Two siblings of the Earth, two different temperaments

In our solar system all four inner planets are different from outer gas giants. Gas giants Jupiter, Saturn and ice giants Uranus and Neptune are more or less very similar to each other in the composition made up of hydrogen and helium and traces of other gases. Inner planets are rocky with some or no atmosphere. Mercury is an airless world with the sunlit side reaching 427 degree C and night side -173 degree C, that is understandable. But the remaining three are a bit eccentric in their climatic temperament. Let’s find the differences and underlying causes for the same. This understanding is going to be quite  important for the future of humanity. 

Let’s start with our home planet, the Earth. It’s the only place in the Solar system that has liquid water on the surface. Water is almost everywhere be it comets, asteroids, dwarf planets, most moons of outer gas giants ts or ice giants plants, Mars, our Moon and on mercury too but only in the form of frozen ice. The only planet where no trace of water is found is Venus. Earth and Mars both have very dynamic weather and regular seasonal cycles.  Venus on the other hand doesn’t have much seasonal variability as its axis though is upside down but has very little tilt of 2.64 degree, compared to its orbital plane. What makes these three ‘sibling’ or ‘sister’ planets so different from each other is distance from the Sun and difference in sizes. 

Venus burning inside 

Temp 438 to 482 with mean of 253 degree C

Atmospheric pressure 93 time of Earth 

‘Air’ composition:

96.5% Carbon dioxide

3.5 Nitrogen

Let’s start with the innermost of the three planets Venus. It is only 638.4 km less in diameter and its mass is 81.5%of the Earth. But its atmosphere differs drastically from the Earth. The Venusian atmosphere is 96.5% carbon dioxide and the remaining 3.5% is nitrogen. Atmospheric pressure is 93 times that of Earth surface pressure. When Venus was formed it must have been very similar to the Earth but being close to the sun made it slightly warmer. Another thing different was its internal geological processes were different from the Earth. Earth’s solid iron core is surrounded by molten iron and nickel core, rotation of this molten core produces Earth magnetic field. Probably Venus’s molten core has solidified much earlier in its history and Venus lost its magnetic field. Or the core of Venus had a liquid iron core but it was slow in rotation and did not have convection current resulting in failure of magnetic dynamo. On the earth this magnetic field shields earth from the solar wind  so on be us lake this magnetic shielding made it loose atmospheric water very early in its history.  It didn’t have an active dynamic crust too which helps in the burial of surplus Carbon dioxide. Venus had little more Carbon dioxide to begin with, this made Venus atmosphere a little more hotter than the Earth.  This little more temperature leads to a little more carbon dioxide released by rocks, a little more carbon dioxide causes more heating of the planet and so on. This led to a ‘catastrophic’ runaway  greenhouse effect so much that the planet named after the Greek goddess of beauty turned into a planetary ‘hell’! Till date only two Russian probes Venara 7, 9 and 13 could land on the surface and return images of its surface.  Both Venera probes were burnt in 23 and 63 minutes but managed to send more data. 

Red Mars out in the cold 

Temp -78.5 to 5.7 degree C

Atmospheric pressure 1/100 of the Earth

‘Air’ composition:

95.97% Carbon dioxide 

1.93% Argon

1.89% Nitrogen

.146% Oxygen

The last of the terrestrial planets is about half in diameter from Earth but  has a ‘real estate’ equal to the total land mass of earth. The most striking feature we can see through any decent telescope is its polar ice caps.  These ice caps periodically grow and shrink from Martian winter to summer that last twice the duration of Earth’s seasonal cycle as Mars takes 686 days to orbit the Sun. Mars’s axial tilt of 25.19 degree is very similar to the Earth’s tilt of 23.5. Most atmospheric carbon dioxide is frozen in the polar ice caps. So there very little greenhouse gases are  available to warm the planet to make it hospitable. Another noticeable feature  that can be seen from the images captured by large telescopes and  sent by various orbital probes patterns on the ground resembling marks left by dried up river and delta made by river falling in the sea. Polar ice cape is made of a mixture of water ice and carbon dioxide ice or  dry ice.  Some earlier visual telescopic observations have mistakenly reported finding canals crisscrossing its surface. This led to wild speculation of discovering advanced  engineering civilization that has built irrigation canals to supply water to dry equatorial areas from frozen ice caps.  Further observation using better telescopes at more suitable observation conditions couldn’t do any such things. Now many orbital probes including our own Mangalyaan have mapped Martian surface in detail matching with terrestrial space surveys . And we didn’t find any evidence of any kind of life forgate the any trace of  technological civilisations. 

If we ever want to build a human space colony on Mars we have to understand how mars has lost its atmosphere and most of the water. Though we have a rough picture of what happened at the dawn of Solar systems planet formation. Mars probably never had any magnate of field to begin with, so solar winds strip away its most atmosphere. Most water was in the early era of heavy bombardment of asteroids and comets and got splashed away by such impacts as Mars has less gravity compared to the Earth. Remaining water was ionised by UV radiation in hydrogen and free oxygen. Hydrogen was lost to space and oxygen combined with surface iron and provided Mars with its signature red colour. 

It reminds me of Earth as our own Earthly ‘Garden of Eden’. It is a classic case of a habitable planet (though we have seen no other habitable planet yet)  with water in all the three forms: solid liquid and gas.  A home planet, not too hot, not too cold, in the so-called Goldilocks zone. But how long it will remain in such comfy  condition is a million dollar question. If we keep pumping carbon dioxide by burning fossil fuel, methane from our agriculture and animal husbandry and nitrous oxide as an industrial waste we might end up as a planet not very unlike Venus. Some extemophile bacteria may evolve to survive in such hot conditions but our civilization will collapse after Earth’s average temperature has risen by 5 degree centigrade.  We have already hit the 1.5 degree C mark and at the  current rate of resisting global temperature, we might reach the 5 degree mark by the end of the current century. On the brighter side we might learn how to warm up Mars quickly to build human settlement but will have no one left to build it. Maybe Robots equipped with AI will be our descendants colonising Mars.