and 39 minutes, so sunrise and sunset follow nearly the same rhythm as they do on Earth. When we eventually establish a base there, astronauts should be able to adjust to the planet’s day-night rhythm with relative ease. Jet lag would be worse.

But sunsets and sunrises offer a different palette of colors than they would on Earth. For starters, the Sun only radiates the equivalent of a partly cloudy afternoon’s worth of light. That’s because Mars’ average distance from the Sun is 141.6 million miles or about half again Earth’s distance. Increased distance reduces the intensity of sunlight.

Not only that, but the solar disk shrinks from the familiar 0.5° across we see from Earth to 0.35° at Mars. Here on the home planet, your little finger extended at arm’s length would cover the equivalent of two Suns. On Mars it would be three!

What about color? 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. When these tints are reflected off clouds, sunset colors are amplified and spread about the sky, making us reach for that camera phone to capture the glory.

Things are a little different on Mars. 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 ruddy hue. At the same time, dust particles in the Sun’s direction scatter blue light forward to create a cool, blue aureole near the setting Sun. If you were 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.

Even robots can’t tear their eyes from a beautiful sunset. NASA