LATEST RESULTS FROM THE MARS
LATEST RESULTS FROM THE MARS PATHFINDER ATMOSPHERIC STRUCTURE
Introduction: The Mars Pathfinder Atmospheric
Structure Investigation (ASI) obtained information on
Martian atmospheric structure from three science
accelerometers, which measured the deceleration of
the probe at all levels in the atmosphere. Entry,
descent, and landing occurred within 850 km of the
Viking 1 landing site and somewhat later in northern
summer. Pathfinder entered at 3 AMÊ Local Mars
Time (LMT), which provided the first opportunity to
study Mars’ nighttime atmospheric structure, and
Viking 1 entered at 4:15 PMÊ LMT. Magalh‹es et al
[1] and Schofield et al [2] have previously reported on
the analysis of accelerometer measurements from the
entry phase, which ended at about 8.5 km. The derived
temperature profile extends from 140 km altitude
down to 8.9 km, with a vertical resolution ranging
from 250 meters to 50 meters, respectively. Here we
report on a refined analysis of the Pathfinder entry
phase ASIÊ data in which the effects of the small
angular motions of the entry vehicle have been
removed, thus enabling a search for small amplitude
and small vertical wavelength structures. In addition,
we will report on the atmospheric structure at altitudes
below 8 km which is being derived from the
accelerometer data acquired during the parachute
descent phase.
Results: The high spatial resolution of the ASI
data makes it well suited to the study of atmospheric
waves and other phenomena with small spatial scales.
The Pathfinder entry profile shows oscillatory
structures with a wide range of vertical wavelengths
and amplitudes. Between 90 and 60 km, large
amplitude (10-20ûK) long wavelength (20-40 km)
oscillations are consistent with the vertical scale
expected for the diurnal tidal mode. These oscillations
show a remarkable correspondance to equivalent
structures found in the Viking 1 profile at the same
pressures. The anticorrelation of the extrema of the
oscillations in the two profiles is consistent with the
phase change expected for a diurnal tidal mode over
the ~ 12 hour local time difference between the two
profiles. Below 60 km a weaker signature of the
diurnal tide appears to be evident in the data as well.
The rate of amplitude growth with altitude indicates
the diurnal tide is saturated above 60 km, suggesting
Òwave breakingÓÊ and energy and momentum
deposition by this tidal mode.
At levels above 60 km, wave-like oscillations with
an apparent vertical wavelength of ~5 km are evident.
The amplitudes of these oscillations show a positive
correlation with the background static stability of the
atmosphere, consistent with an atmospheric gravity
wave interpretation. Comparison with analytical
gravity wave theory indicates that gravity waves with
a true vertical wavelength of ~5 km would likely be
subject to critical-level absorption due to slow phase
speeds and expected wind shears. However since the
motion of the Pathfinder lander was mainly horizontal
at altitudes above ~30 km, the ASI experiment
samples horizontal phase variation as well as vertical
phase variation. Gravity waves with a true vertical
wavelength of ~20-40 km and a true horiztonal
wavelength of ~10-20 km could produce the observed
apparent vertical wavelength and would be more
likely to propagate vertically. The amplitude growth
with altitude of these oscillations appears to be
saturated, suggesting energy and momentum
deposition and turbulent mixing.
References: [1]Magalh‹es, J. A., J.T. Schofield,
and A. Seiff (1999). J. Geophys. Res. 104, 8943-8955.
[2]Schofield, J.T., J.R. Barnes, D. Crisp, R.M.
Haberle, S. Larsen, J.A. Magalh‹es, J.R. Murphy, A.
Seiff, and G. Wilson (1997). Science 278, 1752-1758.
Structure Investigation (ASI) obtained information on
Martian atmospheric structure from three science
accelerometers, which measured the deceleration of
the probe at all levels in the atmosphere. Entry,
descent, and landing occurred within 850 km of the
Viking 1 landing site and somewhat later in northern
summer. Pathfinder entered at 3 AMÊ Local Mars
Time (LMT), which provided the first opportunity to
study Mars’ nighttime atmospheric structure, and
Viking 1 entered at 4:15 PMÊ LMT. Magalh‹es et al
[1] and Schofield et al [2] have previously reported on
the analysis of accelerometer measurements from the
entry phase, which ended at about 8.5 km. The derived
temperature profile extends from 140 km altitude
down to 8.9 km, with a vertical resolution ranging
from 250 meters to 50 meters, respectively. Here we
report on a refined analysis of the Pathfinder entry
phase ASIÊ data in which the effects of the small
angular motions of the entry vehicle have been
removed, thus enabling a search for small amplitude
and small vertical wavelength structures. In addition,
we will report on the atmospheric structure at altitudes
below 8 km which is being derived from the
accelerometer data acquired during the parachute
descent phase.
Results: The high spatial resolution of the ASI
data makes it well suited to the study of atmospheric
waves and other phenomena with small spatial scales.
The Pathfinder entry profile shows oscillatory
structures with a wide range of vertical wavelengths
and amplitudes. Between 90 and 60 km, large
amplitude (10-20ûK) long wavelength (20-40 km)
oscillations are consistent with the vertical scale
expected for the diurnal tidal mode. These oscillations
show a remarkable correspondance to equivalent
structures found in the Viking 1 profile at the same
pressures. The anticorrelation of the extrema of the
oscillations in the two profiles is consistent with the
phase change expected for a diurnal tidal mode over
the ~ 12 hour local time difference between the two
profiles. Below 60 km a weaker signature of the
diurnal tide appears to be evident in the data as well.
The rate of amplitude growth with altitude indicates
the diurnal tide is saturated above 60 km, suggesting
Òwave breakingÓÊ and energy and momentum
deposition by this tidal mode.
At levels above 60 km, wave-like oscillations with
an apparent vertical wavelength of ~5 km are evident.
The amplitudes of these oscillations show a positive
correlation with the background static stability of the
atmosphere, consistent with an atmospheric gravity
wave interpretation. Comparison with analytical
gravity wave theory indicates that gravity waves with
a true vertical wavelength of ~5 km would likely be
subject to critical-level absorption due to slow phase
speeds and expected wind shears. However since the
motion of the Pathfinder lander was mainly horizontal
at altitudes above ~30 km, the ASI experiment
samples horizontal phase variation as well as vertical
phase variation. Gravity waves with a true vertical
wavelength of ~20-40 km and a true horiztonal
wavelength of ~10-20 km could produce the observed
apparent vertical wavelength and would be more
likely to propagate vertically. The amplitude growth
with altitude of these oscillations appears to be
saturated, suggesting energy and momentum
deposition and turbulent mixing.
References: [1]Magalh‹es, J. A., J.T. Schofield,
and A. Seiff (1999). J. Geophys. Res. 104, 8943-8955.
[2]Schofield, J.T., J.R. Barnes, D. Crisp, R.M.
Haberle, S. Larsen, J.A. Magalh‹es, J.R. Murphy, A.
Seiff, and G. Wilson (1997). Science 278, 1752-1758.
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