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Title: What kept early Mars warm enough for rivers and lakes?
Plain: About 3.7 billion years ago Mars had river valleys and lakes even though it is farther from a then fainter Sun. Carbon dioxide and water vapour alone do not seem able to warm it enough, so the question is what else did.
Precise: For Noachian to early Hesperian Mars (about $3.8\ \text{to}\ 3.5\,\mathrm{Gyr}$ ago, insolation $\sim 0.32\,S_0$ relative to modern Earth) determine whether surface runoff required sustained warm conditions (mean $T$ above $273\,\mathrm{K}$) or episodic warming, and identify the forcing: CO2-H2 or CO2-CH4 collision-induced absorption, cirrus clouds, impacts or volcanic outgassing. An answer specifies atmospheric composition, pressure and duration consistent with valley-network and lake-sediment volumes and with surviving carbonate inventories.
Settled by: A climate scenario whose required H2 or CH4 sources, surface pressure and warm-period duration match sediment volumes and the measured carbonate and atmospheric-loss record.
Kind: phenomenon
Answer type: mechanism
References: (empty)
Literature status: contested
Status note: (empty)
Posed since: (empty)