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As I prepare slides & we pack bags for Luxembourg & #AsteroidDay, something strange is appearing in skies over #NewMexico & around the world. Noctilucent clouds—shimmering wisps in twilight sky—are normally only seen on poleward side of 50°. This year they're below 35°. Why!? 
NLCs are easily visible to naked eye, but were never reported before the industrial revolution had kicked into high gear. The first confirmed observations were in 1885. It's tempting to link them to fossil-fuel driven climate change but could that generate clouds 50 miles up?
Human-caused global warming causes stratospheric & mesospheric cooling, which might suggest more condensation of ice particles that make up NLCs. But colder air means crystals are also smaller, with a counterintuitive visibility reduction. earther.gizmodo.com/humans-are-mak…
Research published in GRL last year identifies the culprit is methane pollution, which reacts to form water vapor in the middle atmosphere. Methane also exacerbates global warming, as documented by the acclaimed @HBO program #IceOnFire featuring fellow New Mexican @vivarioarriba
Even in a cold mesosphere, condensation nuclei are needed to turn water vapor into the ice crystals that make NLCs scatter sunlight. Windblown dust from Earth’s deserts or salt from sea spray can’t get that high. The nuclei come from above as dust from meteors.
They don’t have to be small meteors. We know that when comet Shoemaker-Levy 9 struck Jupiter 25 years ago this summer, the ballistic plume formed a visible NLC out of condensed comet vapor when it collapsed (imaged by HST team lead by my friend, @hbhammel).
That HST images led to an “aha!” moment. After the 1908 Tunguska airbursts, a bizarre cloud drifted over western Europe that was so bright it was said that you could read a newspaper under it at midnight. The cloud of meteorite dust was high enough to be sunlit.
Similar NLC-like clouds were documented after airbursts above Tagish Lake (2000), Chelyabinsk (2013), & Bering Sea (2018). Here’s a cool movie that shows the NLC from the Bering Sea bolide (h/t Simon Proud @simon_sat).
June 30 was chosen for #AsteroidDay, because it’s the anniversary of the Tunguska explosion. That date is also notable because it’s near the peak of the Beta Taurids, a daytime meteor shower associated with debris from Comet Encke. My simulations are consistent with a Taurid.
This association has a long history. In 1996 I attended a presentation at a Tunguska conference in Bologna by David Asher, who argued for a Taurid resonant swarm that was in a 7:2 resonance with Jupiter. He suggested could in contain many objects, one of which was Tunguska.
It’s a difficult hypothesis to test, because Tunguska-sized objects (my models suggest ~40m) must be very close fore even most powerful telescopes. The resonant swarm rarely comes close enough. But my colleague @pgbrown pointed out to me last summer that 2019 is an exception.
The center of the swarm (if it exists) is just about to cross Earth’s orbit at a spot only 5° away. The last year it was closer was 1975. Seismometers that had just been placed on the moon by Apollo astronauts recorded a significantly higher rate of impacts that summer.
Peter Brown suggested an observational campaign to look “down the pipe” toward the anti-radiant of the swarm, at a “sweet spot” in space where near-misses would appear to converge. We presented this idea at @theAGU fall meeting.
washingtonpost.com/science/2018/1…
washingtonpost.com/science/2018/1…
We soon realized that a better strategy might be to look for larger objects near center of swarm as it crosses into the night sky beyond the Earth in our orbit. David Clark did the necessary calculations and generated some images of the swarm’s visibility to guide observations.
I told Phil Plait about it. I didn’t want to wait until last minute to get some publicity for opportunity to be known to all astronomers involved in NEO survey efforts. Phil wrote up for his @BadAstronomer column. It's now caught fire in science media.
syfy.com/syfywire/could…
syfy.com/syfywire/could…
Phil included links to Dave's movies & illustrated it with Don Davis’s depiction of Tunguska explosion. By coincidence, I'd just heard from Don a couple days earlier to discuss collaboration on new set of paintings of formation of Meteor Crater (his previous were 1982).
Don Davis is one of my favorite space artists, who’s illustration of the Tunguska event is both technically accurate and aesthetically beautiful. It was based, it part, on my simulations of the event. I use it in talks all the time.
We’d been playing email & phone tag since, & finally had long, wide-ranging, & fun conversation Thursday. Among things we discussed was his plan to try to photograph the glow (like zodiacal light) from the Taurid swarm w wide-angle lens. Something that had never occurred to me.
Then the 1st message I got yesterday was from him with a phot he'd just taken of noctilucent clouds from Joshua Tree. “Something highly unusual,’ he said. “Perhaps meteor dust is being sprinkled in some quantity on us.” He gave me permission to tweet it.
He sent me another message today letting me know that it had set a record for the furthest south NLCs had ever been observed.
spaceweather.com
spaceweather.com
If the Taurid Swarm is a real thing, and if it’s got a lot of dust associated with it, then it may already be here. Here are today’s frames from Dave’s animations of the putative swarm.
NLCs require 2 things. 1) H20 vapor in the mesosphere. 2) Condensation nuclei. Scientists featured on #IceOnFire showed CH4 leaking from everywhere at unprecedented rates so H2O level is high. We know Taurid Swarm (this month) is historically close. cantab.net/users/davidash…
I encourage everyone to go out after sunset or before dawn to look for them & post photographs this summer. They are beautiful, but they also might be serving as our planet’s idiot light. We would should not ignore this warning, even if we are not sure exactly what it means.
Sorry for all the typos. Was in a hurry to meet friends for music & beer so didn’t proofread. It’s just 1st-draft Twitter!
