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Atmospheres
Mercury does not have an appreciable atmosphere. It is small with a low escape velocity, and it is close to
the Sun with very high temperature during its day. As a result, Mercury cannot retain any gas particles to form an atmosphere.
All other terrestrial planets have appreciable atmospheres.
Temperature and Pressure The Earth’s atmosphere
has an average surface temperature of about 14o. Mars has a thin and cool atmosphere of an average surface temperature below zero
celsius degree and average surface pressure of less than 1% of the Earth’s atmosphere pressure at sea level. Venus has
a dense and hot atmosphere with an average surface temperature of over 400o and average surface pressure 90 times Earth’s
atmosphere pressure.
The difference is a result of the distance to the Sun and different greenhouse effects on the three planets.
In terms of the height-temperature profile, in general, the bottom of the atmosphere is warmer than upper layers,
because the atmosphere is transparent to sunlight (dominant in visible wave-lengths) and solar energy deposits at the surface,
heating the atmosphere from the bottom. However, the Earth’s atmosphere also exhibits inversions in the temperature
profile in stratosphere and thermosphere because certain gas elements (O 3, O and N atoms) in those layers absorb shortwave
length radiations from the Sun.
Evolution - greenhouse effect
All three atmospheres started from CO2, water vapor, SO2 etc by volcanoe outgasing.
These are greenhouse gases which trap (outward) infrared radiation by the planets thus keep the planet warmer than without
greenhouse gases. With different initial temperatures, atmospheres on the three planets then evolve differently.
On the Earth, the distance to the Sun is such that the temperature allows for water in liquid state (oceans),
which can dissolve CO 2 or lock CO2 into rocks. Evapouration, volcanoes, and plate tectonics then send these gases into the air. This way, greenhouse
gases are recycled between atmosphere, water, and rocks, and the greenhouse effect is in check. As a result, the surface temperature
is raised by about 40 degrees.
On Venus, the higher surface temperature due to its closer distance to the Sun drives water vapor and CO 2 (from water and rocks)
into the atmosphere, which enhances the greenhouse effect and raises the planet’s surface temperature. The higher temperature
further drives more greenhouse gases into the air to lead to still higher temperature. This is the runaway greenhouse effect
which raises the Venus’s surface temperature by more than 4000
and produces a thick dense atmosphere.
On Mars, the lower surface temperature due to its being farther away from the Sun and quicker cooling of the
planet keeps water as ice and CO 2 trapped in rocks. No drastic geological activities (such as plate tectonics) works to release greenhouse
gases into the air. As a result of this runaway icehouse effect, the atmosphere on Mars is thin and cool.
Composition
At the present, more than 95% of the atmosphere on both Venus
and Mars is of CO2, while the Earth’s atmopshere consists 20% of molecular oxygen and about 80% molecular nitrogen.
At the start, all three planets had atmospheres of similar composition: CO2, water vapor, SO2 etc outgased from volcanoes.
On the cold Mars, water is kept as ice below the surface other than inthe atmosphere. On the hot Venus, water vapor in the
air was cracked down by ultraviolet radiation.
So on both planets, CO2 dominates the air composition. On the Earth, the large body of liquid water allows for
life to develop on the planet. Photosynthesis by plants and biochemical processes by certain bacteria have vastly changed
the atmosphere composition into molecular oxygen and nitrogen gases with very little CO2 remaining in the air.
Surface features
Surface features in the four planets can be compared in terms of cratering, volcanism, plate tectonics, and
water.
Mercury has the oldest surface, which is heavily cratered. Mountains and basins on Mercury are largely features
by impact cratering. Scarps may be produced when the planet was cooling down and shrank. Mercury is small and lack of internal
energy, which cannot drive significant geological activities to renew its surface.
Venus has a relatively young surface characterized by vast gentle smooth plains of lava flows and volcanoe hills.
Geological features like ridges also formed by lava flows. Volcanism has been, and probably still is, active on Venus because
of its hot molten interior. No evidence of plate tectonics is found on Venus probably because Venus is hot, molten, and plastic,
and lacks rigid crustal plates.
Mars’s surface is divided into a cratering old highlands in the southern hemisphere and smooth young lava-covered
lowlands north of the equator. (The exact reason for such crustal dichotomy pattern remains unclear.) Mountains on Mars are
volcanoe or cratering mountains. The presence of large-scale volcanoe mountains and old cratering surface indicates no plate
tectonics on Mars.
Mars, however, exhibits geological features (channels, riverbeds, gullies, flood islands, mud flash etc.) indicative
of water flow or erosion in the past. Mars has polar caps made of dry ice and ice water, and water ice is believed to be present
below the surface elsewhere on the planet. Mars does not have active plate tectonics and strong volcanism (compared with Venus
and Earth) perhaps because of its small size thus less internal heat and thicker crust (compared with the Earth).
The Earth’s surface is young, largely reshaped by abundant geological activities driven by plate tectonics.
Large mountain ranges, ocean rifts, gulfs, earthquake and volcanoe belts, are results of plate tectonics and are located at
plate boundaries (or subduction zones) where plates collide, separate or rub against each other. Most significantly, the Earth’s
surface is dominated by a large body of liquid water, oceans. The Earth’s surface also bears signatures of weathering,
such as wind and water erosion, and life activities such as vegetation.
The Earth is the only planet with active plate tectonics because of its internal heat, molten interior, convective
athenosphere (all these owing to the large size of the Earth), a thin and rigid crust (perhaps because of formation of the
Moon taking away most of the Earth’s rocky crust), and the presence of a large body of liquid water (due to its distance
to the Sun and surface temperature) allowing for plate floating.
In short, Mercury has the oldest surface, followed by Mars, Venus, and Earth, in the same order as their sizes.
The major reason is that larger planets have more internal energy to drive geological activities.
Evidence of past and current water on Mars can be summarized as the following:
(a) erosion - features by past water flow, such as gullies, dried riverbeds, flood-carved islands, and sedimentary
rock layers found from high resolution images of Mars’ surface; (b) minerals dissolved in liquid water, as revealed
by spectral observations; (c) neutron (deficiency) measurements revealing presence of ice water beneath Mars’ surface.
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