Picture a rainbow. Way up above your head in the sky. You probably imagine a bright arching band of colours appear. Yes you should. 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. It's the only kind of bow you've ever known. Perhaps you’ve even chased one, hoping for that elusive pot of gold. Ha ha! 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? That kind of temperature would make you disbelieve that it has a rainbow since rainbows have always appeared to you during cold wet seasons. Venus has it’s own version of rainbow. It is different from the one that our Earth makes. Enter the ‘glory’ – a celestial optical illusion that defies expectations. It is unlike anything you know. On Venus is an entirely different type 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. It is truly marvellous to observe. Yes, it's labelled a “glory” because apparently, even in the most nightmarish hellscapes of the Solar System, Mother Nature can’t resist contributing a bit of celestial artistry. In the searing hot temperatures of Venus, there aren't conditions needed for an earthlike rainbow. 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. That'd be an awesome sight! It’s arguably even more mesmerising than it’s Earthly cousin. Here we cannot even see a complete rainbow from the ground. But how does a glory form in such an extreme environment? Why is it so different from the rainbows we all know? 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. It is a nice learning that you can get. Strap in because we’re about to take a deep dive into the acidic, fiery and oddly beautiful world of Venus. Let's go.Table of contents
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. You will have to drop everything you know about the rainbows on Earth. 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. More on Earth's glory further. A glory is a circular rainbow-like halo of light that forms when sunlight interacts with tiny droplets in a planet’s atmosphere. It depends also on the content of the atmosphere because the atmospheres are different on Earth and Venus (for example). Unlike a traditional rainbow, which forms a vast arc, a glory appears as a perfectly round shimmering halo and sometimes with multiple concentric rings. You can see the whole thing unlike a rainbow.
Glories are often visible from aeroplanes when looking down on clouds here on Earth. But you have to be way up in the atmosphere. It surrounds the shadow of the aircraft like a mystical aura. By that I mean, glories form around the shadows that fall on clouds down below as you fly. On Venus, however, it’s a different ballgame entirely. It's atmosphere is not even remotely Earth-like. A glory is an optical phenomenon caused by the diffraction, interference of light interacting with uniform spherical droplets in a cloud layer and refraction of light. All 3 factors have to come in play for the formation of a glory. 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. The spherical droplets in the clouds causing a glory needn't be water either exactly.
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| 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. It's the only way for us to spot a glory. If there is a natural way, it is presently not known. 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. It was a chance discovery to say. 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. We see it as an arc of 7 colours and call it a rainbow. Being a circle and large, we don't get to it entirely. Hence, we call it a bow.
- Glories – Sunlight interacts with tiny spherical droplets, bending around them, interfering with itself and scattering backwards in a near-perfect circular halo. It involves a more complex mechanism than with the formation of a rainbow.
Earth’s glories appear as a ghostly rainbow-like halo surrounding a plane’s shadow when it flies above cloud cover. They are somewhat faint too. It’s like the universe decided to give your aircraft a saintly aura for a few moments. It can be mistaken for a rainbow if you didn't know of glories. But on Venus, things get really weird. It's almost magical.
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. So, it would be faint. This means glories lack the bright spectral colours of rainbows and instead appear as a soft glowing white halo with faint concentric rings. Without seeing one, you have to imagine in order to understand that. 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. You can easily see a rainbow from anywhere on Earth whereas, you have to really have the right conditions to spot the faint glory.
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”. Whereas a glory is an introverted asocial guy.
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. But changes happen in due time.
- Venus’s clouds are made of sulphuric acid and not of water. Water is the strongest candidate for making wonderful visual displays naturally and Venus has no water due to the extreme hot temperatures.
- The atmosphere is so thick that sunlight barely makes it through to it’s surface. Venus is permanently cloaked in darkness (except for volcanoes) because of it.
- Venus is basically Dante’s Inferno in planetary form and finding something delicate like a rainbow seemed improbable. The atmosphere is heavily polluted and doesn't reflect light.
In 2011, ESA’s Venus Express Orbiter was minding it’s own
business and scanning the planet’s atmosphere. The probe was designed for tracing the harsh climate. It chanced upon a stunning
observation while using it’s Visible and Infrared Thermal Imaging Spectrometer
(VIRTIS). For the first time, a glory was spotted. It detected a gigantic glory in Venus’s thick cloud layer. The probe just happened to be at the right place and at the right time. 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. That's almost 10% of the planet's size!
This discovery confirmed for the first time that Venus’s upper atmosphere contains uniform spherical droplets capable of producing the phenomenon. We discovered that water wasn't really essential to make glories and rainbows naturally. But what makes this particularly fascinating is that Venus’s atmosphere is not composed of water droplets like Earth’s clouds. It was a stunning discovery indeed. Instead, it is rich in sulphuric acid aerosols suspended in a thick swirling haze. It's a boiling hot place unlike the water clouds here on Earth. This means Venusian glories must form under vastly different conditions than here on Earth. The curiousity to understand the mechanics had kickstarted.
Apart from making Venus 10% more aesthetic
Aside from being visually stunning, the discovery of a glory on Venus is scientifically significant. Yes, because we have a new natural phenomena that hadn't been thought to be possible. The detection of a glory on Venus is more than just an astronomical curiosity. New insights was about to come our way now. 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. Theoretically, we had strong reasons to think Venus might have nothing colourful until now. It’s highly uniform cloud droplets provide us valuable insights into aerosol formation and cloud dynamics. Scientists had to find out the physics. 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. Only more research could satiate the answers which we sought.
Potential for life? – Believe it or not, some scientists speculate that the high-altitude cloud layers (where glories form) of Venus could potentially harbour life. By life, I don't mean complex organisms like us but mere microbial life forms. The presence of glories suggests that Venus’s cloud droplets are stable and uniform which are ideal conditions for microscopic life to exist. They could thrive in the upper atmospheres of Venus without sinking into the death below. 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. So, we need to know better. Could the well-behaved sulphuric acid aerosols and droplets be playing a role in some unknown biogeochemical process? Can we trace it, if so? The discovery of Venusian glory triggered questions after questions needing urgent answers.
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| 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. Our cosmic understand would deepen on the makeup of their atmospheres and tracing life via biosignatures. 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. Our understanding of these were based on water's properties earlier.
Clues about atmospheric stability – The existence of a glory means Venus’s cloud droplets aren’t just random blobs. For a liquid droplet to stay spherical meant an environment or surrounding of tranquility. They’re extremely uniform. Although there was a hell below, the upper atmosphere wasn't much disturbed by it. This challenges our understanding of aerosol dynamics. We had to discover how glories were forming on Venus.
Cool factor – Let’s be honest that Venus having a
rainbow-like optical phenomenon made of acid clouds is just objectively
awesome. For a common man, water might be the only thing he knows that could create luminary magic.
This discovery was a big deal. The Venusian glory shook grounds. It meant that Venus’s cloud droplets are incredibly stable and uniform in size which in turn, hinted at weirdly calm atmospheric conditions. Venus' surface has never known calm. 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. It's so large that it would blow your mind. Size-wise, a glory’s diameter depends on the size of the sulphuric acid droplets causing it. The greater the area of calm in the upper atmosphere, the larger is the glory that is going to develop. The Venus Express spacecraft observed a 1,200 km-wide (750 miles) glory making it absolutely massive compared to earthly glories. It'd be the size of a large country. That was a colossal size since earthly glories are typically just a few metres across. Our glories are laughable comparitively.
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. But still, it's never large as the one first found on our sister-planet. The immense size of Venus’s glory suggests that the sulphuric acid
droplets in it’s clouds are remarkably same 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. We can even calculate that. 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. But it still impressive how much large of an area on the upper clouds of Venus is stable.
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. Everything gets crushed before even reaching it's ground. It’s hardly the kind of place where you’d expect to find something as poetic as a rainbow. The heat is strong enough to melt lead which is one of the densest metals in existence. But science as always, loves surprises. There is no predicting what can happen even in impossible locations. The Venusian atmosphere is laden with clouds of sulphuric acid and not exactly the kind of thing that you’d want to breathe in. Sulphuric acid is extremely corrosive which means it starts corroding things very rapidly instantly on contact. When sunlight interacts with these tiny acid droplets, light waves are refracted, diffracted and scattered. The formation of glories can now begin. That ends up creating the rare optical effect known as a glory. The thing should be impossible on a hellish planet but there we have it.
A key ingredient in this process is Mie scattering which is a physics principle governing how light interacts with small spherical particles. Without this process, a glory cannot form on Venus. 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. These droplets have to be either spherical or near-spherical for such an interaction to sustain.
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| 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. Because look, the possibility is bleak. 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. The chance find accelerated curiousity. To understand glories in depth, we need to explore the intricate physics of wave optics — specifically the 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. Remember, it's a very large real estate with this perfectly spherical droplets. In Venusian case, microscopic sulphuric acid aerosols in the upper clouds. That makes each of them smooth and reflective which are perfect for the formation of a Venusian glory. Also, this sunlight encounters microscopic sulphuric acid aerosols that are (oddly) well-behaved and possibly the only well-behaved parties on Venus. Hence, the perfect recipe for a glory.
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. It's a highly internally reflective performance. Some
waves cancel out while others reinforce and goes on creating a circular
multi-ringed glow. When the engineering is going on correctly, the glory starts forming. Diffraction is a wave-based phenomenon in which light bends
around obstacles and creates interference patterns. You can experiment the phenomena yourself at home or a lab. When the light encounters
these tiny droplets, the scattered light waves interfere constructively and
destructively and keeps producing alternating bright and dark rings. They contribute to the faintness of glory.
Backscattering magic and phase shifts – A fraction of
the light is scattered backward toward the Sun in a phenomenon known as retroreflection. The light gets ejected out of the planet. Some of the scattered light is redirected backward toward the source in a
process called backscattering. It's a very important process responsible for lighting up the glory. This is what causes the perfectly symmetrical
halo instead of a fragmented arc. It separates from what could've formed a rainbow. This reflection undergoes phase shifts which
are subtle changes in the light wave’s path that reinforce certain wavelengths
and cancel out others. A bit of a complex process indeed. This keeps creating a structured concentric pattern of
rings around a central bright core. Now you can have a glory from all the above factors at play.
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}. More the real estate of calm and those spherical droplets, the larger and larger that the glory would form.
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. It'd be wonderful to witness it form.
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. The theory tells us exactly where glories would appear. It explains why glories appear only when cloud droplets are highly uniform in size. Without which, we'd end up having a rainbow or nothing at all. Otherwise, interference effects break down and the phenomenon vanishes. There'd be no glory in shapeless vapours. Since Venusian glory was 1,200 km across, this suggests it’s droplets were massively and shockingly uniform. That's an extremely large space on the planet. They measure about 1.0 micrometre in radius which is far more precise than anything we’ve ever engineered. Legitimately, mind-blowing! 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. It's literally either hell or a runaway pollution on display.
- The pressure is 90 times Earth’s which means you’d be squashed flatter than a pancake. That would be equal to you having drowned somewhere near the Titanic with kilometers of water above you. You will never fight the weight upon you and resurface from there.
- The atmosphere is 97% carbon dioxide so your lungs would reject it faster than bad reality TV. You'll never find the remaining 3% of some other gas to breathe in there.
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. The atmosphere is densely dark by which you will never be able to see even the brightest lights as they are. Instead, it would appear above
you in the hazy sulphuric sky as a haunting glowing halo surrounding the Sun
like an ethereal crown. Like a ghost floating around in the blackest darkness. Unfortunately, standing on Venus would be terribly brief, as you’d be burned, crushed and dissolved in mere seconds. The thought-experiment is fascinating anyway. But it’s a
glorious way to go!
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| 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. It isn't that dark up there above the clouds on Venus. Right after that, your craft gets corroded by sulphuric acid and you plummet into oblivion. In the books of astronomy, Venus is the 'You may see but not touch' chapter. 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. Imagine a halo of concentric rings of light going on and on seemingly forever in that 1200 km size. Spotting a perfectly 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. You will never know until you look around and find it.
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| 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. Some day, our scientists are going to accomplish that. 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. It's a small sample of the big thing on our sister-planet. Next time you’re on a plane, glance at the clouds below. You'll fall in adoration with the phenomena. 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's the hottest planet in the system. It somehow manages to produce delicate ghostly glories in it’s sky. That is a magnificent part of a planet with the worst reputation. It’s a reminder that even in the most extreme places, light, physics and science combine to create jaw-dropping beauty. There is no escape from the science.
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| Proposed balloon missions to Venus Pic credit - Universe Today website |
Did you enjoy this deep dive into Venus’s mesmerising optical wonders? Hope you liked what you got to learn. Share your thoughts below and reveal if you’ve ever spotted a glory here on Earth!
Bonus
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| Source - physics.org |
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