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Mars - was it once much like Earth?

First things first

Mars, the 4th planet from the sun, is often misunderstood as being the closest planet to Earth, which Venus in fact takes the crown on (if we measure by closest proximity at any time). If, however, we are talking about average distance, then, surprisingly, that goes to Mercury for reasons that I cover here.

Pretty standard image of Mars, but it's a great one- I'm sure you'll agree with. Image Credit: NASA.

Image of Mars taken by NASA

The martian world is a cold -65 degrees Celsius on average, with the highest recorded temperature being 21 degrees Celsius (sounds nice...), and the lowest being -125 degrees Celsius (not so nice...).

It's a barren world with craters and volcanoes covering much of the surface, the tallest of which is Olympus Mons. You might be wondering how it fairs up against our infamous Mount Everest?


It's 3 times as high, measuring 374 miles top to bottom, and is as wide as France! In fact, it's possibly the tallest discovered mountain in the Solar System. Although there is a dispute about that - there's a mountain on Vesta, an asteroid, that's supposedly around 315 feet taller than Olympus Mons. Suffice to say though that Mount Everest is next to nothing compared to Olympus Mons!

Mars has 2 moons - Phobos and Deimos, with Phobos being the larger of the 2. What's unusal though, is that Phobos orbits extremely closely to Mars, orbiting around the planet 3 times every Martian day (which is very similar to a day on Earth - only 37 minutes longer.) In comparison, the moon takes 27 days to orbit around the Earth, albeit the Earth is about twice the size of Mars.

Mars is particularly small, and is in fact the 2nd smallest planet in the Solar System, after Mercury.

However, while Phobos orbiting around the planet 3 times a day might seem quite innocent, it has some interesting consequences for both the moon and the planet....

It's all to do with tidal forces, and the conservation of angular momentum. You may have heard that the Moon is gradually moving away from the Earth, which is true (it's been measured).

But why?

The Moon takes 27 days to orbit around the Earth, while it takes only 24 hours for the Earth to complete a single revolution about its axis. This means that the Moon essentially lags behind, but doesn't do so without consequences for the Earth.

If you've read my article on tidal forces (if not then you can check it out here), then you'll know that the Moon's gravitational field causes the Earth to "bulge" slightly, which shifts the oceans towards the direction of the Moon, which naturally changes as the Earth rotates.

This movement of tides creates a frictional force that slows the Earth's rotation down, but conservation of momentum means that if the Earth loses momentum (in the form of angular momentum), then something must gain momentum. That "something" is the Moon, and as it gains momentum, its orbit gets pushed out slightly, away from the Earth.

Remember Isaac Newton's 3rd law of motion - for every action, there is an equal and opposite reaction. In this case, the "reaction" is that the Moon speeds up, and, as it does so, it's orbit gets pushed slightly further away.

How this applies to Phobos is that, because it works the opposite way (Phobos races around Mars 3 times a day), Mars' angular velocity increases, meaning that Phobos loses momentum (loses velocity in this case). Hence, the orbit of Phobos is slightly shrinking, albeit extremely minimally, with Phobos becoming 2 meters closer to Mars every 100 years...

Now you can probably tell what this means, but if not, Phobos will eventually crash into Mars, after millions of years (suspected to be around 30 million), or will it...

An alternative scenario

There's another potential situation that could arise from Phobos getting too close to the planet, which is that it could be ripped apart by tidal forces, and stripped down into a collection of debris in the form of asteroids, before it gets to make any contact with Mars' surface. As the moon gets closer, the difference in gravitational field strength that the side of the moon that's closest to Mars experiences compared to the other side increases, due to another one of Isaac Newton's famous laws - his Law of Universal Gravitation, which states that:

Every particle attracts every other particle in the universe with force directly proportional to the product of the masses and inversely proportional to the square of the distance between them.

That sounds complicated, but honestly it's not, so stick with me here. What this essentially says, is that if you have 2 objects, and you double the mass of one, then the force of gravity between them will double. If you double both, it would quadruple. However, if you double the distance between them, you will not simply double the force of gravity.... instead, you will divide it by 2 squared, which is 4. So if you halve the distance, then you multiply the force of gravity by 4.

Now I have 2 separate posts about tidal forces, one that goes into more of a general discussion about them, and how they cause the tides on Earth, and the other one is a short breakdown of the mathematics behind it:

Click here for the general discussion

Click here for the Mathematics behind tidal forces

But for now, let's accept that the this principle of the Law of Universal Gravitation means that as Phobos moves closer to Mars, the difference in the force of gravity felt by either side increases, until it reaches a limit knows at the Roche limit (of mars in this case), where the moon will deform under this tidal force and break up into a collection of smaller asteroids.

So, to summarise, conservation of angular momentum means that the rotational velocity of Mars is gradually increasing, and Phobos is gradually orbiting closer to the Planet. There are 2 potential outcomes of this, with one being that the Moon simply collides with the planet after perhaps 30 million years, or tidal forces strip the moon apart before then.

Moving on to the History of Mars.....

A domino effect of disasters

It's very widely accepted that Mars was once very similar to the Earth, in that it was abundant in liquid water, and coupled with the once acceptable temperature for microbial life, it's also been one of the strongest candidates for having some version of life forms, albeit potentially very different from that what we see on Earth.

But why is Mars so different now? What changed?

Perhaps the most important characteristic of Mars to note here is its apparent lack of a magnetosphere. Mars does produce a magnetic field, but it's extremely weak, far weaker than the Earth's. The reason for this remains theoretical, but the most accepted theory is that, at some point during Mars' lifetime, it was struck by a large body, such as an asteroid. This collision resulted in an upset of some sort in the chemistry of the planet, which hindered the production of a magnetic field via the convection of liquid Iron at the core.

The reason for such an unusual explanation of this, is because of the need to explain why the magnetic field strength in the Southern Hemisphere is stronger than that in the Northern Hemisphere. This abnormality rules out the more obvious explanations, like the Iron at the core is solid, or doesn't move enough to induce a strong magnetic field, as is the case with Venus - It's much more complicated than this, and the answer is still very unclear.

Of course, I would advise you to research as much as you can about this (you might solve the mystery...), but I don't want to spend too much time going over every highly theoretical concept here.

So why is this important?

Why is the (almost) lack of a magnetosphere important when trying to understand what caused the Martian planet to go from harbouring an Earth-like environment to becoming the cold, desolate, dry planet that we see today?

Well, it's for the same reason that the Northern lights happen here on Earth....

It's important to understand that the sun isn't stable by any means (unless you consider stable as meaning it's not imminently collapsing to its death). Due to inter-twisting magnetic field lines (I talk more about it in this article), there are disparities in the temperature at the Sun's surface which is what causes sunspots to form on the Sun (see image below).

But, more importantly, irregular outward expansions of plasma (which are due to the same mechanism as talked above) that travel to the sun's outermost layer (the "Corona"), become superheated, and achieve velocities higher than the escape velocity of the sun.

Hence, they're ejected into the cosmos as a wave or halo of energetic charged particles (essentially forms of charged ions, electrons and protons) that travel at about a million miles per hour! These solar winds as they're known as, can be extremely devastating if they're not deflected, as they contain intense levels of energy.

Fortunately for us, our strong magnetic field interacts with these energetic charged particles, as they're energetically charged and therefore experience a force in a magnetic field, deflecting them to the poles as that's where our magnetic field lines converge at.

These energetic particles are then merely absorbed by the gases that make up the atmosphere at the poles, such as Nitrogen and Oxygen, which in turn release this energy in the form of visible light radiation... sound familiar? The effect of this is what we call the Northern/Southern lights! Although I talk more about this here.

Here we have the sun releasing a solar flare as described above... (middle right) Image Credit: NASA

Image of the sun taken by NASA

But without this magnetic field, we would be completely bombarded by these highly energised particles that would have devastating effects, such as extreme radiation poisoning, and the boiling of our oceans.

Now I've probably given away why Mars is so desolate now.....

A desolate rock

In the absence of a strong enough magnetic field, solar wind from the Sun stripped Mars of much of its atmosphere, and consequently boiled the oceans away. When I say the water "boiled " away, it's important to understand that this isn't due to an increase in temperature. There are 2 factors that can affect a particle's energy state, and hence it's state of matter: Temperature and Pressure.

In this case, because the atmosphere is extremely thin, there's simply not enough pressure to keep water in its liquid state, and so it drops to a higher energy state - a gas. The transition from liquid to water is what we call the boiling phase, and is usually associated with an increase in temperate, but just know that a decrease in pressure will do the same. You can read more about the relationship between pressure, temperature, and state of matter here

In fact, the atmosphere of Mars is only about 1% of the volume of Earth's. This is mostly due to the fact that the planet is a mere 10% of the Earth's mass, and about half the size in volume.

The consequence of such is that the planet's gravitational field strength at the surface is relatively small, with an escape velocity of 5km per second, compared to the escape velocity here on Earth of 11km per second. Consequently, the planet has a very hard time holding on to an atmosphere, as molecules can easily achieve high enough velocities through the absorption of radiation from the Sun to surpass this escape velocity, and essentially run off into space.

The thin atmosphere also means that Mars has a very difficult job holding on to its heat, as the radiation from the Sun reflects off the surface and returns to space, without being largely absorbed by molecules in the atmosphere, as is the case with the Earth and to the extreme in Venus.

Going left to right, we have Mercury, Mars, Venus, Earth.... Here you can see that Mars is considerably smaller in size than the Earth, being far more comparable to Mercury.

size comparison of all the planets in our solar system

In despite of this, however, it's absolutely clear to us that Mars is the only planet in the solar system that we could feasibly survive on, albeit with a lot of human intervention. For one, we would somehow have to increase the amount of oxygen in the atmosphere, as at current levels, we wouldn't stand a chance of surviving.

The landscape

Referring back to the landscape of Mars, as you may have heard before, there's an abundance of craters scattered across the surface of Mars.

These are generally believed to have formed from the same type of events that formed the craters here on Earth - Asteroid collision. It's hard to say whether these craters would've come from collisions in the early life of Mars, or whether they're relatively new, but what we can surmise is that Mars would've formed through asteroids themselves colliding, and gelling together under the elevated temperatures that would have come from such collisions.

Therefore, it's highly likely that Mars, as well as the other 3 terrestrial planets, would have got their craters from these collisions.

This article has involved a few heavy physical concepts, so I apologise if you came here for more Mars-specific factual content, but hopefully you still feel like you learned quite a bit of that, plus some very useful physics.

We have a category of posts called "know your neighbours!" that's dedicated to this solar system, so if you want to learn more about the other planets or the sun, then I'd definitely recommend checking them out (you can go back to the main blog page and select the "know your neighbours" filter)

Thanks for reading!

Where to next, Ryu?


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