Skip to content

Grow a planet without dynamics: the dummy module

accretion.module = "dummy" builds an impact history from scaling laws instead of integrating a system of embryos. It is deterministic: the same configuration always produces the same history, with no Monte Carlo and no optional dependency.

This is a PROTEUS module, not part of Morrigan. It is documented here because it occupies the same slot, answers the same interface, and is usually the right tool when what you want is a growing planet rather than a dynamical history.

When to reach for it

  • Exercise the coupling in a test or a quick run without installing Morrigan.
  • Prescribe the growth you want to study, rather than sampling whatever a dynamical model produces.
  • Sweep an accretion parameter where a single controlled history is worth more than the spread over seeds.

If what you need is a plausible dynamical history, with the scatterings, ejections and the stochasticity that come with it, use Morrigan.

Configuration

[accretion]
    module = "dummy"

    [accretion.dummy]
        mass_accreted    = 0.1       # [M_earth], total delivered over the timeline
        num_impacts      = 3
        timescale        = 1.0e6     # [yr], e-folding time of the growth law
        time_last        = 5.0e6     # [yr], time of the final impact
        eccentricity     = 0.05
        impact_parameter = 0.5
Parameter Type Default Description
mass_accreted float 0.1 Total mass delivered over the whole timeline [M⊕]; the increments are scaled to sum to exactly this
num_impacts int 3 Number of impacts in the timeline
timescale float 1.0e6 E-folding time of the accretion law [yr]
time_last float 5.0e6 Time of the final impact [yr]. Impacts are spaced evenly from time_last / num_impacts up to this time
eccentricity float 0.05 Encounter eccentricity [1], setting the approach velocity added to the mutual escape velocity, and the impactor's orbit
impact_parameter float 0.5 Impact parameter of every collision [1], the sine of the impact angle. Zero is head-on, one is grazing

What the law does

The planet approaches an asymptotic mass exponentially, which is the standard picture of an accretion rate that decays as the feeding zone empties. Impacts are placed at evenly spaced times, and each delivers the mass the law accretes over its interval, so the increments decay with time and the first impact is the largest. The increments are then scaled so they sum to mass_accreted exactly.

Everything else follows from the pair: radii from the Noack & Lasbleis (2020) mass-radius scaling, collision velocities from the mutual escape velocity combined with an encounter velocity set by eccentricity, and the merged orbit from conserving linear momentum.

timescale against time_last is what shapes the history. Short compared with time_last concentrates almost all the mass in the first impact; long compared with it spreads the mass nearly evenly across the impacts.

Traps

timeline_path belongs to the other module

Setting accretion.dummy.timeline_path is refused at configuration load. The field exists only to catch that mistake: it asks to replay a file, and without the check the run would quietly generate a timeline at default settings instead. To replay a file, use accretion.module = "timeline".

The impacts are still real impacts as far as the rest of PROTEUS is concerned: they grow the planet, deliver volatiles under impactor_volatiles, strip atmosphere under atmloss_module, re-melt the mantle and move the orbit. Only the schedule is analytical.

See also