Mercury’s Seasonal Na Exosphere
Data from MESSENGER’s MASCS UVVS instrument
Tim Cassidy, Aimee Merkel, Bill McClintock, Matt BurgerMenelaos Sarantos, Rosemary Killen, Ron Vervack, Ann Sprague
From http://www.ips.gov.au/Category/Educational/Space%20Weather/The%20Aurora/Aurora.pdf
Potter et.al. (2001)
Just how bright is the Na exosphere?
We see the Na exosphere because Na scatters sunlight ~589 nm (yellow)
Messenger limb scan
vs.
Earth-based Na observations
Messenger UVVS data is especially valuable because it gives high resolution vertical profiles (‘limb scans’) of the atmosphere
Potter andMorgan, 1990
Killen et al., 2008
column densityor
radiance
Altitude
For Na, we will focus on near-surface (<1000 km) limb scans
Of particular interest is the slope of the limb scan—which tells us the energy of ejected Na
Gravity acts as an energy spectrometer.
hot
cold
What is a limb scan and why is it useful?
lines of sight
This talk is about the dayside, which is typically probed near the equator:
Note: poles are harder to investigate with MESSENGER’s orbit. MESSENGER also has a lot of tail data, not presented here.
Chamberlain model:density ~ n0e-U/kT where U is the potential energy(times another factor called zeta…)
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−MmG /r +mracos(χ)
Radiation acceleration term, analogous to U = -mgh
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χ
Gravitational potentialsunlight
To get atmospheric properties we have fitted limb scans with a simple function, called a Chamberlain model.
Chamberlain model fits give us two parameters: surface density and temperature
Note: Radiation acceleration is up to ½ Mercury’s gravity
lines of sight
Need to account for line of sight:
line of sight column density =Integral of density over line of sight≈ surface density*2*K*H
where K~Sqrt(pi*r/2H)
H = kT/mg
where g is the sum of gravitational and radial photon acceleration
I am going to focus on the dense lower exosphere in this talk
What is it’s temperature? What is its density? How does it vary?And what do these tell us about the process that launches molecules off the the surface?
Example limb scan fits:
Temperature is roughly constant
Results, part I: Na temperature
This excludes the high temperature ‘tail’ at high altitudes mentioned earlier.Modelers predicted a more variable temperature…
Example: temperature at noon local time
TAA0°180°
Data from over 6 Mercury years
(some data points randomly excluded for clarity)
And temperature is the same across dayside:
Killen et al., 1999:1500 K (at equator)
Compared with ground-based observations of the temperature: 700-1500 K
2008
We can compare with possible ejection mechanisms
Thermal Desorption<700 K(and thermal accommodation)
PSDPhoton Stimulated Desorptionsimilar to ESD, electron stimulateddesorption
Meteorite Impact Vaporization1000s degrees
Sputteringthousands to 10s of thousands of degrees
Molecular dissociation(e.g. CaXCa + X + energy)10s of thousands of degrees
Experimental Data (Yakshinskiy and Madey, 1999 & 2004) 900 K Maxwellian
PSD from ice Johnson et al., (2002)
Conclusion: PSD is the best match to supply the near-surface exosphere temperature
The temperatures we derive are similar to, but slightly colder, than Earth-based observations (Killen et al., 2008)
There is no evidence of thermal desorption
But PSD would quickly deplete surface of Na, Na must be continually resupplied to surface by other processes such as impacts or ion-enhanced diffusion (e.g., Killen et al., 2008).
Results, part II: Na density
Plotting vs true anomaly angle shows pattern:(for this example, we use limbscans at 10:00 local time)
Perihelion
Perihelion
Aphelion
TAA0°180°
Data from over 6 Mercury years
Different local times have similar (but distinct) patterns
(some data points randomly excluded for clarity)
Suggests correlation with radiation acceleration, as some ground based observations suggest
TAA0°180°
Potter et al., 2009
TAA0°180°
Compared with ground-based data (Potter et al 2007)
Lines show model of Smyth and Marconi (2005)
It’s difficult to compare with ground based data, which tends to report disk-averaged quantities. A large effort would be required to do this.
TAA0°180°
Exosphere content from ‘PSD enhanced’ modeling
scenario
Leblanc and Johnson, 2010
But perhaps the closest model:
Conclusions about Dayside Na
•These trends are consistentThese limb scan column densities don’t change much from year to year: 20% standard deviation
•Strong evidence that PSD supplies lower dayside atmosphere-temperature (~1200K)-variation in noon density with TAA like Leblanc predicted in
his ‘enhanced PSD’ simulation-no evidence of thermal accommodation/thermal
desorption
E.g., Noon,TAA 150-170°
• There is no detailed comparison with models.
• Ground-based data does not seem to match our results. Observation geometry?
•Na has abundance comparable to O.
• extra slides
Compared to atomic oxygenEstimate of O vertical column density:If O is hot (10's of thousands of degrees), then the vertical column density is of the same order as the line-of-sight density near the surface, which can found from the observed O emissions, about 4 Rayleighs, and the O g value (~1E-4/sec, Killen et al., 2009):=4 Rayleighs/g value*1E6 (/cm2) = 4E10 cm-2
(regardless of the O temperature, this is an upper limit)
vs Na:
Ca
•Ejected mostly from dawn and density peaks near perihelion—impact vaporization?•High temperature (15,000-25,000 K): molecular dissociation of impact vapor
Mg
•Uncertain mix of temperatures•Nightside source needed
Compared to Other Species
NaMgCa
Observed column densities:Na has a two components, two temperatures. It dominates near surface.Mg and Ca have single temperature. Mg dominates further from the surface.
Others suggested that same correlation
Killen et al. (2008)
Others did not:
--- Chamberlain with photon pressure3000 K1500 K900 K
Test: comparison of Chamberlain model with Matt’s Monte Carlo model, the gold standard
Chamberlain model overestimates densities near dawn and dusk, where the Chamberlain model assumes no photon pressure effects (cos(chi)~0 in previous slide).
validating optical thickness correction
