The Climate of MarsStephen Wood

University of Washington

Why Study Mars?

• Why study mice?• For climate science in particular, Mars is

especially valuable because:– Its climate system is relatively simple and predictable

• No oceans = no long-term “memory”

• Thin atmosphere = less influence on surface temperature

– Its climate is highly variable over time• Huge variations in obliquity

• Evidence for very different climate regimes in the past

Mars ClimatePredictability

This storm system occurred in the same place at the same time of year 4 years in a row!

Artist’s representation of Mars at high obliquity (credit Brown University)

Mars Climate Variations

EARTH MARS

Orbit Distance 1 A.U. 1.52 A.U.

Orbit Period 365 days 687 days

Orbit Eccentricity 0.0167 0.0934

Rotation Period 24 hrs 24.66 hrs

Axial Tilt 23.44 25.19

Diameter 12,742 km 6,786 km

Land Area 148*106 km

2 145*10

6 km

2

Gravity 9.8 m/s2 3.7 m/s

2

Atmospheric Pressure

1013 mb 7 mb

Atmospheric Composition

N2 78% O2 21%

H2O 1% - 4% Ar 1%

CO2 0.0383% CH4 0.0002%

CO2 95.3% N2 2.7% Ar 1.6% O2 0.1%

H2O 0.03% CH4 0.0002%

Mean Surface Temperature

+14 C -25 C

State of Mars knowledge before 1900

Telescopic Observations of Mars

Opposition:When an outer planet is on the opposite side of the Earth from the Sun

Date Distancefrom Earth

(million km)

Apparentequatorialdiameter(arcsec)

2005 Nov. 7 70.35 19.9

2007 Dec. 24 88.64 15.8

2010 Jan. 29 99.38 14.1

2012 Mar. 3 100.87 13.9

2014 Apr. 8 92.94 15.1

2016 May 22 76.19 18.4

2018 Jul. 27 57.72 24.3

2020 Oct. 13 62.63 22.4

Future Mars Oppositions

Giovanni Schiaparelli(1835-1910)Brera ObservatoryMilan, ItalyTelescope: 8.6” refractor

1877 brought the ideal opportunity in the form a particularly favorable opposition of Mars....

Schiaparelli prepared for it almost like a prize fighter, avoiding "everything which could affect the nervous system, from narcotics to alcohol, and especially ... coffee, which I found to be exceedingly prejudicial to the accuracy of observation."

...mapped “cannali”, meaning “channels”, but was translated as “canals”

Percival Lowell(1855-1916)

Lowell ObservatoryFlagstaff, ArizonaTelescope: 24” refractor

Lowell produced intricate drawings delineating "canals“... He concluded that the bright areas were deserts and the dark were patches of vegetation. He further believed that water from the melting polar cap flowed down the canals toward the equatorial region to revive the vegetation. Lowell thought the canals were constructed by intelligent beings

who once flourished on Mars.

excerpt from Mars by Percival Lowell (1895)

Chapter 3

The Polar Cap

After air, water. If Mars be capable of supporting life, there must be water upon his surface; for, to all forms of life, water is as vital a matter as air. On the question of habitability, therefore, it becomes all- important to know whether there be water on Mars.

On the 3d of June, 1894, the south polar cap stretched, almost one unbroken waste of white, over about 55 degrees of latitude. A degree on Mars measures 37 miles; consequently the cap was 2,035 miles across.

... the cap was already in rapid process of melting; and the speed with which it proceeded to dwindle showed that hundreds of square miles of it were disappearing daily.

As it melted, a dark band appeared surrounding it on all sides...it was the darkest marking upon the disk, and was of a blue color...a deep blue, like some other-world grotto of Capri.

But the most significant fact about the band was that it kept pace with the polar cap's retreat toward the pole. As the white cap shrank it followed pari passu so as always to border the edge of the snow.

It thus showed itself not to be a permanent marking of the planet's surface, since it changed its place, but a temporary one, dependent directly upon the waning of the cap itself ...

...instantly suggested its character, namely, that it was water at the edge of the cap due to the melting of the polar snow.

Before going further we will take up here at the outset the question of the constitution of these polar caps...the possibility that instead of ice we have here snow-caps of solid carbonic acid gas (carbon dioxide).

Volatiles: Molecular compounds which experience phase changes within

the range of planetary surface temperatures (and pressures) Ices: Any volatile in the solid phase

Phase Stability Temperatures at P = 1 bar

solid liquid gas

solid liquid gas

EarthPluto Titan

Radiative Equilibrium Temperature

planet

Iin/4 (Iin /4)A TE4

Iin (1-A) / 4 = TE4

Fin = Fout

Incident - Reflected = Emitted

Iin(πR2) - AIin(πR2) = TE4(4πR2)

Iin (1 - A) (πR2) = TE4(4πR2)

TE = { Iin(1-A) / 41/4

TE = { Iin(1-A) / 41/4

Radiative Equilibrium Temperature

Earth MarsSolar Flux Iin 1370 W/m2

Albedo A 0.28Radiative EquilibriumTemperature

TE 257 K-16 C

“Greenhouse Warming” TG +33 CMean SurfaceTemperature

Tavg +17 C

“Polar Cooling” TP -77 CMinimum SurfaceTemperature

Tmin -60 C213 K

TE = { Iin(1-A) / 41/4

Radiative Equilibrium Temperature

Earth MarsSolar Flux Iin 1370 W/m2 593 W/m2

Albedo A 0.28 0.28?Radiative EquilibriumTemperature

TE 257 K-16 C

208 K-65 C

“Greenhouse Warming” TG +33 CMean SurfaceTemperature

Tavg +17 C

“Polar Cooling” TP -77 CMinimum SurfaceTemperature

Tmin -60 C213 K

Inverse Square Law I = Fsun / d2

Fsun = IEarth dEarth2

Fsun = IMars dMars2

IEarthdEarth2 = IMarsdMars

2

IMars = IEarth (dEarth / dMars )2

IMars = IEarth (1.00 / 1.52 )2

TE = { Iin(1-A) / 41/4

Radiative Equilibrium Temperature

Earth Mars Solar Flux Iin 1370 W/m2 593 W/m2 Albedo A 0.28 0.28 Radiative Equilibrium Temperature

TE 257 K -16 C

208 K -65 C

“Greenhouse Warming” TG +33 C 0 C +33 C Mean Surface Temperature

Tavg +17 C -65 C 208 K

-32 C 241 K

“Polar Cooling” TP -77 C -77 C Minimum Surface Temperature

Tmin -60 C 213 K

-142 C 131 K

-109 C 164 K

solid liquid gas

EarthPluto Titan

Mars (est.)

Percival Lowell’s argument for the polar caps being H2O ice instead of CO2 ice:

The dark band that surrounds the retreating polar cap in spring must be due to the ice melting into a liquid phase, and CO2 cannot exist as a liquid at the probable temperatures and atmospheric pressure of Mars.

Although H2O also is not liquid at the temperatures we calculated, Lowell estimated that Mars is “as warm as the south of England”

Percival Lowell’s argument for the polar caps being H2O ice instead of CO2 ice:

The dark band that surrounds the retreating polar cap in spring must be due to the ice melting into a liquid phase, and CO2 cannot exist as a liquid at the probable temperatures and atmospheric pressure of Mars.

Although H2O also is not liquid at the temperatures we calculated, Lowell estimated that Mars is “as warm as the south of England”

Our best guess for the composition of the polar caps based on pre-1900 information and calculations:

The seasonal polar caps are probably not H2O ice, because Mars’ surface temperatures are too cold for H2O ice to melt (or sublimate)

If the dark band around the cap is due to melting, then the most likely composition is NH3 (ammonia) ice

If the dark band has a different cause, then the composition could also be CO2 ice

1937

19371947

Gerard Kuiper uses spectroscopy to measure the amount of CO2 in Mars’ atmosphere – finding it has ~2 times as much as Earth’s atmosphere.

Large yellowish cloud observed over the Hellas-Noachis region of Mars, and in less than a month, spreads to cover the whole planet

Gerard Kuiper suggests that seasonal removal of the dust by wind currents can explain the "wave of darkening."

1956

1959

H. Spinrad, G. Münch, and L. D. Kaplan make improved spectroscopic observations of Mars’ atmospheric composition with the 100-inch reflector at Mount Wilson, CA.

Their findings: Water vapor: ~1% of the amount in Earth’s driest deserts CO2 partial pressure: 4 mb

Total surface pressure: 25 mb

1959 1963

1965 - Mariner IV Mars fly-by mission

• First close-up images of Mars• cratered, moon-like surface• clouds and hazes

• First direct measurement of Mars’ atmospheric density

• surface pressure ~ 4 mb

Key Results of Mariner IV

Total surface pressure = 4 mb (Mariner IV)

CO2 partial pressure = 4 mb

(Earth-based spectroscopy)

Mars’ atmosphere is ~100% CO2

Therefore...

Carbon Dioxide

Lat

itud

e

Days

North polar cap

South polar cap

Summer

Winter

SpringWinter

Summer

Fall

Spring Fall

4 mb

14

5 K

CO2 Vapor Pressure / Frost Pt. Temp.

Region III – large-grained CO2 ice (>10 cm!)

1969 - Mariner 7 fly-by

Region IV – transparent CO2 ice (+ Dirt)

Region II – fine-grained CO2 ice (1 cm)

Region I - cap edge: Dirt + CO2 ice + H2O ice

(Calvin & Martin, 1994)

Near-infrared spectra ofsouth seasonal polar cap confirms that composition is CO2 ice

1976 – Viking Mission2 Orbiters and 2 Landers

MeteorologyInstrument Package(pressure, T, wind)

Camera

Carl Sagan

Viking Lander 1 pressure measurements (1976-1982)

Leighton & Murray 1966 prediction !

Viking Lander measurements (1976-1982)

1980 1981 1982 1983 1984 1985330

332

334

336

338

340

342

344

346

348

350

CO

2 M

ixin

g R

atio

(p

pm

) a

t Ma

un

a L

oa

Time

Monthly Mean CO2 Mixing Ratio Comparison of CO2 Amounts and

Pressure Variations on Earth and Mars

• Mars’ atmosphere has 50 times more CO2 than Earth• The total atmospheric pressure at the surface of Mars is 150 times less than it is on Earth• The variation in surface pressure on Earth associated with weather systems is ~ 20 mb, which is 2%• The seasonal variation in surface pressure on Mars is ~2 mb, which is 29% !• On Earth, a 29% decrease in air pressure would be equivalent to going to the top of Mt. Rainier.• The surface pressure on Mars is equivalent to the pressure at 100,000 ft. altitude in the Earth’s atmosphere

Mauna Loa Atmospheric Observatory

Earth’s CO2 cycle

Dust

T. Schoennagel

Sahel Dust Storm

500 km wide Mars dust storm southern high latitude spring

1971-1972 – Mariner 9 orbiter mission

• First spacecraft to orbit another planet

• Arrived during a global dust storm, which totally obscured the surface for months

• Operated for 349 days in orbit

• Transmitted 7,329 images, covering over 80% of Mars' surface. revealing:

• river beds• volcanoes• canyons • erosion and deposition by wind and water • weather fronts, clouds, and fogs

as the dust settles...

Water&

Past Climate

Valles Marineris Vast canyon system named after Mariner 9

4,500 km long, 200 km wide, and up to 7.7 km deep (~ 10x bigger than the Grand Canyon)

Apparent source location for large outflow channels carved by catastrophic floods, billions of years ago (similar to Channeled Scablands of E. WA)

Evidence for catastrophic floods on early Mars (2-3 billion yrs. ago)

Possible ancient river valley

Evidence of persistent water flow on early Mars: a river delta

10 km

“fossilized” riverbed meanderEberswalde Crater delta formation

MGS MOC Release No. MOC2-1225Malin Space Science Systems

Residual North Polar Capin northern summer season

• Composed of H2O ice• 500 km in diameter• 2.5 km thick• Contains fine-scale layering which may record past climate variations• Spiral trough features are still unexplained• Ice evaporates in summer, providing primary source of global atmospheric water vapor• Surface shows very few impact craters, therefore is very young (< 1 million yrs)

Fine-scale layering in theNorth polar residual water ice cap

MGS MOC image (3 m/pix)

NASA/MSSS

Viking Orbiter image (60 m/pixel)

Morning frost (H2O Ice)at the Viking Lander 2landing site (48°N)

identified as H2O ice because surface temperatures were too warm for CO2 ice

EARTH MARS

Atmospheric Pressure 1013 mb 7 mb

Atmospheric Composition N2 78% O2 21%

H2O 1% - 4% Ar 1%

CO2 0.0383% CH4 0.0002%

CO2 95.3% N2 2.7% Ar 1.6% O2 0.1%

H2O 0.03% CH4 0.0002%

Radiative Equilibrium Temp. -16 C -65 C

“Greenhouse” warming +33 C +5 C

Mean Surface Temperature 14 C -60 C

Min. Temp. (winter pole) -60 C -133 C

Max. Temp. (equator) 35 C 20 C

Summary Slide 1

Atmospheric Composition and Temperature Range

Summary Slide 2

• In comparison to the Earth, Mars has a more variable, dynamic, and predictable climate – thus is valuable for testing climate physics and models

• CO2

– Mars’ atmosphere is 95% CO2, with a surface pressure of 6-8 mb

– Vast seasonal deposits of CO2 ice form each winter in the polar regions

• 1-2 m thick, and extend down to 50 deg. latitude

• decrease total atmospheric mass by ~25%

• H2O

– Tiny amount of water vapor in the atmosphere (10 precipitable microns)

– Lots of water ice clouds, and some water frost forms at higher latitudes

– Residual polar caps consist of water ice (and dust)

• up to 2500 m thick in central portion

• ~500 km in diameter

• contain fine-scale layering which may record past climate variations

Summary Slide 3

• (H2O continued)

– Geologic evidence that liquid water flowed on Mars’ surface in the past

• catastrophic outburst floods

• persistent rivers (delta formation)

• Dust

– Global dust storms can develop, but do not occur every Mars year

– Regional dust storms are common