And now the weather ... on Mars! (Forwarded)
Robert Clark
> ESA Science News
> http://sci.esa.int
>
> 03 Jul 2002
>
> And now the weather ... on Mars!
>
> ...
> Water-ice clouds continue to become more frequent, especially near
> or over all the major volcanoes, forming a thin cover at southern
> mid-latitudes (see 'C'). Under some conditions, dust storm activity
> warms the atmosphere and inhibits water condensation. However, as
> noted last week, present atmospheric temperatures and water
> distributions (probably in the form of diurnally-precipitated
> frosts that sublime each morning), combined with the slightly dusty
> atmosphere, appear to favor nucleation of mid-atmospheric (at or
> slightly above the tropopause) ice clouds.
>
> ...
I think that there will soon be research into the idea that some
of these low level fogs and clouds consist in part of supercooled
liquid water droplets.
The investigation of supercooling on surfaces on Mars is already
occurring:
Capillary Effects on the Stability of Ice in Martian Crust
http://www.lpi.usra.edu/meetings/lpsc2001/pdf/1702.pdf
Low-Temperature Phase Relations in the CO2-H2O System with
Application to Mars.
http://www.lpi.usra.edu/meetings/lpsc2000/pdf/1518.pdf
PTX Phase Equilibria in the H2O-CO2-Salt System at Mars Near-Surface
Conditions.
http://www.lpi.usra.edu/meetings/lpsc2001/pdf/1689.pdf
GEOCHEMICAL MODELING OF BASALT-BRINE INTERACTIONS AS AN ANALOG FOR
MARS NEAR-SURFACE PROCESSES.
http://www.lpi.usra.edu/meetings/lpsc2002/pdf/1211.pdf
See this page on fogs on Mars:
------------------------------------------------------------
NASA SP-441: VIKING ORBITER VIEWS OF MARS
------------------------------------------------------------
- THE ATMOSPHERE -
"Early Morning Clouds in the Tharsis Montes and Valles Marineris
Region. Ascraeus Mons and Pavonis Mons are prominently displayed in
this mosaic, and dense cloud blankets cling to their northern slopes.
High cirrus clouds lie to the west of Tharsis, and waves are visible
in the clouds surrounding the peaks. Bands of clouds appearing to
have a cellular structure extend north from the canyon, and the areas
within and immediately surrounding the chasm exhibit water-ice fogs.
[211-5049; 5°S, 105°W]"
"Early Morning Surface Fog. The presence of morning fogs in some
crater and channel bottoms is a Viking discovery with possible
implications for the future biological exploration of Mars. These
early morning views of the Memnonia region were taken one-half hour
apart using a violet filter to enhance the contrast of the
condensates. The areas marked by arrows are noticeably brighter in
the later picture. The fogs indicate specific spots where water is
exchanged, probably on a daily cycle, between the surface and the
atmosphere. The surface and lower air layers in this region become
unusually cold at night because of the thermal properties of the
surface. When the surface warms in the morning, it seems that a small
amount of water vapor-estimated to be about one-millionth of a meter
thick if liquefied is driven off; this vapor recondenses in the
atmosphere, which warms more slowly, to form a ground fog of ice
particles. [P17487; 13°S,147° W]"
"Early Morning Clouds in Noctis Labyrinthus. Condensate clouds are
seen here in early morning in the canyons of Labyrinthus Noctis,
which lies at the western end of the equatorial Valles Marineris
system. This picture, which covers about 90 000 km2, was made by
combining three frames of the same field taken through violet, green,
and red filters. Although these clouds lie mainly down inside the
canyons, they evidently extend above the walls and spill over some of
the surrounding plateau. Like most condensate clouds in the Martian
troposphere, they are believed to be composed of water-ice crystals.
[P18114, 9°S, 95°W]"
http://history.nasa.gov/SP-441/ch12.htm
See also the large size images of Noctis on this page:
Clouds in Noctis Labyrinthis.
http://planetscapes.com/solar/cap/mars/noctis.htm
It is now recognized that supercooled liquid water droplets are a
big part of cloud formation. This perhaps was not as well recognized
back in the Viking period. Back then it was probably assumed that if
you saw clouds at below freezing temperatures and you knew they were
made of water then they must have been ice.
Now no study of cloud formation on Earth would proceed without
considering the effects of supercooling. I'm suggesting the study of
clouds on Mars has lagged behind in this regard.
CLOUD PHYSICS
Collision/Coalescence; The Bergeron Process.
"Liquid water that is cooler than 0°C is called supercooled. In the
atmosphere, similar to CCN, there exist freezing nuclei. In contrast
to CCN, freezing nuclei are not plentiful in the atmosphere because
there structure must be similar to the structure of an ice crystal.
Most of the naturally occuring freezing nuclei "activate" at about -
10°C. These freezing nuclei allow for the cloud droplets to freeze
around them. Because of the relative sparceness of the freezing
nuclei, ice crystals and supercooled water droplets can coexist at
the same time. This is where the Bergeron's primary fact becomes
important."
http://weather.cod.edu/sirvatka/bergeron.html
And here's a recent paper that discusses it in Earth clouds:
Deep convective clouds with sustained supercooled liquid water down
to -37.5°C.
DANIEL ROSENFELD* AND WILLIAM L. WOODLEY
Institute of Earth Sciences, the Hebrew University of Jerusalem,
Jerusalem 91904, Israel
Woodley Weather Consultants, 11 White Fir Court, Littleton, Colorado
80127, USA
Correspondence and requests for materials should be addressed to D.R.
(e-mail: daniel@v...).
ABSTRACT
In cirrus and orographic wave clouds, highly supercooled water has
been observed in small quantities (less than 0.15 g m-3). This high
degree of supercooling was attributed to the small droplet size and
the lack of ice nuclei at the heights of these clouds. For deep
convective clouds, which
have much larger droplets near their tops and which take in aerosols
from near the ground, no such measurements have hitherto been
reported. However, satellite data suggest that highly supercooled
water (down to -38 °C) frequently occurs in vigorous continental
convective storms. Here we report in situ measurements in deep
convective clouds from an aircraft, showing that most of the
condensed water remains liquid down to -37.5 °C. The droplets reach a
median volume diameter of 17 µm and amount to 1.8 g m-3,
one order of magnitude more than previously reported. At slightly
colder temperatures only ice was found, suggesting homogeneous
freezing. Because of the poor knowledge of mixed-phase cloud
processes, the simulation of clouds using numerical models is
difficult at present. Our observations will help to understand these
cloud processes, such as rainfall, hail, and cloud electrification,
together with their implications for the climate system.
Nature, v. 405, p. 440 - 442 25, May 2000.
http://www.nature.com/cgi-taf/DynaPage.taf?
file=/nature/journal/v405/n6785/abs/405440a0_fs.html
Bob Clark