soil samples could be adversely affected by heating to approximately 50 °C and destroyed with long-term storage in the dark at 10 °C, as data suggest. Critiques James Lovelock argued that the Viking mission would have done better to examine the Martian atmosphere than look at the soil. He theorised that all life tends to expel waste gases into the atmosphere, and as such it would be possible to theorise the existence of life on a planet by detecting an atmosphere that was not in chemical equilibrium. He concluded that there was enough information about Mars' atmosphere at that time to discount the possibility of life there. Since then, methane has been discovered in Mars' atmosphere at 10ppb, thus reopening this debate. Although in 2013 the Curiosity rover failed to detect methane at its location in levels exceeding 1.3ppb. later in 2013 and in 2014, measurements by Curiosity did detect methane, suggesting a time-variable source. The ExoMars Trace Gas Orbiter, launched in March 2016, implements this approach and will focus on detection, characterization of spatial and temporal variation, and localization of sources for a broad suite of atmospheric trace gases on Mars and help determine if their formation is of biological or geological origin. The Mars Orbiter Mission is also attempting —since late 2014— to detect and map methane on Mars' atmosphere. A press commentary argued that, if there was life at the Viking lander sites, it may have been killed by the exhaust from the landing rockets. That is not a problem for missions which land via an airbag-protected capsule, slowed by parachutes and retrorockets, and dropped from a height that allows rocket exhaust to avoid the surface. Mars Pathfinder's Sojourner rover and the Mars Exploration Rovers each used this landing technique successfully. The Phoenix Scout lander descended to the surface with retro-rockets, however, their fuel was hydrazine, and the end products of the plume (water, nitrogen, and ammonia) were not found to have affected the soils at the landing site. Present and future missions The question of life on Mars will probably not be resolved entirely until future missions to Mars either conclusively demonstrate the presence of life on the planet, identify the chemical(s) responsible for the Viking results, or both. The Mars Science Laboratory mission landed the Curiosity rover on August 6, 2012, and its goals include investigation of Martian climate, geology, and whether Mars could have