Overview of the Mars PEM

Overview of the PEM (Planetary Evolution Model)

The objective of the “Planetary Evolution Model” is to simulate and understand the past environments of a planet and it's long term evolution. It's also usefull to explain the observed landforms etc.

PEM schematic view ; credit: Francois Forget

To accurately simulate the climate and the fate of volatiles for thousands to millions of years we must couple physical processes with very different timescale, ranging from clouds microphysics and atmospheric dynamics (represented in the GCM) to the evolution of lakes, glacier accumulation, and subsurface ice evolution.

Given the diversity and the complexity of the Martian paleoclimates, the model use an “asynchronous coupling” between the slow ice and water reservoirs models and the GCM. In practice the Planetary evolution model use a horizontal grid identical to that of the PCM, and include the same representation of the microclimate on slopes. In our case, we run the Planetary Evolution Model with an adaptative time step, depending upon the dynamics of the modeled system (smaller timesteps must first be used so that the different volatile reservoirs reach a quasi-equilibrium, then the timestep will depends on the evolution of the forcing, which is slow in the case of obliquity, for instance) . At each timestep, the inputs from the atmosphere (e.g. mean precipitation, sublimation and evaporation, temperatures, dust deposition) is obtained through a multi-annual run of the Global Climate model using the outcome of the Mars Evolution Model as initial state.

The two modes of simulations

The PEM can works in two modes:

• Mode 1 (M1) : Search of the steady state given an external forcing and an initial situation
• Mode 2 (M2): Realistic following of orbital forcing

For M1, we always apply the same external forcing (the GCM is always called at the same obliquity for example). The PEM will run to find a steady state. Be carefull that it can oscillate around the steady state so all the stopping criterions needs to be choosen wisely. For M1, be also carefull that the uniquity of the steady state is not proven for this model. This means that for a given external forcing it is possible that the steady state is not unique and could depend on the initial condition and other parameters.

For M2, we follow a predefined orbit. It is read in the file ob_ex_lsp.asc. Stopping criterion specific to orbital change can be specified in run_pem.def. The steady state is not necessary reached for all the orbit configuration as it could take more time to reach it than the time spent in this orbital configuration.

Stopping criteria

For both modes, the PEM will run and extrapolate tendencies 'as long as it is meaningfull'. This means that for the different modelled physical processes, a stopping criterion is computed to express how much the system has changed. The PEM compares it to an acceptance rate and decides to stop or continue. The criteria are the following:

• Water ice: Ratio of the surface of water ice that is sublimating at the time step t over the surface of water ice that is sublimating from the GCM run;
• CO2 ice: Ratio of the surface of CO2 ice that is sublimating at the time step t over the surface of co2 ice that is sublimating from the GCM run;
• Pressure: Ratio of the planet averaged surface pressure at iteration t over the planet averaged surface pressure at the end of the last GCM run;
• Orbital parameters: A maximum absolute value of change is set in the PEM for the variation of obliquity, eccentricity and Ls perihelion;
• Maximum number of years: A maximum number of years can be defined.

Overview of a PEM run

In either mode, a run works like this:

• Run 2 years of GCM: From this, we extract data such as daily average pressure, daily averaged vmr of atmospheric components, daily minimum of ice etc... You thus need to use XIOS
• The GCM restart file are renamed to be read by the PEM. They will be modified by the PEM to be able to start a new GCM simulation.
• If we are in a M2 configuration mode, a beginning year needs to be specified in the run_PEM.def, and the file ob_ex_lsp.asc needs to be present.
• The PEM is launched.
• GCM tendencies are computed from the data of the first item. They are applied to the start files and can be adapted to the situation (change of surface pressure etc...). All the physical processes also take place.
• The stopping criterion are checked and it either stop the time iteration or apply again the tendencies and run the physical processes
• GCM like start files are written.
• GCM is run again for 2 years and it loops again from the beginning.

Physical processes modelled

The PEM is still in development, so far, the physical processes modeled are quite specific to Mars at low obliquity.

Current processes that have been validated:

✅ Ice accumulation

✅ CO2 / H_2O adsorption

✅ Soil temperature from the surface to ten's of kms.

✅ Glacier flow for CO2

Current processes that are in validation:

∼ Subsurface Ground Ice at equilibrium

∼ Orbital Forcing (RV & LL)

Future works:

❌ Exchange between subsurface reservoirs and atmosphere / Formation of a lag deposit(will be done by Simon Neviere, Futur intern of Lucas)

❌ Dynamical Ice Table

❌ Hydrology: lake, river, ocean, etc. (Alexandre Gauvain)

❌ Photochemistry

Naming convention of variables

Some variables comes from the GCM run. There are 3 cases:

• Constant variables : Some variable are not changed during the PEM run, they are named var_cGCM. ex:???
• Adapted variables : Some variables are adapted to the current state and recomputed through the PEM run, they are named var_aGCM. ex:???
• Tendencies from GCM : Some variables are tendencies obtained from the GCM run, they are named var_tGCM. ex:???

Some variables coming from the GCM can be shaped as the dynamical grid but needs to be used in the physical grid. The change is done via the subroutine grid_dyn_phys. In this routine, the name is changed:

• Dynamical to physical grid the variable is renamed from var to var_phys

Variables that exists in the GCM but that here belong to the PEM are named:

• Exist in the GCM but belong to the PEM var_PEM

Variables that are specific to the PEM are not named following a special convention.

Index name for loops through:

• physical grid : ig (max ngrid)
• tracer : iq (max nq)
• subslopes : islope (max nslope)