1996/97 Mars Sketch (2)
from CMO #200 (25 February 1998)
-- Movement of Warm, Moist Air from the Arctic Area to the Equatorial Band--
he 1996/97 apparition of Mars was relevant to our observations of the Martian northern hemisphere (NH), and furthermore the season corresponded to the time when the Martian NH was quite sunlit and heated. In fact, the areocentric longitude of the Sun attained 000°Ls, the vernal equinox of the NH, on 16 August 1996 and the autumnal equinox 180°Ls was reached on 12 September 1997, so that the sub-Solar point was at the NH during the almost all the season: The sub-Solar point was above 23°N (25.4°N at the maximum) from 16 January (065°Ls, δ=9.2") to 08 May (115°Ls, δ=10.9"). The summer solstice of the NH, 090°Ls, was attained on 13 March when the latitude of the sub-Solar was 25.4°N. The planet was closest to the Earth on 20 March and hence we should say we could well observe the very summer NH in the 1997 season.
One of the characteristics of the summer hemi- sphere of Mars is that the polar area is warmer than the equatorial band (EB), contrary to the case of the Earth. This is because the Martian atmosphere is so thin that the polar region is more radiated and heated than the band region located along the equator: In fact if the latitude of the sub-Solar point is 20°N, then the Arctic region to the north of 70°N has no night, and the radiation must easily reach the Arctic obliquely but constantly to make the polar area warmer than the remote EB because of the thin atmosphere.
The air mass heated will ascend at the
The heated moist air-mass at the middle latitudes will be often absorbed in the night area and gradually cooled and then made to descend. However this is delayed by the presence of the Coriolis force and the heat will be retained and accumulated more in the high and middle latitudes than the EB. This causes a relatively lower temperature at the EB, and the water vapour will be saturated near the EB and eventually a whitish mist or haze will appear along the EB. This may be identified the so-called equatorial band cloud (EBC), but the matter is not so thick as to be called the cloud, and so we shall consider that it is more appropriate to call here it the "equatorial band mist (ebm)".
We further note that the summer hemisphere is the warmest in the middle latitudes at noon (or just after it), and the lower wind blows towards the noon line, and hence in the morning a lower westerly prevails which blows from the dawn area containing the cooled water vapour or the matter made of frost in the night. The water vapour will appear as a lower mist or haze in the morning and will vanish towards the noon. In the evening, the water vapour in the air will gradually saturated but remain inside because of the lower easterly which blows towards the noon line. Notable is that in the morning area the upper easterly and lower westerly might make sometimes a front.
The easterly will cease as the summer comes to the end. The EB begins to
be warmed up more than the
The above is qualitatively and macroscopically stated, and the local topography and the presence of the local dust storms will give rise to several complicated factors.
In the following we shall see how we observed the phenomena in the
1996/97 apparition. The present writer (Mn) observed already at 051°Ls 13 Dec
1996 at ω=036°W an overall mist lying along the EB from the evening Tymiamata through Chryse to the morning Tharsis, though the
belt was not uniform but thin and thick. The latitude of the sub-Solar point
was already 19°N at that time, and we considered that the moist air mass had
already reached the EB. On 14 Dec at ω=045°W the ebm
was also apparent. On 16 Dec, it was also seen but after ω=016°W and until
then it was not so evident to the naked eyes, implying a topographical effect.
On 15 Dec, the apparent diameter was 7.2". On 26 Dec (056°Ls) at
ω=232°W-- ω= 262°W, the ebm
was clearly visible at the both sides of Syrtis Mj connecting with the evening
and morning haze. Syrtis Mj was green-bluish and so was considered to be
covered by a white mist. On 29 Dec (058°Ls) at ω=272°W, the white belt
Gianni QUARRA and his colleagues from
Next stage must have come around mid-Feb: As reported in CMO #186 p2019, a white band made of mist from the morning Tharsis to the eastward made HIKI (Hk) amazed on 20 Feb (081°Ls) at ω=035°W & ω=046°W. Hk also observed 22 Feb at ω=021°W and so on. The present writer also observed it on 20 Feb at ω=041°W and so on. Tharsis is such a specific area that it is difficult to discuss the wind tendency as well as the distribution of temperature and moisture, but the white mist activity of the area in the morning since 080°Ls must have been enhanced.
Since then, the observations of the so-called EBC augmented. HIGA (Hg)'s
Video images frequently show the ebm
from mid-Feb to March. TROIANI (DTr) observed EBC on
11 Mar at ω=075°W, and SIEGEL (ESg) gave a fresh observation of the EBC on
06 Apr (102°Ls) and also on 18 May at ω= 095°W (cf
CMO #189 p2070, #192 p2109). The mist observed by ISHADOH (Id) on 17 Apr at
ω=258°W (#190 p2082) is nothing but a trail of the ebm
here defined. The B images of
HST's images taken on 30 Dec (058°Ls), 10 Mar (089°Ls), 30 Mar (097°Ls), 17 Apr (105°Ls), 17 May (119°Ls), 27 June (140°Ls) and so on all depict the ebm. A B image on 30 Mar was cited at page 2102 of CMO #191 where the mist is partly thick and complicated.
HST's images on 30 March 1997 (097° Ls) at ω=285°W : Original (left), Red (middle) and Blue (right)
Here we will show another B image (together with R image) taken on 30 Mar at ω=285°W where a typical ebm crossing Syrtis Mj is seen. We also cite the R and B images on 17 May and on 27 June. These make combinations with those already cited in #193 p2137. On 27 June, the sub-Solar point retreated to 16°N, but the evening mist is still present towards Syrtis Mj.
HST's images on 17 May 1997 (119°Ls) Original (left), Red (middle) and Blue (right)
HST's images on 27 June 1997 (140°Ls) Original (left), Red (middle) and Blue (right)
We also cite the images taken successively on 9 July, 10 July and 11 July. On 11 July the season was 146°Ls and the latitude of the sub-Solar point was still 14°N. Notable is the morning cloud/haze which received every- day changes at the rather high latitude. It is considered that the higher easterly and lower westerly sometimes make fronts inside the region still at the season. Note however that the ebm is weaker.
HST's images on 27 June 1997 (146°Ls) Original (top), Red (middle) and Blue (bottom)
Finally we cite HST images on 12 September 1997 (180°Ls) which are important:
HST's images on 12 September 1997 (180°Ls) Original (left), Red (middle) and Blue (right)
Note that the B image does not show the ebm any longer. The time was exactly at the autumnal equinox, and at that time the EB was intrinsically warmer than before.