Thermal anomalies suggest that ongoing clathrate dissociation in icy sediment contributes to martian atmos-

Introduction: The likelihood that dissociating methane clathrate hydrate makes a significant contribution to martian atmospheric methane has been reported by [1-3]. The clathrate might be found in or beneath subsurface permafrost [1, 3-4], in surface ice such as glacial remnants [2], or, as reported here, in icy sediments. These reports suggest that triggers for clath-rate dissociation in recent times may include glacial retreat [1], sublimation of surface ice [2], or`subsur-face movement of brine [3]. The merits of these hypotheses , and geomorphological indicators of clathrate presence and dissociation in various settings are described in [5-8]. In the current work, we report thermal evidence that slope erosion and instability in icy sediment may also trigger clathrate dissociation and methane release. We describe thermal anomalies associated with theater-headed valleys in sediment-filled craters, and hypothesize that the thermal anomalies indicate ongoing clathrate dissociation. Clathrate dissociation: Dissociation of clathrates (gas hydrates) can be caused thermodynamically by decreasing pressure or increasing temperature; or chemically, by introduction of chemical inhibitors such as salts [9]. Mechanical disruption of the clathrate lattice structure can also cause dissociation [10], although in a hydrate stability zone (HSZ), re-association will occur unless the released gas escapes to the surface. Dissociation is endothermic, which implies that areas of active dissociation, and attendant methane release , might be found by locating anomalously low temperatures in association with suggestive surface morphologies, if the thermal effect is not masked by burial under too great a depth of insulating materials. Thermal observations: In the Mamers Valles region , there are tributary channels with theater-shaped heads. They contain lineated valley fill (LVF), and are sometimes not far from degraded valley networks that look like runoff valleys. Theater-headed valleys have been described as analogous to glacial cirques [e.g. 11] or the heads of sapping valleys [e.g. 12]. In most such valleys that have been examined, there is no distinctive thermal signature, but some of the tributaries that are found in filled craters have cold thermal anomalies in the valley head or along the valley walls (Figure 1). These thermal anomalies could be a result of grain size differentiation, but in that case one would expect to see the thermal effect in all similar theater-headed tributaries. A thermal anomaly might also indicate differentiation in the materials on the floor of the valley,