Deep Impact Projects (2)

Deep Impact Projects mfa - 1

Deep Impact Projects (2)

Mike A’Hearn


Problem 1Optical Depth & Albedo of

Ejecta from DI


Optical Depth

• Well known and widely used in astronomy but often ignored in cometary science because the coma is usually optically thin

• I/I0 = e- where = optical depth• For simple scattering and absorption

• = N • where = extinction cross section

• and N = column density• A typical photon travels = 2/3 before being absorbed or scattered


Scattering Function

• A single particle (grain of dust or whatever) scatters light anisotropically

• The phase function describes the distribution of light with scattering angle, both for microscopic particles and for large bodies like cometary nuclei and asteroids

• Phase function is often approximated by many different simple functions of the scattering angle

• For a single particle, – I = Fsun p f( where p = geometric albedo and

f() is the scattering function or I = Fsun**A(– Be careful of confusion between geometric albedo and

Bond albedo - geometric is backscattering and Bond is integrated around the sphere.

– The widely used quantity Af uses Bond albedo as A, even though this is not well defined for microscopic particles


Phase Functions

• Sample phase functions appropriate to comets– There is variation

from one comet to another

– Dust from Ney & Merrill, comet West (1976 VI)

– Nucleus from Lumme-Bowell law

• These are normalized to unity at phase = 0°– correct for use with

geometric albedo– Renormalize to unity

of integral over 4 for use with Bond albedo

Phase Dust Nucleus0 1.00 1.00005 0.92 0.596310 0.85 0.442615 0.77 0.342920 0.70 0.271325 0.65 0.217530 0.60 0.176040 0.50 0.117750 0.45 0.081560 0.45 0.059570 0.45 0.047180 0.45 0.040990 0.45 0.0386100 0.55 0.0382110 0.80 0.0382120 1.50 0.0376130 2.50 0.0357140 4.00 0.0320150 7.00 0.0265160 10.00 0.0191170 15.00 0.0100180 20.00 0


Albedo

• Solving for the albedo from and I is a matter of algebra left to the student– Use the approximation that the optical depth is not large ( < 1) so that I(column) = N*I(particle)

– Be very careful of the units! • I has units of radiance (see the image labels for the exact units)

• Look up solar flux for a typical wavelength (look at a document that describes the bandpass or use the label values for center wavelength and bandpass)

– Remember that phase = 180° - scattering angle


Making the Measurements - 1

• Consider MRI images just before and just after the impact (ID 9000910_050 is before, ID 9000910_080 is after)

• You may wish to register the images first– IDL has, e.g., a cross-correlation function that you can use to determine the offset as long as the ejecta are not too bright

– There are numerous other ways to do this if you are familiar with image processing

– If the images are registered, you can make the measurements in the same pixels on all images

• Alternatively, you can choose a feature on the surface or the limb and make sure that you make all measurements at the same feature


Making the Measurements - 2

• Measure the profile of brightness across the limb of the nucleus at your “feature”

• Extrapolate the coma brightness to the brightness just inside the limb

• Measure the height, I, of the limb above the extrapolated coma– Extrapolation should only be a few pixels– Call it I0 for the pre-impact image (or averaged over a few pre-impact images)

– Use I/I0 to determine – Use I and to determine p*f() and use the table of scattering functions to get p, the geometric albedo


Extending the Project

• Ultimately, the interesting question is the variation of albedo with position around the limb and with time– This can show variations in the type of particles (ice

vs. dust and organics) both with direction of ejection and with depth of excavation

• Optical depth is >1 after about 5-10 seconds– The problem is much messier because one must allow both

for the attenuation of the incident sunlight from one direction and the attenuation of the scattered light along the line of sight

• Any other sharp boundary (a crater or a scarp) on the nucleus can be used to measure the optical depth but – the contrast is lower – One must use features on the sunward side of the impact

site or the “true” brightness of the surface will change


Problem 2Plot the Light Curve of an

Outburst


Scientific Goal

• We do not understand what causes the natural outbursts

• Determine how long the emission of material continues, which is important in choosing a mechanism for producing the outburst, and whether the outflow of material explains the drop in brightness after the outburst


Outbursts on July 1 & 3


Outburst Motion

• July 3 outburst has most data

• Lightcurve best done from MRI data

• Images best done from HRI data, but requires deconvolution


Making the Measurements

• Select the images that include the outburst plus a couple before and after– NOTE!!! This analysis should start with the clear filter and should eventually use BOTH the science data and the navigation data.

• On each image– Locate the nucleus of the comet - usually at the photocenter

– Add up the intensity within a box centered on the comet’s nucleus• Try boxes of several sizes, e.g., 5, 9, 15, 25 pixels

• Plot the intensities vs. time to see the light curve

• A circular aperture is better than a box, but requires a little more programming in IDL or use of procedures in the GSFC library for IDL (probably also in IRAF)


Interpreting the Result

• Calculate the distance (in m or km at the comet) from the nucleus to the edge of the aperture

• Use the time from beginning of outburst to end of outburst to estimate the velocity of the material

• Verify that the deduced velocity is the same for various apertures


Extending the Project

• The HRI images can be used to trace the material in time - see the deconvolved images shown in a previous slide– Not many images during the outburst– Requires subtracting a pre-outburst, registered

image and then doing the deconvolution for the focus problem

– Mapping the intensity variation with time can provide further constraints on the motion of the ejecta - need a true simulation

• Finding all the outbursts and coordinating them with rotational phase of the nucleus

• Tracing the outbursts to specific locations on the nucleus

• Simulating a mechanism to produce the outbursts

Deep Impact Projects (2)

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