Elsevier Editorial System(tm) for Icarus Manuscript Draft Manuscript Number: ICARUS-13370R1 Title: Pluto's Implications for a Snowball Titan Article Type: Special Issue: The Pluto System Keywords: atmospheres, pluto, titan, atmospheric evolution Corresponding Author: Mr. Michael L Wong, Corresponding Author's Institution: California Institute of Technology First Author: Michael L Wong Order of Authors: Michael L Wong; Yuk L Yung; Randy Gladstone Abstract: The current Cassini-Huygens Mission to the Saturn system provides compelling evidence that the present state of Titan's dense atmosphere is unsustainable over the age of the Solar System. Instead, for most of its existence, Titan's atmosphere might have been in a Snowball state, characterized by a colder surface and a smaller amount of atmospheric CH4, similar to that of Pluto or Triton. We run a 1-D chemical transport model and show that the rates of organic synthesis on a Snowball Titan are significantly slower than those on present-day Titan. The primary method of methane destruction—photosensitized dissociation in the stratosphere—is greatly dampened on Snowball Titan. The downward flux of higher-order molecules through the troposphere is dominated not by hydrocarbons such as ethane, as is the case on Titan today, but by nitriles. This result presents a testable observation that could confirm the Snowball Titan hypothesis. Because Pluto's atmosphere is similar to Titan's in composition, it serves as a basis for comparison. Future observations of Pluto by the New Horizons Mission will inform photochemical models of Pluto's atmosphere and can help us understand the photochemical nature of paleo-Titan's atmosphere. Detailed Response to Reviewers Title: Pluto’s Implications for a Snowball Titan Authors: Michael L. Wong; Yuk L. Yung; G. Randall Gladstone Dear Editor, We’d like to thank our reviewers for their helpful and insightful comments and suggestions. In what follows, the reviewer’s comments have been reproduced in italics. Following a vertical bar, our responses are printed in bold. Where we have cited the revised manuscript, it appears in bolded blue. Since the manuscript has been significantly revised, the line numbers in the reviewer’s comments—which refer to the original document—may no longer be accurate. We appreciate the opportunity to resubmit this paper with major revisions, and we look forward to your response and a chance to improve the writing again, if necessary. Thanks, Michael L. Wong, Yuk L. Yung, & G. Randall Gladstone Reviewer #1: This paper is on an interesting and timely topic, however requires substantial extension and revision, particularly in the Discussions & Conclusions section. I like the topic and find the results presented interesting, so I hope it will be re-submitted. | Thanks for your constructive criticisms. We hope that the revisions and extensions made have better prepared this paper for publication. The most pressing issue is that Discussions & Conclusions section is lacking depth and leads the reader hanging. The earlier sections of the paper are well written and present an interesting and novel investigation. But, then the Discussions & Conclusions section leaves the reader hanging without much new insight. It would be interesting and useful to make predictions for what New Horizons will see at Pluto. And/or, to present a few scenarios of what NH might see at Pluto and what that will mean for their paleo-Titan model or our understanding of paleo-Titan. Additionally, it would be interesting to point toward what future Titan observations (or missions) would have the most impact on the understanding presented here and how they could confirm or revise the understanding. Comments & Recommendations: The Discussion & Conclusions section needs to be substantially expanded. | We have added several paragraphs to the Discussion & Conclusions section that covers a comparison with Pluto atmospheric modeling as well as potential results from New Horizons and future missions. We have also bolstered the discussion of the results in the Results section. Please see paper. The abstract needs revising. This would make a solid abstract for a DPS talk, but a paper abstract is most useful when it is a summary of the work, not an advertisement to encourage people to see a talk (or read a paper). e.g. don't say "We discuss what we may learn...", but actually summarize in a sentence or two what we may learn. To the extent possible include the numerical results, e.g. don't just say "rates...shown to be significantly slower", attach a number (or range of numbers) to it. I would also recommend significantly condensing the initial three long introductory sentences down to a shorter sentence or two. There isn't enough room in an abstract to give the full background (that's what the introduction is for) and a non-specialist will go to the introduction to get the background information, while a specialist in the field doesn't need that background. The abstract is ideally a concise listing of the main new results of the paper. A specialist in the field should ideally be able to glean all the important new results simply from the abstract. | The abstract has been significantly revised. It now reads: “The current Cassini-Huygens Mission to the Saturn system provides compelling evidence that the present state of Titan’s dense atmosphere is unsustainable over the age of the Solar System. Instead, for most of its existence, Titan’s atmosphere might have been in a Snowball state, characterized by a colder surface and a smaller amount of atmospheric CH4, similar to that of Pluto or Triton. We run a 1-D chemical transport model and show that the rates of organic synthesis on a Snowball Titan are significantly slower than those on present-day Titan. The primary method of methane destruction—photosensitized dissociation in the stratosphere— is greatly dampened on Snowball Titan. The downward flux of higher-order molecules through the troposphere is dominated not by hydrocarbons such as ethane, as is the case on Titan today, but by nitriles. This result presents a testable observation that could confirm the Snowball Titan hypothesis. Because Pluto’s atmosphere is similar to Titan’s in composition, it serves as a basis for comparison. Future observations of Pluto by the New Horizons Mission will inform photochemical models of Pluto’s atmosphere and can help us understand the photochemical nature of paleo-Titan’s atmosphere.” Citations: While in general the paper does a good job of citing previous work to support its statements, there are a few places needing additional references. (I don't take issue with any of these statements, they just need to be supported.) First two sentences of introduction: reference(s) needed to support the claim that Titan's current atmosphere is probably transient and that Cassini provided the evidence. Further down in first paragraph: reference(s) needed to support the claim that the known lakes are not of large enough volume to be the methane buffer reservoir. First sentence of Model section: needs reference to the Caltech/JPL CTM, or at least other papers where substantially the same version has been used. | We have added citations as recommended. Comment: Paragraph 3 is a particularly nice concise summary of the aspects of the photochemistry that the authors want to convey to the reader. | Thanks! We appreciate the positive feedback. page 7: prediction of ethane fluxes: It's worth noting that although the predicted ethane flux is much less than for modern day Titan, it's still significant. If modern day fluxes predict ethane oceans "hundreds of meters" deep, then at the paleo-rate of 2.7% of modern day there are still multiple meters, or even tens of meters, of ethane to deal with. That's still much more than the upper limit on observed volume of surface ethane in the lakes and seas. So, it'd be worth suggesting that the ethane would need to be absorbed into the surface. | We have added to and rewritten a portion of Section 3.1: “Modern Titan has a downward ethane flux at the tropopause of ~3×109 molecules cm-2 s-1; over ~4 Gyr, this would generate a global ocean of liquid ethane hundreds of meters deep. Our calculations show that on paleo-Titan, the downward ethane flux at the tropopause is reduced to ~8×107 molecules cm- 2 s-1, which produces only a ~2 m global layer of ethane over Titan’s history. Although this is still a significant body of liquid that no longer exists on Titan’s surface, it is much more plausible that it could almost entirely be removed (via absorption into porous regolith and/or in the form of clathrate hydrates, for instance) and result in the ~36 cm globally averaged layer of liquid we observe on Titan. Thus, the absence of a deep global ocean of hydrocarbons is consistent with the supposition that Titan existed in a Snowball state throughout most of its history.” page 8: This page of the results is ripe with items that would be usefully summarized in the abstract: e.g. lines 124-126 on the photodissociation of methane vs. photosensitized dissociation. e.g. lines 137-140 on the mass fluxes through the troposphere | We have included these key results in the abstract. p5: "frostbite" is an odd word here since that refers to a medical condition. "ice age" is not quite right either, but is better. or perhaps "deep freeze"? | We like the frostbite metaphor. Figure 1: Overplotting Titan's present-day profile would make this figure much more informative, particularly for readers less familiar with Titan. | We have added Titan’s present-day temperature profile to the figure. Figure 2: Expand caption to include sources of plotted observational data.| We have expanded the caption to include the sources of plotted observational data.