What might you expect of volcanoes erupting on other planets similar to Earth, such as Mars or Venus?Venus has a thick atmosphere of carbon...

In terms of mass and radius, Venus
is the most similar to Earth. Venus is about 0.82 Earth masses and 95%
the radius of the Earth. Near the surface of Venus, there is very little wind. The
atmosphere on Venus consists of carbon dioxide (96%) and nitrogen (3%). The atmosphere
is so thick that the pressure at the surface of Venus is 90 times the atmospheric
pressure on the surface of the Earth.  Due to the high atmospheric pressure, the winds
on Venus are also relatively slow. Thus, neither wind nor rain can really affect the
surface on Venus. As a result, volcanic features will look freshly formed for a long
time. Venus shows no evidence for plate tectonics. There are no long, linear volcano
chains. There are no clear subduction zones. Although rifts are common, none look like
the mid-ocean ridges on Earth. Also, continent-like regions are rare, and show none of
the jigsaw fits seen on Earth. Thus, where volcanism on Earth mostly marks plate
boundaries and plate movements, volcanism on Venus is much more regional and much less
organized. In addition, volcanism on Venus shows fewer eruptive styles than on the
Earth. Almost all volcanism on Venus seems to involve fluid lava flows. There is
no sign of explosive, ash-forming eruptions on
Venus, and little evidence for the eruption of sludgy, viscous
lavas. This may reflect a combination of several effects. First, due to the high air
pressure, venusian lavas need much higher gas contents than Earth lavas to erupt
explosively. Second, the main gas driving lava explosions on Earth is water, which is in
very short supply on Venus. Lastly, many viscous lavas and explosive eruptions on Earth
occur near plate subduction zones. Thus, the lack of subduction zones should also reduce
the likelihood of such eruptions on
Venus.

Mars, on the other
hand, is the most distant terrestrial planet from the Sun. The mass of Mars is about
one-tenth the mass of the Earth and Mars is about half as large in diameter. Like the
Earth, Mars has polar caps; unlike the Earth, one of these is mostly CO2 ice.
Temperatures on Mars range from 133 K to 293 K, and the atmospheric pressure at the
surface is very low, about 1/100 the pressure at the surface of the Earth. The
atmosphere on Mars is 95% CO2, similar to the atmosphere of Venus. Because the
atmosphere is so much thinner, however, there has been no runaway greenhouse effect on
Mars. The atmosphere is thick enough to support tenuous water ice clouds, and enormous
dust storms, some of which are over a mile high. Although Mars has no plate tectonic
activity, the surface of Mars has been shaped by geologic activity. The Tharsis Plateau
contains a string of volcanoes, the largest of which is Olympus Mons. Olympus Mons grew
to such enormous size because the hole in the crust that brought magma to the surface
stayed stationary over time. Also spectacular is Valles Marineris, whose feature is not
the result of plate tectonics, as it would be on the Earth. Instead, Valles Marineris is
the result of the cooling and shrinking crust splitting open at the equator, much like
the surface of drying clay.