Sky & Telescope - December 2019 - 25

the weather conditions on the surface are far from hospitable.
From blistering cold to sky-darkening dust storms, Mars
could kill you in so many ways.

Typical Martian Conditions
The Martian atmosphere is clear and dry, but it's thin,
bearing down on the surface with only about 1% of Earth's
surface pressure. In composition, it's 95% carbon dioxide
(CO2), which we can't breathe, with traces of nitrogen and
argon and only minuscule amounts of water vapor and oxygen. Most of that oxygen comes from the breakdown of CO2
by harsh levels of ultraviolet solar radiation, which barrages
the surface because the planet doesn't have a protective ozone
layer like Earth does.
Mars is on average about 50% farther from the Sun than
Earth is, so it shouldn't be surprising that the environment
is colder. However, the nature and magnitude of the coldness
would be stunning to even the heartiest Arctic dwellers on
our planet. Near the peak of summer, near the equator, the
surface temperature during the "heat of the day" can briefly
go above freezing, perhaps to 5°C to 10°C (41°F to 50°F) or
more in certain places. But during most other times of the
year, conditions are much more frigid, averaging more like
−25°C to −10°C in the daytime and dipping down below
−100°C on many nights. And that's the temperature at your
feet - as you raise your thermometer up off the surface only
a meter or two, the midday temperature can drop by 20°C to
30°C. Cozy toes but frozen nose.
Just like on Earth, it's colder the closer you get to the poles,
and if you go far enough north or south in the winter, it gets
so cold that the CO2 starts to snow out onto the surface,
forming dry ice. There is no similar analogy on Earth, because
the water vapor that turns to snow and falls to the ground
here is just a tiny fraction of the gas in our atmosphere. For
it to snow on Earth like it does on Mars, the temperatures
would have to drop to around −200°C so that our primary
atmospheric gas, nitrogen, started to snow out onto the surface. Brrrrr . . .
Unprotected on the surface, you would not only suffocate
and get the mother of all sunburns, but you'd also freeze to
death.

:DUPHUWHPSHUDWXUHVSURYLGH
PRUHHQHUJ\WRIXHOVWRUPV˪ERWK
(DUWKOLNHV\VWHPVGRPLQDWHGE\
WKLQZDWHULFHFORXGVDQGGLVWLQFWO\
0DUWLDQGXVWGHYLOVDQGGXVWVWRUPV
approximately twice as long as our planet's because of Mars's
greater distance from the Sun. The more direct sunlight in
the summertime means warmer temperatures, which in turn
provide more energy to fuel storms - both Earthlike systems
dominated by thin, water-ice clouds and distinctly Martian
dust devils and dust storms.
Complicating obliquity's effect, however, is Mars's orbital
eccentricity. Its orbit is far more elongated than Earth's,
which has a significant influence on the weather. Specifically,
Mars's orbital eccentricity of 0.09 means that the planet's
distance from the Sun changes from 1.38 astronomical units
at perihelion (around southern summer/northern winter)
to 1.67 a.u. at aphelion. That change in distance results in a
whopping 45% increase in the amount of sunlight that the
planet receives around perihelion compared to aphelion.
In comparison, Earth's orbital eccentricity of just 0.017
results in only about a 7% change in the intensity of sunlight between January, when we're closest to the Sun, and
July, when we're farthest. The Red Planet's more elongated
orbit also means that the duration of southern summer is
q POLAR CAP This four-image mosaic from the Mars Reconnaissance
Orbiter's MARCI camera shows the perennial north polar cap (white) atop
layered material (light tan) and circumpolar dunes (dark brown).

N ASA / JPL / MSSS

Funky Seasons
Planets have seasons because of the tilt of their rotational axes
- their obliquity - relative to the plane of their orbit around
the Sun. The tilt determines how directly sunlight hits a hemisphere - or, in the extreme case of Uranus, whether sunlight
reaches the hemisphere at all (see page 16). Earth's obliquity of
23.5° results in the winter, spring, summer, and fall seasons
familiar to people living in mid-latitudes, as well as the long
summer days and long winter nights familiar to people living
closer to the poles. Jupiter and Mercury have almost no obliquity, so those planets do not have appreciable seasons.
The obliquity of Mars (25.2°) is currently similar to that of
Earth. Thus the planet has Earth-like seasons, though they're
sk yandtele scope.com * D EC E M B E R 2 019

25


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Sky & Telescope - December 2019

Table of Contents for the Digital Edition of Sky & Telescope - December 2019

Contents
Sky & Telescope - December 2019 - Cover1
Sky & Telescope - December 2019 - Cover2
Sky & Telescope - December 2019 - 1
Sky & Telescope - December 2019 - Contents
Sky & Telescope - December 2019 - 3
Sky & Telescope - December 2019 - 4
Sky & Telescope - December 2019 - 5
Sky & Telescope - December 2019 - 6
Sky & Telescope - December 2019 - 7
Sky & Telescope - December 2019 - 8
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