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