ZEPHYRUS is the atmospheric escape module of the PROTEUS coupled atmosphere-interior evolution framework. It computes the rate at which an exoplanet loses its atmosphere to space under high-energy stellar irradiation, and the fraction of its atmosphere removed by a giant impact.

Given a planet's mass and radius, the radius at which XUV radiation is absorbed, the stellar XUV flux at the planet, and an escape efficiency, ZEPHYRUS returns the energy-limited mass-loss rate. An optional tidal correction after Erkaev et al. (2007) enhances the rate for close-in or eccentric orbits, and two forms of the XUV-heated cross section are available. The stellar XUV flux is typically supplied by MORS evolution tracks. For giant impacts during accretion, ZEPHYRUS returns the eroded fraction of the target's atmosphere from the collision speed, masses, densities, and impact angle, following the scaling law of Kegerreis et al. (2020).

Named after Zephyrus, the Greek god of the west wind and a messenger of spring. Pronounced ZEF-ir-us.

Escape model

Energy-limited (EL) escape: the deposited XUV power divided by the gravitational binding energy of the escaping gas, with an optional tidal enhancement K_tide. The XUV cross section scales as Rp * Rxuv**2 (default) or Rxuv**3, and the tidal factor applies only outside the Roche lobe (R_Hill / R_XUV > 1).

Documentation

Full documentation is at proteus-framework.org/ZEPHYRUS, including:

• Getting started: installation and the quickest path to a first run.
• Tutorial: a first escape calculation for the Sun-Earth system.
• How-to guides: install, run the tests, build the documentation.
• Explanations: model overview, coupling to PROTEUS, limitations, and the testing suite.
• API reference and validation anchors: every public function with NumPy-style docstrings, plus the reference-pinned test inventory.

Installation

pip install fwl-zephyrus

Or, for development:

git clone https://github.com/FormingWorlds/ZEPHYRUS.git
cd ZEPHYRUS
pip install -e .[develop,docs]

The docs extra pulls in Zensical so you can build this documentation locally with zensical serve.

Input data

ZEPHYRUS uses MORS for the stellar XUV flux, which needs a set of stellar evolution tracks from the OSF repository. Set the FWL_DATA environment variable to the directory where the data should live, then download the tracks:

export FWL_DATA=/your/local/path/FWL_DATA   # add to ~/.bashrc to persist
mors download all

Quick start

import numpy as np
import mors
from zephyrus.escape import EL_escape
from zephyrus.constants import au2cm, au2m, ergcm2stoWm2
from zephyrus.planets_parameters import Me, Re, a_earth, e_earth

# XUV flux at Earth from a solar-mass star, over its evolution (MORS tracks)
star = mors.Star(Mstar=1.0, Omega=1.0)
Lxuv = star.Tracks['Lx'] + star.Tracks['Leuv']          # erg s-1
Fxuv = Lxuv / (4 * np.pi * (a_earth * au2cm) ** 2)      # erg s-1 cm-2
Fxuv_SI = Fxuv * ergcm2stoWm2                           # W m-2

# Energy-limited mass-loss rate [kg s-1] across the star's history.
# a, e and Ms only enter the tidal branch (tidal_contribution=True).
mdot = EL_escape(
    tidal_contribution=False,
    a=a_earth * au2m, e=e_earth,
    Mp=Me, Ms=1.0, epsilon=0.15,
    Rp=Re, Rxuv=Re, Fxuv=Fxuv_SI,
)   # aligned with star.Tracks['Age']

See the first-run tutorial for the full walkthrough.

License

Apache License 2.0. ZEPHYRUS is part of the PROTEUS framework.