;+
; NAME:
;  vt_lp09_lam
; PURPOSE: (one line)
;  return lambda
; DESCRIPTION:
;  return lambda (gravitational potential energy/kinetic energy)
; CATEGORY:
;  Volatile Transport
; CALLING SEQUENCE:
;  lam = vt_lp09_lam(M, r, m_g, T)
; INPUTS:
;  M - mass of the body in g
;  r - distance from the center of the body in cm
;  m_g - mass of a gas molecule in g
;  T - temperature, in K
; OPTIONAL INPUT PARAMETERS:
;  none
; KEYWORD INPUT PARAMETERS:
;  none
; KEYWORD OUTPUT PARAMETERS:
;  none
; OUTPUTS:
;  lambda = (gravitational potential energy/kinetic energy)
;  Ratio of the ratio of the gravitational energy of the atmospheric gas to its thermal energy.
; COMMON BLOCKS:
;  None
; SIDE EFFECTS:
; RESTRICTIONS:
;  None
; PROCEDURE:
;  Levi and Podolak 2009. Icarus 202, 681–693. Eq. 1.
;  m_g is the mass in g, or mu * !phys.m_u
; EXAMPLE:
;  Reproducing the statement on P. 681.  
;  "a 300 km object with an average density of 1 g cm−3 and a CO
;    atmosphere at 30 K also has lam ∼ 3."
;  print, vt_lp09_lam( (4/3.)*!pi*(300*1e5)^3 * 1., 300*1e5, 28*!phys.m_u, 30.)
; MODIFICATION HISTORY:
;  Written 2015 May 5, Leslie Young
;-


function vt_lp09_lam, M, r, m_g, T
    
    physconstants

    lam = ( !phys.G * M * m_g)/(r * !phys.k * T)

    return, lam

end