Empirical Earth · Gravity, Buoyancy & the Air

Gas next to vacuum — no dome required

The claim

“Gas can’t touch a vacuum without a container, so we’re sealed under a dome.”

What is true

Gas does expand to fill whatever it can reach. Release it into a vacuum and it disperses, with no exceptions.

What is measured

Air pressure falls off exponentially with height, halving roughly every 5.5 km, which only happens if gravity binds a gas shell to a planet. See also the atmosphere as an open system and the gas laws and gravity.

What would show this is wrong

a measured pressure-vs-altitude profile that did not fall off exponentially under gravity.

Sources

  1. Atmospheric structure, scale height & escape. Pressure falls exponentially, H = kT/(mg) ≈ 8.5 km; no hard boundary (Kármán line is a 100 km convention); thermal/Jeans escape slowly leaks lig link
  2. Falling in a vacuum — NASA’s largest vacuum chamber. In NASA’s Space Power Facility (about 30.5 × 37.2 m, the world’s largest vacuum chamber), a bowling ball and a feather dropped together s link
  3. Atmospheric escape — Jeans escape of hydrogen and helium. Escape is significant only when escape velocity < ~6× mean molecular speed; on Earth only for H and He. atmospheric escape link
  4. Pressure as the weight of air; the ISS in the upper atmosphere. Sea-level pressure (~101 kPa) is the weight of the overlying air; it falls smoothly with height (Everest ~1/3 sea level), and link
  5. The geocorona extends past the Moon. SOHO/SWAN Lyman-alpha mapping (Baliukin et al., 2019) found Earth’s hydrogen geocorona, the outer exosphere, reaching about 630,000 km, some 50 Earth dia link
  6. The largest vacuum chamber ever built. NASA’s Space Power Facility at Plum Brook Station in Sandusky, Ohio, houses the world’s largest vacuum chamber: 30 m (100 ft) in diameter and 37 m (122 link
  7. Rarefied gas flow and the free-molecular regime. The mean free path of air is about 6.5 × 10⁻⁸ m at sea level and rises with altitude as pressure falls, reaching order 10⁻¹ m near 100 km and link
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