Abstract
This paper discusses two possible pathways of loss of the two main gases from Titan's post-accretional atmosphere, methane (CH4) and ammonia (NH3), by the mechanisms of thermal escape and emission from the interior coupled with thermal escape. The results give the decline of initial atmospheric gas masses to their present-day levels of 0.1 bar CH4 and 1.4 bar N2 (or equivalent 1.7 bar NH3, as a precursor of N2). From the published data on planetary and Titan's accretion rates, the accretion temperature was estimated as Tac=355 to 300 K. In the first 0.5-0.6 Myr after accretion, Titan's surface cools to 150 K and it takes about 5 Myr to cool to near its present temperature of 94 K. The present-day internal composition corresponds to the accreted Titan made of two solids, antigorite and brucite, that account for 59.5 wt%, and an outer shell of an aqueous solution of NH3+(NH4)2SO 4 accounting for 40.0 wt%, and methane for a much smaller fraction of 0.6 wt%. In thermal escape of CH4 and NH3, based on the Maxwell-Boltzmann distribution of gas-molecule velocities, the initial gas mass N0 in the atmosphere is lost by a first-order flux, N t=N0 exp(-kt), where t is time (yr) and k (yr -1) is a rate parameter that depends on temperature, gas molecular mass, atmosphere thickness, and Titan's escape velocity. The computed initial Tac=355 K is too high and the two gases would be lost from the primordial atmosphere in several hundred years. However, emissions of CH 4 and NH3 from the interior, at reasonable rates that do not deplete the Titan gas inventory and function for periods of different length of time in combination with thermal escape, may result in stable CH4 and NH3 atmospheric masses, as they are at the present. The periods of emissions of different magnitudes of CH4 range from 6×104 to 6×105 yr, and those of NH3 are 55,000-75,000 yr. At the lower Tac=300 K, thermal escape of gases alone allows their atmospheric masses to decrease from the primordial to the present-day levels in 50,000-70,000 years, when Titan's temperature has decreased to 245-255 K. Below this temperature, the NH3 atmospheric mass is comparable to the present-day N2 mass. Thermal escape does not contradict the existence of the photolytic sink of CH4 in the cooled Titan atmosphere. The thermal escape mechanism does not require arbitrary assumptions about the timing of the start and duration of the gas emissions from the interior.
| Original language | English (US) |
|---|---|
| Pages (from-to) | 41-53 |
| Number of pages | 13 |
| Journal | Planetary and Space Science |
| Volume | 93-94 |
| DOIs | |
| State | Published - Apr 2014 |
Funding
We thank Professors Emile Okal and Craig Bina, Graduate Students Joshua Townsend and Michael Witek (all of this Department), and Professor John Franks (Department of Mathematics) for checking some of our mathematical derivations; Professor Emile Okal for an explanation of pressure–depth relationships in spherical , incompressible , homogeneous , and layered bodies; Professor Steven Jacobsen for a reference on NaCl solutions; Dr. Manuel Conde (M. Conde Engineering, Zürich) for his data on NH 3 solutions and release of copyrighted information ; and Professor Amitai Katz (Institute of Earth Sciences, The Hebrew University, Jerusalem) for the recent density value of the Dead Sea brine. We also thank Anonymous Reviewer for insightful and helpful comments, and the Editor-in-Chief, Dr. Rita Schulz, for her efforts in having this paper reviewed. This work was supported by NASA Headquarters under the NASA Earth and Space Science Fellowship Program – Grant NNX13AO02H and by the Arthur L. Howland Fund of Northwestern University .
Keywords
- Atmosphere
- Cooling
- Kinetic model
- Methane and ammonia
- Thermal escape
- Titan
ASJC Scopus subject areas
- Astronomy and Astrophysics
- Space and Planetary Science