2 The planet Mars 3

Contents

1 Introduction 1

2 The planet Mars 3

2.1 Generalities . . . . 3

2.2 Mars exploration . . . . 7

2.2.1 History . . . . 7

2.2.2 How do we go to Mars? . . . . 8

2.3 The Martian surface . . . . 9

2.3.1 Age estimation . . . . 9

2.3.2 Topography and geography . . . . 9

2.3.3 Albedo . . . . 10

2.4 Seasons on Mars . . . . 11

2.4.1 Solar longitude (L

) . . . . 11

2.4.2 Seasonal asymmetry . . . . 11

2.4.3 Polar caps . . . . 12

2.5 The Martian atmosphere . . . . 14

2.5.1 Chemical composition . . . . 14

2.5.2 Temperature and pressure . . . . 14

2.5.3 Circulation and winds . . . . 16

2.5.4 Dust . . . . 17

2.5.5 Clouds . . . . 19

2.5.5.1 H

O clouds . . . . 19

2.5.5.2 CO

clouds . . . . 20

2.5.6 Ozone . . . . 20

2.5.6.1 Column density . . . . 20

2.5.6.2 Production and destruction cycles . . . . 21

2.5.6.3 Seasonal variations . . . . 21

3 SPICAM instrument and data 25 3.1 Introduction . . . . 25

3.2 Mars-Express orbiter . . . . 26

3.2.1 Objectives and scientific instruments . . . . 26

3.2.2 Discoveries and achievements . . . . 26

3.2.3 Viewing geometries . . . . 27

3.2.3.1 Occultations . . . . 27

3.2.3.2 Limb . . . . 27

3.2.3.3 Nadir and EPF . . . . 28

3.3 SPICAM instrument . . . . 28

3.3.1 SPICAM/UV . . . . 29

i

ii CONTENTS

3.4 Data and observations . . . . 31

3.4.1 Data description . . . . 31

3.4.1.1 CCD Recording . . . . 31

3.4.1.2 Data and calibration . . . . 32

3.4.1.3 Radiance factor and PSF . . . . 33

3.4.1.4 Solar flux . . . . 34

3.4.1.5 Uncertainty and binning . . . . 35

3.4.1.6 Health history . . . . 36

3.4.2 Data selection . . . . 37

3.4.2.1 Observations dataset and coverage . . . . 37

3.4.2.2 Spectral interval . . . . 38

3.4.2.3 Data selection summary . . . . 39

4 Radiative transfer in the ultraviolet 41 4.1 General overview . . . . 41

4.1.1 Extinction by gases . . . . 41

4.1.2 Extinction by aerosols . . . . 41

4.1.3 Surface reflection . . . . 42

4.2 Radiative transfer equations . . . . 43

4.2.1 Beer-Lambert-Bouguer extinction law . . . . 43

4.2.2 Scattering phase function . . . . 44

4.2.3 Radiative transfer general equation . . . . 45

4.3 Description of the radiative transfer parameters . . . . 46

4.3.1 Ozone absorption . . . . 46

4.3.2 Molecular scattering . . . . 47

4.3.3 Dust extinction . . . . 50

4.3.4 Cloud scattering . . . . 51

4.3.5 Regolith surface reflectance . . . . 51

4.3.5.1 Opposition effect . . . . 52

4.3.5.2 Parameters . . . . 53

4.3.5.3 Reflectance equations . . . . 53

4.3.5.4 Reference studies . . . . 55

4.3.6 Ice surface reflectance . . . . 58

5 Retrieval method 61 5.1 Overview . . . . 61

5.2 Models and methods . . . . 65

5.2.1 Radiative transfer model . . . . 65

5.2.2 Inversion method . . . . 66

5.2.3 Code comparison and NOMAD preparation . . . . 67

5.3 Cloud detection algorithm . . . . 67

5.3.1 Variables and characteristics . . . . 68

5.3.2 Principle of the method . . . . 71

5.3.2.1 Detection principle . . . . 71

5.3.2.2 Ice surface flagging . . . . 71

5.3.3 Reference estimate . . . . 72

5.3.4 Method . . . . 76

5.3.4.1 References’ selection . . . . 76

5.3.4.2 Reference ratios . . . . 78

5.3.4.3 Detection criteria . . . . 78

5.3.4.4 Method summary . . . . 80

5.3.5 Results and comparison with OMEGA . . . . 80

5.3.5.1 Comparison over Tharsis and Lunae Planum region . . . . 81

5.3.5.2 Other simultaneous comparisons . . . . 84

5.3.5.3 Conclusions on the method . . . . 87

CONTENTS iii

6 Cloud, dust and ozone climatology 89

6.1 Introduction . . . . 89

6.2 Cloud optical depth climatology . . . . 89

6.2.1 Seasonal cycle and interannual variability . . . . 89

6.2.2 Aphelion cloud belt . . . . 94

6.2.2.1 Spatial and seasonal distribution . . . . 94

6.2.2.2 Diurnal variability . . . . 96

6.2.2.3 Orographic clouds . . . . 97

6.2.3 Polar hoods . . . . 100

6.2.3.1 Northern polar hood . . . . 100

6.2.3.2 Southern polar hood . . . . 101

6.2.3.3 Cloud ”bridge” and morning haze . . . . 103

6.2.4 Errors and uncertainties . . . . 104

6.3 Dust optical depth climatology . . . . 106

6.3.1 Seasonal distribution . . . . 106

6.3.2 Spatial distribution . . . . 109

6.3.3 Profile influence on the retrieved opacity . . . . 110

6.3.4 Opacity comparison . . . . 112

6.3.5 Errors and uncertainties . . . . 113

6.4 Ozone column climatology . . . . 114

6.4.1 Seasonal distribution . . . . 114

6.4.2 Spatial distribution . . . . 116

6.4.3 Comparison with other data . . . . 117

6.4.4 Errors and uncertainties . . . . 118

6.5 Surface albedo . . . . 119

6.5.1 Regular surface . . . . 119

6.5.2 Ice surface . . . . 121

6.5.3 Errors and uncertainties . . . . 122

6.6 Retrieval summary . . . . 122

7 Conclusions and future developments 125 7.1 Conclusions . . . . 125

7.2 Future developments . . . . 126

Bibliography 127

Appendices

A Acronyms 137

B List of publications 139

C Topography of the polar regions 141