Other Temperature and Pressure Profiles
The daily Martian weather reports are prepared from radio occultation
measurements of the Martian atmosphere. Such measurements involve analysis of
the phase perturbations of an ultrastable radio transmission due to refraction
(bending) in the planetary atmosphere. Occultations generally occur twice
per orbit of the MGS spacecraft, and each occultation yields vertical profiles
of the temperature and pressure of the atmosphere of Mars.
Links to other temperature and pressure profiles are available on this
page. These profiles of the atmosphere of Venus were obtained with the
Magellan
spacecraft in October, 1991. Although the primary goal of the Magellan
mission was to map the surface of Venus with a synthetic aperture
radar, the Project kindly supported a number of radio occultation
experiments over a period of three years.
Temperature Profile of Venusian Atmosphere
The temperature profile of the Venusian atmosphere was determined from an
ingress occultation of the Magellan spacecraft on October 5, 1991. The
wavelength of the S-band radio signal used to probe the atmosphere was
13 centimeters. The occultation point was at a Northern Venus latitude of 67
degrees, and the local time at the occultation point was 22:05. The
units of temperature in the profile are Kelvin degrees. Zero
Kelvin degrees is the so-called absolute zero. Water freezes at
273 Kelvin degrees and water boils at 373 Kelvin degrees (on the surface of
the Earth). To convert from Kelvin degrees to the more familiar
Celsius degrees, just subtract 273. The units of altitude are
kilometers above the surface of the planet.
The profile shows that the atmosphere of Venus is extremely hot. Indeed,
the planet suffers from a runaway greenhouse effect. The temperature at
the surface of Venus is in fact sufficiently hot to melt lead! The profile also
shows how the temperature of the atmosphere decreases as the altitude
above the planet increases. The Magellan spacecraft was only able to
probe the Venusian atmosphere to within 34 kilometers of the surface. When the
MGS radio occultations begin, that spacecraft will be able to probe the atmosphere of Mars all the way down to the surface.
You may click on the link below to view the temperature profile. The
profile itself is shown in blue and the 3-sigma error bars (99.7%
confidence level) are shown in red. This means that at the time of the
occultation, the true temperature of the atmosphere was within the red
envelope with a likelihood of 99.7%.
Pressure Profile of Venusian Atmosphere
The pressure profile of the Venusian atmosphere was determined from the
same occultation as the temperature profile. The pressure is plotted on
a logarithmic scale because the pressure decreases exponentially with
height. The units of pressure in
the profile are millibars. The typical surface pressure on
Earth is 1000 millibars (1 bar). The profile shows that the pressure of
the Venusian atmosphere is extremely high. As with the high temperature,
this is due to the runaway greenhouse effect. The Venusian atmosphere is
so dense and cloudy that it is nearly opaque to visible light. That it why the
Magellan spacecraft carried a radar to map the surface rather than a
more conventional visible light camera. Radio signals can penetrate the
thick atmosphere because their wavelength is much longer than that of
visible light.
You may click on the link below to view the pressure profile. Again, the
estimated pressure profile is shown in blue, and the 3-sigma envelope is
shown in red.
Credits
Thanks to Jon Jenkins of the SETI Institute for providing the
Venusian temperature and pressure profiles. For more information, the Magellan
radio occultation results are published in:
J.M. Jenkins, P. G. Steffes, D.P. Hinson, J.D. Twicken, and G.L. Tyler,
Radio Occultation Studies of the Venus Atmosphere with the Magellan
Spacecraft, Icarus, Vol. 110, 79-94, 1994
Last updated: October 11, 1999
Joe Twicken
