PDS logoPlanetary Data System
PDS Information
Find a Node - Use these links to navigate to any of the 8 publicly accessible PDS Nodes.

This bar indicates that you are within the PDS enterprise which includes 6 science discipline nodes and 2 support nodes which are overseen by the Project Management Office at NASA's Goddard Space Flight Center (GSFC). Each node is led by an expert in the subject discipline, supported by an advisory group of other practitioners of that discipline, and subject to selection and approval under a regular NASA Research Announcement.

Amagats


From the ideal gas law we can define the number of particles at standard temperature (273.15 K) and standard pressure (1 atm = 1013.25 milli-bars). The ideal gas law can be written many ways, here we will adopt the notation(s)

P = N k T = rho Na k T/<mw> = rho Rg T
where, N is the number of molecules per unit volume, Boltzmann's constant k = 1.3806 · 10-16 erg/K, the gas constant
Rg = Na · k/<mw>, Avogadro's number, Na = 6.02 · 1023 particles per mole, <mw> is the molecular weight in grams/mole.
This number of molecules at STP is called Loschmidt's number and has the value,
N0 = Pstp/(k · Tstp) = 2.687 · 1019 cm-3

The number of amagats of a gas is given as a ratio of the actual number of particles at the given temperature and pressure to Loschmidt's number. In planetary atmospheres the number density is a function of height. We can calculate the thickness of an equivalent atmospheric column at standard temperature and pressure. This is denoted as cm-amagats or Km-amagats and is given by

Z = (1/N0) integ Ni(z) · dz
Hydrostatic equilibrium relates the pressure and vertical coordinates
dP ident - rho g dz, or dz = -dP/(rho g),
and the number density, Ni can be written as a function of the density,
Ni = qi · rho Na/<mw>, molecules/cm3
where, qi is the volumetric fraction of species i.
Z = Na/(g N0) integ qi/<mw> · dP approx Na/(g N0) · ( qi/<mw>) · P = H0 · P
H0 = 10 · Na qi/(g N0 <mw>) Km-amagat/Bar

EarthMarsJupiterSaturnTitanUranusNeptune
g981.0374.12425.31000.0136.0880.11110.5 cm/s
q(H2)1.001.000.900.961.000.850.85
<mw>28.9744.012.222.1428.002.302.30 gm/mole
H07.8813.6137.45100.5058.8594.0874.56 Km-am/Bar
1000/H0126.973.4926.709.9516.9910.6313.41 mB/Km-am