Thread by @DrPhiltill

I’ve resisted commenting on the tardigrades that crashed on the Moon, but this may be interesting so here goes. Fossil tardigrades have been found in 530 million year old rock. That means they have lived here during some large asteroid impacts. They live all over Earth and... /1

2/ I would guess that almost certainly they have been blown into space by some of these impacts. Around an impact there is a region called the “spallation zone” where the rocks are shocked only lightly and yet ejected all the way into space. The tardigrades in those rocks...

3/...would likely not be too mangled, maybe even survived, and yet they would be blown into space. Since the Moon is *right there*, a lot of these rocks and the tardigrades and other life would land on the Moon. So tardigrades have probably landed on the Moon before. However,...

4/...when I said “landed” on the Moon, well...let’s think about that. These Ancient Tardigrade Astronauts (ATAs) only had rocks for spaceships, so they couldn’t do powered descent to the surface. That means they impacted with at least 2.43 km/s velocity (lunar escape velocity).

5/ Rocks falling from space to the Moon is a lot different than falling to the Earth due to lack of an atmosphere. Rocks entering Earth burn up in the air (“shooting stars”, possibly containing ATAs from another star???👽🤷‍♂️😄) but really big meteoroids burst apart in the air,

6/ and the fragments have a smaller ballistic coefficient than the big rock so they slow down and land on the ground. That’s why we find meteoroids on Earth. [Review: they survive entry by being too big to burn up, then air burst so the fragments can slow down & survive impact!]

7/ None of that happens when rocks fall on the Moon since there’s no atmosphere to make the rocks get hot & airburst, then no atmospheric drag to make the fragments slow down. So all the rocks go super, super fast when impacting the Moon. (That’s why the Moon looks like this!)

8/ Geophysicists have learned that rocks generally VAPORIZE on impact. When huge shock waves compress a rock then un-compresses it, every molecular bond in the rock will be broken. I.e., the rock turns into gas. We see this in experiments like at the NASA Ames hypervelocity gun.

9/ So what happened to all the adorable Ancient Tardigrade Astronauts in their cute little Rock Spaceships when they tried landing on the Moon during the past 500 Million Years? Did the vaporize? ☹️☹️☹️😭

Well, not necessarily. Some might have survived.

10/ Because rocks that were ejected from an asteroid impact on Earth that immediately fall to the Moon may not have gone very fast. They might have hit at little more than lunar escape velocity, just 2.43 km/s. That is quite slow for hypervelocity impacts!

11/ A study in 2013 argued (on the basis of computer simulations) that some even-faster rocks hitting the Moon might survive. So maybe the Ancient Tardigrade spacecraft landing on the Moon didn’t vaporize, but could the cute little moss piglets survive? nature.com/articles/ngeo1…

12/ Scientists shouldn’t speculate when there’s a total absence of empirical data. But if someone wants to provide funding, we can do hypervelocity experiments at White Sands or at NASA Ames, accelerating Tardigrades in a vacuum chamber to 2.43 km/s. (It will likely be a first.)

13/ Anyhow, the reason this matters is because scientists hope to explore the Moon to study the beginnings of life on Earth. The Moon doesn’t have plate tectonics or rain constantly recycling the crust like on Earth, destroying the evidence of what happened billions of years ago.

14/ The Moon DOES have impacts constantly reworking the surface,destroying what lies on top. But those impacts also throw regolith blankets on top of stuff, sequestering it where it can survive. The Moon is Earth’s attic where a selection of Earth rocks and stuff got stored away.

15/ So one of the reasons we want to go back to explore the Moon is to get clues about conditions on ancient Earth. E.g., if we could find some of Ancient Tardigrade Astronauts still be buckled into their little Rock Spaceships from 500 million years ago, it would be A-MA-ZING.

16/ Now this is why there are planetary protection requirements EVEN FOR THE MOON. Yes, although life can’t survive there, we might find things that will help us understand the beginnings of Earth-life. IOW, the stuff in Earth’s attic is valuable so sell it all in a yard sale.

17/ If we find ancient life (or fragments of it) on the Moon, we will compare it to modern life to see how it changed. (How could we find it? By looking inside any Earth rocks we find there.) We can genetically sequence the ancient life and study the chemistry of its rocky home.

18/ This is where I end the thread becoz I don’t want to join the discussion on how much planetary protection is enough/too much, or the legal issues of sending modern moss piglets to mix among the ancient ones, or the technical matter of how we would tell them apart...

19/19 But I hope this provides a little more context to the discussion. And personally, I just find this idea that cute little water bears bumbled along with their stubby legs and managed to land on the Moon hundreds of millions of years before humans ever did FASCINATING 😄😄😄

*so DON'T sell it all in a yard sale. (I hate it when the pivotal word is omitted.)