Seismic noise recorded at the surface of Mars has been monitored since
February 2019, using the seismometers of the InSight lander.
The noise on Mars is 500 times lower than on Earth at night and it
increases during the day. We analyze its polarization as a function of
time and frequency in the band 0.03-1Hz. We use the degree of
polarization to extract signals with stable polarization whatever their
amplitude. We detect polarized signals at all frequencies and all times.
Glitches correspond to linear polarized signals which are more abundant
during the night. For signals with elliptical polarization, the ellipse
is in the horizontal plane with clockwise and anti-clockwise motion at
low frequency (LF).
At high frequency (HF), the ellipse is in the vertical plane and the
major axis is tilted with respect to the vertical. Whereas polarization
azimuths are different in the two frequency bands, they are both varying
as a function of local time and season. They are also correlated with
wind direction, particularly during the day.
We investigate possible aseismic and seismic origin of the polarized
signals. Lander or tether noise are discarded. Pressure fluctuation
transported by environmmental wind may explain part of the HF
polarization but not the tilt of the ellipse. This tilt can be obtained
if the source is an acoustic emission in some particular case. Finally,
in the evening when the wind is low, the measured polarized signals
seems to correspond to a diffuse seismic wavefield that would be the
Mars microseismic noise.