Climate optimum on Mars initiated by atmospheric collapse

Edwin S. Kite1, Michael A. Mischna2, & Peter Gao3

1. University of Chicago (kite@uchicago.edu). 2. Jet Propulsion Laboratory, California Institute of Technology. 3. NASA Ames Research Center.

Abstract.The progressive drying-out of Mars’ surface was punctuated by a dramatic transient increase in fluvial erosion around the Noachian-Hesperian boundary (~3.7 Ga). Standard explanations of this climate optimum appeal to volcano- or impact-triggered climates and imply that individual runoff episodes were brief, apparently inconsistent with evidence for persistent runoff. We examine a scenario in which the duration, intensity and uniqueness of the Noachian-Hesperian climate optimum result from degassing of CH4 -clathrate consequent to Mars’ first atmospheric collapse. Atmospheric collapse causes low-latitude surface water ice to sublimate away, depressurizing and thus destabilizing CH4 clathrate in subglacial pore space. Subsequent atmospheric re-inflation leads to further warming and further destabilizes CH4 clathrate. CH4 -induced warming is efficient, permitting strong positive feedbacks, and possibly raising Mars into a climate optimum. The optimum is brought to a close by photolysis of CH4 , and drawdown of the CH4-clathrate reservoir prevents recurrence. This scenario predicts a 105-106 yr climate optimum, transient connections between the deep hydrosphere and the surface, and strong surface weathering, all of which are consistent with recent observations. Crustal hydrothermal circulation very early in Mars history could yield CH4 that would be incorporated into clathrate on approach to the cold surface. The scenario explains why regional watershed integration on Mars occurred relatively late and only once, and suggests that the contrasts between Noachian versus Hesperian climate-sensitive deposits on Mars correspond to a transition from a never-collapsed atmosphere to a collapse-prone climate, ultimately driven by slow loss of CO2 to space.

1. The problem: an anomalous wet-climate episode on Mars.When Mariner 9 reached Mars orbit in 1971, it was soon realized that ancient Mars terrain is crisscrossed by networks of dry valleys. Today, valley-network formation around the Noachian/Hesperian boundary (3.8-3.6 Ga) is recognized as the biggest anomaly in Mars’ geomorphic record of global drying (Irwin et al. 2008). The valleys fed precipitation-fed lakes covering 2-20% of the ancient terrain (Fassett & Head 2008a, Matsubara et al. 2011). Lakes spilled over to form regionally-integrated watersheds (Warner et al., Goudge et al. 2016). This regional integration of watersheds is unique in the geomorphic record, and in sharp contrast to smaller-scale fluvial integration both before (Howard et al. 2005, Irwin et al. 2013) and subsequently (e.g., Zabrusky et al. 2012, Goudge et al. 2016). Valley depths suggest that, during the climate optimum, column runoff totaled >10-100 km (SI). This water could not have been delivered catastrophically, because some but not all lakes overflowed; a hydrological cycle of likely duration >105 yr (Barnhart et al. 2009) or 105-107 yr (Hoke et al. 2011) is inferred.

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Because more of the planet’s surface was covered by liquid water than at any other time in Mars’ discernable history, we refer to this as a climate optimum.

Given that Mars’ valley-network-forming climate is the most habitable known extraterrestrial surface environment, and widespread interest in the physical basis of sustained surface habitability (Kasting & Catling 2003), it is perhaps surprising that there is currently no physical model that accounts for the distinctiveness, late start, and relative brevity of the Mars climate optimum (Howard et al. 2005, Irwin et al. 2005). The climate optimum must be connected in some way to the evolution of Mars atmospheric CO2, because today Mars’ atmosphere is too thin (atmospheric pressure ≈ partial pressure of CO2, PCO2 = 6 mbar) to permit extensive liquid water, and past PCO2 was much greater, due to gradual and irreversible atmospheric loss by carbonate formation and escape-to-space (Jakosky & Phillips 2001). Because the rate of atmospheric escape-to-space fell off more quickly than the rate of volcanic outgassing, some models yield a broad maximum in PCO2 around the Noachian-Hesperian boundary (Tian et al. 2009). However, CO2–warming alone is insufficient to explain the climate optimum (even with H2O-vapor feedback; Mischna et al. 2013). What was the source of additional warming? Why was it limited to the observed interval around the Noachian/Hesperian boundary? Why is Mars’ surface little-weathered, despite the evidence for lakes and rivers? Why was the uptick in warming not repeated?

Previously proposed drivers include bolide impacts, volcanic eruptions, true polar wander, climate cycles, and atmospheric thickness maxima. However, almost all large bolide impacts on Mars predate the climate optimum (Robbins et al. 2013), and for bolide impact to kick-start a hydrological cycle, the impact must access a metastable climate state for which there is no uncontested evidence (Wordsworth 2016). Volcanic SO2 warming is weak and ephemeral (Kerber et al. 2015). Rapid polar wander (inertial-interchange) is unlikely (Matsuyama & Manga 2010). Climate cycles involving rapid silicate weathering during wet intervals (Batalha et al. 2016) predict >3 × 104 kg/m2 carbonate in Noachian/Hesperian boundary sediments, which is not observed.

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Fig. 1. Upper panel: geomorphic history of Mars. Lower panel: magma flux (red; model) and crater flux (semi-empirical). The VN timing is from Fassett & Head (2008) using the Neukum topography. The pre-optimum erosion rate is from Irwin et al. (2013). The valley network fluvial erosion rate is from Williams & Phillips (109 m average depth, assumed duration 1 Myr) (2001). The Hesperian deposition rate (3 microns/yr) estimate is from Zabrusky et al. 2012, assuming fluvial sediment transport. The Amazonian upper limit is from Golombek et al. 2014. (<10 nm/yr). Golombek et al. 2014 also argue for 1 micron/yr erosion rates in the Middle-Late Noachian. Data and models for magma flux and crater flux are consistent with monotnic decrease, unlike the geomorphic record.

Here we propose a new scenario for the climate optimum, that makes use of recent advances in the understanding of Mars’ CO2 and H2O cycles. The scenario tracks the transient release of CH4 gas, released by depressurization and warming of CH4 clathrate. CH4 clathrates are a driver of climate change on Titan (Tobie et al. 2006), and perhaps during Phanerozoic Earth history (Bowen et al. 2015). Yet CH4 clathrates are rarely invoked as greenhouse agents on Mars, in part because CH4 has a short photochemical lifetime (Sagan 1977, Chassefière et al. 2016). Recent work shows that relatively brief pulses of liquid water are consistent with data, and shows that the warming potential of CH4 on Early Mars has been underestimated (Wordsworth et al. 2016, Kite et al. 2016), indicating that this dismissal may have been premature. The collapse-trigger mechanism explains why valley incision occurred around the Noachian/Hesperian boundary but not thereafter, explains the duration of the event, seems consistent with other orbiter and rover data, and makes several testable predictions.

2. Recent advances in the understanding of Early Mars set the stage for the scenario.The collapse trigger scenario is motivated by evidence for the following:- (i) Noachian water-rock reactions in the deep subsurface; (ii) a 0.5 bar past atmosphere that (iii) ≳drove surface water ice to high ground; and (iv) chaotic obliquity variations. Understanding of all four has improved in recent years.

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(i) Noachian water-rock reactions are recorded by hydrothermal minerals (e.g. Ehlmann et al. 2010). Hydrothermal circulation would cool the mid-crust, reconciling thermal-evolution models with the pressure-temperature stability fields of observed hydrothermal minerals (Parmentier & Zuber 2007, Carter et al. 2013, Sun & Milliken 2015). Reactions between Mars’ mafic/ultramafic crust and waters charged with magmatic and/or atmospheric C should yield H2 and CH4 . We emphasize CH4 here and assume abiotic production (Etiope & Sherwood-Lollar 2013). Kinetic barriers to CH4 production (McCollom 2016) can be overcome by high temperatures, cycling of fluid by hydrothermal circulation, and catalysts (e.g, Co, Ni, Ru). Reaction-rate would slacken as the Noachian progressed and the geotherm cooled: almost all evidence for hydrothermal circulation predates the Noachian-Hesperian boundary (Ehlmann et al. 2011, Fassett & Head 2011). On approach to the cooling surface (e.g. beneath ice sheets or primordial seas), CH4(±C2H6) produced by water-rock reactions would be trapped into clathrates.

CH4 clathrate is stabilized by increasing pressure (stable for P ≥ 21 bar at 273.15K), and by decreasing temperature (stable for P ≥ 0.15 bar at 200 K). Once formed, clathrate could be destabilized by orbital forcing (Prieto-Ballesteros et al. 2006, Kite et al. 2016) or by other mechanisms (this paper). CH4-clathrate destabilization on Mars has been proposed as an explanation for chaos terrain (Rodriguez et al. 2006) and mounds interpreted as mud volcanoes (Oehler & Allen 2010).

(ii) Most estimates of PCO2 at the time of the valley networks are in the range 0.2 – 2 bar (Hu et al. 2015, Kite et al. 2014). This is thick enough that surface temperatures follow the atmospheric thermal lapse rate (Wordsworth et al. 2013).

(iii) Therefore, Tsurf is anticorrelated with elevation. As a result, H2O ice would be located at high ground (Wordsworth et al. 2015). High ground on Mars is mostly at low latitude. Average ice sheet thickness, assuming ice above +1 km elevation, was 400-700 m (Fastook & Head 2015, Carr & Head 2015, Mahaffy et al. 2015). This thickness is sufficient to stabilize CH4 clathrate in the regolith beneath the ice sheet.

(iv) Unlike Earth, Mars undergoes infrequent chaotic large-amplitude obliquity shifts (0-70°, Laskar et al. 2004) on which are superposed quasi-periodic obliquity variations of period 105-106 yr and amplitude 0-20° (10× Earth).

3. An overview of the climate optimum scenario.The proposed scenario is shown in Fig. 2.

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(a)

(b)

(c)

Fig. 2. Overview of the collapse-triggered climate optimum scenario (section 3). (a) Left panel: schematic of the long-term evolution of a column of the Mars highlands. Right panel: obliquity diffusion and slow CO2 loss lead to polar temperatures dropping below a pressure-dependent

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critical value for atmospheric collapse initiation, φc. (b) Below φc, 90-99.9% of the atmosphere will condense in 1-10 Kyr (step 1). This unloads high ground (step 2), releasing CH4 from sub-ice clathrate. Reinflation of the atmosphere leads to climate optimum (step 3). (c) Methane clathrate phase diagram showing pathways to charge-up and release. Phase boundaries shown in dark blue. Mars geotherms shown in red (early, steep geotherm) and cyan (later, shallow geotherm). Early in Mars history, cooling of the geotherm locks-in CH4 as clathrate in regolith beneath ice sheets (AB). Further geotherm cooling and escape of ice-sheet water to space (BC) has little effect on CH4-clathrate stability. Atmospheric collapse and consequent unloading causes CH4 breakdown (CD). Warming of the surface moves the regolith deeper into the CH4-clathrate destabilization region (DE).

Fig. 3. Atmospheric collapse on Mars drives water-ice distribution. Thin black lines show annual-mean polar temperature as a function of atmospheric pressure (calculated using the GCM of Mischna et al. 2013). Thick black line is the condensation curve for CO2; atmospheres below this line are collapsing onto polar CO2 ice caps (e.g., AB). Blue dashes outline the approximate pressures and obliquities below which H2O ice is stable only at Mars’ poles (e.g. Mischna et al. 2003, Wordsworth et al. 2015). Collapse leads to relocation of surface water ice from highlands to poles. For an initial CO2 -substance inventory of 8×1018 kg (= 2 bar), the atmosphere is stable until φ ≤ 15 deg (at A). Rapid collapse then moves the system to point B. Increasing obliquity (over ~1-5×104 yr) moves the ice-cap/atmosphere system along the condensation curve to C, (the highest φ consistent with permanent CO2 ice caps). Further φ rise leads to sublimation of the CO2 -ice-cap (103 yr) and the system returns to A.

Step 1. Collapse of an initially-thick CO 2 atmosphere. As the atmosphere is thinned by escape-to-space and carbonate formation, the remaining atmospheric CO2 becomes increasingly vulnerable to collapse (Fig. 3). Collapse is triggered if polar CO2 -ice caps grow from year to year. Cap growth occurs below a critical polar temperature. Polar

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temperature is set by insolation, which increases with obliquity, and by heating from the CO2 atmosphere (both the greenhouse effect, and equator-to-pole heat transport, increase with atmospheric pressure). Given secular atmospheric-pressure decline, obliquity variations will eventually lower insolation below the threshold for perennial CO2-ice caps. Once nucleated, caps grow quickly (103-104 yrs; Soto et al. 2015) because ice-cap sequestration of CO2 further reduces polar temperatures. Collapse typically sequesters 90-99.9% of the planets’ CO2 . Post-collapse PCO2 is in equilibrium with polar-CO2-ice-cap surface temperature - i.e., very low (Sagan 1973) (Fig. 3). CO2 -ice cap thickness (assuming caps poleward of 80°) is ~1200 m/bar CO2. Atmospheric collapse involves a hysteresis (Fig. 3, Fig. S2), but is ultimately reversible. CO2 trapped as CO2 ice is only sequestered temporarily. Eventually, obliquity rise will trigger rapid re-sublimation of CO2 condensed at low obliquity. Many collapse-reinflation cycles should have occurred in Mars history (Soto et al. 2015), and there is strong radar evidence for collapse <1 Mya (Bierson et al. 2016). However, Mars’ small total CO2 inventory at present precludes the 102-103 fold swings in pressure that should have occurred on Early Mars. Bedforms with wavelengths suggestive of PCO2 < 10 mbar at ~3.7 Ga hint that the first collapse was >3.7 Ga (Lapôtre et al. 2016).

Cooling of the surface due to loss of the greenhouse effect helps to stabilize CH4 -clathrate, outweighing the destabilizing effect of loss of the weight of the atmosphere. Therefore, no CH4 release occurs at this stage. Cold post-collapse conditions are unfavorable for liquid water (Hecht 2002). Correspondingly, we are not aware of any previous suggestion that atmospheric loss and atmospheric collapse could favor surface habitability (Wood 2012).

Step 2: H2O -ice unloading of low latitudes triggers CH4 release. Following Mars’ first-ever atmospheric collapse, the atmosphere can longer provide much heat to the poles. The cold poles are now the stable location for H2O ice. H2O ice condenses at the poles, and sublimates away from low-latitude highlands for the first time in Mars history. The low-latitude ice sublimates in <105 yr (net rate of 5-200 mm/yr; e.g. Madeleine et al. 2009, Fastook & Head 2015, Wordsworth et al. 2015). On this timescale H2O-ice glacial flow is negligible (Fastook & Head 2015). This latitudinal shift in the ice overburden is the first such shift in Mars history, and (in the collapse trigger scenario) it has an effect on CH4 -clathrate decomposition that is unsurpassed. (For an Earth analog, see MacAyeal & Lindstrom 1990). Unloading of the highlands due to removal of the H2O-ice load causes irreversible decomposition of sub-ice within-regolith pore-space CH4 clathrate. The corresponding CH4 (g) release is g × f × 7000 kg/m2, where g is the fraction of surface area initially shrouded by ice. g = 0.X – 0.X in published models (Wordsworth et al. 2015, Mischna et al. 2003). (Although the polar (H2O+CO2)-ice cap overburden and insulation net-stabilizes polar clathrate, this covers <10% of the planet’s surface area and this stabilization is ignored in the calculation). The clathrate is charged up with CH4 produced by water-rock reactions over >108 yr (Fig. 2a). It is released in <103 yr (Methods; Stern et al. 2003). Once in the atmosphere CH4 is

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photolyzed at a rate of 0.1 mbar/Kyr (SI) (Kite et al. 2016). However, CH4 is not effective at warming Mars at this stage. This is because CH4’s principal greenhouse band at 7.7

m is mismatched with the cold-Mars Planck function (which peaks at ~15 m). μ μ CH4-CO2 Collision-Induced Absorption (CIA) is also ineffective because the collapsed-atmosphere PCO2 is low (Wordsworth et al. 2016). The collapsed-atmosphere CH4/CO2 ratio may exceed the threshold for forming haze, which will scatter sunlight and further suppress temperatures.

(a) (b)Fig. 4. (a) Map of pre-collapse depth (below the top of the regolith), in meters, to the CH4 -clathrate stability zone. Blue color shows stabilization of highland clathrate by H2O -ice-sheet overburden. Black line corresponds to 27°E, for which cross-section is shown in (b). (b) Blue line corresponds to a pre-collapse clathrate stability boundary. Red line corresponds to cold, post-collapse conditions, assuming complete H2O movement to the poles. Yellow line corresponds to re-warmed, post-reinflation conditions, assuming H2O has not yet returned from the poles, but not including effect of CH4 warming. Green line corresponds to an additional 10K of warming (e.g., due to the greenhouse effect of CH4-CO2 CIA). Step 3. CO 2 -atmosphere reinflation and further CH 4 release. Mars’s first atmospheric collapse occurs at the bottom of a (20° amplitude) quasi-periodic obliquity cycle (SI, Figure). As the obliquity cycle continues, the collapsed atmosphere is still too thin to warm the poles – a hysteresis effect (Fig. S2). Therefore, PCO2 rises slowly in vapor-pressure equilibrium with polar temperature, and with most CO2 sequestered in ice. PCO2

rises in equilibrium with CO2 -ice temperature because, although stiff H2O -ice may enshroud the CO2 -ice, flow of thick, soft CO2 -ice opens crevasses. When obliquity rises to the point where no value of pressure yields a polar temperature below the condensation point, the remaining polar CO2-ice caps rapidly (103 year) and completely sublimate (Fig. 5). Because the duration of quasi-periodic orbital cycles is known for the post-4.0 Ga solar system, the time between Mars’ first atmospheric collapse and reinflation is 1-5 × 104 yr for reasonable parameter choices. 1-5 × 104 yr is long enough for much or all of the H2O ice on high ground to sublimate away. Loss of the stabilizing H2O-ice overburden exposes clathrate to the CO2-greenhouse warming associated with

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reinflation, and clathrate irreversibly decomposes, releasing CH4. The wait time for CH4 clathrate decomposition is (for 100m depth-to-clathrate) (reinflation time + subsurface conductive-warming time + decomposition time) = (103 yr + 102 yr + <10 yr) = 103 yr. This is much quicker than the 104-105 yr needed for H2O ice to return to the low latitudes. In the simulations, the amount of CH4 released at this point is larger than in Step 2 (Fig. 4, Section 4). In consequence, the atmosphere now contains abundant CO2 , with a 1-5% CH4 – ideal conditions for strong CH4-CO2 CIA warming (Wordsworth et al. 2016).

Step 4. CH 4 + CO 2 = Mars climate optimum. The cocktail of circumstances enabled by Mars’ first atmospheric collapse (a massive CH4 pulse, a thick CO2 atmosphere, low-latitude water ice) now permits low-latitude rivers and lakes. H2O snow falling on the equator (plus any H2O ice that did not have time to sublimate) encounters high insolation and high greenhouse forcing. Seasonal snowmelt runoff forms valleys that drain into perennial ice-covered lakes (McKay et al. 1985). Lakes overspill to form regionally-integrated valley networks. Lake-bottom warming is a further positive feedback on greenhouse warming. High lake-bottom temperatures destabilize sub-lake methane clathrates (Fig. 4). CH4-induced warming is strong to destabilize additional CH4

in the regolith (and under some circumstances this positive feedback can produce a runaway; section 4.2, Figure 6). With or without these feedbacks, CH4 can remain in the atmosphere at radiatively significant levels for 105-106 yr.

(a) (b)

Fig. 5. (a) Pressure evolution. Typical collapse-reinflation follows on from >>100 Myr of continuously high pCO2. CH4 /CO2 does not exceed 0.05. (b) Time series corresponding to the CH4 release shown in Fig. X. Red and blue corresponds to fit to MarsWRF GCM. Temperature evolution due to corresponding X mbar CH4 release. After reinflation, a ~100 Kyr-long, 10K warming occurs, bringing annual mean temperatures in the ±45° latitude, -1 to +3 km elevation zone to the Antarctic Dry Valleys range.

Step 5. CH 4 loss shuts down of wet climate in 10 5 -10 6 yr. The wet climate ends when atmospheric CH4 is destroyed by photochemical processes. So long as the CH4 persists,

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atmospheric re-collapse is unlikely, because the additional heating from the CH4 warms the poles during minima in the quasi-periodic obliquity cycle. As CH4 is destroyed without resupply, eventually the rivers dry up, and atmospheric collapse is again possible. (The number and total duration of atmospheric collapses on Mars is impossible to calculate from first principles, because Mars’ obliquity is chaotic. More geologic information is needed to constrain these parameters). Wet conditions persist for 105-106 yr, consistent with geomorphic estimates of the number of wet years needed to form the VNs. The required valley incision rate is 0.1-1 mm/yr, similar to the mountainous Western United States (≥0.15 mm/yr post-0.6 Ma; Dethier 2001). This duration reconciles the geomorphic data with mineralogic upper limits on the interval of surface liquid water (Olsen & Rimstidt 2007).

4. Results of a time dependent scenario.

4.1. Step-by-step description of one model run (discuss sensitivities and choices made).

[Step 1 para …][Step 2 para …][Step 3 para …]

4.2. Runaway clathrate breakdown solutions.CH4 is released by the warming triggered by CH4 release, and this can permit runaway clathrate breakdown (Fig 6). Runaway clathrate breakdown can only occur following H2O-ice unloading: so long as ice overburden is present, all CH4-clathrate within regolith is stable and so no runaway clathrate breakdown can occur. Runaway clathrate breakdown depends on f, PCO2 and Tinit. The required f for runaway clathrate breakdown increases with decreasing PCO2 (because the CIA warming per unit CH4 increases with PCO2 ), and increases for cooler pre-release temperatures.

We find the runaway clathrate breakdown threshold by comparing the direct and indirect warming. If the indirect warming exceeds the direct warming, then the system will run-away. Otherwise the asymptotic warming (gain, G) is given approximately by

G = 1/1-F, (1)

where F is the feedback factor (Roe 2009). Even when the system does not runaway, the positive feedback on CH4 induced warming can be very significant (Fig. S3).

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Fig. 6. Showing potential for runaway warming on Mars due to CH4 release from an f = (0.15 x porosity) reservoir. Colored dashed lines correspond to CH4 -induced warming from the calculations of Wordsworth et al. (2016). Solid lines correspond to the corresponding CH4 release (single-column model). When the warming for a given pCO2 plots above a CH4 release line, runaway results. When the warming for a given pCO2 plots above a CH4 release line, there is no runaway, but positive feedback still occurs. Circles highlight selected stable equilibria that terminate the runaway. The kink in the Tinit = 260K curve results from subsurface temperatures exceeding 273.15K (which marks a discontinuity in the CH4 -clathrate phase diagram). Asterisk highlights an unstable fixed point in the Tinit = 260K curve.

A runaway usually, but not always, leads to a temperate Mars climate (Fig. 6). However, even a runaway that releases enough CH4 to cause a temperate climate need not fully degas the Mars clathrate reservoir. That is because of the fCH4

1/3 dependence of greenhouse warming on CH4 concentration, a nonlinearity in the feeback that is not accounted for in (1). This may cause the warming-outgassing-warming cycle to self-arrest before the clathrate reservoir is fully depleted (Fig. 6).

A temperate Mars climate lasting ~1 Myr would explain the surface leaching event recorded by Al-clays globally (Carter et al. 2015).

5. Sensitivity to parameters. [not yet fleshed out]The most important parameter is f. f is affected by charge-up history and subsequent loss. Loss is minor. Charge-up: can happen beneath early ocean as well as beneath early ice-sheets.

The atmospheric collapse map is very important too: if trigger at too low a pressure, warming may be insufficient. Both the collapse map and the radiative effect of reducing

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gases in a global climate model are poorly quantified – needs more work (e.g. Kahre et al. 2013).g is less important.Can the hysteresis loop permit collapse persistence? Depends on atmospheric heat transport parameter. Albedo probably less important (seasonal CO2 ice …)

The locations of ice stability are robust and model-independent.The kinetics of ice unloading are model-dependent. The rate that we have used is from Madeleine et al. (2009). The true rates may have approached the energy-limited sublimation rate (>40× higher; Wordsworth et al. 2015). Even incomplete ice unloading (e.g. merely ‘trimming’ the margins of an ice sheet) can still be enough to trigger major change.

H2 produced by X and dissolved in aquifers is an alternative (solubility 10-3), but rapid release of climate-altering amounts of subsurface H2 requires the complete removal of the cryosphere (an impact, or very large-amplitude climate warming). Thus, H2 warming is conceivably of comparable amplitude to CH4 warming, but requires a much bigger kick to start.

6. Predictions.The first-collapse trigger scenario predicts a major shift in Mars’ Cl cycle at the time of valley-network incision. ClO4

-, which is present in Hesperian-to-recent Mars soils and sediments (Archer et al. 2016), builds up to the observed percent levels when Galactic Cosmic Radiation (GCR) is available (Wilson et al. 2016). GCR is not available before the first collapse because GCR are absorbed by the thick atmosphere. Therefore, detection of Noachian ClO4

- would disprove the model.

The collapse trigger scenario predicts an absence of Noachian large wind ripples (Lapôtre et al. 2016), because these form only at low atmospheric density. Detection of such wind ripples in Noachian deposits would disprove the scenario.

The scenario would be falsified by finding a D < 50 m craters that predate the valley networks (Kite et al. 2014). If PCO2 was high before the valley networks as required by the model, small impactors would be screened by the atmosphere. The scenario would also be falsified by detection of more than a few impact craters (of any size) that formed during the main interval of valley network formation, because these would indicate an extended interval of valley network formation (e.g. Hoke & Hynek 2009). Crosscutting relationships can also be used to test the prediction that regional integration of valley networks was globally synchronous. The scenario predicts a climate optimum that wanes gradually (with orbital modulations) and all lake breaches should occur early in the history of the event (Fig. 5).

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The scenario can also be falsified through the following routes:- (2) GCMs: The collapse has to occur at less than 40°, otherwise no major water-ice shift. The collapse trigger atmospheric pressure has to be 1 bar, otherwise no strong warming from ≳ CH4.

A strong prediction of the model is that there should be widespread charge-up of the clathrate reservoir (not just a few methane seeps!). Remnants of the clathrate reservoir should, therefore, persist today and should today be slowly (through diffusion) releasing methane. Shallow-buried clathrate has the greatest chance of surviving the climate optimum near the South Pole (Fig. 4a). However, polar clathrate may be destabilized in subsequent high-obliquity episodes (Kite et al. 2016). There is strong evidence for mud volcanism on Mars, which on Earth is usually powered by CH4 gas, but from orbit it is hard to exclude CO2-driven mud volcanism. Pervasiveness of serpentine surface outcrops (Ehlmann et al. 2010) is a weaker test. This is because reactions that make CH4 can occur at depths too deep to for excavation during complex-crater formation (Carter et al 2013, Sun & Milliken 2015), with CH4-charged waters being swept from the mid-crust to the upper crust by hydrothermal circulation (Parmentier & Zuber 2007).

The model would receive strong support if the Noachian-Hesperian boundary is found to be marked by abiotic photochemical soots. Such soots can only be produced when the atmosphere is reducing (CH4/CO2 1). CH≳ 4/CO2 = XXX-XXX is predicted by the nominal model during the collapse interval. Climate-optimum runoff would wash the soots into river and lake deposits. Complex abiotic organic matter uniquely associated with river and lake deposit is a potential astrobiology false positive. The soot layer should be <103-104 kg/m2, limited by photon supply (Lorenz et al. 2008).

7. Discussion and comparison to observations.

The collapse trigger scenario elucidates the timing of Mars' sulfate-rich rocks that are concentrated at low elevations (Fassett & Head 2011). In a <200 mbar collapsed Mars atmosphere (Hesperian) but not in a ≥2 bar atmosphere before the first collapse (Noachian), the following statements are true. (1) Sulfur species exsolve from magma and dominate volcanic degassing (Gaillard et al. 2013); (2) Sulfur species deposited from the atmosphere (Farquhar et al. 2000) can build up to high concentrations in sediments (McLennan et al. 2005), because reworking by physical erosion and chemical weathering is very limited under a thin atmosphere (Golombek et al. 2014); (3) Sulfate-rich rocks are found at low elevation, the preferred location for snowmelt at low pressure (Kite et al. 2013). Prior to the first collapse, sulfate-rich rocks rarely form. Therefore, combining (1-3), the model predicts that almost no sulfate-rich Light-Toned Layered Deposits (LTLD) predate regional integration of the valley networks, in contrast to the prevailing view that the LTLD and valley networks overlapped in time (Carr & Head 2010, Fassett & Head 2011).

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Previous Mars climate evolution (e.g. Manning et al. 2006) show a 'first collapse' but (because the reinflated atmosphere is the same as the pre-collapse atmosphere) no warming. By contrast, in the collapse trigger scenario, the reinflated atmosphere has a significantly high infrared opacity than the pre-collapse atmosphere (due to CH4). Collapse kicks in when obliquity is dropping – i.e., on 107 yr timescales, the collapse will be followed by deeper and more frequent collapses. The percentage of time that post-Noachian Mars spends in the collapsed state is set by the competition between CO2 escape-to-space (which favors collapse) and increasing solar luminosity (which raises polar temperatures, disfavoring collapse).

The collapse trigger scenario has implications for Mars astrobiology. Climate-optimum fluviolacustrine deposits represent candidate landing sites for astrobiology rovers that are attractive both in terms of taphonomy and habitability (Summons et al. 2011, Mustard et al. 2013). However, in the collapse trigger scenario, the climate optimum is immediately preceded by temperatures ~200K and >100 kRad surface radiation doses (Hassler et al. 2014), both hitherto unprecedented. This would likely sterilize the surface, although ponds fed by hot springs might provide near-surface refugia. Radiation-resistant spores could conceivably endure the evolutionary bottleneck. The sub-lake taliks predicted by the model would vertically integrate the hydrosphere, allowing deep-subsurface biota (if any) to inoculate the lakes. Deep-subsurface methanogens could boost methane production (Sinha et al. 2016)

For exoplanets as for Mars, reductions in atmospheric pressure are usually considered to detract from habitability (Tian et al. 2013). However, in the collapse trigger scenario, atmospheric shrinkage plays a twofold role in triggering Mars’ climate optimum. First, a >3 bar atmosphere, which corresponds to a plausible initial inventory for Mars, is essentially invulnerable to runaway collapse. Second, the collapse triggers the optimum. Therefore, in our scenario, atmospheric loss is required for Mars’ climate optimum.

In addition to explaining the lack of correlation between Mars’ climate optimum and the impact and volcanism forcings, the collapse trigger hypothesis reveals a precise physical basis for the timing of valley network formation (Fig. 7). Valley network formation occurred 0.8-1 Gyr after solar system formation (Hartmann 2005). In the great unloading scenario, valley network formation is triggered by Mars’ first atmospheric collapse which in turn is caused by the interplay between atmospheric loss and chaotic obliquity (Fig. 7). Using 912 realistic obliquity trajectories we find that any constant CO2

inventory of >0.8 bars can match the data for the Forget et al. (2013) GCM output; for the Mischna MarsWRF GCM output, ≥2 bar CO2 inventory is favored. Realistically, CO2 inventory decreases over time due to escape-to-space, so the observed wait time for valley network formation may consist of a long time at high but decreasing PCO2 (for which collapse is improbable), then a short time at low and decreasing PCO2 (for which collapse is increasingly certain). Because the timing of VN formation is sensitive to

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paleoatmospheric pressure, improvements to solar system chronology (Boehnke & Harrison 2016) and improvements to GCMs may allow the timing of valley networks to be related to Early Mars atmospheric loss rates in a time range that is poorly sensed by other techniques (e.g. 129-Xe, and extrapolation of present-day loss rate) (Phillips & Jakosky 2001).

The Mars climate optimum appears to be unique in the orbitally-imaged geomorphic record, consistent with the hypothesis of triggering by Mars’ first atmospheric collpase. This is despite the likelihood that very early basin-forming impacts (e.g. Hellas), and chaos formation in the Hesperian-Amazonian, also released CH4 . The absence of a prolonged Hellas-initiated climate optimum could be explained by some combination of more intense UV in the pre-/Early Noachian [look for Robbins+ 2013 dates here] (), post-impact thermochemical destruction of CH4 , and an impact paleo-latitude that was deficient in CH4 -clathrate. Alternatively, a Hellas climate optimum might have occurred but the associated valley networks have been subsequently buried by sluggish but prolonged fluvial transport in the middle Noachian.

8. Conclusions [needs to be fleshed out].Mars’ climate optimum “appears to be fundamentally different than the fluvial environment characteristic of [still earlier times]” (Howard et al. 2005). Since 1971, there has been extensive exploration of connections between mineralogy, atmospheric loss, and surface temperature that could account for regionally-integrated valley networks on Mars. We examined a scenario in which the climate optimum is triggered by atmospheric collapse. The scenario anchors these connections and makes them precise (and predictive). Specifically, we infer that the valley networks of Mars are the surface expression of an irreversible loss of reducing power from the shallow-subrface of Mars, a temporary re-integration of the surface and deep-subsurface hydrospheres, and [TBD]

Finally, the scenario explains why Mars’ climate optimum also marks a global shift in sedimentation patterns and mineralogy.

Previously proposed triggers for Mars’ climate optimum may supplement the scenario discussed here and are not inconsistent with it.

Therefore, the results underline the need to consider planetary history when evaluating habitability.

Materials and Methods.[this section will mostly follow the corresponding section of Kite et al. 2016]

Model of CH4 release and feedbacks.

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The goal is to compute the total CH4 release consequent upon Mars’ first atmospheric collapse and reinflation. Total CH4 release can be decomposed into:

- CH4 release from atmospheric collapse (loss of confining pressure from collapsed portion of the atmosphere, combined with T shift consequent on P loss). In practice we find that this is very minor.

- CH4 release from H2O -ice unloading following atmospheric collapse (direct+feedback). Step 2 in Fig. 4b

- CH4 release from warming+repressurization from P gain during reinflation (direct+feedback). Steps 3 and 4 in Fig. 4b.

- CH4 release due to sub-lake taliks (direct+feedback)-

These terms are shown as a function of P just before collapse, mean T just before collapse, and f, in Fig. S1. They are not very sensitive to the obliquity at collapse. The total amount of CH4 -induced warming is also shown. The duration of the climate optimum is approximately 10 Kyr/mbar (Kite et al. 2016).

Set-up: To test the scenario described above, we use a self-consistent obliquity model to drive a simple model of surface temperature as a function of latitude, atmospheric pressure, solar luminosity, and obliquity. Using this simple energy balance model, atmospheric collapse and reinflation can be treated self-consistently. The surface energy balance model drives a model of subsurface methane clathrate stability. Once released, CH4 -clathrate is destroyed by photolysis. We found the photon-limited direct-destruction rate assuming photons are destructive out to X nm. Supporting full-photochemistry modeling shows that this underestimates the CH4 destruction rate by only X%.

A. Atmospheric Collapse.Previous work includes:G&T 1973. N&T 2002 say ‘essentially 0-D’. No GH effect.McKay 1991. Regolith, no ice reservoir.James & North 1982 – 1D EBMStone 1972 – diffusivity parameterizationNakamura & Tajika 2002. Reviews Annual-mean model from N&T 2001. Extent of ice-sheets controls possibility of atmospheric reinflation. Seasonal cycle expands the unstable region (no-residual-caps). If the amount of CO2 gets really low then you will just have 1 seasonal cap.

To calculate atmospheric collapse, we use the Mars Weather Research and Forecasting General Circulation Model (MarsWRF GCM) to build a look-up table for Mars annual-mean temperatures as a function of obliquity, log of atmospheric pressure, and latitude. We used a polynomial fit to these results (RMS error = X.X K) to determine the boundary of atmospheric collapse (coldest temperatures at the CO2 condensation point). To show that the conclusions are insensitive to GCM choice, we also fit a simple energy balance

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model of polar temperatures (Manning et al. 2006) to the binary collapse/no-collapse results of Forget et al. (2013) employing the greenhouse parameterization of Ozak et al. (2016). During collapse, we use a ~1 mbar/yr collapse rate (Soto et al. 2015) – a pace that is controlled by the ratio of the surface longwave radiation at the CO2-condensation temperature, to the latent heat of CO2 ice. We use the same (energy-limited) rate for reinflation. We force the model with obliquity forcing from realistic obliquity histories (Kite et al. 2015), because the dominant control on polar insolation is obliquity. Collapse takes ~1 Kyr and is followed after a collapsed interval of 10-50 Kyr duration by reinflation which also takes ~1 Kyr. Example tracks are shown in Fig. 5. We assume a critical obliquity of 15°, for which the (model-dependent) atmospheric collapse pressure is ~1-2 bar. The sequence of events is not sensitive to the choice of critical obliquity (see Fig. 7 for details of trade-offs between initial CO2 inventory, critical obliquity, and wait-time).

In the model, the distal trigger for Mars’ atmospheric collapse is atmospheric loss, which is slow relative to quasi-periodic orbital cycles. The proximal trigger for Mars’ first atmospheric collapse is always an obliquity minimum caused by constructive interference between ~100 Kyr and 2 Myr quasi-periodic obliquity cycles. As a result, the amplitude of the 100 Kyr cycle that includes Mars’ 1st atmospheric collapse is always >20°.

CH4 that escapes the clathrate cage is assumed to be geologically-instantly released to the atmosphere. We found the CH4-induced warming using Wordsworth et al. (2016). We plugged this back into the energy balance model.The initial H2O-ice distribution is very simplified, uses modern MOLA topography, and follows the modeling results of (e.g.) (Forget et al. 2006, Kite et al. 2013, Mischna et al. 2013).. Ice stability zones are shown as >77.5° lat (both hemispheres) for p<250 mbar and obl<40 deg; as <30° lat for obl≥40° and p<250 mbar; and as elevation> 1 km for p>250 mbar. Unstable ice is reallocated to the stable zone at a rate of 5 mm/yr (e.g., Madeleine et al. 2009). H2O radiative feedbacks are neglected.

B. Temperatures and clathrate stability.We set porosity = 0.2 (consistent with Wieczorek et al. 2013), assume 120 kg CH4 per m3

of clathrate (assuming a composition CH4 .6H2O ); lithospheric heat flow of 0.03 W/m2, regolith thermal conductivity of kcond = 2.5 W m-1 K-1; density of 2000 kg m-3 for regolith and 910 kg m-3 for ice, and Mars gravity = 3.7 m s-2.

The vertical thermal conduction timescale is <X Kyr, which is shorter than the other timescales of interest in this problem, so throughout we approximate vertical thermal reequilibration as rapid. (This is totally fine because [the over-sophisticated thermal model from Paper I was overkill; scaling]. Even when the low thermal diffusivity of CH4 clathrate and the latent heat of clathrate decomposition are included.)

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C. Methane release temperatures feedbacks. [To be written, will be similar to Kite et al. 2016]D. Methane destruction. [To be written, will be similar to Kite et al. 2016]

Fig. 7. Effect of chaotic diffusion of obliquity on timing of valley network formation. Dark gray bar corresponds to observed timing of VNs. The lines show the spread of outcomes for hundreds of different randomly-initialized obliquity simulations. Asterisks and circles correspond to the median wait time for atmospheric collapse for different pCO2 s, for different models of polar temperature (asterisks: Forget; circles, Mischna; symbols are vertically offset for clarity). Effects of luminosity change and atmospheric escape are not included in this figure.

Acknowledgements. We thank Colin Goldblatt and Yuk Yung for their significant contributions to a closely related paper (Kite et al. 2016). We thank Robin Wordsworth and Francois Forget for sharing their LMD GCM code. We thank Alan Howard, Ross Irwin, Bob Craddock, Alejandro Soto, Tom McCollom, and Chris Oze for discussions. All of the scripts and input files used to do the science described above are attached as a zip

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file. After unzipping, to reproduce all of the science, open a Matlab command prompt and type “go”.

Supplementary Information.

[Compare to the amount of CH4 released by an impact]

[Not yet done] Controls on the magnitude of CH4 release (Pcollapse, T_init, f)

Fig. S1. Controls on the magnitude of CH4 release.

Fig. S2. Hysteresis loop. Explain why albedo is not important to the hysteresis loop.

More details on obliquity tracks: [Text from proposal to be inserted here]

Charge-up pathways

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C source is outgassing from intrusions. CH4 production by reduction of DIC is doubted (McCollom PNAS 2016). See also Kite et al. 2016.

Where ice cover was lacking, pore-ice would initially form but could then be converted to CH4 -clathrate by diffusion from deeper layers.

Atmospheric collapse parameterizationTest points: Fastook & Head 2015

The wind caused by collapse is minor, as it is for the seasonal CO2 cycle today (Read & Lewis 2004).

CH4 release parameterization.(1) If there is continuous pore-filling ground ice above the within-regolith CH4 -clathrate stability zone then that could trap the destabilized CH4 gas and prevent it from getting to the atmosphere.

Geologic dataValley-networks represent 0.4m – 4m of area-averaged erosion (104-105 yr of erosion at Earth rates) (Hoke et al. 2011, Luo et al. 2015). Consistent with this, valley long-profiles are far from topographic equilibrium, indicating much less erosion than on Earth (Aharonson et al. 2002). The rarity of delta entrenchment suggests abrupt shutdown of the climate optimum (Irwin et al. 2005). Climate physics predicts synchroneity of wet conditions across the planet, broadly consistent with valley-network crater counts (Fassett & Head 2008; but see Hoke & Hynek 2009 for an alternative view). Overall, the challenge to modelers is to explain a ~3.7 Ga wet climate that persisted for >104 yr but was geologically brief, marked by precipitation-fed rivers and overspilling lakes.

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Fig. S3. Gain map.

Fig. S4. Simulations that incorporate CO2-ice clouds predict the obliquity at first-collapse is less than 40°, and the pressure at first-collapse is >500 mbar, consistent with a collapse-triggered climate optimum.

Notes / scrapbook--1-1.2 bar, up to 25 deg, Soto C, Forget NCSoto: mostly obliquity-driven.

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Forget: mostly luminosity-driven.

---The pCH4 feedback is negligible.-- f can be interpreted as a fraction of gas released (if some is trapped beneath permafrost)

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