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Dr. Phil Metzger Profile picture
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7 May, 4 tweets, 2 min read
What I find cool about successfully hearing this sound isn't just the low density of the atmosphere, but also the fact that the atmosphere is almost pure carbon dioxide. CO2 is unusual among gases for having very high bulk viscosity, which vastly reduces sound propagation.
2/ Viscosity tells how momentum diffuses through a gas. Honey has high shear viscosity, meaning if flows slowly. Water has low shear viscosity, so it flows easily. In addition to shear viscosity, there is the lesser-known "bulk viscosity" related to the rotation of molecules.
3/ In most gases, the bulk viscosity is ~ zero, but in carbon dioxide the bulk viscosity κ is 1000 times greater than the shear viscosity η. The attenuation of sound is described by this equation. So (order of magnitude) sound attenuates about 1000x more on Mars than on Earth.
4/4 That equation came from this paper, a good (but technical) introduction to bulk viscosity. But beware of the fonts, because the manuscript font mixes χ with κ (at least the way it displays on my Mac). So if you see χ, understand it to be κ. aip.scitation.org/doi/10.1063/1.…

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Dr. Phil Metzger Profile picture
8 May
I love this quote for so many reasons, I’m going to translate it into a modern writing style. 1/n
“It used to be there weren’t many people bold enough to cross the ocean, but now it’s so easy even fearful and faint-hearted people can cross it. So maybe someone will invent a way to travel to the Moon, even though it seems like such a terrible voyage across vast, empty sky! /2
[I’m translating a mix of two versions of his book to get his entire thought.]

“And without a doubt there will be bold people to take that venture, just like crossing the sea!

You might be thinking we have no way to sail into the sky, unless the poet’s fantasies were true.

/3
Read 9 tweets
Dr. Phil Metzger Profile picture
21 Apr
From the talk I gave at the ASCE Earth & Space conference today. When you land on the Moon, your rocket exhaust is faster than lunar escape velocity and there is no atmosphere to slow down the dust you blow. We need to worry about damaging things in orbit.

Short thread... /1
We've done a lot of experimental work to understand how much lunar soil will blow because of the rocket exhaust during a landing. The work included reduced gravity flights measuring soil erosion in lunar gravity. /2
As you would expect, erosion rate is faster when gravity is lower. That part of the physics is easily understood, at least. Erosion of soil on another planet scales as 1-over-gravity. /3
Read 9 tweets
Dr. Phil Metzger Profile picture
10 Apr
Since this Guardian writer has recycled the super-uninformed claim "we ought to be spending the money on Earth instead," it is now time to recycle the informed responses. (1) Hardly *any* money is spent on space. E.g., the US spends 4X as much on tobacco as on space. 1/n
2/n US consumers spend 5X more on credit card interest and fees as we spend on NASA. Every year the US spends $1.77 Trillion on retail food, and about 30-40% is thrown away, which is equal to 31X the amount we spend on NASA.
3/n Every year the US spends 12.5X as much on alcoholic drinks as we do on NASA, and 77% of that is for binge drinking. In other words, the US spends 10X as much on binge drinking as we do on NASA. (NASA makes you feel better in the morning, by the way.) cdc.gov/alcohol/featur…
Read 22 tweets
Dr. Phil Metzger Profile picture
3 Feb
😟😟😟 This chart is SO untrue, though, as the published records show. Asteroids were NOT removed by scientists from being planets until the 1950s. Moons were classified by astronomers as planets until the 1920s. I’m so sad this nonsense chart is being published because... 1/
2/...it is going to amplify the presentism fallacy that has plagued astronomers over the past few decades. Presentism is when you take a view that developed in recent times and use it to interpret past events as if that view had existed back then. voicesandimages.com/presentism-don…
3/ The idea that planets only include the primaries (no moons) and doesn’t include minor planets (no asteroids) is a relatively recent view among astronomers. But the public has held that view since the mid-1800s because it came from 1800s astrology, not from scientists.
Read 11 tweets
Dr. Phil Metzger Profile picture
30 Jan
I have worked with lunar samples but I don’t remember smelling anything. The astronauts who were there reported that it smelled like gunpowder. Our belief is that on the Moon the minerals have broken chemical bonds on their surfaces that activate our smell sensation, BUT... 1/2
2/ ...when lunar samples are exposed to air, the molecules bond with those locations on the minerals, passivating their surfaces, so they lose the gunpowder smell. In Houston the samples are stored in dry nitrogen but no gas is perfectly pure so passivation is inevitable I guess.
3/ This is the story we tell, but I’m not totally sure about the details. It raises the question how did the dust retain its smell as the Lunar Module was re-pressurized and the crew removed their helmets to smell it? Was the air still dry enough, even as sweat was evaporating?
Read 13 tweets
Dr. Phil Metzger Profile picture
29 Jan
Great article explaining why there’s no reason to assume anything beyond natural process for ‘Oumuamua. The preprint (not paywalled) is here: arxiv.org/pdf/1907.01910…
Not an expert here, but I asked astronomers in our faculty group and they tell me that the extreme light curve might not be entirely due to ‘Oumuamua’s shape. For example, it could be a contact binary where one lobe is a brighter material than the other lobe. Compare Arrokoth:
3/ If different parts of the object have different albedos, then it might be less than 6-to-1 in elongation. So the light curve that suggests it has extreme elongation shouldn’t be considered so strange. But even if it does have > 6:1 elongation, even that is not so strange.
Read 5 tweets

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