Early Mars was Warm

James F. KastingDepartment of Geosciences

Penn State University

• Question: Why should we be interested in early Mars’ climate?

• Answer: Two reasons1. Mars itself may have harbored life

early in its history (and perhaps today, deep underground)

2. Mars gives us an idea of the extent of the liquid water habitable zone around the Sun and, by extension, around other stars as well

Mars Pathfinder(Twin peaks view)

• Today, the surface of Mars is a frozen desert• Mean surface temperature: Ts = 218 K (55oC)

Fresh gully in Centauri Montes region(image from Mars Global Surveyor)

http://www.nasa.gov/mission_pages/mars/images/pia09028.html

• There is evidence for a limited amount of fluvial activity today• This may be caused by the sudden release of very salty brine • But, in the distant past, there is lots of evidence for much more, and presumably much fresher, liquid water…

Martian ‘outflow’ channel (Viking)

From: J. K. Beatty et al., The New Solar System, 4th ed

~ 200 km

• Note the much larger scale of the Viking images• This outflow channel is more than 20 km across!

Ares Vallis(from Viking)

From: J. K. Beatty et al., The New SolarSystem, 4th ed.

~200 km

• This is another outflow channel, or flood feature• Note that it formed after the crater at the lower left

Nirgal Vallis (Viking)

• These valleys have short stubby branches, thought to form by sapping of subsurface groundwater

• One still needs to recharge the groundwater aquifers with rainfall or snowfall

• Perhaps rainfall percolated directly into the soil instead of flowing over it, as it does on Earth?

200 km

From: J. K. Beatty et al., The New Solar System, 4th ed.

From: J. K. Beatty et al.,The New Solar System, 4th ed

WarregoVallis

(Viking)

~200 km

• This valley network more closely resembles river systems on Earth

How did the martian valleys form?

(At least) Three different models:

1. Warm early Mars modelMars’ early climate was essentially Earth-like. The water needed to carve the valleys came from rainfall.

2. Cold early Mars model 1Mars was cold most of the time, but it warmed up sporadically due to impacts (Segura et al., Science, 2002)

3. Cold early Mars model 2Mars was cold all of the time. The groundwater reservoirs were recharged by vapor diffusion from a subsurface aquifer (Clifford, JGR, 1993)

• Let’s test the impact hypothesis (Cold early Mars model 1)…

Nanedi Vallis(from Mars Global Surveyor)

~3 km

River channel

• The Grand Canyon on Earth required ~17 million years to form• During this time, roughly 5106 m of rain should have fallen on the Colorado plateau (assuming 30 cm/yr of rainfall)• By contrast, those who have argued that the martian valleys could have formed in a cold environment, following large impacts (Segura et al. Science, 2002) assume much lower volumes of precipitated water, 50-500 m• The terrestrial analogy suggests that these estimates are too low by a factor of 104-105!

• Universal soil loss equation

A = RKLSCPwhere

A = average annual soil lossR = rainfall indexK = soil erodibility factorLS = topographic factorG = cropping management factorP = conservation practice factor

Soil erodibility factorSegura et al. (2008)

Schwab and Frevert (1985) Elementary Soil and Water Engineering

Bottom line: The impacthypothesis for valley formation (Cold earlyMars model 1) would onlywork if Mars’ surface consisted of loamy sand,rather than rock.

So, I will assume for this talk that early Mars was indeed warm most of the timein order to produce the valleys

Question: When did these large-scale fluvial features form?

Possible clue: They are all found on heavily cratered terrain

Opportuni

ty

Meridiani Planum

Spirit Gusev Crater

• Mars’ southern highlands are heavily cratered• The northern plains may be cratered as well, but they are lower and are filled in with dust• How can we tell when the craters formed?

Lunar cratering record

• We know a lot about cratering history from having studied the Moon

• Most Moon rocks have ages of about 3.8-3.9 Ga (billion years ago)

• Was this a pulse, or was it the end of a continuous period of bombardment?

1200 km

Imbrium

Serenitatis

Tranquillitatis

Tycho

Copernicus

Procellarum

The “Nice model”

• There is a way to get a pulse of bombardment relatively late in Solar System history (~3.9 Ga)

• In the Nice model, Jupiter and Saturn migrate through the 2:1 mean motion resonance at this time, flinging Neptune and Uranus into the outer Solar System and triggering a flurry of impacts on the terrestrial planets Gomes et al., Nature (2005)

Uranus

Neptune

Jupiter

Saturn

Implications for Mars’ climate history

• Implication: Most of the fluvial features on Mars are at least 3.8 b.y old

• Solar luminosity should therefore have been no more than 75% that of today– This makes it very

difficult to explain why early Mars was warm!

D. O. Gough, Solar Physics (1981)

Time of the LateHeavy Bombardment

How could early Mars have been kept warm?

• To answer this question, we first need to think about how Earth remained habitable early in its history

• Low solar luminosity was a problem for the Earth, as well

• Earth has stabilizing feedbacks, though, most importantly the one between CO2 and climate, acting through the carbonate-silicate cycle…

The carbonate-silicate cycle

(metamorphism)

• This cycle regulates Earth’s atmospheric CO2 level over long time scales and has acted as a planetary thermostat during much of Earth’s history, because CO2 builds up as the climate cools• CH4 may also have contributed to the greenhouse effect on early Earth, but that is a different story

CO2 vs. time if CH4 was absent

J. F. Kasting, Science (1993)

Ice ages

Today’sCO2 level(1 PAL)

ArcheanCO2 level(>300 PAL)

Back to Mars…• Question: Could this same

feedback have happened on Mars?

• Answer: Yes, but only early in its history when the planet was more volcanically active

• Early Mars had lots of volcanoes

• Carbonates could also have been remobilized by impacts

• There is a problem in warming early Mars with CO2, though… Olympus Mons

SURFACE PRESSURE (bar)

0.001 0.01 0.1 1 10

SU

RF

AC

E T

EM

PE

RA

TU

RE

(K

)

180

220

260

300

MARS

0.7

0.8

0.9S/S0 = 1

J. F. Kasting, Icarus (1991)

Martian surface temperature vs. pCO2 and solar luminosity

• Previous calculations showed that greenhouse warming by CO2 (and H2O) could not have kept early Mars’ mean surface above freezing

S/S0 = 0.75 at 3.8. b.y. ago, when most of the valleys formed

Freezing point of water

Why is it difficult to warm early Mars?

• Two reasons:– Condensation of CO2

reduces the tropospheric lapse rate, thereby lowering the greenhouse effect

– CO2 is a good Rayleigh scatterer (2.5 times better than air), so as surface pressure increases, the increase in albedo outweighs the increase in the greenhouse effect

CO2

condensationregion

Ref.: J. F. Kasting, Icarus (1991)

Rayleigh scattering

When the sun is high in the sky, theshorter wavelengths are preferentiallyscattered, making the sky appearblue

When the sun is near the horizon,the blue wavelengths are scatteredout of the beam, making the sunappear orangish-red

New theories for warming early Mars

• Greenhouse warming by CO2 ice clouds (dry ice) – Forget and Pierrehumbert, Science (1997)– But this requires nearly 100% cloud cover– Needs to be studied with 3-D climate models

• Greenhouse warming by SO2 (sulfur dioxide)– Geochemical calculations: Halevy et al., Science

(2007)– Volcanic SO2 builds up when the climate is cold

(similar to CO2 feedback on Earth)– Climate calculations: Johnson et al., JGR (2008)

3-D GCM modeling of early Mars climate and SO2

• This model predicts equatorial surface temperatures above freezing even without SO2

• Adding 245 ppmv of SO2 yields an additional 50 K of warming!

• But there are lots of problems with this model– No Rayleigh scattering!– Questionable CO2

absorption coefficients– No sulfate aerosols

Johnson et al., JGR (2008)(S/S0 = 0.75, 0.5 bar CO2)

Testing Halevy’s hypothesis

• To test the SO2 hypothesis of Halevy et al., we first repeated the climate calculations of Kasting (1991)– Get about 10 K extra

warming at high CO2 levels because of increased grid resolution (and different absorption coefficients)

F. Tian, J. F. Kasting, J. D. Haqq-Misra, and M. Claire, submitted

Testing Halevy’s hypothesis

• We then repeated the calculations for S/S0 = 0.75 (3.8 Ga), using different amounts of SO2

• Get significant warming for fSO2 > 1 ppmv

• But, there are no aerosols in this model. So, we repeated these calculations with aerosols included– A reduced (low-O2)

atmosphere was assumed so as to minimize production of sulfate

F. Tian, J. F. Kasting, J. D. Haqq-Misra, and M. Claire, submitted

Testing Halevy’s hypothesis

• Bottom line: When sulfate aerosols are included in the calculation, SO2 cannot warm early Mars up, even under the most optimistic assumptions• So, how can we do this?

Tian et al., submitted

With aerosolsRealistic SO2 concentration

(~1 ppmv)

• Idea: Instead of trying to increase the greenhouse effect on early Mars, maybe we should think about how to reduce its albedo

• Need something that absorbs in the visible (but not too strongly or one would create an anti-greenhouse effect)

• NO2 absorbs strongly in the near-UV and in the blue part of the visible• It is at least partly responsible for Denver’s “brown cloud”

http://vpl.astro.washington.edu/spectra/no2hitranuv.htm

(nm)

Absorption bynitrogen dioxide

(NO2)

Effect of NO2 on albedo and surface

temperature

J. F. Kasting et al., manuscript on hold

65o !

• Warming peaks at about 300 ppb of NO2 •10-15 ppb of NO2 is enough to keep early Mars warm• Would this have been available on early Mars?

Problems with the NO2 model

• One can get NO from impacts, but to get NO2, one needs O3

NO + O3 NO2 + O• To get O3, one needs an

oxidizing early atmosphere– Can get this either from

photosynthetic life or from escape of C from CO2

• But a high concentration of NOx (NO + NO2) destroys O3, so this warm solution can exist only transiently

http://www.astronomy.com/asy/default.aspx?c=a&id=5941

• How about the Halevy et al. model? Is there a way to get warming out of sulfur compounds?

Calculations by Megan Smith (PSU)

Optical propertiesof crystalline S8

• Elemental sulfur particles should have formed if the early martian atmosphere was reduced • S8 absorbs strongly below about 0.5 m• This is not quite enough to produce warming in our climate model, even if we make lots of S8 particles

• Brian Toon (Theresa Segura’s thesis advisor and coauthor) may have provided a better answer to this question almost 30 years ago

• Amorphous sulfur forms when elemental sulfur condenses in an atmosphere, and it absorbs more strongly in the visible than does crystalline S8

Optical properties of amorphous sulfur

• Can amorphous sulfur particles warm the early martian climate?

• Calculations are in progress. Stay tuned!

Toon et al., Icarus (1982)

Conclusions• Early Mars was warm. The amount of

rainfall needed to carve the martian valleys far exceeds the amount that could have been generated by impacts

• The most promising way to warm early Mars is with particles of amorphous elemental sulfur, which absorb visible sunlight, along with generous amounts of CO2

• Early Mars did have lots of liquid water, and so it could have supported life. If it did, life could still be present today