Expert: Philip Stahl - 2/16/2005

Question
Hi Philip

I was wondering: Exactly how dense is the atmosphere of Neptune? I have tried to find out on my own but haven't been able to locate that information. Could you help me? Thanks!

amanda

Answer
Hello.

Unfortunately, the density of Neptune's atmosphere is not exactly known. To do this, calculate such an "exact" figure - one would need to know precisely how the atmosphere of Neptune scales with altitude (height) above some defined surface level. No one knows this. Though we do know the rough proportions of gases in Neptune's atmsophere: hydrogen (> 84%); helium (> 12%); methane (2%) and ammonia (0.01%).

There have been also been estimates made of its atmospheric pressure - anywhere from 100kPa (kilo-pascals), or pretty close to Earth stndard atmospheric pressure at sea level - to nearly 300 kPA at the upper limit.

By way of comparison (and please bear in mind I am not giving a realistic answer to your question here!) for Earth's atmosphere at sea level, the corresponding density of air (at a temperature of 20C or 68F) would be:

0.075 pounds per cubic foot.

Thus- IF Neptune's atmospheric pressure estimate were made at the level of EARTH'S conditions (standard temp. & pressure, at Earth sea level) it would translate to a density of anywhere from 0.075 to 0.225 pounds per cubic foot.

But, of course, this figure is useless in terms of Neptune's OWN conditions! I.e. we've no idea exactly what temperatures, pressure obtain from where we're making the measurements!

What is needed, therefore, is to actually launch a sophisticated measuring space probe INTO Neptune's atmosphere and take continuous measurements all the way to its (presumable) crashing onto the surface. Relaying those measurements back to us before it crashes of course!