EPSC Abstracts,
Vol. 3, EPSC2008-A-00427, 2008
European Planetary Science Congress, © Author(s) 2008
Nature of the Venus thermosphere derived from satellite drag measurements
(solicited paper)
G. Keating (1), M. Theriot (1) and S. Bougher (2)
(1) The George Washington University, Newport News, VA, USA, (2) University of Michigan, Ann Arbor, MI,
USA ([email protected] / Fax: 757-874-5648)
Abstract
From drag measurements obtained by Pioneer Venus
and Magellan, the Venus upper atmosphere was
discovered to be much colder than Earth’s, even
though Venus is much closer to the Sun than the Earth.
On the dayside, exospheric temperatures are near
300K compared to Earth’s of near 1200K [1]. This is
thought to result principally from 15 micron excitation
of carbon dioxide by atomic oxygen resulting in very
strong 15 micron emission to space, cooling off the
upper atmosphere [2]. On the nightside the Venus
upper atmosphere is near 100K [3], compared to Earth
where temperatures are near 900K. The nightside
Venus temperatures drop with altitude contrary to a
thermosphere where temperatures rise with altitude.
As a result, the very cold nightside is called a
“cryosphere” rather than a thermosphere. This is the
first cryosphere discovered in the solar system [1].
Temperatures sharply drop near the terminator.
Apparently, heat is somehow blocked near the
terminator from being significantly transported to the
nightside [4].
Recently, drag studies were performed on a number of
Earth satellites to establish whether the rise of carbon
dioxide on Earth was cooling the Earth’s
thermosphere similar to the dayside of Venus. Keating
et al. [5] discovered that a 10 percent drop in density
near 350km at solar minimum occurred globally over
a period of 20 years with a 10 per cent rise in carbon
dioxide. This should result in about a factor of 2
decline in density from 1976 values, by the end of the
21st century brought on by thermospheric cooling.
Subsequent studies have confirmed these results.
Thus we are beginning to see the cooling of Earth’s
upper atmosphere apparently from the same process
cooling the Venus thermosphere.
Fig. 1 VIRA Exospheric Temperatures
Atmospheric drag data from the Pioneer Venus
Orbiter and Magellan were combined to generate an
improved version of the Venus International
Reference Atmosphere (VIRA) [6], [7]. A “fountain
effect” was discovered where the atmosphere rises on
the dayside producing adiabatic cooling and drops on
the nightside producing some adiabatic heating. (See
figure 1).
The thermosphere was discovered from drag
measurements to respond to the near 27-day period of
the rotating Sun, for which regions of maximum solar
activity reappear every 27 days. The increased euv
emission from active regions increased temperatures
and thermospheric density, (See Figure 2).
Fig. 2 Exospheric Temperatures Compared to 10.7cm Solar Index
Second diurnal survey (12/5/79 – 3/6/80) Pioneer Venus Orbiter
measurements (OAD) 11 day running means [2]
Estimates were also made of the response to the 11year Solar Cycle by combining the Pioneer Venus and
Magellan data. Dayside exospheric temperatures
changed about 80K over the solar cycle, [8]. Earlier
estimates of temperature change gave 70K based on
Lyman alpha measurements. The responses to solar
variability were much weaker than on Earth due
apparently to the much stronger O/CO2 cooling on
Venus which tended to act as a thermostat on
thermospheric temperatures.
Another discovery from drag measurements was the 4
to 5 day oscillation of the Venus thermosphere [3],
(See figure 3). These oscillations are interpreted as
resulting from the 4-day super-rotation of the
atmosphere near the cloud tops. Other indications of
the super-rotation of the thermosphere come from
displacement of the helium bulge and atomic
hydrogen bulge from midnight to near 4AM.
Fig. 3 Four to Five Day Oscillations in Thermospheric Densities
Magellan 1992.
During 2008, the Venus Express periapsis will be
dropped from 250km down to approximately 180km
to allow drag measurements to be made in the North
Polar Region, [9]. Drag measurements above 200km
have already been obtained from both Pioneer Venus
and Magellan so measurements near 180km should be
accurate. In 2009, the periapsis may be decreased to a
lower altitude allowing accelerometer measurements
to be obtained of drag as a function of altitude, to
determine density, scale height, inferred temperature,
pressure, and other parameters as a function of altitude.
The risk involved in the orbital decay and
accelerometer measurements is minimal. We have not
lost any spacecraft orbiting Venus or Mars due to
unexpected thermospheric drag effects in over 30
years.
The Venus Express accelerometer drag
experiment is very similar to accelerometer
experiments aboard Mars Global Surveyor, Mars
Odyssey, and Mars Reconnaissance Orbiter which
orbit Mars. The Venus Express drag measurements of
the polar region will allow a global empirical model of
the thermosphere to emerge.
Previous drag
measurements have been made principally near the
equator. The experiment may help us understand on a
global scale, tides, winds, gravity waves, planetary
waves, and the damping of waves. Comparisons will
be made between low and high latitude results;
between the middle and upper atmosphere; and with
other instruments that provide information from
current and previous measurements. The character of
the sharp temperature gradient near the day/night
terminator needs to be studied at all latitudes. The
cryosphere we discovered on the nightside needs to be
studied at high latitudes. The rotating vortex dipole
over the North Pole surrounded by a colder “collar”
needs to be analyzed to identify how wave activity
extends into the polar thermosphere. We have already
discovered super-rotation
in
the
equatorial
thermosphere, but we need to study 4-day superrotation at higher latitudes to obtain a global picture of
the thermosphere. The super-rotation may affect
escape rates and the evolution of the atmosphere.
References:
[1] Keating, G. M., et al: Venus Thermosphere and
Exosphere: First Satellite Drag Measurements of an
Extraterrestrial Atmosphere. Science, Vol. 203, No.
4382, 772-774, Feb. 23, 1979.
[2] Keating, G. M. and Bougher, S.W.: Isolation of
Major Venus Cooling Mechanism and Implications
for Earth and Mars, Journal of Geophysical
Research, Vol. 97, 4189-4197, 1992.
[3] Keating, G.M.; Taylor, F.W.; Nicholson, J. V. II;
and Hinson, E.W. : Short-Term Cyclic Variations and
Diurnal Variations of the Venus Upper Atmosphere,
Science, Vol. 205, No. 4401, 62-64, July 6, 1979.
[4] Bougher, S. W.; Dickinson, R. E.; Ridley, E. C.;
Roble, R. G.; Nagy, A. F.; and Cravens, T. E.: Venus
mesosphere and thermosphere, II, Global circulation,
temperature, and density variations, Icarus, Vol. 68,
284-312, 1986.
[5] Keating, G. M. et al.: Evidence of Long-Term
Global Decline in the Earth’s Thermospheric
Densities Apparently Related to Anthropogenic
Effects, Geophysical Research Letters, Vol. 27, No.
10, 1522-1526, 2000.
[6] Keating, G. M. et al.: Models of Venus Neutral
Upper Atmosphere Structure and Composition: The
Venus International Reference Atmosphere (Edited
by A. L. Kliore, V. I. Moros, and G. M. Keating)
Advances in Space Research, Vol. 5, No. 11, 117171,1985.
[7] Keating, G. M.; Hsu, N.C., and Lyu, J.: Improved
Thermospheric Model for the Venus International
Reference Atmosphere, Proceedings of the 31st
Scientific Assembly of COSPAR, Birmingham,
England, 139, 1996 (Invited)
[8] Keating, G. M. and Hsu, N. C.: The Venus
Atmospheric Response to Solar Cycle Variations,
Geophysical Research Letters, Vol. 20, 2751-2754,
1993.
[9] Keating, G.M. et al: Future drag measurements
from Venus Express. Advances in Space Research,
Vol. 28, July 2008.