The twin Earth almost had

It’s hard to imagine Mars as a wet place, but that’s exactly what the data and images coming in from the Curiosity rover in Gale Crater are telling us. In fact, Mars is the twin Earth almost had.

Wet Mars.

3.5 billion years ago Gale Crater was filled with ponds of water, with streams cascading down the ancient basin’s walls, down to its wet centre. Eventually these watercourses dried up, but then perhaps the whole cycle repeated numerous times. Of course the water did eventually go for good.

Why?

Unlike Earth, which has a powerful magnetic field protecting its atmosphere, Mars has no such advantage. The solar wind — all those energised particles — just ram straight into it, knocking molecules right out of its atmosphere into space. Which molecules go first? The lightest ones. The hydrogen, the water, the oxygen. What is left is the heavier molecules like carbon dioxide, purely by virtue of the balance between gravitational attraction and the applied force of that solar wind. But that’s planet formation!

How do scientists conclude that there may have been these super wet and dry periods from the geology? By evidence left in the rocks, specifically high concentrations of mineral salts, deposited during periods of evaporation. This is not the first time Curiosity has found evidence of water here. The rover has also unearthed evidence of freshwater lakes.

Gale Crater: Source NASA JPL

The Gale crater itself started life with a bang, and is thought to have been formed by one massive impact. Sediment on the floor of the crater was built in layer upon layer of alluvial deposits, drying into a substantial formation over time. This layered rock was later wind-eroded to form the current Mount Sharp, which Curiosity is busily climbing.

So, we know there was water there, and likely there for long periods of time. The 64 million dollar question is, was this wet environment capable of supporting microbial life at the surface, and if so, for how long? How long were evolution’s engines allowed to turn, working to transform that life? And is that life still present?

If not for the weak magnetic field of Mars, we could have had a celestial twin. A planet in our own solar system with water-based life. Now that is something to think about!

Studies like this are invaluable in understanding our own home. As a SF writer, they provide invaluable insights when it comes to building your own planets! Check out my own world-building in The Tau Ceti Diversion.

With the crew dead, and the starship’s fusion drive held back from a lethal explosion, Karic and the surviving officers reach a habitable planet – the last thing they expected was to find it already occupied . . .

Get it now on Amazon!

Near Future SF

Try some Near Future SF! With the crew dead, and the starship’s fusion drive held back from a lethal explosion, Karic and the surviving officers reach a habitable planet – the last thing they expected was to find it already occupied . . . #TheTauCetiDiversion @ChrisMcMahon111 #ScienceFiction #NearFuture Check it out on Amazon!

Near Future SF

Try some Near Future SF! With the crew dead, and the starship’s fusion drive held back from a lethal explosion, Karic and the surviving officers reach a habitable planet – the last thing they expected was to find it already occupied . . . #TheTauCetiDiversion @ChrisMcMahon111 #ScienceFiction #NearFuture https://amzn.to/2k8k1Vx

Atmosphere on a Fictional Planet

So you’ve got your story working, but how do you sketch out the atmosphere on a fictional planet? Maybe you have some idea of the mass, radius and gravity and you’ve got the orbit in the ‘sweet spot’ goldilocks zone where liquid water can be present on the surface, but what will conditions on the surface actually be like?

What sort of factors go into whether that planet, presumably an Earth-like rocky world, will have an atmosphere that can support terrestrial life?

Planets above a blue planet

The gravity of the planet is one key variable, along with surface temperature, and the strength of the planet’s magnetosphere, which can protect against atmospheric stripping due to solar wind.

The surface temperature of a planet will determine how much kinetic energy, and so velocity, the gas particles will have. If that temperature, and velocity, is high enough it will exceed the planet’s escape velocity and the molecules will fly off into space like tiny spaceship explorers. Earth has lost most of its very light gases like hydrogen and helium in this way, whereas the gas giants have enough gravity to retain them. We kept our water, and we’ve got a lot of it! If Earth was sitting where Venus is things would be different, the additional temperature would give those lighter gases like water vapour enough energy to escape, and also prevent any being trapped on the planet’s surface itself (whereas some is ‘sequestered’ on Earth as water and ice at our lower surface temperature). But beyond the early, settling down period where the lighter gases are lost, any world larger than Earth, orbiting in that goldilocks zone, will not continue to lose a significant proportion of its atmosphere through thermal processes.

Here’s a cool pictorial on thermal escape (source: Wikipedia).

Solar_system_escape_velocity_vs_surface_temperature.svg

Beyond that thermal stripping process, is where the magnetosphere comes into its own, deflecting the solar wind – one of the main non-thermal processes leading to atmospheric loss. The very thickness of a planet’s atmosphere (retained due to its gravity, and as a function of surface temperature), will also protect a planet from the solar wind, even in the absence of a magnetosphere. It’s thought that Venus’ thick atmosphere, ionized by solar radiation and the solar wind, produces magnetic moments that act out to 1.2-1.5 planetary radii away from the planet to deflect the solar wind, much like a magnetosphere (but an order of magnitude closer to the planet). In fact, it’s thought the dominant non-thermal atmospheric loss process on Venus is actually from a type of naturally induced electrical acceleration. On Venus, the stripping of the lighter electrons from the atmosphere causes an excess of positive charges, accelerating ions like H+ out of its atmosphere.

Our explorers need a breathable atmosphere, but they also need an atmospheric pressure like our own Earth’s.

My fictional planet of Cru, in the Tau Ceti Diversion, has comparable surface temperatures to Earth, but a higher surface gravity. The higher surface gravity, and its lower density, allowed me to assume a lighter atmospheric composition, and allow an atmospheric pressure, or weight of atmosphere, close to surface much like Earth’s. That atmospheric composition is crucial to having a reasonable atmospheric pressure – its not just the gravity of the planet. Venus, even though it has slighter lower gravity than Earth, has a crushing atmospheric pressure of 90 times Earth’s due to its heavier  atmosphere of CO2.

Check out what my my intrepid explorers found in my novel The Tau Ceti Diversion when they touched down on the planet!

Read it now on Amazon!

Tau-Ceti-Diversion-severed-ebook-cover (Medium)

SpaceX Claims the Title of World’s Most Powerful Rocket

This week’s launch of the Falcon Heavy Booster on Tuesday (February 6) means that Elon Musk’s SpaceX claims the title of world’s most powerful rocket. The Falcon Heavy can carry twice the payload as its nearest competitor, the United Launch Alliances Delta IV Heavy – and at a lower cost.

Every time I see footage of the SpaceX boosters touching down on reentry I get a shiver down my spine. This is really some revolutionary technology, driven by revolutionary thinking. All based on the simple premise that the most expensive thing about spaceflight is the hardware – not the fuel. If you can reuse the booster that gets you to orbit, then the whole ball game changes.

Watch the Falcon Heavy launch footage here.

 

The two smaller side-boosters completed their vertical reentry landing without a hitch, but the much larger central booster missed its drone-ship landing and crashed into the ocean. Still, the test is considered a success.

Falcon Heavy can lift an impressive 64 metric tons, certainly more than adequate for the astronaut-come-space-dummy and Tesla Roadster that Musk send into orbit around the sun, which is expected to orbit for hundred’s of millions of years! That’s a hell of a time capsule!

Falcon Heavy launches come at an estimated cost of $90 million, with the Delta IV launching 29 metric tons for between $300 and $500 million per flight. It’s easy to see how SpaceX’s paradigm is changing the future of space travel.

 

There are two more Falcon Heavy launches scheduled for this year. The first is a communications satellite, and the second a Space Test Program for the US Air Force that will also launch a solar sail for the Planetary Society. As well as the possible launch of two passengers in a trip around the moon. To apply for a ticket, click here – no, just kidding! – but wouldn’t that be awesome?

And this isn’t the end for the development of SpaceX’s reusable launch systems. SpaceX’s BFR (Big F- Rocket), a megarocket capable of a single-stage to orbit fully fuelled, will potentially launch a spaceship carrying up to 100 passengers, taking us further on a development path that might lead to the establishment of a city on Mars – one of Musk’s ultimate goals.

Space exploration is at the heart of my novel The Tau Ceti Diversion! But they got a little further than the asteroid belt!

Read it now on Amazon!

Tau-Ceti-Diversion-severed-ebook-cover (Medium)

Estimating Surface Gravity on a Fictional Planet

So you want to estimate surface gravity on a fictional planet? Easy!

One of the things I had to do as part of the rework of my novel The Tau Ceti Diversion, is to try and work out the surface gravity of my fictional planets. From the Kepler data, there are two exoplanets located in the Tau Ceti system that are likely to be in the system’s habitable zone, or where there is the possibility of liquid water on the surface, and perhaps life as we know it.

To play around with my estimates of gravity, I used ratioed rearrangements of Newton’s law of gravity (law of universal gravitation) and a simple formula relating the density of a spherical planet to its mass and radius (these are at the bottom of the post in the ADDENDUM).

WARNING: MATHS CONTENT!!!

Here’s Newtons famous law:)

law of gravity

The two planets thought to be in Tau Ceti’s habitable zone are denoted Tau Ceti e and Tau Ceti f. What is known about these two planets is their likely orbit, eccentricity, and their mass. All of these properties have been derived by calculation, based on observed data, so are all known to within appropriate error bounds, but I’m leaving the error off my scribblings so things don’t get too messy.

Tau Ceti e is thought to be around 4.3 Earth Masses, or Me (i.e. 4.3 times as heavy as Earth), while Tau Ceti f, the planet that orbits a bit further out, is thought to be around 6.67 Me. For the astronomically minded, these two planets orbit at around 0.55 and 1.35 AU from Tau Ceti respectively.

So, here’s where I cheated a bit, like any good engineer. I started with the answer I wanted and calculated backwards to see if the answer I wanted led to reasonable base assumptions. This is not as cheeky as it sounds, because when you have an insoluble problem (i.e. not enough data is known for an explicit result), an iterative approach is often used.

For my story to work, I needed a surface gravity on my planet of no more than 1.2g – that’s twenty percent higher than Earth’s. But how could I get a gravity that low on a planet that was over 4 times the mass of Earth? The answer is that surface gravity is a function of mass and radius, or going a step further along the calculation path, mass and density.

I used a ratioed form of Newton’s law that allowed me to relate the ratio of two planets gravitational forces to the ratios of their masses and radii. I already knew the ratio of the gravities ( assumed at gTCe/gE= 1.2) and the ratio of the masses (MTCe/ME =  4.3), so could calculate the ratio of radii (rE/rTCe) at 1.89.  Using another formula that related the ratio of the two planet’s densities to their ratioed mass and radii, I could then calculate their ratioed densities (dens TCe/ densE) at 0.63. So at the end of all that, to have a surface gravity of 1.2 g, Tau Ceti e would have to have a density of 63% of Earth’s. Is that reasonable?

The density of Earth is 5.514 g/cm3, not too much different from the density of a rocky planet like Mercury (5.427 g/cm3), but a lot higher than other solar system planets like Jupiter and Uranus (1.326 g/cm3 and 1.27 g/cm3 respectively), comprised of lighter materials. A surface gravity of 1.2g on Tau Ceti e would put its density at around 3.5 g/cm3, less dense than our own rocky planets, but certainly in a feasible range.

So what sort of densities would you expect for the Tau Ceti system? One clue is the metallicity of the system, which is a measure of the ratio of iron to hydrogen in the star’s makeup. In the case of Tau Ceti, this is estimated to be around one third of our own sun. This indicates the star is likely to be older than the Sun, made up of stellar remnants left over from less evolved stars that have not had time to form as much of the heavier elements in their internal fusion factories.

So Tau Ceti is made up of lighter elements. Based on this, it was reasonable to assume that the planets in the Tau Ceti system would also be made up of proportionally lighter elements, and quite possibly in the range I had estimated. Tau Ceti e and Tau Ceti f are also large planets – much larger than our own Earth – so having a density in between Earth and our own gas giants also made sense to me.

Using the same planetary density I had calculated for Tau Ceti e, for the larger Tau Ceti f, gave me a surface density of around 1.4g for the bigger planet – just a little too high for feasible human colonisation – and that fit nicely with my story as well.

It was a lot of fun playing with these calculations, and thankfully the known science fit with my story, at least with some comfortable wiggle room!

Check out what challenges that increased gravity provided for my intrepid explorers in my novel The Tau Ceti Diversion!

Read it now on Amazon!

 

 

 

 

 

 

 

 

ADDENDUM

For those interested in the maths. . .

Density formula:  densp= Mp / (4/3*pi()*rp^3)

Where:

densp= Density of Planet (kg/m3)

Mp = mass of planet (kg)

rp = radius of planet (m)

In ratio form: densp1/densp2= Mp1/Mp2 *(rp2/rp1)^3

 

Ratio of Newtons law relating gravity, mass and radius of two planets:

gp1/gp2= Mp1/Mp2 *(rp2/rp1)^2

 

When It Rains On Mars

It’s been raining in Brisbane this week, but does it rain on Mars? Is it raining on Mars right now?

Hardly. 

Things on the Red Planet are a little different. Here’s some background.

mars

Mars has around one third of Earth’s gravity, around one hundredth of Earth’s atmospheric pressure, and its atmosphere is almost entirely composed of carbon dioxide. So far we have not found any trace of water. There is ice at the poles, but it’s dry ice – frozen carbon dioxide.

That hasn’t always been the case. The various Mars probes, orbital surveyors and buggies that are still roaming about the terrain have not found any water, but they have found ample evidence that water existed on Mars in the past. There are plenty of geological features on Mars that are consistent with the movement of large bodies of water, and secondary rocks that have been observed that are almost certain to have formed inside ancient lakes. It seems certain that our smaller solar system neighbour had a Warm Wet past.

So where is all that water now?

Very early in its history, things on Mars may have been very similar to conditions on nascent Earth, but striking differences between the two planets led to major changes.

For a start, Mars lacked the powerful magnetic field that could shunt away the effects of the solar wind. Like our other neighbour Venus, which also lacks a strong magnetic field, that means that lighter molecules are knocked right out of its atmosphere. Carbon dioxide is more than twice as heavy as water. The molecular weight of CO2 is 44, while H2O is 18. That means a lighter grip on the molecule by Mars’ already lower gravity. So Mars’ early water has likely to have been irrevocably lost to space.

So it does not rain on Mars, we can be sure of that, but do we want it to?

I think the answer to that question should be a resounding “Yes!”

We live on a small, single planet in a vast, unwelcoming universe. Our planet-evolved bodies just don’t do too well in space. Even trying to orbit the planet in a tiny capsule a few kilometre above our heads is problematic. We have to take all our food and water. There is exposure to harsh radiation, and the threat of cold as heat radiates away into space. The lack of gravity itself is a major threat to our health. We just weren’t built for space, but that’s fine because we have Earth, right?

Well, there might have been an intelligent dinosaur that thought the same thing as it watched a 100 mile wide asteroid plunge into South America 65 million years ago, sending the Earth into a decades-long winter that saw most life die.

That was not the only mass extinction that Earth has experienced. There have been many. Life survived, sure, but every time it was knocked orders of magnitude back down the ladder of complexity. Sentience requires stability. Shelter.

If humanity wants to protect the precious flame of its civilisation, we need to look outward.

The astronomical programs looking to other solar systems are geared to finding Earth analogues. Other Earth-sized planets with similar gravity, with water, and that are the right distance from their suns for life. But its going to be a long, long time before we have the technology to cross the vast distances of space to these new places. Think about this – at the same velocity as the Voyager 1 probe it would take an astronaut 70,000 years just to get to our closest star, which is only 4 lightyears away.

So what do we do?

We can terraform. We take a planet like Mars and make it habitable for humans. We can thicken the atmosphere, releasing chemicals with high global warming potential that heat the atmosphere. We can add water and oxygen by diverting asteroids with these resources and crashing them into Mars’ surface. Maybe we could even do some genetic tampering to help the new crop of Mars humans cope with the lower gravity.

So when will it rain on Mars?

I hope it wont’ be too long, because when rains on Mars humanity will have taken its next, great step into the future, ensuring its survival.