Sky Matters: Just as people change and age, so to do planets

Climate change and its impacts are constant, and not just on Earth. We only have to look at our two nearest neighbour planets to see examples of how rampant climate change has transformed them into inhospitable wastelands

You might expect Venus to be hotter than Earth. But not as hot as it actually is – surface temperatures sit in and around 400C. Picture: iStock

Climate change is not a new phenomenon. It has happened before, it is happening now, and it will happen again. And not just on the Earth. We only have to look at our two nearest neighbour planets to see examples of how rampant climate change has transformed them into inhospitable wastelands.

Venus may well have started off on the same evolutionary path as the Earth, some 4.5 billion years ago. It’s a similar size, though being closer to the Sun it receives more sunlight. You might expect it therefore to be hotter than our home planet. But not as hot as it actually is – surface temperatures sit in and around 400 degrees centigrade. Somewhere in the past Venus’ atmosphere was injected with greenhouse gases on scales thankfully never seen on Earth, and the inevitable greenhouse catastrophe ensued.

By contrast, the planet Mars (which is a little farther away from the Sun than the Earth), is now a mostly frozen planet. It wasn’t always that way. Mars had a thick atmosphere when it was young. There were rivers and oceans, with huge canyons which dwarf some of Earth’s largest, such as the Grand Canyon in Arizona. Mars is about one third the size of the Earth and it has no magnetic field. The lack of gravity and magnetism meant Mars couldn’t hold onto its atmosphere. And without an appreciable atmosphere things get progressively colder and colder.

Mars had a thick atmosphere when it was young. There were rivers and oceans, with huge canyons which dwarf some of Earth’s largest, such as the Grand Canyon in Arizona. Picture: iStock

There has been a lot of talk recently about colonising Mars. It’s not really an accurate depiction. For sure, we will likely be able to send people to Mars, to live inside enclosures, exploring the outside only from within the protection of high-tech spacesuits. But future Mars will never be like current Earth. That window of opportunity has passed. It was always fleeting. Mars was always going to lose its atmosphere. Just as people change and age, so do planets. You can ultimately keep both on life support, but you can’t reclaim their youth.

On June 21st we will all experience the annual Summer Solstice when the Sun reaches its apparent highest northerly latitude. This is due to the Earth’s axis being tilted at 23.5 degrees and it’s largely responsible for the cycle of the seasons. Mars, with an axial tilt of 25 degrees, experiences its own Summer Solstice. However, simulations suggest that this tilt varies over a few million years, and is sometimes at zero degrees. During those periods the Sun traverses the same path in the Martian sky day-after-day. The seasons on Mars would only then happen because its distance to the Sun varies – when it’s closer it would be warmer. By contrast, Venus has an incredibly stable axial tilt of only 2.7 degrees (though it rotates the “wrong way round” with the Sun rising in the Venusian west and setting in its East). Venus therefore experiences no seasons – just perpetual temperatures high enough to melt lead!

The solar system, including the sun and each of the eight planets — Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune — and a representation of their orbit. Picture: iStock

If you look up at the stars during June you’ll notice how the sky never really looks dark, as it does during winter. This is due to the Earth’s atmosphere scattering sunlight combined with the fact that the Sun doesn’t dip very far below the horizon at this time of the year. The skies above are still beautiful to look at, perhaps not quite as crisp in the depths of winter.

On Mars, with its very tenuous atmosphere, scattering of sunlight is much less pronounced and the summer solstice nights would be just as dark as during the Martian winter (more or less). On Venus, the thick blanket of cloud means the stars are never visible, so the point is somewhat moot.

Interestingly, the Moon keeps the Earth’s axial tilt stable at around 23.5 degree and has done so for billions of years. It’s almost like the “Man in the Moon” is not just watching us, but rather watching over us.