Angular-Momentum-Constrained Polar Descent and Composition Sensitivity: Discriminating Signatures for Venus's Cloud-Top Circulation

Angular-momentum conservation imposes a sharp, mechanism-independent constraint on Venus's cloud-top circulation: any air parcel advected poleward from the low-latitude topside heating pool while conserving specific angular momentum would reach zonal winds of ~200 m s⁻¹ by 60° latitude and ~385 m s⁻¹ by 75°, far exceeding the observed 100–130 m s⁻¹ cloud-top maximum. Relieving this excess requires organized descent inside ~60°, which necessarily depresses the cloud top and generates a warm polar core. We quantify this descent pathway and show it reproduces the observed ~8 km polar cloud-top depression, the ~6 km cloud-base depression, and a 20–30 K warm-core contrast — all within Venus Express and Akatsuki ranges — without tuning. We also derive a second, independent testable signature: in a mass- and gravity-anchored reference state built on the sulfuric-acid lifting-condensation level (LCL) and the planetary emission boundary, surface temperature responds to lower-atmosphere N₂ fraction at roughly +2 K per percent — a factor-of-four larger than the ≤0.5 K per percent warming expected from pressure broadening in conventional radiative-convective models. We propose a re-reduction of Pioneer/Venera/Vega entry data on a common pressure grid as a decisive test. Energy, mass, and angular-momentum budgets are shown to be consistent with a mean-meridional pathway at factor-of-two precision; eddy transport, while likely present, is not required for closure. All predicted signatures are consistent with existing observations, and none by itself falsifies the conventional greenhouse picture — we present these as discriminating tests, not as a replacement theory.