Mercury’s exosphericsodiumShingo Kameda
Planetary Exploration Research Center
Chiba Institute of Technology, Japan1

--土星までの惑星の中で唯一周回衛星による観測が行われていない惑星--金星・火星にはない
弱い固有磁場を持つ↑流体核の存在[Margot et al., 2007]--大気は非常に薄く無衝突
NASA

水星大気
• 水星固体表面と水星環境の相互作用--大気圏・電離圏がなく、太陽風・磁気圏粒子が地表に衝突--太陽光・隕石ダストの衝突
表面物質が大気中に放出され、無衝突のまま惑星間空間に散逸していく
Solomon et al. 2008Collisionless atmosphere P~10-12 atm
Released from the surface ・Impact to the surface (SP, SW, IPD)
・Loss to the Interplanetary Space

Kameda et al., 2008

Remote Sensing: H,He,O,Na,K,Ca,Mg,Ca+,Al?,Fe?
In Situ : H+,He+,Na+(Mg+),O2+,K+(Ca+,Ar+), etc.

Zurbuchen et al., 2008

4Concentration at High latitudes andTemporal variability
 Effect of Solar wind impact??
Potter et al., 2008

Leblanc et al., 2009

Dec 2005 & Aug 2008
(OAO in Japan)
Temporal variability of
Average density<~10%
SW variability should behigher.
Kameda et al., 2007

Normalised Intensity

432100246hour
25 Aug 2008

81σ~9% [Leblancet 2009al. 2009]
Leblanc et al.,For Reference
Lunar Na ion observed by PACE-IMA on Kaguya
Count/Sample

流星群

Inside Earth’s
Magnetosphere

Yokota et al.

For Reference
Lunar Na ion observed by PACE-IMA on Kaguya
Count/Sample

流星群

Inside Earth’s
Magnetosphere

Solar Wind sputtering is not dominant source process, but“Gardening” is possibly effective.
↑SW particles sticks into the surface and diffuses material.

Yokota et al.

Solar Wind-Magnetosphere-Surface
Simulation has alreadybeen done based on
the data obtained in
Messenger Flyby.
Benna et al., 2010

Burger et al., 2010

*Ion flux to thesurface*Sodium density onthe dayside9Long-term variabilityAverage column density, 10 11atoms/cm 25
1998-2003
2005-2009
432

10060 120 180 240 300
True Anomaly Angle, degree

360●Potter et al. (2007)○Kameda et al. in prep.
Okayama and Haleakala

dave=Φsource τlifetime
ΦPSD∝r-2
τPI ∝r2
 dave=const.

In 1 Mercury year,
the averagedensity changes by
a factor of ~4.
10

What makes sodium released from thesurface?
Average Na density vs1, F10.7 solar flux
2, Solar flux (0-200nm)
3, Solar wind proton flux
4, Distance from ecliptic plane
5, Distance from symmetry planeof Interplanetary Dust distribution
6, Tidal force11• F10.7 solar flux UV/EUV flux
• F10.7 at Mercury is
estimated considering theheliospheric distanceandthe rotation of the sun
assuming F10.7 isdependent on longitude
and independent on time.

12• F10.7 solar flux UV/EUV flux  PSD
Average column density, 10 11atoms/cm 25
1998-2003
2005-20094

No correlation321
r=-0.1140010002000
F10.7 Intensity(the same as shown by Kameda
et al. (2009)
using the data obtained in1998-2003 and 2006)13

What makes sodium released from thesurface?
Average Na density vs1, F10.7 solar flux  No
2, Solar flux (0–200nm)
3, Solar wind proton flux
4, Distance from ecliptic plane
5, Distance from symmetry planeof Interplanetary Dust distribution
6, Tidal force
14

LASP

Solar EUV Experiment (SEE)
on TIMED measured thesolar flux at thewavelength of 0-200nm.
The flux at Mercury wasestimated in the same
way as F10.7.

15

Average column density, 10 11atoms/cm 25
1998-2003
2005-2009

4No correlation

3(SEE Observation was started inFeb 2002.)

21r= 0.005
00.2

0.3
0.4
0.5
0.62
Solar flux (0-200 nm), W/m

PSD threshold wavelength is ~300 nm [Yakshinskiy andMadey, 2004].
16

What makes sodium released from thesurface?
Average Na density vs1, F10.7 solar flux  No
2, Solar flux (0–200nm)  No
3, Solar wind proton flux
4, Distance from ecliptic plane
5, Distance from symmetry planeof Interplanetary Dust distribution
6, Tidal force
17

LANL

Solar Wind Electron Proton
Alpha monitor (SWEPAM) onACE measured the solar windproton flux at L1.
SW flux is estimated to be:dependent on longitude∝ r-2
velocity is constant18

Average column density, 10 11atoms/cm 25No correlation

1998-2003
2005-20094

32

1
r=-0.152002

4Sunspot number in
1998-2003 ismore than 20052009

68
Solar wind proton flux, particles/cm /s [1010]
219

What makes sodium released from thesurface?
Average Na density vs1, F10.7 solar flux  No
2, Solar flux (0–200nm)  No
3, Solar wind proton flux  No
4, Distance from ecliptic plane
5, Distance from symmetry planeof Interplanetary Dust distribution
6, Tidal force
20

Solomon et al., 2008

Kameda et al., 2009 (GRL)

Mercury’s orbital plane is tilted (7deg) against theecliptic plane and the interplanetary dust (IPD) isconcentrated near the ecliptic plane.
 Frequency of IPD impact is higher at B and D.21
Potter et al., 2008

Average column density, 10 11atoms/cm 2

5Weak inverse
correlation

1998-2003
2005-200943

with absolute valueof distance fromecliptic plane.

21
r=-0.314
0-0.04 -0.02
00.02 0.04
Distance from ecliptic plane, AU

What makes sodium released from thesurface?
Average Na density vs1, F10.7 solar flux  No
2, Solar flux (0–200nm)  No
3, Solar wind proton flux  No
4, Distance from ecliptic plane  Weak
5, Distance from symmetry planeof Interplanetary Dust distribution
6, Tidal force
23• Symmetry plane is tilted against theecliptic.

Ω: Ascending node, i: inclination
Ω=-31 deg, i=2.9 deg [Kameda et al., 2009]*Mercury: Ω= 48 deg, i=7 deg
24

Average column density, 10 11atoms/cm 25
1998-2003
2005-20094

Correlated with the
distance from IPD
symmetry plane.

32

1
r=-0.598
0-0.04 -0.02
00.02 0.04
Distance from IPD symmetry plane, AU

 IPD impact iseffective for the
release of sodium
atoms from thesurface.
25

What makes sodium released from thesurface?
Average Na density vs1, F10.7 solar flux  No
2, Solar flux (0–200nm)  No
3, Solar wind proton flux  No
4, Distance from ecliptic plane  Weak
5, Distance from symmetry planeof Interplanetary Dust distribution
 Correlated
6, Tidal force
26

Mercury’s orbital is eccentric (e ~0.2).
 Tidal force at perihelion is twice stronger thanthat at aphelion.
27
Potter et al., 2008

Average column density, 10 11atoms/cm 251998-2003
2005-2009

4

Atmosphere isdense nearperihelion andaphelion (butnot always).

32

10

3

4
5
6
Tidal force, EN728

Tidal force is always outward.
Relative tidal force is inward at aphelion andoutward at perihelion.
Distortion is greatest at perihelion andaphelion.
29
Potter et al., 2008

Average column density, 10 11atoms/cm 251998-2003
2005-2009

4 ● r= 0.805
321
r= 0.556
001
2
delta Tidal Force, EN

Tidal force is nota releaseprocess,however,,,Tidal force will
possibly
increase thesource rate,e.g., refreshing
the surfacelayer.
30
Potter et al., 2008

What makes sodium released from thesurface?
Average Na density vs1, F10.7 solar flux  No
2, Solar flux (0–200nm)  No
3, Solar wind proton flux  No
4, Distance from ecliptic plane  Weak
5, Distance from symmetry planeof Interplanetary Dust distribution
6, Tidal force  Animation
31

IPD impact? Tidal force?

The perihelion and aphelion is near theecliptic plane.
It is difficult to know which is more
effective and explain sodium concentration
at high latitudes.
Mercury Dust Monitor on BepiMMO
and Mercury lander in the future.
32

Super Earth Super Mercury

ESO

CoRoT-7b: 0.017 AU (5ME)(CoRoT-7c: 0.046 AU (9ME))
ここなら熱脱離(?)が支配的

What makes sodium released from thesurface?
Average Na density vs
2, Solar flux (0–200nm)  No
MASCS data for UV reflected onsurface
3, Solar wind proton flux  No
Precipitation detected using FIPS
 Long-term monitoring from March
18, 2010 will provide us something new.
34

Solomon et al., 2008

Kameda et al., 2009

McClintock et al., 2008

Baumgardner et al., 2008

After released from the surface, sodium atoms move toward
antisunward direction due to solar radiation pressure.
 Photoionized in ~3 hours  Loss to the interplanetary space.

35

Solar Radiation Acceleration

Solomon et al., 2008

300
Kameda et al., 2008

200
10000100

200
TAA

300

Solar radiation pressure depends on True Anomaly Angle.
Gray line shows the result of calculation by Smyth and Marconi, 1995.
Black shows the result of ours.
Na tail will be longest at TAA of ~40.
[Kameda et al., 2009 GRL]

36

Solomon et al., 2008

Kameda et al., 2008

McClintock et al., 2008

Baumgardner et al., 2008

No correlation between Sodium density and solar radiation acceleration.
(Solar radiation acceleration depends on orbital position of Mercury.)37

Observation at Haleakala
Observatory
Preliminary
results
TAA=345, 23, 40
2010/09/19
Distance of thetail varies withTAA.

2010/09/25

Velocity
resolution is~
1km/s.
We plan to do
In 2011-2012.

2010/09/28
38

BepiColombo/MMO/MSASI• 日欧共同水星探査計画--2014年打ち上げ2020年軌道投入

ESA

BepiColombo/MMO/MSASI• 日欧共同水星探査計画--2014年打ち上げ
2020年軌道投入--惑星表面探査（ESA)
磁気圏探査（JAXA)
輸送（ESA)MSASI onboard BepiColombo MMO• MSASI is “Mercury Sodium Atmosphere
Spectral Imager.”• Spectral resolution of ~85,000 (7pm) enables
us to observe distribution of Na exosphere onthe dayside.

Line width of NaD is 5pm
Potter et al., 2008

Mercury Sodium Atmosphere
Spectral Imager (MSASI) onboard
BepiColombo MMO

MMO is a spin-stabilized spacecraft. MSASI scans Mercury withmoving mirror and S/C spin.
One pix corresponds to 2 msec.  The CMOS image sensor can take
500 images per sec.
42
The platescale is 1.3 km/pix at periapsis and 40 km/pix at apoapsis.

Fabry-Perot Interferometermλ=2ndcosθ  Transmission

43

Wavelength map on the detector

Transmit wavelength Cos θ (Etalon)
 Eq. above (Filter)
Tilt angle
0.51 deg (Etalon)
1.39 deg (Filter)
44

Wavelength map on the detector
500
400

Obs
Surface
NaD2Counts300
200
1000-0.04-0.02
0Wavelength-NaD2, nm

0.02

Transmit wavelength Cos θ (Etalon)
500Obs Eq. above(Filter)
Surface400
NaD2Counts300

Tilt angle
2000.51 deg (Etalon)
1.39 deg 100(Filter)
0-0.04-0.02
0Wavelength-NaD2, nm

0.0245

Wavelength map on the detector
500
400

ObsSurface
NaD2

Counts300
200
100
0-0.04-0.02
0Wavelength-NaD2, nm

0.02

Transmit wavelength Cos θ (Etalon)
500
Obs Eq. above(Filter)
Surface400
NaD2Counts300

Tilt angle
2000.51 deg (Etalon)
1.39 deg 100(Filter)
0-0.04-0.02
0Wavelength-NaD2, nm

0.02

46

Thermal Vacuum Test-20 degC ~ +60 degC
d=1.49 mm (Zerodur) δd<+/- ~15 nm TEC~ 10-7
δCWL < 6 pm
(Center Wavelength)47

光学設計 (2003年1月の太陽系シンポジウム)

左：光学系の設計図
左上：横から見た図
左下：上から見た図
上：像面要求される性能に従って区画化した図
開口部φ3mmに入射する光量は、上図の１区画毎に、８．３×１０４[photons/100msec/25mÅ]

Laboratory Test Model
Moving Mirror(Russia)

Fabry-Perot Interferometer& Optics (UK)

Detector (Japan)

Structure, Thermal design(Portogul)

Laboratory Test Model
Moving Mirror(Russia)

Fabry-Perot Interferometer& Optics (UK)

Detector (Japan)

Structure, Thermal design(Portogul)

MSASI Observation

LTM on equatorial mounting(The detector is not LTM.)

Moon and NaD2 line
(We turned on Na lamp wason for a short time duringexposure for moon.)51

MMO-MSASISAMURAI_poster4r.jpg

End