Abiotic Oxygen Buildup: Biosignature False Positive

Star & Planet

XUV flux 50× quiet-Sun

Planet mass 1.00 M⊕

Initial water inventory 1.00 oceans

Time speed 15 Myr/s

Live Readouts

Evaluating escape regime…

Elapsed time

0 Myr

Water remaining

1.00 oceans

Abiotic O2 buildup

0.00 PAL

H escape rate

– kg/s

Crossover mass m_c

– amu

O retained fraction

–

How it works

Young M dwarfs stay locked in a high-activity phase for up to a billion years, bathing close-in habitable-zone planets in strong XUV radiation. That radiation photolyses upper-atmosphere H₂O into H and O. Because H is 16× lighter than O, it escapes to space far more readily — and if the heavier O is left behind, it can build up into a thick abiotic O₂ atmosphere that mimics a photosynthesis biosignature with no life involved (Luger & Barnes 2015; Wordsworth & Pierrehumbert 2014).

Escape rate (energy-limited, XUV-driven hydrodynamic loss):

dM/dt = ε · F_XUV · π R_p³ / (G M_p)

Whether O is dragged out with the escaping H, or left behind to accumulate, is set by the Hunten (1973) crossover mass — the heaviest species the outflow can still carry:

m_c = m_H + (k_B T φ_H) / (b · g)

φ_H is the hydrogen number flux at the exobase, b is the H–O binary diffusion parameter, g is surface gravity, and T is the exobase temperature. If m_c > m_O (16 amu), oxygen is entrained in the outflow and escapes with the hydrogen — no buildup. If m_c < m_O, oxygen is left behind and accumulates.

XUV flux — sets the photolysis and escape rate. Too low and almost nothing photolyses; too high and the outflow becomes strong enough to drag oxygen out too — the false-positive risk peaks in between.
Planet mass — sets surface gravity and escape radius; a deeper gravity well raises the crossover mass needed to entrain oxygen.
Water inventory — the O2 source runs out once the ocean is fully photolysed; buildup then stops.
Time speed — this plays out over 10s–100s of millions of years in reality; the slider fast-forwards it.

Simplification: this model tracks only the water-derived O–H photolysis branch and ignores crustal/magma-ocean oxygen sinks, which real planets likely have and which would cap the buildup below what's shown here.

XUV photons Escaping H Accumulating O2

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