### Mars

Mars is one of the most observed primary flux calibrators at NOEMA.

In Fig. 1–5 we compare the Mars spectrum provided by the ASTRO Legacy implementation and by the CASA model on a single date (01-Jan-2010).

In the Mars physical models time variations are not driven by heliocentric distance only, but also by atmospherical circulation processes.

In Fig. 6–9 we compare the time variations of the Mars surface average brightness temperature as computed by ASTRO Legacy and CASA at different frequencies between 80 and 360 GHz.

We cannot show the output of the Lellouch model because the implementation scheme is not yet defined. However we expect it to be very similar to the CASA model, since it is another implementation of similar physical and radiative transfer model.

There are several obvious differences:

- The long scale (period of 2 years) variations in the physical models do not follow a sine-like curve as in the ASTRO Legacy model. The temperature increase occurs in two phases separated by a plateau, while the decrease is quicker and steady.
- The amplitude of the long scale variation is also larger in CASA than in ASTRO Legacy.
- In addition to the long term variation there is in the physical model a daily variation of the temperature, with an amplitude of 2–3%.
- As seen in Fig. 1 the temperature spectra is not flat in the physical model.

In summary the CASA model (i.e. physical model) produces brightness temperatures 10% lower than the ASTRO Legacy values around the minimum of the long term period for frequencies between 80 and 230 GHz. For the rest of the period, the difference is much lower. At higher frequencies (260–360 GHz) the minima and maxima of the CASA time variations are respectively 5% lower and 5% higher than the ASTRO Legacy values.

The irregular time variations of Mars brightness temperature might affect the flux monitoring of the secondary calibrators, especially at 3mm.