Sky and Telescope - October 2016 - 25

Seasonal Temperature Ranges at Gale Crater (with temperatures in Los Angeles at equivalent seasonal points)
40°C
B

JUL AU

Los Angeles
JAN

N
R PR MAY JU
MA A

JUL

100°F

JAN

0°C

80°F
60°F

Daily temperature ranges
in Los Angeles are much
smaller than at Gale Crater.

40°F
0°F

Gale Crater
−20°C

−20°F
−40°F

−40°C
−60°F

Mars's elliptical orbit makes for longer
winters and shorter summers at Gale Crater.

−80°F

−60°C

−100°F
−80°C

−120°F
Spring
Sol 0

Summer
Sol 200

Autumn

Winter

Sol 400

Sol 600

Spring

Summer

Sol 800

Autumn
Sol 1,000

Winter
Sol 1,200

Days on Mars

Curiosity Sees Seasonal Trends on Mars

NOT A MUGGY WINTER The air in Gale is
most humid in winter, even though the water
content is lowest at that time of year (see text).
The average water content of 40 ppm is 1/500 that
of the 20,000 ppm that is typical on Earth.
S&T: GREGG DINDERMAN, SOURCE: NASA / JPL-CALTECH / CAB (CSIC-INTA)(2)

The rover has detected methane, too. Methane is the most abundant hydrocarbon in the
solar system, but it must be replenished -
either by biotic or abiotic activity - because
it is easily destroyed by sunlight's ultraviolet
rays. So when methane was ﬁrst found on
Mars in 2004, scientists were skeptical.
But when Curiosity detected a spike of
7 parts per billion by volume (ppbv) during
its ﬁrst autumn (S&T: Apr. 2015, p. 14), it
conﬁrmed methane's presence in the atmosphere. Scientists are still unsure where the
compound came from.
The rover has also measured a methane
"background." The levels over the last two
Martian years fall mostly between 0.3 and
0.8 ppbv, and possibly follow seasonal pat-

HIGHS AND LOWS Much like those in Los
Angeles, temperatures on Mars follow a seasonal pattern. These data are for Gale, which
lies just south of the equator. Each bar is an
average over 1/12 of the planet's year.

terns. Although scientists checked carefully,
the methane spike that happened during
the ﬁrst autumn did not repeat during the
second autumn. This indicated it was an
episodic event that might not occur again.
The mission continues to monitor methane levels, which appear to be even lower in
autumn than in other seasons. If conﬁrmed,
the low autumnal level could be a delayed
reaction to summer's high doses of ultraviolet radiation from the Sun.
Ana V. Aceves is S&T's editorial intern.

Ranges of Atmospheric Water Content & Relative Humidity at Gale Crater
70%

70 ppm
On cold winter nights,
relative humidity is
high, even with very
little water.

60%
50%

60 ppm
50 ppm

Water
content

40%

40 ppm

30%

30 ppm

20%

20 ppm

Relative
humidity

10%
0%
Sol 0

10 ppm
0 ppm

Sol 200

Sol 400

Sol 600

Days on Mars

Sol 800

Sol 1,000

Sol 1,200

Water content (parts per million)

Curiosity recently completed its second
Martian year, even though it landed in Gale
Crater almost four years ago. That's because
the Red Planet takes 687 days to complete
one orbit around the Sun. During this time,
the rover has detected seasonal changes
much like Earth's yearly rhythm.
Curiosity has measured temperature variations ranging from about 60°F (16°C) on a
summer afternoon to −148°F (−100°C) on a
winter night. Daily ranges are much larger
than on Earth. In addition, the air in Gale is
very dry, with water content around 40 parts
per million (ppm), or a few hundredths of a
percent by volume, compared with Earth's
typical reading of 20,000 ppm (a couple of
percent). During the southern hemisphere's
summer, relative humidity - a measure of
how close water vapor is to saturating the
air - is low. But in the winter, even though
there is little water content in the air, the
temperature drops so much that humidity
actually goes up to 70%. That's because for
a given amount of water vapor, the colder
the air is, the closer it is to being saturated,
making the relative humidity higher.

Relative humidity

Air temperature

20°C

N
R PR MAY JU
MA A

Sky and Telescope - October 2016

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