Limits on the trapping of atmospheric CH4 in martian polar ice analogs

Recent detection of methane (CH4) on Mars has generated interest in possible biological or geological sources, but the factors responsible for the reported variability are not understood. Here we explore one potential sink that might affect the seasonal cycling of CH4 on Mars – trapping in ices deposited on the surface. Our apparatus consisted of a high-vacuum chamber in which three different Mars ice analogs (water, carbon dioxide, and carbon dioxide clathrate hydrates) were deposited in the presence of CH4 gas. The ices were monitored for spectroscopic evidence of CH4 trapping using transmission Fourier-Transform Infrared (FT-IR) spectroscopy, and during subsequent sublimation of the ice films the vapor composition was measured using mass spectrometry (MS). Trapping of CH4 in water ice was confirmed at deposition temperatures <100 K which is consistent with previous work, thus validating the experimental methods. However, no trapping of CH4 was observed in the ice analogs studied at warmer temperatures (140 K for H2O and CO2 clathrate, 90 K for CO2 snow) with approximately 10 mTorr CH4 in the chamber. From experimental detection limits these results provide an upper limit of 0.02 for the atmosphere/ice trapping ratio of CH4. If it is assumed that the trapping mechanism is linear with CH4 partial pressure and can be extrapolated to Mars, this upper limit would indicate that less than 1% is expected to be trapped from the largest reported CH4 plume, and therefore does not represent a significant sink for CH4.