8/3/2019 Density and Temperatures of the Upper Martian Atmosphere Measured - F Forget Et Al 2007

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European Space Agency

European Mars Science and Exploration Conference: Mars Express & ExoMars

ESTEC, Noordwijk, The Netherlands, 12 - 16 November, 2007

THE DENSITY AND TEMPERATURES OF THE UPPER MARTIAN ATMOSPHERE MEASURED

BY STELLAR OCCULTATIONS WITH MARS EXPRESS SPICAM

F. Forget1, J.L. Bertaux

2, F. Montmessin

2, E. Quemerais

2, F. Gonzlez-Galindo

1, S. Lebonnois

1, E. Dimarellis

2,

A. Reberac2

, M.A. Lpez-Valverde3

1Laboratoire de Mtorologie Dynamique, IPSL, Universit Pierre &

Marie Curie, BP99, 4 place Jussieu, 75252 Paris, Cedex 05, France,2Service dAronomie, IPSL, Verrire le

Buisson, France, 3Instituto de Astrofsica de Andaluca, Granada, Spain, forget@lmd.jussieu.fr

The observation of numerous stars rising or

setting through the Martian atmosphere as seen by

the SPICAM UV spectrometer aboard Mars Express

allows to retrieve the atmospheric density and

temperature from 60 km to 130 km [1, 2]. This part

of the atmosphere was previously almost unknown

since very few measurements were available (a few

entry and aerobraking profiles). Moreover, General

Circulation Model simulations had shown that this

part of the atmosphere should present a very active

and interesting dynamic.We present one Martian year of observations

with a total of 616 profiles retrieved at various

latitudes and longitudes. The profiles are analyzed

in details, and compared to the predictions of a

General Circulation Model (GCM) [3, 4].

We studied the seasonal, diurnal and spatial

variations. The atmospheric densities exhibit large

seasonal fluctuations mostly due to variations in the

dust content of the lower atmosphere which controls

the temperature below 50 km, and thus the

atmospheric scale height (Fig.1). In particular, the

year observed by SPICAM was affected by an

unexpected dust loading around Ls=130 whichinduced a sudden increase of density above 60 km.

The diurnal cycle could not be analyzed in details

because most data were obtained at nighttime,

except for a few occultations observed around noon

during northern winter. The corresponding mean

profile slightly differ from the mean profile obtained

at the same locations around midnight, and the

observed differences are consistent with propagating

thermal tides and variations in local heating in the

upper atmosphere (Fig. 2).

Comparison with GCM simulations help to

explain the variations. However, the observed

temperatures are found to be significantly colder

than predicted by the GCM above the 0.01 Pa level

(~90 km altitude). The homopause is higher and

colder than expected (Fig.3). In some locations and

seasons, especially during southern summer,

temperature profiles with homopause temperatures a

few kelvins below the CO2 condensation

temperatures are detected, confirming the possible

presence of CO2 ice clouds in the upper martian

atmosphere at low latitudes [5]. .References: [1] Bertaux et al., JGR 111, CiteID

E10S90 (2006) [2] Qumerais et al. JGR 111, CiteID

E09S04 (2006) [3] Forget et al. JGR. 104 , 24,155-24,176

(1999) [4] Gonzalez-Galindo et al., this issue. [5]Montmessin et al., this issue.

Figure 1. Seasonal cycle of the density at 100 km

observed by SPICAM.

Figure 2. Two mean temperature profiles obtained at

about the same latitude and season, but different local

time, illustrating the diurnal cycle

Figure 3. An average of SPICAM temperature profiles

compared to GCM predictions for various dust and EUV

conditions.