From C Martin GASKELL
@. . . . . . Subject:
The
inspiring article by Thomas Dobbins and William Sheehan in the May issue of Sky
& Telescope is wonderfully researched and well written. However there are
some problems in the predictions that might mislead observers into being much
too restrictive in when they look for brightness fluctuations. The good new is that the best time is still
to come. As in 1954, it will be in July,
not early June.
Assuming
that reflecting ice crystals are suspended horizontally, the condition for
specular reflection around local
The sudden
1953 July 24
I urge
anyone who doesn't understand this to draw diagrams, run planetarium programs,
and try to think it through.
Incidentally, the bright spot people see out of aircraft windows below
the aircraft is usually the narrow backscattering that occurs with small
aerosols in clouds, not specular reflection off ice, and I suspect that the
picture on p. 122 in S& T might really be of the former.
(
@. . . . . . .Note the following correction to my
post earlier today:
"The
specular reflection conditions will actually be met most closely at the end of
July, one half of the phase angle AFTER each central meridian transit."
[I was
getting confused with it being before local
Tom Dobbins, Rick Fienberg, and I are currently
discussing an updated ephemeris.
(
@. . . . . . .The recent revised ephemeris is for an
idealized horizontal reflector in the Schiaparelli crater. Observers are cautioned that analysis of the
recent observations by the Florida Keys observers and of historical
observations of sudden Edom brightening imply that the effective reflecting
planes can be inclined at several degrees both in latitude and longitude (note,
for example, the long time period during which the June 7 and June 8 flaring
were each seen). Observers should
therefore start watching up to an hour and a half before the predicted
times. Although the revised ephemeris
runs only into August, conditions will remain favorable through September, and,
fortunately, the apparent size of Mars remains large for a long time this apparition. Flaring before early June was also possible
so observations of Edom Promontorium earlier in the apparition are of interest.
Although
obviously less exciting for the observer than positive detections, negative
reports are also of great value in characterizing the phenomenon.
(
@. . . . . . . . .For analyzing positive sightings
of Martian flares, crude estimates of the brightness are great value. Comparisons can be made with the typical
brightness of the polar caps and, for intense point brightenings, with the
naked-eye brightness of stars.
The temporal
changes are of great interest since they contain information on the size-scales
of the reflectors. Individual concentrations of ice crystals a couple of km
across could can produce fluctuations on timescales of a few seconds as Mars
rotates. For visual observations a tape
recording of descriptions of brightness fluctuations would be of great value,
particularly if commentary on changing seeing conditions is included.
(
@ . . . . . . . . Yes, I recorded yellow dust on the
morning limb on 2001 June 23
I drew the SE
end of Mare Cimmerium (and Mare Sirenum), but not the NW end. Given that it was
towards the morning terminator this not necessarily significant.
Based on my
scanty visual observations so far, yellow dust in the atmosphere seems to have
been a lot more common this apparition than in the previous few.
(
@ . . . . . . . . . .I think "global" is a
bit of an overstatement at this stage. On Friday night from
(
@. . . . . . . .Subject: A Ghostly Mars:
thought a
little description of the visual appearance of Mars from the
I
looked at Mars last night (2001-July-9 UT) with my favorite planetary 20-cm
Newtonian. The CM was around 18
degrees. I think it is important to look
at Mars through a familiar system to appreciate what is going on right
now. The subtle colors reveal a
lot. The first obvious thing is the lack
of contrast. I've seen more contrast on
Mars when it has only been 5 arc seconds across. Normally my 20-cm Newtonian shows the colors
on Mars quite vividly; now it is a pale whitish-yellow. The lack of contrast might not be obvious in
CCD images where the contrast has been artificially enhanced. Tonight (2001-July-10 UT) I think the contrast
is even lower.
Last
night (2001-July-9) for a long time I thought I could not see Sinus Meridiani
at all. Then the seeing suddenly got
better for a moment and I clearly saw a ghostly presence of Sinus Meridiani! It was quite eerie and a remarkable experience.
The
line between Mare Erythraeum and the bright Aurorae Sinus dust storm was quite
sharp. The M. Erythraeum region was
probably the highest contrast part of the planet.
Niliacus
Lacus and Mare Acidalium were visible, but at much reduced contrast. Interestingly, the poorly-resolved
Protonilus/ Deuteronilus/Dioscuria/Cydonia complex was almost as dark as the
Niliacus Lacus/Mare Acidalium region.
The
strongest centers of dust activity were a deeper yellow than the rest of the
disk (they seemed almost brown), especially when they were towards the
limb. Optical depth effects are causing
all of the limb to be a bright yellow, except at the N pole (see below). The terminator is noticeably more yellow than
the evening limb. This will be because
the larger dust particles have a less forward-scattering phase function than
the high-altitude cloud particles.
The
structure of the NPC is interesting.
During rare moments of good seeing it was exceptionally bright so there
would seem to be less general diffuse dust down there. However, the NPC seemed to be cut in two
by a small brown cloud sitting almost exactly at the
pole! A W25 filter showed neither cap to
be bright so the poles must be showing mostly blue haze rather than white ice.
Tonight
(2001-July-10, UT) I could see "the ghost of Syrtis Major". It is a little hard to tell with variable
seeing from night to night, but I wonder if anything at all will be visible
anywhere in a few more days.
(
C Martin GASKELL (NE,