Figures 10 and 11 illustrate how we propose that climate, mantle, and the core interact on Earth and Venus. Venus, being inward of the inner edge of the habitable zone, cannot have liquid water at its surface. As a result, silicate weathering cannot draw CO 2 out of the atmosphere, so a hot, CO 2 ‐rich climate forms via volcanic degassing and/or primordial degassing during accretion and magma ocean solidification. The hot climate prevents plate tectonics, and in turn the long‐lived operation of a core dynamo due to a low core heat flux (Figure 11). For the Earth, being in the habitable zone allows liquid water, and in turn silicate weathering. Thanks to a temperate climate plate tectonics can operate, and the resulting high core heat flow powers the geodynamo (Figure 10). Jellinek and Jackson [2015] invoke a similar series of couplings to explain the Earth‐Venus dichotomy, though they argue that the lack of plate tectonics on Venus is caused by high rates of radiogenic heating in the mantle, and that Venus' hot climate stems from the lack of plate tectonics. Their hypothesis contrasts with our interpretation, that Venus' orbital position is the key factor explaining its evolution.

If surface‐interior coupling is responsible for the divergent evolution of Earth and Venus, the runaway greenhouse climate, loss of plate tectonics (if it ever existed), and death of the magnetic field should all be correlated in time. Thus, determining the climate, magnetic, and tectonic history of Venus is a vital test. Unfortunately, our current knowledge of Venusian history is poor. The trace amount of water in the atmosphere requires a minimum of about 500 Myr for the H to escape to space [Donahue, 1999], so the runaway greenhouse must have occurred by at least Ga, but could have taken place much earlier. In fact, Venus may have entered a runaway greenhouse and lost its water during formation [Hamano et al., 2013].

The Venusian tectonic and magnetic histories are similarly uncertain. There is evidence for a massive resurfacing event at 0.5–1 Ga. Surface features related to this event are most consistent with volcanism and lava flows rather than subduction of old crust and creation of new crust at ridges [Smrekar et al., 2013; Ivanov and Head, 2013]. The planet may still be volcanically “active” today [Smrekar et al., 2010], but eruptions are likely sporadic. The style of tectonics before the resurfacing event is unknown, with stagnant lid convection, episodic overturns, or even Earth‐like plate tectonics all possibilities. Unraveling the Venusian magnetic history is challenging because the preservation and in situ measurement of any remanent magnetization on Venus' hot surface is unlikely. However, another possible line of evidence that may imply the presence of a paleo‐Venusian magnetic field would be the loss of H+, He+, and O+ along polar magnetic field lines to the solar wind, a process known on Earth as the polar wind [Moore and Horwitz, 2007]. This ion escape mechanism relies on the presence of an internally generated magnetic field, and could potentially leave a chemical fingerprint in the Venusian atmosphere [Brain et al., 2014]. However, our present‐day knowledge provides no evidence for a strong, internally generated magnetic field on Venus. Future exploration of Venus is needed to place tighter constraints on its evolution.

6.1 Evolution of Rocky Exoplanets

Venus demonstrates one likely evolutionary scenario for planets lying inward of the habitable zone's inner edge. However, cooler climates, that still allow plate tectonics and a magnetic field, are also possible for such planets if they have a significantly smaller CO 2 inventory than Venus. In this case, a temperate climate can still exist, even without liquid water and silicate weathering to regulate atmospheric CO 2 levels, simply because there is not enough CO 2 to cause extreme greenhouse warming. In some cases, a planet that experiences a runaway greenhouse could even retain some water at the poles, and potentially remain habitable [Kodama et al., 2015]. Planets lying beyond the habitable zone's outer edge will likely be cold, and thus can plausibly sustain plate tectonics and a magnetic field as well. However, whether complex life can develop on any of these planets is unclear.

Likewise, an Earth‐like evolution is one likely scenario for planets lying within the habitable zone, but a number of factors could lead to a different evolution that is unsuitable for life. Hot, CO 2 ‐rich climates are expected after planet formation due to degassing during planetesimal accretion and magma ocean solidification [Abe and Matsui, 1985; Zahnle et al., 2007; Elkins‐Tanton, 2008]. If a planet's initial climate is so hot that liquid water is not stable (i.e., temperature and pressure conditions are beyond the critical point for water), then developing a temperate climate is probably not possible. Initial surface temperature and pressure conditions in excess of the critical point imply that silicate weathering would not occur, with or without plate tectonics (though some limited reaction between atmospheric CO 2 and the crust is possible). As a result, the climate would remain extremely hot, preventing both plate tectonics and a core dynamo. However, a climate hot enough to exceed the water critical point is an extreme case, requiring bar of CO 2 for a planet with an atmosphere composed of CO 2 and H 2 O and receiving the same insolation as the Hadean Earth [Lebrun et al., 2013]. Earth's total planetary CO 2 budget is estimated at bar, so even with complete degassing during accretion or magma ocean solidification, liquid water was still stable [e.g., Sleep and Zahnle, 2001; Zahnle et al., 2007]. Thus, a planet would need a significantly larger total CO 2 budget than Earth for initial atmospheric makeup to exceed the liquid water critical point.

Another possibility is a climate where liquid water is stable, but is still too hot for plate tectonics. In this case, low land fractions or erosion rates can potentially prevent plate tectonics, and a cool climate, from ever developing. High rates of volcanism would be needed to avoid this fate, by creating a sufficient supply of fresh rock at the surface for silicate weathering to cool the climate. Finally, even when climate conditions are amenable to plate tectonics, an uninhabitable state can be reached if plate tectonics does not initiate before increasing insolation warms the climate to the point where it is no longer possible (Figure 12). Before plate tectonics initiates on a planet, weathering may be supply limited and the atmosphere CO 2 rich. Thus, surface temperature will increase with increasing luminosity, and can potentially become hot enough to preclude plate tectonics from ever starting. If plate tectonics does not initiate before this divergence point is reached, a planet could become permanently stuck with a hot, uninhabitable climate, stagnant lid convection in the mantle, and no protective magnetic field.

Figure 12 Open in figure viewer PowerPoint Schematic diagram of the divergence point in planetary evolution involving the initiation of plate tectonics. Before plate tectonics, a planet's weathering rate may be supply limited, such that surface temperature climbs over time as a result of increasing luminosity. Once plate tectonics initiates, silicate weathering is enhanced by higher erosion rates and continent formation, and climate cools.

Conversely, on a planet where plate tectonics does initiate before reaching this divergence point, continental growth and orogeny will increase both land area and erosion rates, enhancing the ability of silicate weathering to establish and maintain a temperate, habitable climate. If a large enough land area forms, weathering may even be capable of maintaining a temperate climate without plate tectonics to elevate erosion rates. Likewise, plate tectonics means that long‐lived dynamo action, and therefore, volatile shielding from the solar wind is possible. The divergence point involving the initiation of plate tectonics could be particularly important if high mantle temperatures are a significant impediment to plate tectonics through low convective stresses or the creation of thick buoyant crust (see section 2.3). Mantle temperature may need to cool before plate tectonics can begin, even if surface temperatures are not hot enough to preclude plate motions. However, the increase in luminosity during a star's main sequence evolution is relatively gradual (the sun's luminosity was only % lower at 4.5 Ga [Gough, 1981]), so there is a large time window, on the order of 1 Gyr, for sufficient mantle cooling to occur before climate becomes too hot for plate tectonics.

Another divergence point involves magnetic shielding and planetary water loss. The magnetic field can limit H escape, and thus help preserve surface water, when atmospheric escape transitions from being hydrodynamically limited to diffusion limited (see sections 4.3 and 4.4.) If no magnetic field is available to shield the solar wind then massive amounts of H may be lost, leaving the planet desiccated and unable to regulate atmospheric CO 2 levels. As discussed in section 4.4, if the transition to diffusion limited escape is early in a planet's history, then magnetic shielding is possible regardless of tectonic regime, and many planets will likely be able to keep their volatiles and possibly develop habitable climates. However, if the transition occurs after the mantle's thermal adjustment period, then plate tectonics is likely necessary for magnetic shielding, and fewer planets will be able to retain surface water.

Additional divergence points are possible later in planetary evolution if climate, tectonics, and the magnetic field are tightly coupled. For example, if the carbon cycle and plate tectonics act as a self‐sustaining feedback, where high erosion rates, supplied by plate tectonics, are required to maintain kinetically limited weathering and thus keep surface temperatures cool enough for plate tectonics (see section 3.3), then the loss of plate tectonics would lead directly to a hot climate state that likely precludes plate tectonics from reinitiating. Without plate tectonics to enhance erosion, the inhospitably hot climate that results from supply limited weathering would likely be permanent. Such tight coupling between plate tectonics and the carbon cycle is most likely on planets with small exposed land areas or high total CO 2 budgets. Another divergence point is possible for planets orbiting very active solar mass stars or very close to active small mass stars where stronger stellar winds can more efficiently strip atmospheric volatiles. For these planets, cessation of the core dynamo could cause significant water loss, in turn halting silicate weathering and, due to the ensuing hot climate, likely shutting down plate tectonics as well (see Figure 13 for a schematic illustration of the divergence points discussed in this paragraph).

Figure 13 Open in figure viewer PowerPoint Schematic diagram of possible scenarios where failure of the feedbacks between climate, plate tectonics, and the magnetic field leads to divergent evolution of terrestrial planets within the habitable zone.

Size is also a major factor in terrestrial planet evolution. Large planets are expected to have wider habitable zones due to the influence of higher gravity on atmospheric‐scale height and the greenhouse effect [Kopparapu et al., 2014], but also shallower ocean basins and thus less exposed land, unless feedbacks can regulate ocean volume such that continents are always exposed [Cowan and Abbot, 2014] (see also section 6). The influence of size on plate tectonics and magnetic field generation is also unclear. Previous studies have found that plate tectonics is more likely on larger planets [Valencia et al., 2007; Valencia and O'Connell, 2009; van Heck and Tackley, 2011; Foley et al., 2012], less likely on larger planets [O'Neill and Lenardic, 2007; Kite et al., 2009; Stein et al., 2013; Noack and Breuer, 2014; Stamenković and Breuer, 2014; Miyagoshi et al., 2014; Tachinami et al., 2014], or that size is relatively unimportant [Korenaga, 2010a]. Different studies reach very different conclusions because of the large uncertainties in the rheological mechanism necessary for generating plate tectonics, how key features such as internal heating rate scale with size, and how mantle properties are affected by pressure and temperature. The influence of size on magnetic field strength is likewise debatable. Several studies have found a weak dependence of field strength and lifetime on planet and core size, and possibly a peak in strength for Earth‐sized planets [Gaidos et al., 2010; Tachinami et al., 2011; Driscoll and Olson, 2011; Van Summeren et al., 2013]. Generally, larger dynamo regions are expected to produce stronger magnetic fields [e.g., Christensen et al., 2009], but variations in more subtle properties, like mantle and core composition, likely play a fundamental role. The coupling between plate tectonics, climate, and the magnetic field is expected to apply to planets of different size, but future work is needed to place tighter constraints on the influence of size on this coupling and on planetary evolution.

The issue of size highlights the many uncertainties remaining in our knowledge of planetary dynamics and evolution. Each aspect of planetary dynamics that is important for magnetic, tectonic, and climate evolution needs to be better understood before more rigorous predictions or interpretations can be made. In particular, the interactions between different components of the planetary system, specifically interactions among surface tectonics, mantle convection, and the long‐term carbon cycle, between mantle convection and the core dynamo, and among the magnetic field, atmospheric escape, and climate, deserve significant attention. With a large number of rocky exoplanets already discovered, many of which are in their respective habitable zones [Batalha, 2014], and more certain to follow, improving our knowledge of planetary evolution is an important goal.