Venus is often called Earth’s twin planet due to their similar sizes, densities, and compositions. However, there are some stark differences between the two planets. One major difference is the makeup of Venus’ atmosphere compared to Earth’s atmosphere. This results in some spectacular variations between an Earth sunset and a Venus sunset.
Venus’ Atmosphere
Venus’ atmosphere is vastly different than Earth’s atmosphere. Venus’ atmosphere is 96.5% carbon dioxide, 3.5% nitrogen, with trace amounts of other gases like sulfur dioxide. Earth’s atmosphere is 78% nitrogen, 21% oxygen, and 1% other gases like argon and carbon dioxide. The high levels of carbon dioxide in Venus’ atmosphere cause a strong greenhouse effect that heats the surface of Venus to nearly 900°F (480°C).
In addition to carbon dioxide, Venus’ atmosphere contains thick clouds of sulfuric acid that completely envelop the planet. These clouds are thought to be around 30 miles (50 km) thick and located 25-30 miles (40-50 km) above the planet’s surface. The clouds on Venus not only obscure viewing of the surface from space, but they have a significant effect on the appearance of the Venusian sky.
Visibility on Venus
The dense sulfuric acid clouds covering Venus absorb visible light but allow some infrared and ultraviolet light to filter through. This would make the surface of Venus appear dark to human eyes. However, above the cloud layers the Venusian sky would appear hazy but bright to humans during the day due to scattered sunlight. As the sunlight starts to fade at sunset, the illumination of the clouds would create colorful, diffuse patterns across the sky.
Sunset Hues
The composition and structure of Venus’ atmosphere results in sunsets that look quite different from Earth’s colorful sunsets. The exact colors a human would see at sunset on Venus depends on what altitude they are viewing it from:
- From the surface of Venus, the sky would turn from light to dark around sunset, but there would be no reds, oranges, or blues like on Earth. The thick sulfuric acid clouds obscure the sun itself from being visible as it drops below the horizon. However, the night sky might take on a diffuse red glow due to the illumination of the upper cloud layers from the sun that has just set below the horizon.
- From an altitude within the sulfuric acid cloud layers, around 30 miles (50 km) up, the sunset colors would be muted and diffused. The dense clouds scatter light in all directions, obscuring the direct sunlight. However, the illumination of the thick clouds could create soft yellows and oranges as the sun disappears from view.
- From high above the cloud layers in the upper atmosphere, the sunset would look more familiar. Bright reds, oranges, and yellows would be visible looking towards the horizon as direct sunlight illuminates the high altitude sulfuric acid ice crystals. The sun itself would still be obscured by the clouds below, but the vivid colors of the sunset would be more visible from this vantage point.
Comparing Venus and Earth Sunsets
To summarize the key differences, sunsets on Venus lack definitive shadows, bright colors, and clear views of the sun descending below the horizon that are characteristic of Earth sunsets. This is due to the extensive cloud cover and scattering of light by sulfuric acid droplets and ice crystals. Here is a table comparing sunset features on Venus vs Earth:
| Feature | Venus Sunset | Earth Sunset |
|---|---|---|
| Direct sunlight visible | No, obscured by clouds | Yes |
| Sun disk visible | No, obscured by clouds | Yes |
| Colors visible | Subdued and diffuse reds, oranges, yellows filtered through clouds | Brilliant reds, oranges, purples, blues in clear sky |
| Shadows visible | No, lighting diffused by thick clouds | Yes, clear shadows visible |
| Duration | Very short, under an hour | Up to 2 hours |
Venus sunsets offer a muted view of the daily setting of the sun, whereas Earth sunsets are generally vibrant displays lasting for an extended period as the sun dips below the horizon. Both arise from the interplay of sunlight with the planets’ atmospheres, but the results are quite different!
Mechanics of the Venus Sunset
The mechanics of the Venus sunset are driven by the speed of Venus’ rotation and the structure of its atmosphere. Here are some key facts about the Venus sunset:
- Venus rotates very slowly on its axis, with a Venus day (the time it takes to fully rotate once) equal to 243 Earth days.
- This means that sunrise to sunset on Venus takes around 58 Earth days!
- However, the sun would still appear to drop below the horizon quickly at sunset, taking around 20-40 minutes.
- This is due to refraction of sunlight by Venus’ atmosphere, bending the path of sunlight over the horizon by a few degrees relative to Venus’ surface.
- The thick sulfuric acid clouds also prevent a visible build-up to sunset on Venus’ surface since the sun is obscured from view all day.
In essence, even though the Venus day is incredibly long, the final sunset stage where direct sunlight fades behind the horizon still occurs relatively rapidly compared to Earth. The visual effect from various altitudes is of a diffuse, muted coloring growing darker over tens of minutes.
Observing Venus Sunsets
Observing the elusive sunsets on Venus is very challenging, but would provide insights into Venus’ atmosphere and the scattering of light by the sulfuric acid cloud layers. Here are some notes on observing Venus sunsets:
- No direct observations of a Venus sunset have ever been made.
- UV observations by Venus Express provided some clues, detecting a brightening of cloud tops in UV as the sun went down, indicating scattering of light.
- The Akatsuki orbiter around Venus can detect infrared radiation, which may allow it to directly observe the illuminated nighttime cloud layers after sunset.
- Future Venus missions could deploy balloons or aircraft to get above the cloud layers and attempt to capture images of the sunset colors and light scattering through the atmosphere.
- However, the extremely hot surface conditions make missions to Venus difficult to sustain for long periods.
Advanced future probes will be needed to directly validate the sunset predictions based on Venus’ atmospheric models. This will improve our understanding of light propagating and scattering through dense, sulfuric acid clouds.
Conclusion
Venus sunsets offer a unique view of a familiar phenomenon on an alien world. While Earth sunsets dazzle with brilliant hues in clear skies, Venus sunsets would appear markedly different. The thick sulfuric acid clouds obscure any direct sunlight from reaching the surface during the day or seeing the sun set at dusk. However, refraction of sunlight through the clouds would create a muted, diffused sunset in yellows, oranges, and reds when viewed from high altitudes. Direct observations of the elusive Venus sunset have not yet been achieved but would provide new insights into the workings of Venus’ perplexing atmosphere.
