H P

Fig. 4.15 The thermal profile of Venus' atmosphere, compared with that of the Earth (Encrenaz etal.,2004)

4.4.3.3 Comparisons Between the Evolution of the Terrestrial Planets

Why have the terrestrial planets evolved in such different ways? Mercury's case is simply explained: the escape velocity at its surface is too high (its gravity being weak and the temperature too high) for the planet to retain a permanent atmosphere. Initially, the other three planets had an atmosphere consisting of CO2, N2, and H2O. On Earth, a vast quantity of water is present in liquid form, but it appears, as water vapour, in only trace amounts in the atmospheres of Venus and Mars. How do we know that abundant water was originally present? That information comes from measurement of the D:H ratio, determined by infrared spectroscopy from the HDO:H2O ratio. Observations have revealed an important result: the D:H ratio on Venus is enriched by a factor close to 120 relative to the terrestrial value, and the same ratio on Mars is about 6 times the terrestrial value. Interpretation of this enrichment is as follows: on Venus, water, which was probably extremely abundant initially, has largely escaped; heavy water, HDO, escaping less easily than H2O, has

100 1 50 200 250

Temperature (k)

Fig. 4.16 The thermal profile of Mars as derived by the Viking probes (Encrenaz et al., 2004)

100 1 50 200 250

Temperature (k)

Fig. 4.16 The thermal profile of Mars as derived by the Viking probes (Encrenaz et al., 2004)

accumulated over the course of the planet's history. The same reasoning applies to Mars to a lesser extent, and seems to indicate that the primitive atmosphere of Mars was denser than it is today. The presence of traces of fluid flow on the surface of Mars is another indicator of the presence of liquid water during the planet's history.

A broad outline of the history of the terrestrial planets may thus be given. The primitive atmosphere of Venus, dominated by CO2 and H2O, was sufficiently hot (because of the planet's heliocentric distance, which is slightly less than that of the Earth) for water to be in the form of vapour. As CO2 and H2O are both particularly effective greenhouse gases, the temperature of the surface and of the lower atmosphere rose and, in the absence of any regulatory mechanism, there was a runaway greenhouse effect, leading to the surface conditions that we see today. The water vapour has disappeared, probably through photodissociation and subsequent loss of hydrogen. The Earth's heliocentric distance was such that water occurred primarily in liquid form. Most of the carbon dioxide was therefore absorbed, and the greenhouse effect remained at a moderate level over the Earth's lifetime.

The case of Mars is different from those of Venus and the Earth, because the planet, farther away from the Sun and thus colder, is also only about one tenth of the mass. Its internal energy sources and its gravitational field are thus much weaker: two factors that prevented it from acquiring a primitive atmosphere that was as dense at those of Venus and the Earth. The planet appears to have had a magnetic field early in its history, but that this decayed within the first thousand million years. The primitive atmosphere must have been denser than today (without ever being comparable with that of Venus), but this has not been confirmed. What happened to the water on Mars? Although it is almost completely absent from the atmosphere, it seems that a significant quantity may be trapped beneath the surface, in the form of ice in the polar caps, and perhaps in the form of permafrost at lower latitudes. Understanding the past climate of Mars, from its origin to the present day, is one of the major issues to be tackled by the exploration of Mars.

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