Alzheimer's Precursor Protein and the Use of Bathocuproine for Determining Reduction of Copper(II)

the species to the total cross section of the entire atmosphere does. "Total cross section" refers to the product of the number density and the actual cross section (consider the trivial case of a mono-species atmosphere where changing the cross section of the species has no effect because only that species can be sputtered). At the altitudes of interest, CO2 is the dominant (75%) constituent. Decreasing its cross section will also decrease significantly that of the entire atmosphere, and the net effect of the decrease will be much smaller. For channel I, the effective decrease is only about a factor of 2; for channel II, it is about a factor of 4. The effect of changing the cutoff energy for dissociation does not linearly affect the collision frequency. With regard to the results of our model, 85% of the 0 in collisions that are above the dissociation threshold stated in our report ( I ) are above that of channel I, and 40% are above that of channel II. In the affected collisions, most of the energy goes into breaking the molecule, so almost none of the particles from such collisions have sufficient energy to escape, further reducing the effects of changing the cutoff energies. Our revised model indicates that the higher cutoff energies have no effect in channel I and results in a factor of two decrease in yield for channel II. While channel I does not produce C directly, it is still relevant to the process. First, the mass of CO is less than a third that of CO2. It thus has a significantly lower escape energy, and even partially dissociating the CO2 increases the sputtering yield. Second, the mean number of collisions that an escaping CO2 fragment undergoes is about four (and over 75% undergo at least two collisions). Thus, while the CO2 will not be completely dissociated in its first collision, subsequent ones will finish it. The C will end up escaping in elemental form as our original model assumed, giving it the significant mass advantage that accounted for so much of the difference between our results and those made earlier (2, 3). This advantage allows channel I (which does not directly produce C) to contribute significantly to the final sputtering yield. These two effects allow channel I to be less efficient than complete dissociation by only a factor of 2. As in our report, we used the impacting ion flux used by Luhmann (2) and Jakosky (3). There is a feedback between the escaping flux and the impacting flux, but this does not affect the actual sputtering yields. Calculated total CO2 loss is affected, but there are other, larger uncertainties in the impacting flux. There are two typographical errors in table 2 of our report (1). The Exospheric 0 value at 1 EUV should read 8 x 1025 and the Escaped H20 at 6 EUV should read 1.9 x 1028. Neither error affects the calculations or figure 1 of our report (which were done with the correct values). With both channels contributing to the sputtering of C (either as C or as CO), our revised model implies a loss of about 1 bar of CO2 (not -3 bar, as we originally stated). This decrease can be attributed to the decrease in effective yield of channel I by a factor of four and that of channel II by a factor of eight relative to the cross section stated in our report (1). This is only a small part of the total difference between our results and those of Luhmann (2) and Jakosky (3). One bar of CO2 is the canonical amount needed by greenhouse models to create the warmer, wetter early Martian atmosphere (4). Our revised results imply that, because of dissociation during collisions, sputtering is an important loss mechanism. D. M. Kass Y. L. Yung Division of Geological and Planetary Sciences, 170-25 Califomia Institute of Technology, Pasadena, CA 91125, USA