Summary- We are building a model of orbitally-paced hydrologic activity on Early Mars. The model assumes seasonal snowmelt runoff. Our long-run goal is to make a probabilistic assessment of the Early Mars climate state (pCO2, T).- Initial results show a strong focus of orbitally-paced hydrologic activity near the Equator, in agreement with the sedimentary rock record.- We predict that mean annual surface temperatures at the Meridiani landing site were 50C colder than in the global groundwater model.

Inevitability of melting at low latitudes on Mars: Implications for the sedimentary rock record & MSL

Edwin Kite* (UC Berkeley), Michael Manga (UC Berkeley), & Itay Halevy (Caltech)

Conclusions

*Poster author. kite@berkeley.edu

Seasonal snowmelt looks viable as a water source for sedimentary rocks on Early Mars. Consistent with fundamentally water-limited and orbitally paced sedimentation and near-surface aquous geochemistry.Tbar ~ 220K, not 273K. - Some change in atmospheric composition is required to melt flatlying equatorial snowpack over orbital timescales on Early Mars; null hypothesis of no change can be rejected.- Multibar atmosphere not required: relatively small O(10K) changes.

Motivating Observations:Distribution of Sedimentary RocksFormation of sedimentary rocks usually requires liquid water. Malin et al (Mars J., 2010) catalogue ~4000 sedimentary rock locations mapped by MOC NA. We exclude FHA/SP/AB orbits. To calculate sedimentary rock densities, we mask out Late Hesperian and Amazonian materials (from Nimmo and Tanaka, AREPS, 2005), about 40% of the planet. This excludes just ~4% of the sedimentary rock data, mainly Medusae Fossae layers and opal SW of Melas Chasma.

Water source for sedimentary rocks on MarsTop-down or bottom-up?

H.O.M.E. : no cold traps

Overview of results for zero elevation

Under certain low-probability orbital conditions (High Obliquity, Moderate Eccentricity, perihelion occurs at equinox), there is no latitude at which dusty snow will not melt - even under the faint young Sun, even with a weak greenhouse effect similar to today. That is, Mars' orbitalparameters can diffuse into a region where latitudinal cold traps do not exist.

FLOODEDAREA

MER

-B P

anca

m 1

P194

8533

27EF

F646

BP2

547R

1C1

Main result: Extraordinary concentration of sedimentary rocks <10 degrees lat (EQ). Not the result of Valles Marineris. Not the result of mantling by Amazonian material.

Probabilistic model of snowmelt runoffForward model as a first step towards a Bayesian assessment of Early Mars climate state

1. 1D column thermal model. Assumption: The columns are independent (there is little seasonal memory, and latitudinal energy transport is small). 28 mbar pCO2, uniform 55 W/m^2 greenhouse. Changes in atmospheric composition (weathering, escape to space) are assumed to be slow relative to changes in orbital forcing. The runs presented here assume a constant atmosphere-surface temperature offset.

2. Snowpack stability model: Interannually-persistent snow is assumed to accumulate only at the latitude of minimum ablation (approximately, the 'coldest latitude.') We use the sublimation model of Dundas & Byrne (Icarus, 2010) and assume 25% humidity.

3. Orbital conditions model: Obliquity, eccentricity, and Lp are assumed to not be strongly correlated. This is the case for 250 Myr solutions at http://www.imcce.fr/Equipes/ASD/insola/mars/mars.html. Age-dependent PDFs are from Laskar et al. (Icarus, 2004).

FLOODEDAREA

Secondary observations: "Wings" of high sedimentary rock abundance at +/-25 degree latitude. Strong preference for low elevations. Robust to exclusion of VM rocks, non-EQ rocks, or both.

1. Run a 1D column model of snowpack temperature over the full range of latitude, season, solar luminosity, orbital conditions, and elevation sampled by Mars.

MeridianiT > 273 K ?

MeridianiT ~ 220 K ?

2. Determine snowpackstability by minimizingsublimation losses overthe year (~coldest locations.)

3. Multiply by age-dependentorbital conditions pdf.

NOW: polar cold traps

Eccentricity changes at the time of the LHB?Laskar's model does not include Solar System disruption suggested by the NiceModel (Tsiganis et al., Nature, 2005) of solar system evolution. Disruption should produce transiently higher Mars eccentricity (secular sweeping resonances increase the eccentricity, and dynamical friction with LHB impactors decreases eccentricity).

a= 0.283.0 Gya

55 W/m^2

no c

old

trap

s - m

elt e

very

whe

re

snow at equator

Andrews-Hanna et al. (Nature,2007; JGR, 2010a, 2010b).

Global groundwater sourceMean temperature above freezing required to preventdevelopment of impermeablecryosphere

Local/regional seasonal snowmelt

Groundwater flowconfined to thin layerabove cryosphere

Perennial lakes arepossible - drainageacts as a solar concentratorvia latent heat

Sedimentary rocks (black dots)Alluvial fans (Kraal et al., Icarus, 2008; red dots)Deltas (Hynek et al., Nat. Geo., 2010; magenta)Young materials (Nimmo and Tanaka, AREPS, 2005; black lines)Equatorial band containing 64% of sedimentary rocks (Malin et al., Mars, 2010; blue lines)Topographic contour interval 1000m.

Predicted locations of snowpack stability (minimum annualaverage free sublimation rate) at e = 0.15, obliquity = 60 degrees, Lp = 0, solar luminosity 3.5 Ga. Thick black contour lines enclose the 2, 5 and 12 percent of Mars surface areamost favorable for perennial snow under these melt-optimal conditions. This is a snapshot from Mars' time-varying orbital parameters, and so should not be compared with the ensemble of Mars sedimentary rock locations. Also note that most of the low-lying plains (darkest blue contours) have been resurfaced by younger lavas and outflow channel activity.

*Under orbital conditions that are optimal for seasonal melting, Gale is within the 1% of Mars surface area that is most favorable forperennial snow*.

Here, orbital parameters have been chosen to produce ice stability in the 'alluvial fan belt'. These conditions are slightly less favorable for melting than the optimal case. The red contour is the 2% of Mars most favorable for perennial snow. It encloses two of the three alluvial fan clusters identified by Kraal et al (Icarus, 2008), plus Bakhuysen and the floor of Valles

Marineris .*Under orbital conditions that are chosen to produce perennial snow in the 'alluvial fan belt', Holden is within the 1% of Mars surface area that is most favorable for perennial snow. Eberswalde is within the 2% of Mars surface area that is most favorable for perennial snow.*.

Orbital plots:Black - T>283KRed - T>278KCyan - All melt eventsBlue - T>268KGreen - all snow.

Latitude plot:Blue: All snowGreen: T> 268KRed: T>278K

Stratigraphic wet fraction

a= 0.283.0 Gya

55 W/m^2

zero elevation zero elevation

Effect of elevation

The snowmelt model predicts ice willaccumulate, melt will occur, and sedimentary rocks will form mainlyat low elevation. This is becausesublimation losses and evaporative cooling are minimized by higher pressure (for p < O(0.1) bar).

The snowmelt model predicts Mars never underwent >= 10^5 yr of continuous wet years. This is because orbital cycles create and destroy latitudinal cold traps on this timescale, unless the greenhouse effect was much stronger (as required by the groundwater model).

Provided that obliquityis high enough to driveice to the equator, eccentricity is the main control on melting.

However, with greenhouse warming unchanged from today, snow temperatures are only just above freezing even under optimal orbital conditions.This is insufficient to explain the observations, so the null hypothesis of no change is rejected. Additional greenhouse forcing (or a radically higher eccentricity) is needed. For the remainder of this poster, we assume a modest. time-indepdendent additional greenhouse forcing of 6K.

Evaluated atlatitude of perennialsnowpack stability