picture of the day
One of the last images from
Phoenix. Credit: NASA/JPL/University of Arizona
Nov 03, 2008
The Phoenix Expires
Winter means the end of life for the Phoenix Mission.
Have its mission objectives been fulfilled or is more
continue to fall as Mars enters its winter months. A frigid -96
degrees Celsius was recorded a couple days ago, with continued
drops expected until the northern latitudes begin to precipitate
carbon dioxide snow. Eventually, the lander will be buried
completely in dry ice, but its functions will cease long before
that time as the batteries drain due to a lack of sunlight.
state that Mars was once a “warm, wet world millions of years
ago.” Due to some slow geological changes or alterations in its
orbit, Mars changed from a hospitable planet to the cold, barren
wasteland that it is today. However, planetary scientists hoped
that water might remain in the polar regions because the Odyssey
orbiting observatory detected the signature of hydrogen in that
region, although it was probably neutrons and gamma rays
released by cosmic rays and not water vapor. It is an assumption
that the detected hydrogen came from water molecules escaping
beneath the surface crust. The hydrogen may have been
"implanted" electrically by the solar wind, or molecules other
than water were transferred to Mars from another celestial body.
Phoenix landed in
the high Martian latitudes on May 25, 2008, while the northern
Sun was above the horizon all day. It touched down in what was
supposed to be a patch of ice so that the lander could dig
several shallow trenches to find and analyze water just below
the surface. The experiments were designed to uncover evidence
for life by revealing carbon-based chemistry within the soil
samples. At present, no biochemistry has been found, despite
baking the samples at almost 1000 degrees Celsius so that
volatile compounds would be driven off and detected by the
On June 19, 2008,
small chunks of a white substance were seen in the bottom of a
shallow excavation dug by Phoenix and remained for two days
before disappearing. In a Picture of the Day about the
discovery, it was
predicted that the material was some kind of
mineral—magnesium sulfate perhaps, or silicon dioxide—because
those compounds are found in abundance in other areas on Mars.
The Martian rovers Spirit and Opportunity both found deposits of
white minerals mixed with iron oxides and sulfur.
The fact that the
small white pieces vanished means they were not mineral
compounds, but the question of whether they were water ice
remains open. On July 31, NASA announced that they had
discovered water in the soil samples that had been delivered to
their ovens. However, readings taken on September 4, 2008, by a
conductivity probe stuck into the ground indicated that the soil
is completely dry.
The key evidence for
water ice on Mars is those small chunks of sublimating material.
Mark Lemmon of Texas A&M University, College Station told
reporters that the disappearing chunks could not have been
carbon dioxide ice because that material would not have been
stable for even one day as a solid.
Since four different
probe measurements found no conductivity consistent with water
in the soil, then it might have been solid carbon dioxide, or
dry ice, that was initially dug up by the lander. Carbon dioxide
is a dielectric insulator, so no current would flow through it,
leaving the prongs of the soil analyzer reading no electricity.
Aaron Zent, team
leader for Phoenix's thermal and electroconductivity probe
wrote: "If you have water vapor in the air, every surface
exposed to that air will have water molecules adhere to it that
are somewhat mobile, even at temperatures well below
freezing.... All the measurements we've made so far are
consistent with extremely dry soil. There are no indications of
thin films of moisture, and this is puzzling."
Dry ice melts like
regular ice at -57 Celsius if the vapor pressure at its melting
point is about five atmospheres. At a vapor pressure of one
atmosphere dry ice sublimes at -78 Celsius, which is lower than
its melting point, so it changes from solid to vapor because the
additional four atmospheres of pressure is missing. The average
daytime temperature on Mars during the last three months has
been -31 Celsius and the lowest nighttime reading has been -80
Celsius with extremely low pressure of around 8.5 millibars
(Earth sea level pressure is approximately 1013 millibars).
Could other environmental factors
have influenced the time it takes for dry ice to sublime? Making
it difficult to determine the rate of sublimation? If that was
the case, then the initial white chunks might have been
something other than water, but with no definite recourse to dry
ice. Both NASA and Electric Universe proponents must wait for
another mission and additional information since Phoenix will
cease operations within a few weeks.
By Stephen Smith
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