High resolution IR and UV imaging of Venus
P.I. Ian Griffin (STScI) griffin@stsci.edu
Co-I’s Melissa McGrath, Keith Noll, Anthony Roman (STScI)
Abstract
We request 2 orbits of Directors time to obtain near coeval ACS UV and NICMOS IR
observations of Venus at its next western elongation, during an already planned GO visit
to the planet.
Scientific case:
Venus has been observed by HST just once in the last 12 years, in January 1995. On that
occasion, in light of the historic significance of a Venus observation by HST, a DD
request was made to add imaging to what was a purely spectroscopic GO program. The
DD request was granted and 2 UV images were obtained using WFPC2.
In this proposal we request 2 orbits to obtain UV and IR images of Venus using the full
power of HST’s imaging capabilities. We contend that in addition to enhancing HST’s
science legacy by providing the first ever comprehensive high resolution UV/IR images
during only the second HST Venus observation, the proposed observations will offer new
insight into the dynamics of the upper and lower atmosphere of Venus which remain
relatively unexplored, even by orbiting spacecraft.
Uniquely, Hubble’s UV and IR capabilities will allow near simultaneous observations of
the day time cloud features via wide band ACS observations in the near UV, and the
night side lower atmosphere via NICMOS observations through the IR atmospheric
holes.
We note that no planned spacecraft, nor ground based observatory has the capability to
perform the observations we are proposing, and the dataset obtained will be uniquely
important to the planetary science community. We also note that a tremendous amount of
planning has gone in to ensuring the safety of the GO program, and that by piggybacking
the proposed observations we can significantly increase the benefits of this investment.
We plan to acquire ACS/HRC and NICMOS images with a variety of filters. Based on
past imaging (especially from Mariner 10, Pioneer Venus, and Galileo spacecraft) and the
work of Esposito (1980) it is known that the natural variability of Venus clouds declines
with phase angle. For the elongation in January 2003, the phase angle is about 90
degrees, so the predicted contrast at 2000 and 3500 angstroms is between 20 and 30%.
Thus we will use the ACS filters that are expected to show the most contrast in the
Venusian clouds, namely F220W, F250W and F330W. We also plan to use the NIC3
Camera to obtain near IR observations through the F108N, F110W and F240M which
will reveal information about the spatial and thermal properties of the planets lower
atmosphere.
The first spatially resolved near IR images of Venus at 1.74 and 2.3 microns (Allen and
Crawford 1983) revealed high contrast markings produced when thermal radiation from
the hot lower atmosphere escapes through differing optical depths in the planet shrouding
cloud layers. Further space borne and ground based studies revealed further spectral
windows between 1 and 5 microns which have been used to learn more about the
structure of the Venusian atmosphere. (see review by Taylor et.al 1996)
Remote sensing in the near IR probes the properties of the lower cloud layers on Venus.
To date, the highest resolution map of the 2.3 micron emission from the Cytherian
atmosphere was made by the NIMS instrument aboard the Galileo spacecraft. (Carlson
etal. 1993). With a resolution of 25km per pixel, the NIMS data have approximately five
times the resolution of the proposed Hubble observations. However, as can be seen from
Figure 1 below, even degraded to the expected resolution of Hubble, significant structure
will be resolved.
Galileo view at 2.3 microns
Simulated HST view
Since the Galileo encounter with Venus lasted only 2 days, the NIMS data give no
information about the long term evolution of global structure in the middle and lower
atmosphere of the planet. With no space missions to the planet imminent, and with the
difficulty of observing Venus from the ground (even with AO) we feel this is an
important opportunity to break new ground in the study of Venus.
Coupled with the already approved cycle 9 spectroscopic program, this entire imaging
and spectroscopic dataset will be of lasting value to the planetary science community, and
will put the already approved GO program into spatial context. We also note that given
the tremendous amount of planning that has been involved to ensure that the cycle 9
spectroscopic observations can be executed safely it seems logical to take advantage of
all this planning and obtain some extra imaging data.
The UV and IR observations will be used to study the stratosphere and the thermosphere
of Venus. They will allow the GO observations to be put into spatial context, while also
providing important observational constraints for multi-dimensional dynamical modeling
of the planet’s thermosphere and the stratosphere.
Additional information:
While we believe we have built a compelling scientific case above, we note that there is
an obvious and strong public and education case in support of this proposal.
On January 26th 2003, for only the second time in the history of the Space Telescope
Program, HST will be pointed at Venus, when program GO program 8659 is executed..
This program (whose PI is Mark Bullock) aims to obtain a high resolution STIS spectrum
of Lyman-a and will assess the abundance of deuterium and constrain models of the
planet’s atmosphere and climate evolution. Despite the rarity of HST Venus visits, no
imaging was planned for this program, perhaps because it was originally proposed to
execute in cycle 9, when only WFPC2 was available.
Things have changed. With a completely different, more extensive and fully functioning
suite of imaging instruments now working on HST, and a significantly better
understanding of how to handle Venus observations thanks to the hard work of the
planning team, it is our contention that this well planned and safe HST visit to Venus
offers an unparalleled opportunity for the HST program to add to its very small inventory
of Venus images. Given that this may well be the last ever Venus pointing made by the
telescope, the HST project owes it to history to document this visit in the most
comprehensive manner possible.
We assert that the proposed legacy images of Venus while being of important science
interest will also be of immeasurable and lasting value to the public and education
communities. With Venus blazing in the eastern sky before sunrise in January, how better
could we demonstrate the power of HST than by sharing 3 color UV and IR images of the
closest planet, and telling the fascinating story of how the telescope uses “Dracula
mode” to creep up and snap images of its prey then hurries to safety before the sun rises?
To summarize, we feel that this proposal would have wide public appeal since it would
be the first planetary target imaged by the ACS, it will provide the highest resolution
images of Venus ever obtained from Earth, and the NICMOS imagery promises to show
Venus in a "new light". In short we feel a wonderful education vignette can be built
around the proposed observations and the story of how they are obtained by the skilled
team of HST planners.
Observing strategy:
We will take advantage of all of the planning involved in obtaining GO program 8659,
which gives an 11 minute window in each orbit for Venus observation. At the epoch of
observation, the planet will be just over 50% illuminated, with the following physical
parameters:
Sun angle: 46.00 degrees
Apparent magnitude: -4.4
Surface brightness: +1.5 (average for 1 square arc second of illuminated portion of disk)
Apparent diameter: 21.12 arc seconds
Phase angle: 81.24 degrees
The UV data:
We will observe Venus using the HRC on ACS, taking care to avoid the fastie finger!
The ACS observations, which will cover the entire illuminated Venus disk, will allow the
STIS spectra to be put into context relative to bright and dark UV markings.
Observations through 3 filters wavelengths (F220W, F250W and F330W) will enable a 3
color image of the planet to be made, detailing cloud structure on the illuminated
hemisphere.
We will obtain 2 dithered images using each of the F220W and F250W filter in filter
wheel 2. We request the HRC Clear filter (HRC CLEAR1S) in filter wheel 1. Using the
ETC on the ACS webpage, we find that a s/n ratio of 750 can be reached with an
exposure of 60 and 6.6 seconds through the 220W and 250W filters respectively. We will
follow these exposures with a two 1 second exposures using the F330W filter in filter
wheel 2. These calculations are appended below.
The IR Data
For NICMOS we would use NIC3 SCAMMER multi accumulation sequence with 10
exposures in the F108N, F240M and F110W filters. These data, obtained in pass bands
close to the Venusian atmospheric windows, will provide spatial information on the
structure of the planet’s lower atmosphere.
References:
Allen, D. & Crawford, J. 1984, Nature 307:222-224
Carlson, R. et. al, Planet. Space Sci. 41(7):475 (1993)
Esposito, L. JGR 85 8151-8157 (1980)
Taylor, F. W. et. al, Venus II, University of Arizona Press, 1996
Exposure time calculations: Note that these assume a solar analogue spectrum with
Venus being an extended source with V=1.5 magnitude per square arc second. We note
also that during the last visit to Venus, in 1995, the exposures used were 2 seconds at
2550 angstroms and 20 seconds at 218 angstroms respectively.
F220W Filter
S/N Ratio and Exposure time:
Exposure time = 60 seconds
S/N = 786.9
Optimal S/N = 0.0
WFC/HRC Integrated Countrate Analysis
The brightest pixel in a single image would have 7.74e+04 electrons (3.87e+04
ADU). Breakdown of Detected Counts
Origin Signal
Source 6.194e+05eSky background 0.02065eDetector dark current 0.6eThe S/N ratio calculations are based upon counts within a square aperture of 2x2
pixels
The observational parameters for this calculation were:
Detector = hrc
Filter = f220w
Gain = 2 e-/ADU
CR-split (Total number of images) = 2
Target was an extended source.
Source spectrum: Standard Star Spectrum Solar
Source Flux: v/arcsec2 = 1.5
Average Galactic Extinction of E(B-V) = 0.0
The Zodical Light is average
The Earthshine is average
F250W filter
S/N Ratio and Exposure time:
Exposure time = 6.599 seconds
S/N = 750
Optimal S/N = Not used.
WFC/HRC Integrated Countrate Analysis
The brightest pixel in a single image would have 7.03e+04 electrons (3.52e+04 ADU).
Breakdown of Detected Counts
Origin
Signal
Source
5.627e+05e-
Sky background
0.006573e-
Detector dark current 0.066eThe S/N ratio calculations are based upon counts within a square aperture of 2x2 pixels
The observational parameters for this calculation were:
Detector = hrc
Filter = f250w
Gain = 2 e-/ADU
CR-split (Total number of images) = 2
Target was an extended source.
Source spectrum: Standard Star Spectrum Solar
Source Flux: v/arcsec2 = 1.5
Average Galactic Extinction of E(B-V) = 0.0
The Zodical Light is average
The Earthshine is average
F330W filter
S/N Ratio and Exposure time:
Exposure time = 1.007 seconds
S/N = 750
Optimal S/N = Not used.
WFC/HRC Integrated Countrate Analysis
The brightest pixel in a single image would have 7.03e+04 electrons (3.52e+04 ADU).
Breakdown of Detected Counts
Origin
Signal
Source
5.628e+05e-
Sky background
0.002268e-
Detector dark current 0.01eThe S/N ratio calculations are based upon counts within a square aperture of 2x2 pixels
The observational parameters for this calculation were:
Detector = hrc
Filter = f330w
Gain = 2 e-/ADU
CR-split (Total number of images) = 2
Target was an extended source.
Source spectrum: Standard Star Spectrum Solar
Source Flux: v/arcsec2 = 1.5
Average Galactic Extinction of E(B-V) = 0.0
The Zodical Light is average
The Earthshine is average
The IR Data
For NICMOS we would use NIC3 SCAMMER multi accumulation sequence with 10
exposures in each of the following filters
F108N
S/N and Exposure time:
•
•
Exposure time = 0.3412 seconds
S/N for the brightest pixel = 1000
The observational parameters for this calculation were:
Target was an extended source. S/N was calculated for one pixel.
•
•
•
•
•
•
•
•
•
•
•
•
•
Detector = NIC3
Detector temperature (K) = 77.10
Filter = f108n
Spectrum: G2V (Model Spectrum)
Source Flux: v/arcsec**2 = 1.5
Average Galactic Reddening ( E(B-V) ) = 0.0
The Zodiacal Light is average
The Earthshine is average
Total counts from the source(e per s) = 7.33e+07
Telescope thermal background (e per s per pixel) = 0.000
Sky background (e per s per pixel) = 0.018
•
•
•
•
•
•
Dark current (e per s per pixel) = 0.300
Number of Multiple Initial and Final read(s): 1
Read noise with 1 end-point read (e): 26
Saturation in electrons = 185000
Exposure time to saturate (s) = 0.063
F240M
S/N and Exposure time:
Exposure time = 0.0002893 seconds
S/N for the brightest pixel = 100
The observational parameters for this calculation were:
Target was an extended source. S/N was calculated for one pixel.
Detector = NIC3
Detector temperature (K) = 77.10
Filter = f240m
Spectrum: G2V (Model Spectrum)
Source Flux: v/arcsec**2 = 1.5
Average Galactic Reddening ( E(B-V) ) = 0.0
The Zodiacal Light is average
The Earthshine is average
Total counts from the source(e per s) = 9.19e+08
Telescope thermal background (e per s per pixel) = 497.603
Sky background (e per s per pixel) = 0.285
Dark current (e per s per pixel) = 0.300
Number of Multiple Initial and Final read(s): 1
Read noise with 1 end-point read (e): 26
Saturation in electrons = 185000
Exposure time to saturate (s) = 0.005
F110W
S/N and Exposure time:
Exposure time = 5.85e-05 seconds
S/N for the brightest pixel = 100
The observational parameters for this calculation were:
Target was an extended source. S/N was calculated for one pixel.
Detector = NIC3
Detector temperature (K) = 77.10
Filter = f110w
Spectrum: G2V (Model Spectrum)
Source Flux: v/arcsec**2 = 1.5
Average Galactic Reddening ( E(B-V) ) = 0.0
The Zodiacal Light is average
The Earthshine is average
Total counts from the source(e per s) = 4.55e+09
Telescope thermal background (e per s per pixel) = 0.000
Sky background (e per s per pixel) = 1.130
Dark current (e per s per pixel) = 0.300
Number of Multiple Initial and Final read(s): 1
Read noise with 1 end-point read (e): 26
Saturation in electrons = 185000
Exposure time to saturate (s) = 0.001