Episodic Warming of Early Mars by Punctuated Volcanism

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LETTERS PUBLISHED ONLINE: 17 NOVEMBER 2014 | DOI: 10.1038/NGEO2293

Episodic warming of early Mars by punctuated volcanism

Itay Halevy1* and James W. Head III2

The widespread evidence for liquid water on the surface plains seem to have effused predominantly from wide fissures rather of early Mars is dicult to reconcile with a dimmer early than central edifices, and the observed dimensions of the fissures Sun. Many geomorphological features suggestive of aqueous imply effusion rates of 105–106 m3 s−1 (ref. 19). Dividing estimates activity, such as valley networks and open-basin lakes, date of the total volume of the basaltic plains (3.3 × 107 km3; ref. 17) to approximately 3.7 billion years ago1–5, coincident with by the lower end of this range of effusion rates gives an upper a period of high volcanic activity5,6. This suggests that limit of ∼10,000 years on cumulative eruptive duration, which is volcanic emissions of greenhouse gases could have sustained only ∼0.01% of the total emplacement time of the basaltic plains a warmer and wetter climate on early Mars. However, (100–200 Myr; ref. 18). Second, the thick, laterally extensive and models that consider only CO2 and H2O emissions fail to topographically flat morphology of the basaltic plains, as well as the produce such climates7,8, and the net climatic eect of the scarcity of central edifices, suggest an analogy to terrestrial flood 9–11 sulphur-bearing gases SO2 and H2S is debated . Here we basalts that may guide estimates of eruption rates. Radiometric investigate the atmospheric response to brief and strong ages and volumes of flow packages in the Columbia River Flood volcanic eruptions, including sulphur emissions and an evolving Basalts and the Deccan Traps indicate that individual eruptive population of H2SO4-bearing aerosols, using a microphysical episodes lasted one to ten years, and were separated by quiescent aerosol model. In our simulations, strong greenhouse warming periods lasting up to 10,000 years20,21. Again, active eruption only by SO2 is accompanied by modest cooling by sulphate aerosol ∼0.01% of the time is implied. The effusion rates implied by both formation in a presumably dusty early Martian atmosphere. approaches (105–106 m3 s−1) are more than 1,000 times higher than The simulated net positive radiative eect in an otherwise cold those estimated for earlier volcanic activity associated with the climate temporarily increases surface temperatures to permit emplacement of the Tharsis rise (100–300 m3 s−1; ref. 22). above-freezing peak daily temperatures at low latitudes. We With typical terrestrial basaltic volatile content, such brief, conclude that punctuated volcanic activity can repeatedly high-rate (‘punctuated’) effusion implies instantaneous per-area lead to warm climatic conditions that may have persisted for outgassing rates up to several hundred times the terrestrial global decades to centuries on Mars, consistent with evidence for average rate. The higher sulphur content of Martian magmas23, transient liquid water on the Martian surface. together with the large area and volume of the ridged plains relative Geomorphological evidence for the presence of liquid water to terrestrial flood basalts (Fig. 1), means that transient sulphur on the surface of early Mars includes valley networks1–3, open- emission rates as high as a few thousand times the terrestrial basin, occasionally interconnected lakes4,5, meandering channels12, global average were probable during plains emplacement. Thus, the deltaic features13, and relatively rapid erosion rates14. Furthermore, average volcanic emission rate was probably composed of episodes hydrated minerals, especially various phyllosilicates and sulphates, of punctuated eruption separated by long quiescent periods. During 15 are ubiquitous on ancient terrains . Although many of the eruptive phases, large amounts of SO2 were rapidly injected into mineralogical observations do not strictly require a warmer climate, the atmosphere, converted to H2SO4 with e-folding times of several the requirement for surface runoff to explain the geomorphological centuries24, and incorporated into aerosols. The net climatic effect features suggests that liquid water was present, at least transiently, is debated. Strong greenhouse warming by SO2 (ref. 10) may be on the early Martian surface. By inference, it has been widely countered by comparably strong cooling due to scattering of solar suggested that the climate was warmer16, but the latest generation radiation by aerosols11. of high-order climate models cannot sustain near-melting mean If a CO2-dominated atmosphere were unable to sustain warm annual temperatures anywhere on the planet even with a multi-bar temperatures7,8, early Mars is expected to have been cold. The 7,8 7 CO2 atmosphere . spatial distribution of ice under these conditions leaves much of the A clustering of valley networks and open-basin lakes in the Late planet’s surface exposed and desiccated—and, therefore, a potential Noachian and Early Hesperian1–5 (∼3.7 Ga) overlaps with a long source of atmospheric dust. A modern-day dust lofting efficiency maximum in volcanic activity5,6. Most of the lava erupted during and atmospheric loading, augmented by fine-grained volcanic this time occurs as basaltic plains (‘Hesperian ridged plains’ and ash during explosive eruptions, carries important implications for related units)6,17,18, with thicknesses up to hundreds of metres, and Mars’ early climate. First, in addition to nucleating new, highly 11 occasionally one to two kilometres, and an estimated area greater reflective H2SO4–H2O aerosols ,H2SO4 would condense onto than 30% of the planet’s surface17. Two independent approaches pre-existing dust and ash particles. Unless the coating makes up suggest that the basaltic plains were emplaced by a series of brief much of the particle’s mass, its optical properties resemble those eruptions, characterized by extreme effusion rates, and separated of dust, which is less reflective than homogeneous H2SO4–H2O by millennial-scale hiatuses in eruptive activity. First, the basaltic (Supplementary Fig. 1). Second, the H2SO4 coatings, as well as any

1Department of Earth and Planetary Sciences, Weizmann Institute of Science, Rehovot 76100, Israel. 2Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, Rhode Island 02912, USA. *e-mail: [email protected]

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a 180 ) −2 170

lateau 160 p = 273 K (W m s 150 T

Hesperian ridged plains Subsolar zonal average incoming solar flux North Atlantic igneousOntong-Java province 140 Parana andSiberian Etendeka traps traps ColumbiaEmeishan River Karoo−Ferrarflood traps basaltsAfro-Arabian provinceDeccan province traps 0.2 1 1 0.6 4 159 17 2−3 Global Subsolar Volume 10 130 CO2 + H2O CO2 + H2O + SO2 Area 120 CO2 + H2O + clean dust CO2 + H2O + SO2 + coated dust 110 Global average incoming solar flux 3 3 −1

Terrestrial global average effusion rate: 0.005−0.020 10 km yr Total outgoing radiation with 100 0.001 0.01 0.1 1 10 | Volume, surface area and instantaneous eusion rate of the Figure 1 SO2 mixing ratio (ppmv) Hesperian ridged plains17 compared to terrestrial flood basalt Outgassing rate 30 109.0 109.5 1010.0 1010.5 1011.0 1011.5 provinces . The size of the dark teal spheres and light teal circles (molec cm −2 s−1) corresponds to relative volume and area, respectively. The white numbers Martian denote instantaneous eusion rates (103 km3 yr−1), using estimates of plains volcanism emplacement durations and assuming active eruption 0.01% of the time. Terrestrial Terrestrial Given uncertainties in the emplacement duration of the ridged plains, long-term average flood basalts actual eusion rates may have been several times higher. The terrestrial b 290 global average eusion rate is shown for comparison. 1 Aerosol scavenging 2 Decay of atmospheric SO2 270 Desiccation and dust lofting homogeneous H2SO4–H2O aerosols formed, would be added to an 3 already dusty atmosphere, and their scattering effect may be minor relative to coexisting dust. To explore the effects of punctuated 250 2 eruptions into a dusty early Martian atmosphere we developed a 1 microphysical aerosol model, which dynamically tracks populations 3 ? of both homogeneous H SO –H O aerosols and H SO -coated 2 4 2 2 4 230 dust (Methods). We used the calculated optical properties of these particle populations in a radiative transfer model to obtain the steady-state and time-dependent radiative effect of volcanic sulphur 90-year decay

Average surface temperature (K) 210 emissions (Methods). 30-year eruption At the atmospheric steady state, higher SO2 concentrations (resulting from higher volcanic outgassing rates) generate net pos- 190 itive radiative forcing up to ∼27 W m−2, despite the formation of 0.001 0.01 0.1 1 10 SO mixing ratio (ppmv) H2SO4-bearing aerosols (Fig. 2a). This occurs because some aerosols 2 are composed of a moderately absorptive dust core and a vari- ably thick reflective coating of H2SO4 (Supplementary Fig. 1), and Figure 2 | Radiative forcing by SO2 and H2SO4-coated dust. a, Global also the H2SO4 coatings and homogeneous H2SO4–H2O aerosols (dark and light blue) and subsolar zonal (red and orange) average outgoing are added to an already dusty atmosphere. Globally, their average radiation at the steady state, compared with the incoming solar flux (black negative radiative effect (8–17 W m−2) is approximately two to four and grey). b, Global and subsolar zonal average surface temperature at the times the effect of scattering by pre-existing dust (∼4 W m−2), and same steady states as in a, and during a ∼30-year punctuated eruption is outweighed by SO2 greenhouse forcing, leading to net warming (triangles, see Methods). Volcanic emission rates corresponding to the (Fig. 2b). Near the subsolar latitude, where the incoming solar flux steady-state SO2 mixing ratios on the horizontal axis are shown in the is higher, scattering by H2SO4 coatings and homogeneous aerosols is centre, along with estimated emission rate ranges of terrestrial and Martian more pronounced, and leads to net cooling of up to 7 K at SO2 mix- volcanism. Numbered arrows show a possible positive feedback, described ing ratios between ∼5 and ∼500 ppb (Fig. 2b). At higher SO2 levels, in the text. greenhouse forcing dominates, warming the surface by up to ∼23 K. During punctuated eruptions, H2SO4 production and aerosol In the absence of SO2, the mean annual temperature (MAT) dynamics largely keep up with the rapid increase in SO2 levels, globally and at the subsolar latitude (low latitudes at normal resulting in near-steady-state radiative forcing throughout the erup- obliquity) is ∼194 and ∼232 K, respectively (Fig. 2b), in agreement tion (Fig. 2b, triangles). Although punctuated eruptions do not with high-order climate models8, even though our simple model temporarily decouple positive forcing by SO2 from negative forc- does not account for heat transport and clouds. During punctuated 11 ing by H2SO4-bearing aerosols , they allow high transient atmo- eruptions, and for decades after particularly strong eruptions, SO2 spheric concentrations of SO2, which would be impossible to reach mixing ratios reach ppm levels (Fig. 3), but the global MAT does not with lower long-term outgassing rates. For example, maintaining exceed ∼220 K (Fig. 2b). At the subsolar latitude, however, a higher ∼10 ppm SO2 requires high long-term average volcanic outgassing solar flux results in a zonal MAT as high as ∼250 K for the eruptions rates ∼100 times the global terrestrial average (Fig. 2). A series modelled here, and possibly higher for stronger or longer eruptions, of brief eruptions, however, similar in effusion rates to terres- which lead to higher SO2 concentrations. Given the seasonal and trial flood basalts, and accompanied by instantaneous outgassing diurnal temperature cycle in high-order models8, this implies peak rates a few thousand times the terrestrial average, will elevate SO2 daily temperatures at low latitudes that comfortably exceed 273 K concentrations to such values, and induce the associated warming. for several months out of the year. Together with an invigorated

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a 2 b 2

iii ii mixing ratio (ppmv) 2 SO 1

i 1 c mixing ratio (ppmv)

2 0.54 SO

4 Coated dust SO 2

0.50 i ii iii i Pre-eruption steady state

ii Punctuated eruption Fraction of total H

iii Post-eruption decay H2SO4−H2O 0 0 5 10 15 20 0.46 Time (kyr)

Figure 3 | Atmospheric eects of punctuated eruptions. a, Surface SO2 mixing ratios during a series of punctuated eruptions (Methods). b, SO2 mixing ratio during a ∼30-year eruption (thin black box in a). c, Fraction of total atmospheric H2SO4 in coated dust (solid) and homogeneous H2SO4–H2O (broken) aerosols. The lower super-saturation required for heterogeneous condensation of H2SO4 initially raises the fraction of H2SO4 in coated dust, which then settles owing to growth by H2SO4 condensation. Ongoing production of H2SO4 as SO2 levels increase ultimately leads to homogeneous nucleation and an increase in the fraction of H2SO4 in pure aerosols. hydrologic cycle, such conditions lead to intermittent generation of (∼10 mm yr−1; Supplementary Information). As the mass of surface runoff for up to hundreds of years following very strong sulphates grew and as the potential for wet conditions decreased, eruptions. Transient invigoration of the hydrologic cycle may also sulphates would get increasingly preserved, consistent with their lead to scavenging of aerosols from the atmosphere, to a temporary occurrence mostly on Hesperian rather than Noachian terrains15. switch from a dusty to a clean atmosphere (Fig. 2b), and to further Imperfect remobilization of sulphates during wet episodes may warming due to reduced scattering by aerosols, which would last explain the occurrence of sulphates interfingered with or basal to until the surface cooled and dried enough to reinitiate efficient insoluble phyllosilicate-bearing units28. Sulphur that was removed dust lofting. from the atmosphere by dry deposition in extra-tropical regions The occurrence of eruptions capable of elevating SO2 concen- would not form massive, low-latitude sulphate deposits, and may trations to ppm levels once every 1,000–10,000 years, as suggested instead explain the uniform enrichment of sulphur in Martian soils. by the deposit morphology and the analogy to terrestrial flood We suggest that Mars’ early climate was cold during quiescent basalts, implies intermittent surface runoff for ∼100 years out periods7,8. Most of the water would have been locked as ice on the of every 1,000–10,000 years, or ∼1–10% of plains emplacement south pole29, as snow and ice cover in the southern highlands7, time. A total emplacement time of 100–200 Myr (ref. 18) then im- and within the soil and shallow crust. During the Late Noachian plies ∼106–107 years of intermittent surface runoff, in good agree- and Early Hesperian, large volumes of lava were emplaced on the ment with estimates of the duration required for valley network surface of Mars, mostly in the form of flood-basalt-like plains. By formation25,26. Furthermore, the occurrence of these conditions, analogy to terrestrial flood basalts, eruptions are likely to have not globally, but near the subsolar latitude, is consistent with the been brief and voluminous. Rapid emission of sulphur volatiles predominantly lower-latitude distribution of fluvial and lacustrine during such punctuated eruptions warmed the surface through the 1–4 features . Finally, the decay of SO2 concentrations to sub-ppm combined greenhouse effect of SO2 and the subdued scattering levels between punctuated eruptions, and the associated decline to effect of H2SO4 coatings on dust and ash grains. The duration of cold and icy conditions, is consistent with mounting evidence for warming was limited to tens or perhaps hundreds of years after transient, rather than sustained, wet episodes on early Mars1,26,27. volcanic eruptions and would be sufficient to melt surface ice and Between episodes of punctuated volcanism, as the planet snow at low latitudes, but insufficient to melt through the subsurface refroze and became desiccated, the volcanically emitted sulphur cryosphere. Between eruptions, the atmosphere returned to a cold would form an assemblage of sulphate and sulphite minerals and icy state. Soluble minerals, such as sulphates, which deposited in local topographic depressions. Remobilization of these salts as the planet dried, were remobilized in hydrologically active regions during subsequent episodes of volcanically driven melting and during subsequent wet periods and deposited where water collected. hydrologic activity would require very modest precipitation rates As volcanic activity on Mars weakened, the potential for warmer and

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atmosphere: Temperatures and CO2 ice clouds. Icarus 222, 81–99 (2013). permissions information is available online at www.nature.com/reprints. 9. Halevy, I., Zuber, M. T. & Schrag, D. P. A sulfur dioxide climate feedback on Correspondence and requests for materials should be addressed to I.H. early Mars. Science 318, 1903–1907 (2007). 10. Johnson, S. S., Mischna, M. A., Grove, T. L. & Zuber, M. T. Sulfur-induced Competing ﬁnancial interests greenhouse warming on early Mars. J. Geophys. Res. Planets 113, E08005 (2008). The authors declare no competing financial interests.

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