HELIUM IN THE MARTIAN ATMOSPHERE

Citation
Va. Krasnopolsky et al., HELIUM IN THE MARTIAN ATMOSPHERE, J GEO R-PLA, 98(E8), 1993, pp. 15061-15068
Citations number
40
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Geosciences, Interdisciplinary","Astronomy & Astrophysics
Journal title
JOURNAL OF GEOPHYSICAL RESEARCH-PLANETS
ISSN journal
2169-9097 → ACNP
Volume
98
Issue
E8
Year of publication
1993
Pages
15061 - 15068
Database
ISI
SICI code
2169-9097(1993)98:E8<15061:HITMA>2.0.ZU;2-4
Abstract
A simple two-reservoir degassing model for the Earth decribes rather w ell the current degassing rate of He-4 which is equal to (3 +/- 1)x10( 6) cm-2s-1 according to data for the helium polar wind and the measure d He-3/He-4 ratio. This value of the helium degassing and loss rate pr ovides an important constraint in modeling of noble gases, and some re cent models do not fit this constraint. Scaling this value to the know n amounts of Ar-40 in the atmospheres of Mars and the Earth and to U/K ratios in their surface rocks, this results in a crude estimate of th e helium degassing rate on Mars, which is equal to 2.2x10(5) cm-2s-1. Nonthermal escape of He is calculated using the daytime mean atmospher ic models for low, mean, and high solar activity with He mixing ratio f(He) = 1 ppm. Three processes contribute to He escape: (1) electron i mpact ionization and photoionization above the ionopause followed by s olar wind sweeping away of the ions formed (1.3x10(5) cm-2s-1), (2) co llisions with hot oxygen atoms formed mainly by recombination of O2+ ( 2.8x10(4) cm-2s-1), (3) charge exchange of He+ and CO2, N2, and CO bet ween the exobase and ionopause (6x10(3) cm-2s-1). The derived mixing r atio for He is f(He) = 1.4 ppm for the adopted degassing rate. The He 584 angstrom airglow intensity is equal to 37 R and 67 R at low and hi gh solar activity, respectively. The intensities are 105 R and 240 R f or f(He) = 7 ppm, and 10 R and 16 R for f(He) = 0.3 ppm. These brightn esses should be detectable using the Extreme Ultraviolet Explorer sate llite. The measurement of helium on Mars is important for the determin ation of the planetary abundance of U and Th, element differentiation in the primordial nebula (by comparison with the Earth and Venus) and in the interior of Mars (by comparison with the U and Th fraction in t he surface rocks on Mars measured by the Mars 5 and Phobos orbiters). This measurement makes possible an estimate of the radiogenic heat flu x which is important for the thermal balance of Mars' interior.