What does the sun look like from Mars?

[gazza]

I have a few questions about Mars I hope you can enlighten me about.
Firstly,what does the sun look like from Mars,that is the comparative size of the disk with that on Earth? I saw a very poor picture somewhere on the web but couldn't really get a handle on it,perspective/lack of points of reference etc.The picture made it look very tiny indeed , not much bigger than say Venus as viewed from Earth.This suprised me.
The pictures so far shown by the latest lander are enhanced so I'm wondering does it show the true brigtness of the surroundings up there?.Are the light levels very poor in reality ,equivalent to say dusk here on earth?
My last question is about terraforming Mars into a habitable Earth like planet.I keep reading a lot about this here and there as if its a forgone certainty.I know little about the processes intended but am I correct in thinking there is a major flaw which will make this dream impossible? Namely that Mars has a very weak and patchy magnetosphere,so that any artificially generated atmosphere will be stripped away by the solar wind action overtime? Therefore any settlement by humans will be restricted to greenhouse-like accomodation at best.Any thoughts on this?

 

[LURCH]

I think it all comes back to plate tactonics. If the mantle could be heated enough to re-start plate tactonics, the volcanoes would belch out the greenhouse gasses necessary to maintain that heat, and the activity in the core would produce the magnetic field needed for protection of the atmosphere (and any potential residents!) from Solar radiation.

 

[Nereid]

Welcome to Physics Forums gazza! You're not, by any chance, from Melbourne (Australia)?

As to the size of the Sun as seen from Mars, you can work that out quite easily yourself.

If the average distance of the Earth from the sun is 150 million km, and that of Mars 230 million km, then how much smaller will the Sun appear on Mars than on the Earth?

As to the brightness: remember that the Sun's light spreads out in a sphere, so the number of photons falling on a square km (normal to the line to the Sun!) is proportional to the square of the distance. Of course, the Sun's light has to go through the Earth's atmosphere and Mars' - and that does make a difference! - but you can get a good first approximation.

 

[gazza]

Thanks for your welcome and replies folks.
Not from Aus,I'm from England Nereid.I'm sorry Nereid but you will have to paint me a more lucid picture as I'm not an academic or anything.Not good at Maths either.As we say here in England "a bit thick".
So am I right in saying from what your infering that if I held up a ruler at arms length and measure safely the suns disk diameter here on Earth it would be at approx 3cm, then as veiwed from Mars it would be reduced in size by approx a third to 2cm?
As to the atmosphere on each planet,which allows more light to pass through,Mars'? Therefore negating the fact that less photons fall per sq/m on mars? Or would midday there still be like twilight here?

As for plate techtonics,does the amount of energy from the sun falling per sq/m on mars' surface have any effect on the mechanism I wonder? Maybe the energy absorbed however negligable tipped the balance in halting the processes?

 

[russ_watters]

Originally posted by gazza
As to the atmosphere on each planet,which allows more light to pass through,Mars'? Therefore negating the fact that less photons fall per sq/m on mars? Or would midday there still be like twilight here?

The atmosphere of Mars is very very thin. Something like 1/100th of ours (iirc).

Vision is not linear as far as brightness goes and your eyes (and cameras) can make large adjustments to get the brightness to a reasonable level. As a result, the difference in brightness for you standing on Earth vs Mars would be barely perceptible. With other things that affect brightness (clouds, the atmosphere, color/reflectivity of objects) you wouldn't know the difference in the actual brightness of the sun unless you saw the size of the disk.

Ever see a solar eclipse? The only one I have ever seen was 96% and the sky was eerie - it was like a partly cloudy day except that there were no clouds. At 96% the difference was very small, only enough to be a little eerie.

 

[Nereid]

Or would midday there still be like twilight here?

Russ pointed out a number of factors; we could go on with a longer list, such as:
- the amount of dust in the Martian sky; during the Martian dust storms, there can be quite a lot, and for a while after the storms have died down, there's still a lot of very fine dust. This both dims the Sun's light and colours the sky an ochre-pink colour (but colour perception is very complicated!)
- the brightness of the sky; this is what makes 'daylight' seem so different on different worlds, even with much the same incoming light from the star - think of astronauts on the Moon. The Martian sky is also bright (not black, like on the Moon), though perhaps not so bright as in the tropics on Earth
- glare

As for plate techtonics,does the amount of energy from the sun falling per sq/m on mars' surface have any effect on the mechanism I wonder?

No

 

[Jimmy]

Originally posted by gazza
So am I right in saying from what your infering that if I held up a ruler at arms length and measure safely the suns disk diameter here on Earth it would be at approx 3cm, then as veiwed from Mars it would be reduced in size by approx a third to 2cm?

Greetings Gazza!

You are correct. There is a linear relationship between the apparent diameter and the distance from the sun. If you were to double the distance from the sun, it's apparent diameter would decrease by a factor of two. In math speak, the apparent diameter of an object is inversely proportional to the distance from that object.

If you are talking about the apparent size of the sun in terms of area, then the relationship is different. The area of a circle equals pi * R^2. So if the radius were to decrease by a factor of 2, that is, if we were twice as far away, the apparent area of the sun would decrease by a factor of four. Double the distance, area decreases by a factor of four. This is known as the inverse-square law. The intensity of radiation from a source also follows this law.

 

[gazza]

Thanks all for taking the time to reply.Some interesting info for me to digest & mull over.

How would you get plate techtonics started on Mars again? Any theories?
Maybe altering the orbit/trajectory of a suitably large asteroid or some such object to smash into mars? Perhaps one made of ice to reintoduce water on a large scale?

 

[Nereid]

gazza wrote: How would you get plate techtonics started on Mars again? Any theories?

not easily! AFAIK, plate tectonics arise from convection currents in the plastic mantle. On Mars, the mantle has been frozen for at least 3 billion years, and the core is now too cold to start stirring things up again.

Shake things up by moving another solar system object - such as Pluto, Charon, Ceres, Rhea, Iapetus, Titania, Oberon, or Triton - so they collide with Mars? We'd have to wait until the magma ocean that becomes Mars' surface cools, but that'd be only a few million years.

Pour ~10 to 100 million tonnes of molten iron, laced with lots of thorium and uranium, into the Hellas basin? Then wait for it sink to the core and start heating up the core and restarting tectonics? Also have to wait many a million year (and may take 1 to 10 billion tonnes of iron)

 

[selfAdjoint]

Put a big object (Titan?) in close noncircular orbit around Mars and wait for gravitational stresses to heat things up?