1

Volatile Exchange on

Mars

Maria T. ZuberMIT

David E. SmithNASA/GSFC

16th International Workshop on Laser RangingPoznan, Poland13 October 2008

NASA/MRO/HiRISE

2

-2 10 15

0 10 0

2 10 15

4 10 15

6 10 15

8 10 15

1 10 16

0 45 90 135 180 225 270 315 360

Total IceS.H. IceN.H. Ice

Ice

Ma

ss

(k

g)

Ls

Tillman [1985] Smith et al. [1999]

Viking Surface Pressure Measurements

GCM Simulated Seasonal Mass Variation

Seasonal Variation of Surface Pressure

Smith et al. [1999]

NASA/Viking

Tillman [1985]

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Neumann et al. [2003]

CO2 Condensation During South Polar Night

Channel 1 - 3 m (BLACK)Channel 2 - 9 m (RED)Channel 3 - 27 m (GREEN)Channel 4 - 81 m (BLUE)

4

Neumann et al. [2003]

Temperature Profiles from Radio ScienceDuring Polar Winter Night

Hinson et al. [1999; 2001]

Near-surface temperatures buffered by CO2 ice, hovering near CO2 saturation with a lapse rate of -0.85 K km-1. CO2 clouds nucleate spontaneously at 2 K below saturation, possibly as snow. Equilibrium restored as clouds release latent heat and lower PCO2.

Neumann et al. [2002]

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Neumann et al. [2003]

Cloud Density Averaged by Latitude and Ls

Cloudreturnsas % of shots

Noise level varies with threshold and laser output.

Dark curves show limits of along-track day/night terminator.

6

• Model seasonal CO2 mass exchange between Martian atmosphere and polar caps.

• Treat season caps as “mascons” and solve for mass within specified geometric shapes every 5 days. • Use Mars Global Surveyor (MGS) thermal emission (TES) and altimetry (MOLA) data to model latitudinal extent of condensed CO2 and MOLA altimetry to approximate the vertical dimension of shape of anomalous masses.

• Estimate mass of material exchanged with atmosphere from perturbations of orbit of MGS spacecraft from X-band tracking data.

Approach

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30S

Ls=0 Ls=24 Ls=48 Ls=72

Ls=104 Ls=128 Ls=152 Ls=176

Ls=180 Ls=204 Ls=228 Ls=252

Ls=284 Ls=308 Ls=332 Ls=356

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• Model season polar caps, and seasonal variations in atmospheric mass. • Treat seasonal polar caps as cones that overlie topography with radial extent coming from TES bolometric observations and elevation from MOLA.

• Model variable component of seasonal atmospheric mass as a surface layer overlaying the topography. - Model 1 assumes atmosphere is a surface layer between the polar caps. - Model 2 assumes atmosphere is a global surface layer.

Details

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AT

M

AT

M

AT

M

Cap sizes from MGS-

TES

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

60 65 70 75 80 85 90

NH minNH maxSH minSH max

y = -1.908 + 0.0315x

Latitude

Ele

vati

on

, m

north polar cap

0

10

20

30

40

50

0 50 100 150 200 250 300 350

Cap S

ize, degre

es

Ls Values

-4

-3

-2

-1

0

1

2

3

4

0 60 120 180 240 300 360

Ls

101

5 kg

A prioriatmosphere from Ames GCM

CO2

snow depthfrom MGS-MOLA

Simple Model of Mars’ Seasonal Polar Caps of Mars

Cap Model

south north1 2

3 4

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-8-7-6-5-4-3-2-10020040060080010001200mass, 1015 kg Ls*mass, 1015 kg Ls*323mass, 1015 kg Ls*GCM

Seasonal Mass Changes over 4 Mars Years

-2

0

2

4

6

8

0 300 600 900 1200 1500

10^1

5 kg

North Polar Seasonal Cap

-2

0

2

4

6

8

0 300 600 900 1200 1500

10^1

5 kg

South Polar Seasonal Cap

-8

-6

-4

-2

0

0 300 600 900 1200 1500

10^1

5 kg

Ls

Atmosphere Variation

GCM------ Best fit to observed changes

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-8-7-6-5-4-3-2-10020040060080010001200mass, 1015 kg Ls*mass, 1015 kg Ls*323mass, 1015 kg Ls*GCM

Mean Atmospheric Pressure @ V1 and V2

➔ Mean atmospheric pressure derived from global variation in atmospheric mass and used to infer pressure at the two Viking lander sites taking into account their altitudes.

-150

-100

-50

0

50

100

150

0 45 90 135 180 225 270 315 360

Obs Pr @ V2; H=8GCM @V2; H=8V2 pressure varaition

Ls

-150

-100

-50

0

50

100

150

0 45 90 135 180 225 270 315 360

Obs Pr @ V1; H=9GCM @ V1; H=9V1 pressure variation

Pa

Ls

12

-8-7-6-5-4-3-2-10020040060080010001200mass, 1015 kg Ls*mass, 1015 kg Ls*323mass, 1015 kg Ls*GCM

The Future

Abshire et al. [2008]

• Laser ranging would improve s/c position & ephemeris of Mars.

➔ reduce systematic errors ideally enabling detection of subtle longterm effects.

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• Analyzed >4 Mars years (~8 Earth-years) of X-band tracking data from MGS.

• Excellent agreement on magnitude of signal with NASA/Ames GCM, but differences also exist:

– more rapid accumulation in Fall season– non-zero “summer” mass

• MRO is extending time series and will eventually reduce systematic errors in gravity field recovery, but challenge to merge different spacecraft observations.

• Goal is to detect interannual (decadal) variability in seasonal mass exchange.

- laser ranging would help

Summary

NASA/MRO/HiRISE

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0 15 30 45 60 75 90

Recession of NP Seasonal Icecap

Ls=0Ls=30Ls=40Ls=50Ls=60Ls=70Ls=80Ls=90

Latitude, deg

20

30

40

50

60

70

80

90

100

-90 -75 -60 -45 -30 -15 0

Recession of SP Seasonal Icecap

Ls=180Ls=190Ls=200Ls=210Ls=220Ls=230Ls=240Ls=250Ls=260Ls=270

Rad

ianc

e

Latitude, deg

Passive radiometry data provides variation in radiance with latitude averaged over all longitudes.The edge of the cap is taken to have a radiance of 50 and used to monitor the size of each seasonal icecap.