The Missing Zap and Force Field On Venus

"Reading gives us someplace to go when we have to stay where we are" - Mason Cooley


The older I get, the more I've come to relish moments of unexpected childhood nostalgia ("Oh shut up Hans, you're only 26!"). When I was a kid, my mom would bring me and my sisters to the local branch of the San Antonio library in the summertime to make us read. Although my sisters were the more avid readers - they were the ones always taking home the top Accelerated Reader prizes in school each year - I had my little niche at the public library: the shelf with the books about the solar system! While they were reading Harry Potter and Twilight and Eragon, their dorky little brother was off in some corner memorizing the atmospheric composition of Neptune.

Well a few months ago I visited the Los Angeles Central Library for the first time, and of course I dragged my long-suffering girlfriend to the astronomy section to tell her all about the solar system! Patient and wonderful as she may be, eventually even she tired of my planetary perseverations and abandoned me to my own devices in the library. So I sat there cross legged alone on the floor like a child, reading a book about Venus, and there I learned about TWO unsolved mysteries about our sister planet: Venus is unexpectedly missing both the presence of lightning and a detectable magnetic field! 

Doesn't matter if I'm 6 or 26, or if it's my local library in San Antonio (left)
or the flagship branch in Los Angeles (right), as long as there's space books, I'm all for it!

Lightning has been confirmed on other planets with thick atmospheres like Jupiter and Saturn, so planetary scientists have long expected Venus to be the same. Yet multiple spacecraft from many different countries have carefully studied Venus, and still the evidence of lightning, visual or otherwise, has been circumstantial at best. The early Soviet Venera landers detected curious pings on its sensors before the hellish atmosphere crushed the probes; the American Pioneer Venus Orbiter noticed some energetic bursts, but the Cassini spacecraft came up empty when it passed Venus on its way to Saturn a decade later; the European Venus Express heard "whistler" radio waves that can be generated by lightning on Earth, but also by many other atmospheric instabilities seen both on our planet and on others. Now our best bet is the Japanese Akatsuki probe, the only spacecraft actively orbiting Venus right now. So what has Akatsuki seen?

Unlike the dazzling display that Jupiter treats us to, Venus' lighting has been far more reclusive, assuming it exists at all!

Akatsuki has observed a single atmospheric flash of light over Venus. Just one. Does this count as lightning? Well it's hard to say; we'd expect lightning to occur in bunches, so maybe it was just a cosmic ray happening to strike the atmosphere, or a meteor burning up, or even a one-off instrumentation glitch. All of those seem pretty unlikely, so unfortunately it looks like we'll have to just keep watching.

But if it were lightning, what could've caused it? Let's back up and consider what lightning is in the first place. Lightning occurs when an electrical charge builds up in the atmosphere and causes a spark, just like when you accidentally shock yourself with static electricity. On Earth this occurs when water vapor in clouds circulates due to convection; the water near the top of the cloud freezes into ice crystals that bump into each other and generate a positive charge, while negatively charged particles sink. When the charges get big enough, they generate a giant spark - lightning! But Venus' clouds are different. They're made of sulfuric acid and are pretty good electrical conductors, so it's harder for these large positive and negative charges to build up. Furthermore, it's unclear whether the same convective cycles exist in Venus' clouds in the first place.

There is one other potential trigger for lightning on Venus though: volcanoes. Though we haven't observed one directly due to Venus' opaque atmosphere, we're pretty confident that these eruptions are occurring, and the enormous release of ashy plumes could generate the static electricity necessary to spark Venus' atmosphere! While no one knows for sure yet, it's an interesting question to think about because the energy release from lightning can break apart and recombine molecules in ways that would be otherwise improbable. The presence of lightning over the primordial soup of ancient Earth likely contributed to the development of life. 

Venus is scary enough, maybe an intense lightning storm like this would simply be overkill!

So now the other mystery: what happened to Venus' magnetic field? Every instrument we've ever pointed at Venus has unequivocally confirmed that it has no magnetic field to speak of. How could this be, when Venus and Earth are roughly the same size and density and made of the same rocky materials? Shouldn't Venus also have a molten metal interior, whose convective motions create the necessary internal dynamo? And has Venus always lacked it, or did the magnetic field fade away at some point in the past? 

There's no solid consensus yet, though many theories exist. For a long time the most common explanation was that Venus' rotation is too slow to promote the internal dynamo, but this doesn't really hold up to scrutiny since the interaction of heat from the core countered by the cooling of plate tectonics on the surface is more important to driving convection. More recently, an alternate theory has arisen that perhaps Earth is the anomaly! Maybe planets of Earth and Venus' size normally shouldn't have strong magnetic fields, but Earth's impact with a Mars-sized object 4.5 billion years ago (the same collision that formed the Moon) literally shook our planet to its core and jumbled up the stratified layers of the Earth's interior. The theory argues that it's this de-homogenization that has helped promote the convective motion that powers our magnetic field to this day!

Our understanding of Venus' interior would be hugely advanced if we could build a long-duration lander that could survive the hellish surface

This begs the question then, how has Venus' atmosphere not been completely stripped away? While it's true Venus doesn't have an intrinsic magnetic field, it still has an induced magnetic field caused by ionizing solar winds striking the thick upper atmosphere, just like Earth's magnetosphere. Furthermore, Venus' atmosphere is mostly carbon dioxide, a heavier molecule than the oxygen and nitrogen in our atmosphere, so it takes much more energy to dislodge Venus' atmosphere. Those thick, mysterious clouds aren't going anywhere anytime soon!

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