2 differences in Earths habitability

[willstaruss22]

How would life be different if Earths density was 1/2 what it currently is?

Would there be changes to the atmosphere, plate tectonics, volcanic activity or the magnetic field? How would life have evolved in such conditions?

[eburacum45]

How would life be different if Earths density was 1/2 what it currently is?

Would there be changes to the atmosphere, plate tectonics, volcanic activity or the magnetic field? How would life have evolved in such conditions?

The main problem is that a planet with 1/2 Earth's density would need to be made of something much less dense than silcate rock. The most likely candidate is water; you are basically describing a water-world with a deep ocean and a thick high-pressure ice mantle.
See this calculator;
http://www.transhuman.talktalk.net/iw/Geosync.htm
a planet with a density half that of Earth would have a density of 2.75 g/cm3, only a little bit more than the density of a pure water planet. Such a world would have a small silicate core, or perhaps an even smaller iron core.

I'm not sure life could start on such a world, but maybe there would be enough trace elements dissolved in the ocean to support life once it had found its way there somehow. With a very small silicate core there might be some sort of magnetic field, but plate tectonics would be more or less irrelevant on such a world, I think. Any volcanic activity would have to fight its way through hundreds of kilometers of warm, high pressure ice to even reach the bottom of the ocean.

[willstaruss22]

Ok but if we brought up the density to say 4.4 g/cm3 would life far better or would it still be barley habitable? How would this density effect the atmosphere and the magnetic field?

[eburacum45]

Ok but if we brought up the density to say 4.4 g/cm3 would life far better or would it still be barely habitable? How would this density effect the atmosphere and the magnetic field?

You need to define this hypothetical planet a little more clearly. Are you talking about a low density planet with the same radius as Earth, or the same mass as Earth, or neither? A very small planet is more likely to have a low density, but it would also have much lower gravity and a thinner atmosphere, like Mars. In fact it seems likely that very few types of planet would be even marginally habitable by humans.

[JohnBStone]

Ok but if we brought up the density to say 4.4 g/cm3 would life far better or would it still be barley habitable? How would this density effect the atmosphere and the magnetic field?

Lower density implies a much smaller iron core, so a weaker magnetic field. Oxygen could be held onto OK from a temperature point of view (as shown in the calculator referenced above), but with a weaker magnetic field, atmosphere would be more likely to be stripped by the solar wind - timescale depending on how weak the field is but might be long enough for life. Probably best case is moderate core and very, very deep ocean planet. Worst case is tiny core and mostly silicate planet.

Likely much less tectonic and volcanic activity too, which will have atmosphere conequences.

The process of evolution not affected. The outcome of evolution will be relevant to the local environment - that is what evolution does.

[willstaruss22]

Earth type planet same radius same distance from the sun basically earth exept it has 4.4 g/cm3 earth density.

[willstaruss22]

Would that 4.4 g/cm3 density really make the magnetic field be weak enough to completely strip the atmosphere? and would it be completely a water world?

[JohnBStone]

Earth type planet same radius same distance from the sun basically earth exept it has 4.4 g/cm3 earth density.

Which makes it 80% the mass of Earth.

80% of Earths mass should be enough to ensure a liquid core for billions of years and therefore at least some sort of magnetic field - according to another ongoing thread.

Would that 4.4 g/cm3 density really make the magnetic field be weak enough to completely strip the atmosphere? and would it be completely a water world?

To meet your density the planet must either have a much smaller core and smaller magnetic field. Or a much less dense mantle implying a much higher water percentage - a water world. Or a combination of both.

So you either have a relatively strong magnetic field and a water world, or a relatively weak magnetic field and a more earth like mantle.

Note that Venus has no magentic field and it has a significant atmosphere. The reduced magnetic field may therefore just imply a thinner atmosphere. Though note that some of Venus atmosphere is water vapour which is in liquid form on Earth and a lot is CO2 which is mostly sequestered in sedimentary rocks on Earth.

Does a water world mean even more of the CO2 is sequestered out of the atmosphere?

[willstaruss22]

By smaller magnetic field do you mean like mercurys which is 1% the strength of earths or would it be stronger than that?

Yea i would think because its a water world it would have less Co2 freed up because rocks to hold onto a significant amount of Co2 on earth. So less of it would enter the atmosphere because the rocks underneth the water would just essentially hit water and not make it to the atmosphere. Volcanoes eruptions would probably be the only source of Co2

[Ara Pacis]

Ganymede has a magnetic field and it's density is less than 2 gcm³, with a radius of ~41% of Earth's.

[Jeff Root]

Note that Venus has no magentic field and it has a
significant atmosphere. The reduced magnetic field may
therefore just imply a thinner atmosphere. Though note
that a lot of Venus atmosphere is water vapour which is
in liquid form on Earth and a lot is CO2 which is mostly
sequestered in sedimentary rocks on Earth.

I have:

Atmosphere composition near surface, by volume

Carbon dioxide (CO2) 96.5%
Nitrogen (N) 3.5%
Sulfur dioxide (SO2) 150 ppm
Argon (Ar) 70 ppm
Water vapor (H2O) 20 ppm
Carbon monoxide (CO) 17 ppm
Helium (He) 12 ppm
Neon (Ne) 7 ppm

So if the water vapor in the upper atmosphere is anything
like at the surface, Venus is very, very dry. I think the
water molecules were broken apart by UV light and the
hydrogen escaped.

-- Jeff, in Minneapolis

[willstaruss22]

So would this weaker magnetic field be able to deflect solar winds and protect the atmosphere from being stripped away or would it be too weak to do so?