LtE in CMO #246,247,248

From  C Martin GASKELL

@. . . . . . Subject: Edom brightening predictions


  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 noon when Mars is in opposition is that the latitude of the reflecting region equals the mean of the declination of the sun (D(S)) and the declination of the earth (D(e)).  Although this is stated correctly on p. 122 of T. Dobbins & W. Sheehan (2001, Sky & Telescope, 101, 115), their table on p. 123 is instead for the condition D(S) = D(e) around opposition.  The mean of the declinations of the earth and sun in this table are +2 degrees while the crater Schiaparelli in Edom Promontorium lies at a latitude of -2 degrees. The specular reflection conditions will actually be met most closely at end of July, one half of the phase angle before each central meridian transit.


  The sudden 1953 July 24 Edom brightening did not take place under ideal conditions for specular reflection since the right ascensions of the earth and sun were wrong for the reported time of observation.  The historical observations and the recent observations of brightness fluctuations more than an hour away from the Dobbins & Sheehan predicted times show that brightenings are possible over a wide range of dates and times. Conditions for specular reflection comparable to those of the last few days will persist until the end of August.  Observers are urged to continue monitoring for brightness fluctuations until this time.


  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. 

(10 June 2001 eamil)


@. . . . . . .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 noon on Mars].

Tom Dobbins, Rick Fienberg, and I are currently discussing an updated ephemeris.

 (10 June 2001 email)


@. . . . . . .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.

 (11 June 2001 email)


@. . . . . . . . .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.

(11 June 2001 email)


@ . . . . . . . . Yes, I recorded yellow dust on the morning limb on 2001 June 23 05:35 UT (CM = 174) despite bad seeing. From my sketch I estimate that it extended in longitude from the terminator (long about 260) to 220 or 230 long and in latitude it went from the equator, or maybe a bit N. of the equator to around +50 (where it merged with limb and polar haze).

 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.      

           (28 June 2001 email)


@ . . . . . . . . . .I think "global" is a bit of an overstatement at this stage. On Friday night from Linocln, NE, USA we saw Noachis looking yellow as it approached the evening limb, but the storm has not spread to Margaritifer Sinus and Mare Acidalium etc. Noachis looked about the same as the (Martian) E. side of the storm had looked two weeks earlier.

(2 July 2001 email)


@. . . . . . . .Subject: A Ghostly Mars:


 thought a little description of the visual appearance of Mars from the US might be of interest since I think there are things that can be appreciated visually but which can get lost in the processing of CCD images.

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. 

(10 July 2001 email)

  C Martin GASKELL (NE, USA)

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