wind driven hydrodynamic escape intensify atmospheric escape processes 57 This

Wind driven hydrodynamic escape intensify atmospheric

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wind-driven hydrodynamic escape intensify atmospheric escape processes [ 57 ] . This is consistent with an early shutdown of the Martian dynamo (~4.1 Ga; [ 58 , 59 ]). Although other work suggests that the dynamo may have lasted into the early Hesperian (~3.7 Ga; [ 60 ]), these authors had linked the oldest visible surface age of volcanism to the magnetization age of the crust, which may be a less accurate technique [ 59 ]. It is thought that Mars never had plate tectonics, and even if it did, it did not last very long [ 61 , 62 ]. Whereas Mars is likely too small (mass ~10% of that of Earth’s) to have supported plate tectonics over billion-year timescales, plate tectonics may be favored for planets between one and five Earth masses [ 63 ]. However, whether normal or shear stresses govern the initiation of plate tectonics may complicate the above picture [ 64 ]. At even larger masses, increased resistive forces under high gravity may reduce subduction tendency (e.g., [ 63 , 65 ]), although this remains highly debated (e.g., [ 66 ]). The lack of plate tectonics and (possibly) a carbonate–silicate cycle on early Mars would seem to contradict the widespread evidence of surface fluvial features suggestive of a once long-lived warmer and wetter climate (e.g., [ 67 , 68 ]) that was possibly capable of supporting surface oceans (e.g., [ 69 71 ]). However, it is possible that other volatile recycling mechanisms operate on other planets. For instance, CO 2 on early Mars could have been cycled through vigorous volcanic outgassing and subsequent melting and remobilization of surface carbonates [ 72 ]. Plate tectonics may not have even been present on the early Earth ~3.5 Ga [ 73 ] and yet our planet may have been habitable by ~4.4 Ga [ 74 ]. During the Archean, Earth may have had a form of tectonics dominated by plumes [ 73 ]. Also, although the study was performed for Earth-sized planets, if radiogenic production or volcanism is high enough, habitable conditions lasting billions of years may be sustained on stagnant lid exoplanets [ 75 ]. Thus, the possible lack of plate tectonics on early Mars may not be inconsistent with surface conditions capable of supporting standing bodies of liquid water.
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Geosciences 2018 , 8 , 280 9 of 48 4. Limit Cycles Limit cycles are another idea related to the cycling of CO 2 on planets [ 76 , 77 ]. Assuming that CO 2 is efficiently transferred between the surface and mantle through a global CO 2 cycling mechanism, warm planetary periods occur when volcanically-outgassed CO 2 outpaces drawdown, silicate weathering, and incorporation into rocks. Once the opposite occurs, and these removal processes outpace volcanic outgassing, the planet freezes instead. The process of cycling into and out of such warm conditions is called a “limit cycle” (Figure 4 ). Planets that receive low levels of stellar insolation, like those near the outer edge of the habitable zone, and with low enough CO 2 outgassing rates, are susceptible to these unstable climates [ 78 , 79 ]. Limit cycle frequency is a strong function of the soil CO 2 pressure and the volcanic outgassing flux [ 79 ]. Recent 3D calculations [ 80 ] also confirm
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