Earth, Venus and Mars

Earth, Venus and Mars
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					Updated on 05/05/2015

Water is abundant on the Earth, but not on Venus and Mars. This is because of their significantly different atmospheres and because of the presence or absence of plate tectonic activities.

Considering the vast distances in Solar System, Venus, Earth and Mars are in about the same vicinity relative to the Sun, compared to the Jovian and the outer planets. Venus is closer to Sun than Earth, of course, and Mars is farther. But the resulting difference in Solar heat input is not what makes the Venutian surface too hot, or the Martian surface too cold, to have water. Also, judging from their comparable overall densities, they have similar chemical and mineral compositions. Neither of these explain what makes Earth unique in having the abundance of water that Venus and Mars lack.

Solar irradiance, or the heat received from Sun, decreases inversely with the square of distance. The amount of this heat received by the Earth is about 1.37 kW/m², for instance, whereas Venus, located at 0.72 AU (1 AU = Mean Sun-Earth distance), receives about twice this amount (= 2.55 kW/m²) and Mars, located 1.52 AU from the Sun, receives barely one-half the solar irradiance that reaches the Earth, or about 0.55 kW/m². Only about two-thirds of the solar heat received at the Earth's distance actually reaches the planetary surface, however, as one-third of it is reflected away by Earth's atmosphere. The much thicker atmosphere reflects more than two-thirds of this irradiance and the extremely thin atmosphere reflects very little.

Effectively, therefore, solar heat received on the surfaces of the three planets do not vary as significantly as the solar heat received at their locations relative to the Sun.

The three planets have vastly dissimilar atmospheres. Earth’s atmosphere, a ~110 Km thick gaseous halo that encases the Earth, now comprises ~78% Nitrogen and ~21% Oxygen, but was nearly 90% CO₂until about 1.25 Ga ago.

	Planetary Composition	Atmospheric
	Planetary Composition	Compo-sition	Density (Earth = 1)
Venus	Rocky, with metallic core	C, O	90
Earth		N, O	1
Mars		N, O?	0.01

The atmosphere of Venus has 90 times the density of Earth’s atmosphere, and is ~95% CO₂. Since this traps the Solar heat that is received on that planet’s surface, Venus is also called the “Greenhouse Planet”. Mars, on the other hand, has a very thin atmosphere.

Surface temperatures on Venus are very high, therefore, and the average day-time temperature is about the same (~450ºC) as its mean night-time temperature. This is because the lengths of day and year are about the same on Venus, i.e., the planet takes about the same time to complete one spin on its axis as it does to complete one orbit about the Sun and therefore has the same face turned towards Sun all the time. The planet’s thick atmosphere retains and distributes evenly this Solar heat that one side of the planet thus receives constantly, so producing the intense temperatures that preclude any possibility for even the water molecule to exist. This renders moot any question of hydrological cycle on Venus.

As for Mars, there is a good possibility that running water was once abundant enough on the Martian surface to have produced the land-forms we now see but now remains confined, at best, to the subsoil and the polar ice cap. Two questions then arise:

Did oceans comparable to the earth ever exist on Mars?

Where did all this running water go?

3	Two of the many images of Martian surface, obtained by Pathfinder and subsequent missions, that point to the role of running water in shaping the Martian landscape.


	NASA's 2001Mars Odyssey spacecraft returned this view of the south pole of Mars in intermediate energy or epithermal neutrons. Soil enriched in hydrogen is indicated by the deep blue colors, where a low intensity of epithermal neutrons is found. This view of Martian south pole comes from measurements made in the first week of Mars Odyssey's mapping, in February 2002.
		4

NASA's Mars Exploration Spirit, as it started sending back images from the surface of Mars in an engineering feat that has been particularly remarkable
because of problems in receiving signals from European Space Agency's British craft Beagle 2 . The overall scientific goal here has been to look for possibility of life, and the immediate goal has been to look for water, on Mars. Click on the picture for updates from JPL or here for the updates from BBC.

That water must have existed on Mars is also suggested by the discovery of jarosite, a potassium iron sulfate hydroxide mineral that was first found in Jaroso, Spain, and clearly points to the planet's watery past. The problem is that this is only an evidence of water-limited environment. The carbonate minerals that would clearly suggest the existence of either an ocean or ocean-like conditions on Mars remain as yet undiscovered.

These images, taken by cameras on the Mars rover Opportunity, show a close-up of the rock outcrop dubbed "El Capitan," located in the rover's landing site, a crater at Meridiani Planum. Inset, a detail of the rock showing one of the tiny spherules, nicknamed "blueberries." NASA/JPL

As for plate tectonics on Mars (also click here),
scientists have discovered magnetic stripes on the surface of Mars similar to fields in the sea floors of Earth that may be a result of ancient crustal spreading. The discovery is likely to revolutionize current thinking of the red planet's evolution.

l This is because plate tectonics once occurred on Mars, but no longer does. Mars thus lacks hydrological cycle because it no longer has the plate tectonics to create new ocean basins to replace the ones flattened by the “run-off” component of hydrological cycle*.

l The presence of hydrological cycle on the Earth, and its absence on Mars, is therefore due as much to the planetary atmospheres as to plate tectonics, while its absence on Venus is entirely ascribable to the structure and composition of Venutian atmosphere.

* Seeking to answer this question by appealing to low density of Martian atmosphere, and argue that its temperature gradient is too gentle to have prevented the escape of atmospheric moisture (unlike Earth’s tropospheric thermal gradient that is steep enough to have retained the hydrological cycle), ignores the fact that a vigorous hydrological cycle may have once existed on Mars.

		l The three planets have vastly dissimilar atmospheres. Earth’s atmosphere, a ~110 Km thick gaseous halo that encases the Earth, now comprises ~78% Nitrogen and ~21% Oxygen, but was nearly 90% CO₂until about 1.25 Ga ago. The atmosphere of Venus has 90 times the density of Earth’s atmosphere, and is ~95% CO₂. Since this traps the Solar heat that is received on that planet’s surface, Venus is also called the “Greenhouse Planet”. Mars, on the other hand, has a very thin atmosphere.
l
that preclude any possibility for even the water molecule to exist. This renders moot any question of hydrological cycle on Venus. l As for Mars, there is good evidence that water was
	l This is because plate tectonics once occurred on Mars, but no longer does. Mars thus lacks hydrological cycle because it no longer has the plate tectonics to create new ocean basins to replace the ones flattened by the “run-off” component of hydrological cycle*. l The presence of hydrological cycle on the Earth, and its absence on Mars, is therefore due as much to the planetary atmospheres as to plate tectonics, while its absence on Venus is entirely ascribable to the structure and composition of Venutian atmosphere.

l