Encyclopedia of the Solar System 2nd ed

664 Encyclopedia of the Solar System

FIGURE2a Mars dayglow as measured by the

Mariner 9UVS. (Figure reproduced with

permission from Elsevier.)

A, and the CO (A-X) at fourth-positive bands. All these UV ̊
features of the martian airglow are products of processes in-
volving Mars’ CO 2 atmosphere. The CO+ 2 band systems are
the result of a combination of photoionization excitation of
CO 2 and fluorescent scattering of CO+ 2 , and the CO (a-X)
and (A-X) bands are due to photon or electron dissocia-
tive excitation of CO 2. The presence of escaping hydrogen
(Ly-αat 1216A), suggested atomic hydrogen within the at- ̊
mosphere, and also suggested the accumulation of oxygen
and loss of water.
More recent (1995)HUTobservations (820–1840A) ̊
confirmed these earlyMarinerresults (Fig. 2b), andFUSE
measurements detected molecular hydrogen (H 2 ) emis-
sion features at 1070 and 1167A for the first time. ̊ EUVE

provided the first measurements of helium (584A) within ̊

the martian atmosphere. These helium observations have

been used to set constraints on outgassing processes. Re-

cent UV observations from theMars Expressspacecraft

have made the first detection of the martian nightglow, re-

vealing nitric oxide emission features similar to those seen

on Venus, auroral emissions associated with crustal mag-

netic field features, and high altitude CO 2 ice clouds.

Mariner 6and 7 UVS observations revealed the presence

of the Hartley band of ozone (O 3 ), centered near 2550A. ̊

The feature was detected at the south polar cap, through ra-

tios of south polar spectra to low latitude spectra (Fig. 2c).

Further observations with the UVS on theMariner 9or-

biter revealed that the north and south polar ozone amount

FIGURE2b Mars spectrum at EUV–FUV

wavelengths as measured byHUT. (Figure

reproduced with permission from AAS

Publications.)