Anybody with lungs and a diaphragm!

References
www.pnas.org/doi/full/10....
www.nature.com/articles/375...
www.pnas.org/doi/10.1073/...

Further reading
genent.cals.ncsu.edu/bug-bytes/ci...

Larger insects than we see today are known to the fossil record—but their size has been demonstrated to track the amount of atmospheric oxygen! Slow diffusion and smaller amounts of available oxygen prevent insects from getting too big—a perk of being alive today rather than in the Paleozoic.

Insects mostly rely on these tubes to provide oxygen to their organs. Oxygen must diffuse though these tubes into insect organs, but diffusion is famously slow. By contrast Mammalian lungs first push fresh air in, and only rely on diffusion across very small distances, allowing us to grow larger.

Insect blood is important for carrying nutrients to different tissues, but in most insects it doesn’t transport oxygen the way our blood does. Instead the insect respiratory system is separate; openings to the outside throughout the insect’s body ("spiracles") let air into a dense network of tubes.

Their circulatory systems are “open”, meaning they have only limited, open-ended blood vessels. They have pumps in the back of their abdomens that are responsible for pushing blood throughout their body cavities, recirculating blood only when it drips back and settles near the pumps again.

Insects are very different from humans and mammals, in more ways than are obvious just by looking. Their circulatory and respiratory systems are quite alien compared to ours. They don’t have lungs, or veins!

I can't believe this is allowed

References

ui.adsabs.harvard.edu/scan/manifes...
arxiv.org/abs/1405.4310
iopscience.iop.org/article/10.3...
arxiv.org/abs/1604.00958
www.pnas.org/doi/10.1073/...
iopscience.iop.org/article/10.3...

Life on earth has been around almost as long as Earth itself; there’s no telling how many times our distant ancestors looked up toward the stars and got radioactive material in their eyes instead.

And boy is there Iron-60 on earth. There are distinct layers of this stuff deposited in sediments all around the planet, indicating *at least two* nearby supernovae in the last ***~0.25% of the Earth’s lifespan***.

Iron-60—a nucleus with 26 protons and 34 neutrons—is radioactive and tends to decay in less than ~3-4 million years. The Earth, being billions of years old, has certainly lost its original supply of Iron-60 by now. If there’s *any* iron-60 on earth, it must have come from something more recent.

Fortunately, radioactive atoms form in supernovae and come with a built-in expiration date. Various factors related to the structure of the nucleus can cause some radioactive atoms to decay quickly, while others can remain intact for millennia.

Laden as it is with gold, zinc, uranium, and other heavy elements, the Earth is made of the remnants of supernovae and other astrophysical events. So how do we know whether what we see today is from the ancient supernovae that formed Earth, or more recent events?

Good question! It'll depend on the size of the moon--Charon, for example, being so small, is completely solid. A lot of moons have subsurface oceans as well; Ganymede is big enough where it has both an ocean and a molten layer in its core!

References
www.nature.com/articles/nge...
www.sciencedirect.com/science/arti...
web.archive.org/web/20141221...
Shout out to this gr-qc paper using these properties of the moon to test general relativity: arxiv.org/pdf/gr-qc/04...

Incidentally this is some of the strongest evidence I can think of for the moon being at least partially constituted by cheese. Hot pizza burns the roof of your mouth because cheese is a terrible heat conductor—it holds on to heat. A cheese core definitely would have retained its heat this long…

The story is complicated by some of the details of planet formation and the moon’s specific circumstances; for example, Earth’s gravity leads to tidal heating just outside the moon’s core, which can blanket the liquid core and trap heat longer than diffusion alone might suggest.

The moon has held on to this heat for so long because diffusion of that heat requires order, contact, regularity, and/or simplicity. Rock has few of these properties; coupled with the sheer size of a body like the moon, this makes the diffusion time quite long.