Detailing Surface areas of worlds

[PoorMerchant]

Originally Posted by SteveS

We have only two planets and a few moons that are geologically active: Venus appears to have active volcanoes but no signs of tectonic activity. Earth is active both ways. It appears that Mars was once very active, but any remaining activity pretty much vestigial. Io is not only geologically active, it's the most active world we know about -- but its volcanism is driven by tidal stress; gravitational interaction with Europa keep its orbit elliptical, and Jupiter stresses it differentially as its distance varies

Venus appears to have volcanoes but no signs of tectonic activity - surely volcanoes are signs of tectonic activity in and of themselves - don't active volcanoes happen because of tectonic activity? (It's been almost forty years since I did O-level geography so I may well have forgotten a lot).

[Agemegos]

Originally Posted by PoorMerchant

Venus appears to have volcanoes but no signs of tectonic activity - surely volcanoes are signs of tectonic activity in and of themselves - don't active volcanoes happen because of tectonic activity? (It's been almost forty years since I did O-level geography so I may well have forgotten a lot).

There are volcanoes at many of the joins between tectonic plates, and those would be a form of tectonic activity. On the other hand there are also volcanic hot spots in the middles of some plates, like for instance Hawai'i in the Pacific Plate and Yellowstone in the North American Plate, that aren't caused by the movement of plates (indeed the plates seem to move around over them).

The volcanoes on Mars seem to be all of the second sort. There is believed to be no active subduction zones on Mars, nothing equivalent to sea-floor spreading, and no large-scale mantle convection with crustal plates floating around on top of it.

[Malenfant]

Venus has plenty of evidence of tectonic activity - it's got fault and rift zones all over the place. The question is whether they're still active or not, but given that Venus is pretty much the same size and density as Earth there's no reason to assume that it's not tectonically and volcanically active today. We just don't have real-time monitoring of the surface to be sure about it either way.

[Drifter]

Originally Posted by Malenfant

Venus has plenty of evidence of tectonic activity - it's got fault and rift zones all over the place. The question is whether they're still active or not, but given that Venus is pretty much the same size and density as Earth there's no reason to assume that it's not tectonically and volcanically active today. We just don't have real-time monitoring of the surface to be sure about it either way.

Isn't the surface of Venus thick with craters? To me that would say a lack of erosion, uplift and subsidence. Or at least a lack of rain and wind. That is, if they are meteoric and not volcanic. What's the current thinking on that?

[Anthony]

Originally Posted by Drifter

Isn't the surface of Venus thick with craters?

No, Venus has a similar number of craters to Earth.

[Drifter]

Originally Posted by Anthony

No, Venus has a similar number of craters to Earth.

Looking at a map I can see a lot of craters. Warning, super large picture.

At least they are circular features that I assume to be craters. They might be lava domes or sports arenas for all I know.

[Anthony]

Originally Posted by Drifter

Looking at a map I can see a lot of craters.

That's not really very many craters; it's just that craters on Earth tend to be obscured by vegetation. This site gives an estimate of around 900 craters for both Earth and Venus.

[Malenfant]

Originally Posted by Drifter

Isn't the surface of Venus thick with craters? To me that would say a lack of erosion, uplift and subsidence. Or at least a lack of rain and wind. That is, if they are meteoric and not volcanic. What's the current thinking on that?

As Anthony points out, Venus (like Earth) actually has very few craters on it (900 or so, instead of the thousands that the Moon or Mercury or Mars have).

The reason is twofold - one is that the surface has been resurfaced on a huge scale and most of it is covered in lava fields, faults and volcanoes, so any ancient craters have long since been buried or resurfaced away. The other reason is the thick atmosphere - most asteroids just burn up before they hit the ground (in some places you can even see airburst patterns on the ground where the asteroid blew up before it hit the ground). Many asteroids that could make it to the ground on Earth simply wouldn't make it to the ground at all on Venus.

[Drifter]

I didn't realize Earth had so many, or that they were so obscured. You learn something new every day, eh?

[Kilmore]

Regarding the hydrographic discrepancies between the original surveys and Behind the Claw, what's the time difference between the two? A lot can happen in just a few short years...