Early Mars Oceans

The compositions of the crusts of the Moon and Mars are distinct - one is dominated by feldspathic cumulates from an early magma ocean, and the other by basaltic lavas. Regional patterns reflect differences in subjacent mantle compositions. The compositions of the mantles and cores of these bodies can be constrained by chemical analyses of mantle-derived basalts. The interiors of both bodies have remained geochemically isolated, because of the absence of plate tectonics. 

On Mars (a tenth of Earth mass and 38% of its radius), the present water inventory is much less, enough to cover the planet to a few tens of meters puddle oceans (Carr, 1996). On Venus, which must have been very nearly Earth s twin prior to the giant impact on Earth (0.815 modern Earth mass, 95% of its radius), the atmosphere evolved to its present runaway CO2 greenhouse. There has been much speculation about early Venusian oceans, perhaps some kilometers deep, but possibly only a few meters if Venus formed too close to the Sun to inherit a large water inventory (see Taylor, 2001 for a brief summary of this dispute). 

Hydrogen isotopic compositions, expressed as molar D/H ratios, of solar system bodies. The relatively low D/H values in the atmospheres of Jupiter and Saturn are similar to those in the early Sun, whereas D/H ratios for Uranus and Neptune are intermediate between the Jupiter-Saturn values and those of comets and chondrites. The Earth s oceans have D/H shown by the horizontal line. Mars values are from SNC meteorites. Modified from Righter et al. (2006) and Lunine (2004). 

The current surface of Mars is cold and dry (Fig. 5.13). Early oceans or lakes would have dried out either by evaporation or freezing. Solution C was allowed to dry by either (a) evaporation at 0°C (Fig. 5.14a) or (b) freezing to the eutectic (Fig. 5.14b). Both equilibrium-mode evaporation and freezing lead to six precipitated salts (Fig. 5.14) because the number of independent salt components is equal to the number of cations (4) + the number of anions... 

Depending on the rate at which the Sun loses mass on its way to becoming a white dwarf, there are different fates for the Earth. If mass is lost early on, the dry Earth could escape being vaporized as its orbit expands in response to the Sun s lower mass. As a result, the Earth would orbit near Mars s current position as a cenotaph to humankind s past glory. It might be a featureless cenotaph, and I wonder if we could trace some of Earth s ancient shorelines and ocean basins. On the other hand, if the mass loss occurs late in the Sun s evolution, the Earth may actually orbit inside the outer solar atmospheres. In this case, the Earth would be burnt to a crisp and pulled deeper into the Sun. 

The committee surveyed the inventory of environments in the solar system and asked which non-Earth ones might be suited to fife of the terran type. Such locales are few, unless there are laws not now understood that could govern the early stages of the self-organization of biochemical structures and processes that could lead inevitably to evolving life forms.15 Subsurface Mars and the putative sub-ice oceans of the Galilean satellites are the only locales in the solar system (other than Earth itself) that are clearly compatible with terran biochemistry. 

As we have seen (Chapter 6), Mars is rich in iron peroxides but their abundance in Martian soils tells us nothing about how quickly they were formed on the early Earth. While they were almost certainly formed on Earth (which is, after all, closer to the Sun, and so more drenched in ultraviolet rays), the abundance of hydrogen peroxide on Earth would have depended on its rate of formation and destruction — and these in turn are dependent on atmospheric and oceanic conditions. While the existence of catalase implies that hydrogen peroxide was indeed abundant, the story is suggestive but far from conclusive. Luckily, there are other ways to answer the question, and they not only support the notion that photosynthesis evolved in response to oxidative stress, but they also explain a few other long-standing paradoxes. 

Studies of Mars by spacecraft have indicated that it was once a wetter, more habitable world than the cold desert planet of today. Dried stream beds and river channels attest to liquid water flow on the surface. There is even evidence that the northern lowlands may have held an ocean. The geological data indicate that the main epoch of liquid water activity was very early in Mars history, although some limited level of liquid water activity may have continued even to the present time. 

The detection of atmospheres on extrasolar planet is a very difficult task. 71% of the Earth is covered by oceans but up to now it is the only planet with water in liquid form on its surface. Venus might have had water on its early history, on Mars water may exist in a frozen state near the surface and climatic changes have occurred and formed river-like structures that are observed on its surface. There exists the possibility to find condensed water in the atmospheres of Jupiter and Saturn and in deeper layers of Uranus and Neptune. Subsurface oceans may exist on several satellites of the giant planets. But how can we detect water on extrasolar planets, how can we detect whether these objects have even an atmosphere ... 

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