For the last couple of weeks, I've been doing atmospheric probe studies for Jupiter. The task was to design some atmospheric probes to follow-on the success of the Galileo Probe. As you probably know, the Galileo Probe had the misfortune of landing in a 5-micron hot spot on Jupiter. Those regions are anomalous for Jupiter and consequently it is difficult to derive global conclusions about Jupiter's atmosphere based only on Galileo Probe data.

Anyway for me to do my thing, I had to cookup a deep atmospheric model for Jupiter. The standard atmospheric model for Jupiter was developed by Al Seiff based upon the Galileo Probe in-situ data. That model only goes down to an altitude of -132.4 km (22 bar pressure). The zero altitude for a gas giant like Jupiter is define as the altitude where the ambient pressure is one bar. We'd like to drop a probe down to an altitude of 200 bars. Extrapolating down to that altitude is easy. Just assume that temperature is a linear function of altitude. Extrapolate from the lower part of Al Seiff's model where the temperature dependence is roughly linear. Assume the atmosphere is isentropic and then use the isentropic chain as a function of temperature to get density and pressure. It get's damned hot when you get down to 200 bars, i.e. 804 K.

With the extrapolated atmospheric model, I was able to complete the parachute descent calculations which is what the task required. Then just for laughs, I looked into the issue of real gas effects. Using the isentropic chain assumed the atmospheric gas was perfect. I found a paper by W.J. Nellis titled "Hydrogen at High Pressure and Temperature". This paper is or was on the web. There's a section in the paper about Jupiter's atmosphere. There's a figure in the paper showing Jovian temperature as a function of pressure. The pressure was in giga-pascals. Based upon the figure it appeared that the temperature at 20 giga-pascals was over 3000 K. I then had a brain fart and calculated 20 giga-pascals to be 200 bars (confused giga for mega). I compounded my error by pushing the panic button and e-mailing coworkers that real gas effects might make the ambient temperature too hot for parachute descent. Someone then took pity on me and pointed out that 20 giga-pascals is actually 200,000 bars. Jupiter is so damned big! It's possible to miscalculate the pressure by three orders of magnitude and not immediately see the error. I'm reminded of the "Hitchhikers Guide to the Galaxy" where Zaphod Beeblebrox was put into the Total Perspective Vortex. For an instant he could see his true relationship with the entire universe and it was supposed to destroy his mind. Of course, Zaphod Beeblebrox was such an egomaniac that he liked his relationship with the universe and it had a positive effect on him.

Gary