Measuring martian organics and methane: The Curiosity rover has been sampling on Mars for the past 5 years (see the Perspective by ten Kate). Eigenbrode et al. used two instruments in the SAM (Sample Analysis at Mars) suite to catch traces of complex organics preserved in 3-billion-year-old sediments. Heating the sediments released an array of organics and volatiles reminiscent of organic-rich sedimentary rock found on Earth. Most methane on Earth is produced by biological sources, but numerous abiotic processes have been proposed to explain martian methane. Webster et al. report atmospheric measurements of methane covering 3 martian years and found that the background level varies with the local seasons. The seasonal variation provides an important clue for determining the origin of martian methane. Science , this issue p. 1096 , p. 1093 ; see also p. 1068

The upper bound of 50 parts per trillion by volume for Mars methane above 5 km established by the ExoMars Trace Gas Orbiter, substantially lower than the 410 parts per trillion by volume average measured overnight by the Curiosity Rover, places a strong constraint on the daytime methane flux at the Gale crater. We propose that these measurements may be largely reconciled by the inhibition of mixing near the surface overnight, whereby methane emitted from the subsurface accumulates within meters of the surface before being mixed below detection limits at dawn. A model of this scenario allows the first precise calculation of microseepage fluxes at Gale to be derived, consistent with a constant 1.5 × 10⁻¹⁰ kg·m⁻²·sol⁻¹ (5.4 × 10⁻⁵ tonnes·km⁻²·year⁻¹) source at depth. Under this scenario, only 2.7 × 10⁴ km² of Mars's surface may be emitting methane, unless a fast destruction mechanism exists.

The migration of water molecules across the lunar surface from sources sufficiently small, so as to not generate a transient collisional atmosphere, was examined using a Monte Carlo simulation. Previous work using similar models is extended by examining a realistic distribution of large south polar Permanently Shadowed Regions (PSRs) to examine the heterogeneities observed in their water content—frost in Haworth and the nearby lowlands, but not in neighboring Shoemaker, and anomalously high hydrogen abundances from neutron data—and to determine how the pattern of water delivery can be distinguished from that predicted for transient collisional atmospheres. While the Haworth lowlands were seen to accumulate 3.9 times as much water as Shoemaker, the additional water delivered to Haworth crater was only 47% greater than Shoemaker, possibly due to shielding by Faustini and the lowlands. This relatively small difference is likely inadequate to explain the difference in frosting. More broadly, latitudinal trends in delivery dominate over meridional trends. Despite the heterogeneity in water delivery, fractionation effects from ballistic migration were small for the PSRs with equal D/H ratios seen in all PSRs within confidence limits. Finally, a diffusive barrier was observed starting at approximately 15° from each pole that hampers the poleward progress of equatorial particles. This barrier is the result of a reduction in the migration diffusivity from 1.0 × 10⁷ m² s⁻¹ near the equator to 5.5 × 10⁵ m² s⁻¹ within 10° of the pole and finally to 6.8 × 10³ m² s⁻¹ within 4° of the pole.

Experimental measurements of collision‐induced absorption (CIA) cross sections for CO₂‐H₂ and CO₂‐CH₄ complexes were performed using Fourier transform spectroscopy over a spectral range of 150–475 cm⁻¹ and a temperature range of 200–300 K. These experimentally derived CIA cross sections agree with the spectral range of the calculation by Wordsworth et al. (2017) however, the amplitude is half of what was predicted. Furthermore, the CIA cross sections reported here agree with those measured by Turbet et al. (2019, 2019). Additionally, radiative transfer calculations of the early Mars atmosphere were performed, and showed that CO₂‐CH₄ CIA would require surface pressure greater than 3 bar for a 10% methane atmosphere to achieve 273 K at the surface. For CO₂‐H₂, liquid water is possible with 5% hydrogen and less than 2 bar of surface pressure.