ASTR 1210 (O'Connell) Study Guide
15. MERCURY AND VENUS
Radar map of Venus' surface, from the
Magellan Mission. The red color is artificial,
intended to represent the
effects of Venus' thick clouds. Click for enlargement.
A. The "Inferior" Planets
Mercury and Venus are called "inner"
because they are closer to the Sun
than is Earth.
Both revolve around the Sun in shorter times than the Earth (88 and 225 days,
- Elongation is the angular distance of a
planet from the Sun as viewed from Earth. The
term "configurations" refers to the various characteristic
elongations possible for planets as shown in the figure above.
- As viewed from the Earth, the two planets inside the Earth's
orbit can never appear at large angles from the Sun. See the
illustration above. Mercury and Venus always stay within
27o and 48o, respectively, of the Sun. These
are their "greatest elongations."
- Consequently, Venus and Mercury are visible in the
sky only near sunset or
sunrise. Venus is the most
common evening or morning
- An immediate implication of Copernicus' heliocentric model was
that the sizes of the orbits of Mercury and Venus (relative to
Earth's orbit) could be deduced from their greatest
elongations. In the Ptolemaic model, there was no simple geometric
method for determining the sizes of the planetary orbits.
- Because Venus' orbital period is similar to Earth's, it tends
to linger in the sky near the horizon for many weeks at a time. [Recall
the planetarium simulations shown during our discussion of the Maya obsession
- Because of its proximity to Earth and the high albedo (~70%)
produced by its thick cloud layers, Venus is the brightest object in
the sky other than the Sun or the Moon. Its
and white color make it look artificial.
- ===> Venus is the classic "Unidentified Flying Object" (UFO). See
Guide 18 for more discussion.
- The planets outside Earth's orbit ("superior" planets),
starting with Mars, can be seen at up to 180o from the
Sun. At that point they are highest in the sky at midnight and are
said to be at "opposition" with respect to the Sun.
- As the figure shows, when a planet is at opposition, it is also
nearest the Earth and therefore brightest. It will also be
undergoing its fastest "retrograde motion" at that point.
Image of the Caloris Basin on Mercury taken by the
Color coding is for different mineral types.
Mercury is hard to observe from Earth because it is above
the nighttime horizon only for brief periods.
It has been less well studied
than most other planets. Until 2007
there had been only 2 spacecraft visits, both flybys, in
contrast to Venus, which has been a major destination of space
view of Mercury from Mariner 10 (1974).
MESSENGER was an
elaborate mission (ending in 2015) to study Mercury at close range in
3 flybys followed by
long-term in-orbit observations.
Mercury has a high average density
of 5.4 grams/cc, like Earth,
but Mercury's mass (& therefore gravity compression) is smaller. That
implies Mercury is more rich in heavy elements
Mercury's surface is similar to Earth's Moon
but with important differences
(e.g. shallower craters) due to
slower cooling and higher gravity. See the image at right and compare to
Earth's Moon topography
Mercury's orbit is an important test of General
, the revised interpretation of gravity proposed by
Einstein in 1915. Mercury's perihelion (the closest point to the Sun
in its elliptical orbit) shifts 43 arc-seconds/century more than
predicted for Newtonian gravity. This is only 1/10 millionth of its total orbital
motion per orbit, but it can be measured highly accurately over many
orbits. The extra shift is predicted exactly
by Einstein's GR
Venusian cloud layers in UV/optical bands (image from Mariner 10, 1974)
C. Venus: Introduction
Venus is a near "twin"
of Earth in global properties: diameter (95%); mass (82%); distance
from Sun (0.7 AU)
But unlike Earth, thick cloud layers completely obscure its
. See image above (click for enlargement).
- The surface of Venus cannot be
studied from outside its atmosphere at optical wavelengths.
- Clouds in planetary atmospheres are composed of liquid droplets or
ice crystals and are distinct from the atmosphere (gas) in which they
- Therefore, it is difficult to determine cloud composition by
spectroscopy (easy only for vapors).
- The naive presumption until the 1960's, given Venus' appearance
and overall similarity to the Earth, was that the clouds were made of
water and that the planet probably hosted a flourishing, wet,
USSR & USA space missions to Venus started in 1961 and have included
flybys, orbiters, atmospheric probes, and
Results from these missions, as well as Earth-based radio-wave observations,
the notion of a Venusian tropical paradise:
Radio and infrared measurements from early flyby and lander
missions (1962-72) showed that Venus' surface temperature was almost
500o C (900o F) and the lower atmosphere
was crushingly dense.
Landers returned images of a bleak, lava-covered surface:
Above is a wide angle color image of Venus' surface
returned by the USSR Venera 13 lander (1982).
It shows a
lava-strewn plain, extending to the horizon at right. The reddish
color is produced
by the thick cloud layer, which absorbs blue
light. The lander lasted only two hours
in the heat and pressure of
Venus' surface. Click for enlargement.
D. Venus: Surface/Topography
For Venus, the only feasible surface mapping technique was to
to penetrate the thick clouds.
Radar systems emit a short burst of radio waves
and then detect the reflected burst to determine a target's distance
and (through the Doppler effect) motion.
Top: elevation maps of Venus and Earth compared.
Bottom: Radar map of Venus with main features labeled. (Pioneer Mission,
The image above is a relief map of Venus derived from radar
observations with the Pioneer mission. Best mapping coverage was from
(radar orbiter, 1990-94).
The overall topography is flatter than Earth's. There are only two "continent"-like
(Ishtar and Aphrodite in the map above).
Continents and domelike features are evidence of modest
tectonic activity, but this is much less conspicuous than
on Earth, as can be seen in the comparison images above. There are no
Given the surface temperature, there are obviously no oceans on Venus.
Vast lava flows
cover 85% of the surface, but there are no
large basins, either impact (like the Moon's marias) or
tectonic-related (like Earth ocean beds). Most flow regions are
smooth. There is little current eruptive activity (no changes
in the surface features, for example).
There are many dormant volcanoes
, from 500 km diameter to tiny
vents; 3000 over 20 km diameter; 100,000 altogether! Over 160 larger
than the largest volcano on Earth (Hawaii).
This radar image shows four overlapping volcanic domes.
They average about 16 miles in diameter
with maximum heights of 2,500
feet. They were produced by eruptions of thick lava coming from vents
on the relatively level ground, allowing the lava to flow in an even
Click for enlargement.
There are also many impact craters, but fewer per unit area
than on the Moon or Mercury. This implies a younger surface
than those planets.
- Shown at right is a radar image of a 30-mile diameter impact
crater surrounded by a bright "splash blanket" of ejecta. Lighter-toned
regions on radar images are rougher; darker-toned are smoother. Click
for a larger view.
Surprisingly, Venus shows a uniform distribution of craters
This situation is unique in the Solar System (see discussions
of the Moon and the outer satellites in
It implies the whole surface formed at one time. Judged by the
density of impact craters, the surface is relatively young---only
about 500 Myr old, unlike the 4+ Byr-old surfaces of the
Moon, some outer satellites, etc.
The combined evidence indicates that the entire planet underwent
sudden catastrophic melting & resurfacing about 500 Myr years
ago. This event was possibly induced by a thick lithosphere which
trapped heat generated in the interior until it built up to a critical
level. This process could be
cyclic, repeating after sufficient interior heat builds up.
- Venus' surface history will be discussed in the video "Venus
E. Venus: Atmosphere
Venus' atmosphere is dense, hot, dry, and corrosive
surface levels, it is entirely hostile to Earth-like life.
- The bulk of the atmosphere is carbon dioxide (CO2)
- Water is almost absent on Venus. H2O vapor has only
1/10000 of its abundance on Earth, and there is no liquid water on the
surface. A dessicated planet/atmosphere.
- We will find later (Study
Guide 19) that the absence of water is a key to the bizarre
properties of the Venusian atmosphere.
- Lack of liquid water, which on Earth is a lubricant for the outer
layers of the interior, may also act to inhibit tectonic activity on Venus.
- The Venusian cloud decks? The clouds are sulfuric acid(!) droplets
- They originate from volcanic outgassing in the absence of rainfall
- See the atmospheric profile chart at right:
- There are other remarkable differences from Earth's atmosphere
- Temperatures and pressures like those at Earth's surface occur at
an altitude of 50 km in Venus atmosphere. Below that, pressures
and temperatures are much higher than on Earth.
- The surface temperature is ~ 750oK (480oC or 900oF)!
- Venus' surface is hotter than Mercury's, despite its
larger distance from the Sun!
- The surface pressure is 90x higher than on Earth. Since gravity
at Venus' surface is almost the same as on the Earth, this implies
Venus' atmosphere is 90 times more massive than Earth's!
- In September 2020, astronomers announced a possible detection
of phosphine gas (PH3) in Venus' atmosphere, which could
be an indicator of biological activity. See
Guide 23 for the latest views on this controversial result.
Despite the dense and corrosive atmosphere, there is little weathering
of surface features on Venus because windspeeds are very low (and the
sulfuric acid rain evaporates at high altitude before reaching the ground).
The Greenhouse Effect
Venus would be warmer than the Earth simply because it is nearer the
Sun. But the extraordinarily high Venusian temperature is not caused
by higher solar input. Instead, it is produced by
, an atmospheric process which
- The surface temperature is determined by the equilibrium
point, where the heating rate balances the cooling rate. If
the cooling rate (i.e. the escape of radiation to space) falls below
the heating rate, the temperature increases until the two match.
- The main heat input to any planetary atmosphere (including
Earth's) is from the Sun. This occurs mainly at visible
wavelengths, where the Sun is brightest.
- Cooling from the surface is by
(electromagnetic) radiation to space. Because the temperature
of planetary surfaces is (fortunately for us!) much lower than the
Sun's, this occurs not at visible but instead at infrared
II and III to remind yourself of the characteristics of EM radiation
from dense objects like planets.)
Because all of the surface cooling must take place in the
infrared, any gas that can impede IR radiation acts to
increase the surface temperature. We call these
- The most important Greenhouse agents are H2O,
CO2, CH4, and O3. Although these
gases are transparent at optical wavelengths, they are
large parts of the IR spectrum. They preferentially absorb
infrared radiation and reflect it back to the planetary
surface, thereby reducing radiative cooling. They act like a blanket
to "trap the heat." See the sketch above right (click for
- This causes a significant temperature rise to the point where the
surface can radiate as much energy to space (despite the Greenhouse
blocking) as it receives from the Sun. The situation is like the level
of a lake adjusting to the increased height of its outlet dam.
- Even tiny amounts of Greenhouse gases
can have a big effect because they choke the cooling channel.
is a chart that shows the radiative input, output, and Greenhouse gas
blocking as a function of wavelength for the Earth's atmosphere.
- On Earth, where the Greenhouse gases are only "trace"
constituents of the atmosphere (e.g. CO2 totals only 0.04%
of the atmosphere's mass), the long-term Greenhouse temperature
increase was a modest 30o C (or
54o F), which is just enough to keep Earth's
surface "comfortable" by human standards and prevent the oceans
from freezing over.
- But on Venus, where the atmosphere is almost pure CO2
and massive enough to block large regions of the infrared spectrum,
temperature rise is 400o C.
- The existence of a Greenhouse Effect for Earth's atmosphere was
first recognized almost two centuries ago, in the 1820's. The first
quantitative discussion was published in 1896 by the Swedish
Arrhenius, who predicted that human industrial activity might be
able to produce enough CO2 to increase the temperature of
Earth's surface. Evidence of human-induced increases in both atmospheric
CO2 and temperature would not be clear for another 100 years
(see Guide 19).
F. Venus and Earth
Venus is a sobering lesson
in comparative planetology.
The incredible differences between terrestrial and Venusian conditions
were a great shock to astronomers. How can the atmospheres of Venus
and Earth, despite their similarities in size and mass be so
different? The culprit is probably the seemingly small difference
in distance to the Sun
(30%), as we will see
in Guide 19
Venus is unsuitable for a biosphere for two entirely different
reasons: its hostile atmosphere and its episodes of catastrophic
resurfacing (both related to heat-trapping).
It is ironic that this horrific world was named in many cultures for
the Goddess of Love. The Maya, who believed it was a vicious god bent
on destruction, were closer to the truth.
But Venus provided another astronomical touchstone for human societies. It was
the recognition of the power of the Greenhouse Effect on Venus that
first led atmospheric scientists to become concerned about
global warming on Earth.
Spaceman Spiff zooms past Venus on his way to Mars --- next
Reading for this lecture:
Bennett textbook: pp. 203-204; Secs. 9.3, 9.5.
Study Guide 15
Viewing: video shown in class: "NOVA: Venus Unveiled"
If you missed the class, the video can be viewed
in Clemons Library. Its call number is VHS 13769.
Reading for next lecture:
Bennett textbook: p. 206; Sec. 9.4.
Study Guide 16
December 2020 by rwo
Venus images copyright © 1997, Calvin J. Hamilton.
Atmosphere profile copyright © Harcourt, Inc. Greenhouse effect
drawing copyright © Toby Smith. Text copyright © 1998-2020
Robert W. O'Connell. All rights reserved. These notes are intended
for the private, noncommercial use of students enrolled in Astronomy
1210 at the University of Virginia.