Crustal Fields in the Solar Wind: Implications for Atmospheric Escape Dave Brain LASP University of...

Crustal Fields in the Solar Wind:

Implications forAtmospheric Escape

Dave Brain

LASP

University of Colorado

July 24, 2003

Atmospheric Escape to Space

• Evidenceisotope measurementsspacecraft (Phobos, MGS)ionosphere models

• Why do we care?Climate historyAtmospheric chemistry

• Loss estimates.15 - 1 bar CO2 (over history) 1025 - 1026 particles / s ( today )

Atmospheric Escape to Space

• Main issues

#1 Loss over Martian historyAmountTiming

#2 Today’s lossAmountSpecies

Processes

To improve estimates of #1, must improve knowledge in many areas, including #2

MGS Measurements

• Ancient dynamo( early protection for atmosphere )

• Strong crustal sources( affect loss after dynamo turn-off )

this was an animation

Loss Today

Relevant Loss Processes

• Photochemical loss

• Ion pickup • Sputtering

• Bulk removal

neutralion

(Contemporary)

Relevant Loss Processes

• Photochemical loss

• Ion pickup • Sputtering

• Bulk removal

Crustal sources affect these processes through:atmospheric shieldingfield topologyopen field lines

neutralion

(Contemporary)

Atmospheric Shielding

Atmospheric Shielding

Implications of shielding:

1. Reduced ionization charge exchange electron impact

2. Photo-ion motion changed

Atmospheric Shielding

LS = 0 SW Pressure = 5e-9 dynes cm-2

Theoretical Martianpressure balance obstacle

to the solar wind

PSW = Pcrust + Pionosphere

this was an animation

Atmospheric Shielding

With crustal sources

Without crustal sources

The volume of protected atmosphere is larger (by a factor of 2-8) when crustal sources are considered.

How many (more) neutrals are protected?

CO2: < 10% O: 10-60 %

Field Topology

Field Topology

Implication of altered topology:

Modification of charged particle motion

Escape could be enhanced or diminished depending upon orientation of crustal fields relative to solar wind flow

Field Topology

With crustal sources Without crustal sources

Crustal sources severely alter the field topology close to Mars

MHD simulations(courtesy Y. Ma and A. Nagy)

Field Topology

these were animations

Open Field Lines

Open Field Lines

Implications of open field lines:

1. Access to lower atmosphere for SW charged

particles

2. “Escape hatches” for planetary ions

Open Field Lines

Open field lines exist near crustal “cusps” of near-vertical field

Dayside Data

Open Field Lines

Estimates of quantity of open field lines

• Simple models~7% by area at exobase (~200 km) in region of strong crustal

sources

• MAG Data~ 1-2% by area on Martian dayside (2pm local time) at 400 km

Loss over Martian History

Loss over Martian History

Magnetic History

• Mars forms

• Dynamo on

• Surface strongly magnetized

• Dynamo off

• Large impacts - magnetization erased

• Northern resurfacing - magnetization erased

• Relaxation of crustal magnetization to present

Time

Loss over Martian History

Magnetic History

• Mars forms

• Dynamo on

• Surface strongly magnetized

• Dynamo off

• Large impacts - magnetization erased

• Northern resurfacing - magnetization erased

• Relaxation of crustal magnetization to present

Impact

~3.5 Gya

Shielding from global

field

Shielding by crustal sources

Outgassing

Needed Measurements

Concurrent particle and field measurements!

Low-altitude (and surface) measurements of crustal magnetization

Atmospheric/ionospheric density and temperature at high altitude

Measurements of carrier and grain size of magnetization

Solar wind measurements at Mars

Time history of ionization processes at Mars

Coverage in local time, SZA, altitude, and geographic location

Summary

• Atmospheric escape to space has important implications for Mars’ past climate and current atmospheric chemistry

• Crustal magnetic sources might effect present loss rates:• shielding of the atmosphere• alteration of particle trajectories through field topology• particle exchange along open field lines

• Crustal effects have persisted since Mars’ dynamo ceased