Late Noachian Icy Highlands: Spatial Distribution of Top-down Melt- Ing and Volumes of Meltwater for Single-year Warming

Introduction: We present here the next level of detail for the general calculation of top-down melting on supply-limited ice sheets for the Late Noachian Icy Highlands (LNIH) [1], in this case looking specifically at the spatial distribution of meltwater produced by PositiveDegree-Day melting at the ice sheet surface that results from a single-year warming. Knowledge as to where this meltwater is produced in relationship to the existing valley networks (VN) and open-basin lakes (OBL) can provide insight into possible mechanisms for their formation, as well as climatic conditions necessary to produce the required volumes of water thought to have fed the VN and OBL. Reasonable supply limits on the amount of water available to grow LNIH ice sheets range from 1 to 5 times the present available water [2]. Using a lapse-ratedominated climate with an ice-stability elevation predicted by GCM results for a denser Martian atmosphere [3], ice sheets are cold-based [4] and incapable of producing the large volumes of liquid water necessary to form VN or OBL, features and deposits known to have existed in this region. We investigate how such water might be released by surface melting during short-lived climatic warmings due to large meteor impacts (e.g., [5]) or periods of intense volcanism (e.g., [6]). For the minimum estimate of the magnitude of meltwater required, we assume that all the OBLs were filled simultaneously, and adopt as our target the total volume of all of the filled OBLs measured by [7], ~0.42 Mkm, a value less than 10% of the current (1X) surface/near-surface ice volume [2]. Results: With our relatively simple climate parameterization, short-lived warmings are handled by shifting the base temperature, which produces a uniform shift of the global temperature field. Simultaneously, the mass balance as a function of elevation is adjusted by a combination of a specified lapse rate (2.4 K/km) and a positive-degree-day factor [8.9] of 1.05 mm/PDD. Here we investigate the distribution and amount of melting one might obtain from a single-year melting event, using our target value of 0.42 Mkm to assess plausible scenarios. Extracting and calculating temperature from our climate parameterization for all points that are ice covered for our 1X, 2X, and 5X ice sheet simulations, we impose various degrees of warming by offsetting the base temperature in the climate parameterization. We impose a seasonal swing of 40 K (+/-20), analogous to that observed in the McMurdo Dry Valleys, where the mean annual temperature is 253 K and where seasonal melting is known to occur [10-11]. Applying this seasonal cycle we count PDDs and compute a melting for each point for various PDD factors ranging from high albedo snow (1.05 mm/PDD) to low albedo ice (2.55 mm/PDD) [9]. Summing these over the area of our simulated ice sheets we obtain potential mass losses for a single year with warmer temperatures for various warmings and PDD factors. Figure 1 shows volume losses for our three supplylimited ice sheets as a function of warming. Also shown is the target volume of 0.42 Mkm [7], representing the total volume of all open-basin lakes. Clearly, with a snow surface (PDD factor of 1.05 mm/PDD, the bottom solid line in each figure) it is difficult to obtain our target volume for any of the simulated ice sheets. The 2X ice sheet reaches the target, but only PDD factors typical of an ice surface, and even with that, it requires unreasonably large warmings (36, 38, and 41 C for PDD factors of 2.55, 2.25, and 1.95 mm/PDD respectively). For the 5X ice sheet, an ice surface PDD factor of 2.55 mm/PDD obtains the target volume loss with the least warming, 31 K, yielding an average temperature for the ice sheet surface close to the melting point. These results are summarized in Table 1, which also includes the average melt, temperature over the ice sheet, and number of PDDs, as