Passage III

Introduction

Methane (CH₄) has been detected recently in Mars's atmosphere at an average concentration of 10 parts per billion by volume (ppbv). Above localized areas near Mars's equatorial region, higher CH₄ concentrations up to 250 ppbv have been detected. (On Earth, CH₄ is present in the atmosphere at an average concentration of 1,700 ppbv. )

In Mars's atmosphere, intense UV radiation promotes the reaction of CH₄, with hydroxyl radicals (OH) to form CO₂ and H₂O. However, Mars's thin atmosphere means that OH is scarce. Consequently, a molecule of CH₄ can remain in Mars's atmosphere for up to 340 years. Therefore, the CH₄ recently detected must have entered Mars's atmosphere within the past 340 years.

Four students discuss the possible source of the CH₄ in Mars's atmosphere.

Student 1

CH₄ is generated by a hydrothermal process operating a short distance beneath the surface of Mars. Hot fluids circulate through basalt (an igneous rock) and alter it to produce a mineral known as serpentine. This alteration produces H₂, which then reacts with CO₂ to produce CH₄. The CH₄ rises slowly to the surface, where it enters the atmosphere.

Student 2

CH₄ is put into Mars's atmosphere by comets. Comets are composed of several frozen materials, one of which is CH₄. Friction heats the comets as they enter the atmosphere. This heat vaporizes the frozen materials, releasing CH₄ and other gases into Mars's atmosphere.

Student 3

Large deposits of the material known as methyl hydrate are present at specific locations beneath Mars's surface. In this material, CH₄ molecules are held inside a cage of water ice molecules. When the deposit is disturbed, or heated slightly, the CH₄ is released from the cage. Once released, the CH₄ rises slowly to the surface, where it enters the atmosphere.

Student 4

CH₄ is produced through the activity of anaerobic bacteria that are present a short distance below Mars's surface. These bacteria, which do not require oxygen, generate CH₄ from CO₂ and H_2, or from CO and H₂O. Most of the CH₄ in Earth's atmosphere is produced in the same ways by the action of anaerobic bacteria.