These evenings the western sky is dominated by two planets. The brightest one, closer to the horizon is Venus. The other, a bit fainter and more yellowish is Jupiter. Later in the evening a bright, reddish object rises in the east, Mars. That makes three; the other planet is the one weíre standing on.
All the planets, including Venus, Earth, Mars and Jupiter share similar beginnings. They all formed along with the Sun some 4.5 billion years ago, in the collapse of a huge cloud of cosmic gas and dust. However, their subsequent histories differ dramatically. Venus, the second planet out from the Sun, is very slightly smaller than Earth, which is the third planet. Mars comes next, and is just over half the diameter of the Earth. Jupiter is the fifth planet out from the Sun, with a diameter well over ten times that of our world. However, most of that diameter is its atmosphere. In the case of Venus, Earth and Mars, their atmospheres form a thin skin.
We believe that all four planets started with more or less the same ingredients. However, being further from the Sun and much colder than Venus, Earth and Mars, Jupiter managed to retain more of those ingredients substantially unchanged. This primordial mixture is interesting for two reasons: firstly if one passes electrical discharges through it, like lightning on a young world, the mixture forms aminoacids, important building blocks for life. Secondly it contains strong greenhouse gases.
Billions of years ago the Sun was significantly fainter than it is now. If the Sun suddenly became like that today, our Earth would freeze solid. However, the young Earth had that rich greenhouse mixture which made our world suitable for life to get under way. The new living creatures gradually removed that original atmosphere, replacing it with what we have today. The decreased greenhouse effect compensated for the brightening Sun, keeping conditions on Earth comfortable.
The situation probably started the same way on Venus and Mars too. Venus is closer to the Sun and warmed up faster. If life appeared, it did not remove the greenhouse gases in time. The greenhouse effect ran away and the surface of the planet became hot enough to melt lead and tin. Mars is further from the Sun than the Earth, and could benefit with a lot more greenhouse effect. However, the combination of being a smaller world with weaker gravity, and having no significant magnetic field sealed the planet’s fate. The weak gravity allowed the atmosphere to extend far into space and evaporate away, and the absence of a magnetic field allowed the solar wind to scrub the top of the atmosphere away even faster. The greenhouse effect weakened and Mars became the frozen desert we see today.
There could be life on Jupiter, floating around in the atmosphere. If there is still life on Venus, it won’t be anything like us, and it won’t be easy to go there to look. There could still be life on Mars, hiding under the ground. If it is now extinct there could be fossils in the rocks. Out of the four worlds, Mars is the planet that is now most like ours and also the one easiest for us to travel to and explore.
Venus lies in the southwest after sunset, with Jupiter higher in the sky. Mars rises around 6 p.m., Saturn around 10 p.m. The Moon will reach First Quarter on the 28th, and be Full on the 6th.
Ken Tapping is an astronomer with the National Research Council’s Herzberg Institute of Astrophysics, and is based at the Dominion Radio Astrophysical Observatory, Penticton.