maps of ozone and water on Mars reveal
unexpected ozone minimum at sub-solar latitudes.
The first simultaneous meridional maps of Mars ozone and
water have revealed an unexpected minimum in ozone at sub-solar latitudes (R.E.
Novak, M.J. Mumma, M.A. DiSanti, N. Dello Russo, K. Magee-Sauer 2002, Icarus, in press).
The measurements were made during late northern spring on Mars using
CSHELL on NASA's Infrared Telescope Facility atop Mauna
Kea, Hawaii. The observing geometry is shown in Fig.
1. Water was found to increase
monotonically from South to North, as expected for this season, and the
retrieved abundance agrees with independent measurements from the ground and in
space (Fig. 2). However, ozone showed
maxima in both hemispheres along with an unexpected minimum in the sub-solar
region. The detected ozone column
burdens are considerably lower than the detection limits achieved on the
Mariner 9 and Viking space missions (ozone has not been detected by the Thermal
Emission Spectrometer on Mars Global Surveyor).
Mars ozone has been measured remotely with other techniques (HST, infrared heterodyne spectroscopy), but neither
approach can provide simultaneous measurements of water. The capability to map middle-atmosphere ozone and total water throughout its orbit represents a
unique and powerful tool for investigating photochemical cycles on Mars.
In these observations, O2 was used as a tracer for the ozone (O3) and HDO was
used as a tracer for water (H2O).
Novak, R.E., M. J. Mumma, M.A. DiSanti, N. Dello Russo, K. Magee-Sauer
2002. Mapping of Ozone and Water in the Atmosphere of Mars
near the 1997 aphelion: detection of an ozone minimum at sub-solar latitudes.
Icarus, in press.
Fig. 1. Aspect of Mars on UT 21 Jan 1997.
The coordinate system is centered at the sub-earth point; the directions
are on the sky with tick marks indicating intervals of 1.0 arc-seconds. The spectrometer slit (0.5 arc-sec wide) is
centered on the sub-earth point and is aligned North-South on Mars. The sub-solar point is indicated. Diameter of Mars is ~9.7 arc-sec. (diagram
courtesy of F. Espenak)
Fig. 2. Column abundances of
apparent ozone and total water versus latitude on Mars on 21 Jan 1997. The observed ozone
is located at altitudes greater than 20 km and is measured by observing O2 at 1.27
microns as a tracer of ozone. The increase in water column abundance in the
north reflects the release of water from the polar cap during late northern
spring. The water is located primarily at altitudes below 10 km. The photolysis of water produces atomic
hydrogen which then diffuses to higher altitudes. The ozone column abundances are slightly
asymmetric about the sub-solar latitude (smaller in the north), suggesting that
destruction of ozone by atomic hydrogen in the northern polar region has
already begun. An unexpected minimum in
ozone is observed at sub-solar latitudes; the cause is not known but it could
be associated with clouds (gas-grain interaction could catalyze ozone
destruction or could quench the O2 emission).
Further investigation is needed.