Glory's optical effect as seen on Venus
Pic credit - BBC Science Focus Magazine

Rainbows but make it Venusian

Picture a rainbow. You probably imagine a bright arching band of colours appear. When you think of rainbows, you always think of a gentle drizzle followed by a burst of sunshine revealing a seven-coloured arc in the sky. Perhaps you’ve even chased one, hoping for that elusive pot of gold. But what if I told you that Venus which is a scorching world where surface temperatures exceed 475°C (900°F) and the air is thick with carbon dioxide and sulphuric acid, has something even more astonishing? Venus has it’s own version of rainbow. Enter the ‘glory’ – a celestial optical illusion that defies expectations. On there is a different kind of rainbow and we call it a glory.

And not just any rainbow, but a perfectly circular hauntingly beautiful optical illusion known as a glory. Yes, a “glory” because apparently, even in the most nightmarish hellscapes of the Solar System, Mother Nature can’t resist a bit of celestial artistry. Unlike Earth’s familiar rainbows, a Venusian glory isn’t an arc but a perfectly circular multi-ringed halo of light that keeps shimmering high in the planet’s dense toxic clouds. It’s arguably even more mesmerising than it’s Earthly cousin. But how does a glory form in such an extreme environment? Why is it so different from rainbows? And what does this tell us about Venus’s mysterious atmosphere? Why does Venus get the XXL version while we settle for puny little glows around aeroplane shadows?

Prepare for an in-depth exploration of the physics, planetary science and the optics behind this extraordinary phenomenon. Strap in, because we’re about to take a deep dive into the acidic, fiery and oddly beautiful world of Venus.

What is a Glory? (And no, it’s not just a fancy rainbow)

Before we get too carried away with the idea of floating through a Venusian sunset, let’s clarify what a glory actually is. While it may sound like something out of ancient mythology, it’s a very real optical phenomenon that happens both on Earth and in other planetary atmospheres too. A glory is a circular rainbow-like halo of light that forms when sunlight interacts with tiny droplets in a planet’s atmosphere. Unlike a traditional rainbow, which forms a vast arc, a glory appears as a perfectly round shimmering halo and sometimes with multiple concentric rings.

It’s often visible from aeroplanes when looking down on clouds here on Earth. It surrounds the shadow of the aircraft like a mystical aura. On Venus, however, it’s a different ballgame entirely. A glory is an optical phenomenon caused by the diffraction, interference of light interacting with uniform spherical droplets in a cloud layer and refraction. Unlike rainbows that arise from light bending through large water droplets and dispersing into it’s constituent colours, a glory emerges due to wavefront diffraction and backward scattering.

Example of a glory on Earth. Hikers and mountaineers on foggy snowy mountains cast shadows into the skies like this.
Pic credit - Ecole de sports

On Earth, glories appear as a circular halo surrounding the aircraft’s shadow on the clouds when flying above them. But for a long time, astronomers weren’t sure if such an effect could exist elsewhere — until an extraordinary discovery in the atmosphere of Venus. If that sounded like a lot of technical mumbo-jumbo, let’s break it down…

Rainbows – Sunlight enters a raindrop, bends, reflects and then exits the raindrop at an angle which makes the sunlight spread into it’s familiar spectrum.
Glories – Sunlight interacts with tiny, spherical droplets, bending around them, interfering with itself and scattering backwards in a near-perfect circular halo.

Earth’s glories appear as a ghostly rainbow-like halo surrounding a plane’s shadow when it flies above cloud cover. It’s like the universe decided to give your aircraft a saintly aura for a few moments. But on Venus, things get really weird.

Battle of the atmospheric optics – Glory v/s Rainbow

One of the most striking differences is the absence of refraction-based dispersion in a glory. This means glories lack the bright spectral colours of rainbows and instead appear as a soft glowing white halo with faint concentric rings. Think of it like this – a rainbow is the cosmic equivalent of a rock concert – dramatic, full of colour and loud while a glory is a subtle mystical glow like a candle-lit dinner with the universe.

In short, a rainbow can be compared to the Las Vegas neon sign of atmospheric optics while a glory is a divine halo and Venus’s way of saying “I may be a death trap but I’ve got class”.

Discovery of Venusian glories – Clouds of sulphuric acid are photogenic

For years, scientists weren’t sure if glories could even form on Venus because they had strong reasons mentioned below.

Venus’s clouds are made of sulphuric acid and not of water.
The atmosphere is so thick that sunlight barely makes it through to it’s surface.
Venus is basically Dante’s Inferno in planetary form and finding something delicate like a rainbow seemed improbable.

In 2011, ESA’s Venus Express Orbiter was minding it’s own business and scanning the planet’s atmosphere. It chanced upon a stunning observation while using it’s Visible and Infrared Thermal Imaging Spectrometer (VIRTIS). It detected a gigantic glory in Venus’s thick cloud layer. A 1,200 km-wide (750-mile) glory was sitting there and chilling in Venus’s thick sulphuric acid clouds as if the planet had just pulled off the greatest flex in atmospheric optics.

This discovery confirmed for the first time that Venus’s upper atmosphere contains uniform spherical droplets capable of producing the phenomenon. But what makes this particularly fascinating is that Venus’s atmosphere is not composed of water droplets like Earth’s clouds. Instead, it is rich in sulphuric acid aerosols suspended in a thick swirling haze. This means Venusian glories must form under vastly different conditions than here on Earth.

Apart from making Venus 10% more aesthetic

Aside from being visually stunning, the discovery of a glory on Venus is scientifically significant. The detection of a glory on Venus is more than just an astronomical curiosity. It has far-reaching implications for our understanding of planetary atmospheres.

Clue to Venusian climate and atmospheric composition – The existence of a glory helps researchers determine the exact makeup and size of cloud particles which in turn tells us more about Venus’s extreme climate. It’s highly uniform cloud droplets provide us valuable insights into aerosol formation and cloud dynamics. Since Venus has a runaway greenhouse effect, studying it’s cloud layers can help scientists refine climate models — including those which can be applied to Earth.

Potential for life? – Believe it or not, some scientists speculate that the high-altitude cloud layers (where glories form) of Venus could potentially harbour microbial life. The presence of glories suggests that Venus’s cloud droplets are stable and uniform which are ideal conditions for microscopic life to exist. The discovery of phosphine (a potential biosignature) in Venus’s clouds reignited debate about the habitability of it's upper atmosphere and studying glories might help determine whether biological processes play a role in it’s cloud chemistry. Could the well-behaved sulphuric acid aerosols and droplets be playing a role in some unknown biogeochemical process?

Proposed balloon missions that'd study the clouds of Venus after discovering Phospene in it
Pic credit - Forbes

Expanding our understanding of atmospheric optics – Studying glories on Venus can help us understand similar atmospheric phenomena on Earth, Jupiter, Saturn and even exoplanets orbiting distant stars. Venus’s glory challenges current models of how light interacts with exotic planetary atmospheres, helping astronomers predict optical phenomena on exoplanets with thick cloudy atmospheres.

Clues about atmospheric stability – The existence of a glory means Venus’s cloud droplets aren’t just random blobs. They’re extremely uniform. This challenges our understanding of aerosol dynamics.

Cool factor – Let’s be honest that Venus having a rainbow-like optical phenomenon made of acid clouds is just objectively awesome.

This discovery was a big deal. It meant that Venus’s cloud droplets are incredibly uniform in size which in turn, hinted at weirdly stable atmospheric conditions. However, these wild new questions are the most intriguing for us now…

Could Venus’s upper atmosphere be home to microbial life?
How do sulphuric acid clouds stay so consistent?
Is Venus secretly trying to one-up Earth in celestial aesthetics?

How large is a Venusian glory?

One of the most surprising aspects of Venus’s glory is it’s sheer scale. Size-wise, a glory’s diameter depends on the size of the sulphuric acid droplets causing it. The Venus Express spacecraft observed a 1,200 km-wide (750 miles) glory making it absolutely massive compared to earthly glories. That was a colossal size since earthly glories are typically just a few metres across.

On Earth, glories typically range from 5 to 20 metres across when viewed from an aeroplane but it still depends on the altitude and cloud composition. The immense size of Venus’s glory suggests that the sulphuric acid droplets in it’s clouds are remarkably uniform in size which is a fascinating detail that helps scientists better understand the planet’s atmospheric composition and refine models of Venus’s atmospheric chemistry. Their size is determined by the droplet radius (rr), following the relationship…

θ≈1.38λr\theta \approx \frac{1.38\lambda}{r}

where…

θ\theta is the angular size of the glory
λ\lambda is the wavelength of light
rr is the radius of the droplets.

Since Venus’s droplets are composed of concentrated H₂SO₄ aerosols and are significantly smaller than typical water droplets, the resulting glory is much larger.

Magic and physics collide – Science behind a Venusian glory

Venus is a hellish world with surface temperatures of around 475°C (900°F) and a crushing atmosphere thick with carbon dioxide. It’s hardly the kind of place where you’d expect to find something as poetic as a rainbow. But science as always, loves surprises. The Venusian atmosphere is laden with clouds of sulphuric acid and not exactly the kind of thing that you’d want to breathe in. When sunlight interacts with these tiny acid droplets, light waves are refracted, diffracted and scattered. That goes on to create the rare optical effect known as a glory. A key ingredient in this process is Mie scattering which is a physics principle governing how light interacts with small spherical particles. Unlike a standard rainbow which is formed by large water droplets bending and splitting light into it’s constituent colours, a glory is produced by diffraction and interference within uniform droplets.

European Space Agency (ESA) tracks Venusian clouds to detect glories

For a long time, scientists weren’t even sure if a glory could form on Venus. Then in 2011, the European Space Agency’s (ESA) Venus Express orbiter spotted a circular rainbow-like glow in the planet’s dense cloud cover which confirmed that Venus does indeed, have it’s own version of a rainbow. To understand glories in depth, we need to explore the intricate physics of wave optics—specifically Mie scattering, wavefront interference and backscattering resonance effects.

Step-by-step formation of a glory

Sunlight hits cloud cover or sulphuric acid droplets – Sunlight which is composed of different wavelengths of light, travels toward a layer of spherical liquid droplets. In Venusian case, microscopic sulphuric acid aerosols in the upper clouds. Also, this sunlight encounters microscopic sulphuric acid aerosols that are (oddly) well-behaved and possibly the only well-behaved thing on Venus.

Diffraction and interference or resonance effects – Instead of bending and dispersing like in a rainbow, light waves diffract around the droplets and keeps interfering with each other like ripples in a pond. Some waves cancel out while others reinforce and goes on creating a circular multi-ringed glow. Diffraction is a wave-based phenomenon in which light bends around obstacles and creates interference patterns. When the light encounters these tiny droplets, the scattered light waves interfere constructively and destructively and keeps producing alternating bright and dark rings.

Backscattering magic and phase shifts – A fraction of the light is scattered backward toward the Sun in a phenomenon known as retroreflection. Some of the scattered light is redirected backward toward the source in a process called backscattering. This is what causes the perfectly symmetrical halo instead of a fragmented arc. This reflection undergoes phase shifts which are subtle changes in the light wave’s path that reinforce certain wavelengths and cancel out others. This keeps creating a structured concentric pattern of rings around a central bright core.

Size determines the glory’s appearance – The wavelength (λ\lambda) of the light and the droplet size (rr) determine the glory’s angular radius according to θ≈1.38λr\theta \approx \frac{1.38\lambda}{r}

Multi-ringed circular structure – The result? A near-perfect multi-ringed circular halo known as a glory with a bright core and fading coloured bands surrounding it.

This process is governed by the Mie theory of scattering which describes how electromagnetic waves interact with particles roughly the same size as their wavelength. It explains why glories appear only when cloud droplets are highly uniform in size. Otherwise, interference effects break down and the phenomenon vanishes. Since Venusian glory is 1,200 km across, this suggests it’s droplets are shockingly uniform. They measure about 1.0 micrometre in radius which is far more precise than anything we’ve ever engineered. Mother Nature, take a bow.

Would you see a glory if you stood on Venus?

Theoretically? Yes.

Realistically? Absolutely not!

The surface temperature is hotter than a pizza oven so you’d be extra crispy before you even looked up.
The pressure is 90 times Earth’s which means you’d be squashed flatter than a pancake.
The atmosphere is 97% carbon dioxide so your lungs would reject it faster than bad reality TV.

If you could somehow survive the crushing 90-bar pressure, the suffocating carbon dioxide and the acidic rain, you might witness a glory but not in the same way we see them from an aeroplane. Instead, it would appear above you in the hazy sulphuric sky as a haunting glowing halo surrounding the Sun like an ethereal crown. Unfortunately, standing on Venus would be decidedly brief, as you’d be burned, crushed and dissolved in mere seconds. But it’s a glorious way to go!

Photo from European Space Agency of having Venus mapped against the inky black space

However, if you floated high above Venus in a stratospheric balloon, you might catch a glimpse of one of these glories on the cloud top too. Right after that, your craft gets corroded by sulphuric acid and you plummet into oblivion. So…maybe admire them from afar.

A rainbow by any other name…

In the end, a glory is technically not a rainbow, but it is it's own kind of magic. Seeing a perfect, glowing halo of light in the sulphuric skies of Venus is a reminder that even the harshest places in our Solar System can hold beauty.

A glory seen from a hot air balloon around another hot air balloon at cloudtops
Pic credit - EarthSky

And who knows? Maybe one day, when humanity finally sends a balloon or aircraft into Venus’s skies, we’ll get to see a Venusian sunset complete with a glory…in real time. We might be the first humans to see a glory from another world before we hastily retreat back to Earth, because, well…Venus.

Until then, keep an eye out for Earthly glories. Next time you’re on a plane, glance at the clouds below. You might just spot a little piece of Venusian magic, right here on our own home planet.

Venus may be a deathtrap but it’s a beautiful one

Venus is the Solar System’s resident pressure cooker of doom. It somehow manages to produce delicate ghostly glories in it’s sky. It’s a reminder that even in the most extreme places, light, physics and science combine to create jaw-dropping beauty.

Proposed balloon missions to Venus
Pic credit - Universe Today website

Did you enjoy this deep dive into Venus’s mesmerising optical wonders? Share your thoughts below and reveal if you’ve ever spotted a glory here on Earth!

Bonus

The following is an actual image of the first glory ever captured on an exoplanet! This glory is captured on the exoplanet WASP-76b. This planet is a hellish planet just like Venus (not by size) spammed with super hot gases and is at a distance of 637 light years away from us. It is 12 times closer to it's star than our Mercury is to our Sun. By size, this planet is 2 times larger than Jupiter.