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Water on Earth's Moon by David Goldstein


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Hello. I would like to talk to you today about our work using a Lunar Prospector impact on the South Pole of the moon to detect water ice. I'm David Goldstein. The University of Texas team that proposed this idea also included Dr. Steven Nerem of the Department of Aerospace Engineering and Dr. Edwin Barker of the Department of Astronomy and my student, Victor Austin from Aerospace Engineering. Allen Binder was the Principal Investigator of the Prospector mission and William Feldman at Los Alamos developed the instrumentation which originally suggested that water would be likely at the South Pole.

We have a particularly good Web site that you might want to take a look at indicated at the bottom of the slide. The spacecraft itself seen in this overview slide looked like this. It was about a meter tall and a meter in diameter and had three long booms and struck the South Pole in this cartoon fairly much as you see.

Let's begin by discussing why one might expect to find ice at the poles of the moon. Volitiles like water ice might be stable over geologic time, that is over billions of years. And volitiles might have several sources. It could be steady sources. For example, from the rock itself, the solar wind, the hydrogen in solar wind might be combined with oxygen in the rock and produce water. Sunlight might bake water out of the soil. Micrometeoroids or meteorites striking the surface of the moon contain water. It's been known for a long time that certain types of meteorites contain water when they're rock. Or there might be gradual degassing from the interior. For example, volcanos on earth produce steam among other chemicals when they explode or when they erupt.

There are other sources however which are not so steady. For example, comets are made of perhaps 70% or more water ice and they are very large. They might have a mass of 1013 or 1014 kilograms. But they only arrive and would strike the moon every few million years and so they would produce a periodic input of wate to the moon.

Let's talk about what would happen to any water molecules which found themselves on the surface of the moon. The surface of the moon gets very hot during the daytime and so any molecule of water which finds itself on the sunlit side of the moon would bounce off the surface much as you might think of a water droplet spattering off a hot frying pan and would bounce around on the sunlit surface until it happens to find itself either on the dark side of the moon or in a crater at the South or North Pole. I've schematically shown a molecule hopping over the surface and finding its way to North Pole of craters. Any molecule though which finds itself on the dark side of the moon would stay there until the moon rotates around and the molecule finds itself again in the sunlit side and then it starts hopping around again.

If you look at the lower portion of the figure you'd see that schematically certain craters, certain depressions near the poles of the moon never see direct sunlight, that is regardless of whether the sun is coming from the left or the right or wherever. Those craters if they're deep enough have a bottom which is in permanent shadow.

Let's talk about why we might be interested in water ice near the poles of the moon. This is a map showing the illumination or the percent of illumination that this South polar region of the moon sees during the year so any region which is white is illuminated by the sun greater than 70% of the lunar day and different colors represent different fractions of a day that the moon is illuminated. You can see certain of those craters have a large fraction of their day has nearly permanent illumination, that is they stick up above the ground and so they're almost always in sunshine.

On the other hand, some of these dark craters that you see are candidates for containing ice and if you could build a moon base say on the rim of one of these craters it would almost always be in sunlight and there would be a nearly permanent source of water right next to the moon base. The water could be used to build buildings, it could be used to drink, it could be used to irrigate crops, it could be broken down into hydrogen and oxygen and used as rocket fuel. So a polar crater that has a nearly permanent illumination and is right next to a big frozen lake of ice would be perhaps an ideal place.

There has been a great deal of work in the last decade or over the last decade searching for evidence of water ice at the poles of the moon. I'd like to tell you about some of that work, some of the results and that will lead us into a discussion of our proposed mission to crash the Prospector vehicle. There had been a military spacecraft called Clementine which used radar to bounce it off the poles of the moon and look at the signal in the back scattered radar to try to determine whether there was large quantities of frozen water. Clementine also photographed most of the moon and from those photographs it was possible to look for areas of permanent shadow. Earth-based radar results which I'll talk briefly about in a minute also were obtained but they found no ice at the poles. The problem with earth-based radar results is that the radar dish is obviously on the earth and there's no direct view from the earth into those deep craters, or very rarely is there a view into those deep craters and so we can't really see the bottom of those craters from earth.

The Lunar Prospector vehicle itself had an instrument called a Neutron Spectrometer which was used to search for hydrogen in the lunar soil. This slide shows the schematic of how Clementine bistatic radar experiment was done. The Clementine spacecraft sent a beam of radio energy towards the surface of the moon and that beam was scattered back to receive by radio antennaes on earth. The polarization of the radar beam indicates whether or not the surface contains ice and in particular when the angle beta goes towards zero a preferred polarization of the radar signal indicated that there was ice likely at the South Pole according to the work by Nozette et al indicated in the figure.

I already indicated earlier in slide 4 the photographic data obtained by Clementine which indicated areas of permanent shadow and areas where the surface was raised sufficiently so that there was a large fraction of the year in which the surface was exposed to sunlight.

Let's go on now and talk about the earth-based radar results. These are images from the paper by Stacey et al which represent radar back scattering images taken by the Arecibo large radar dish in Puerto Rico. The main point here is that there are no substantial bright regions of strong back scattering indicating the likelihood of water ice on or near the surface or at least large chunks of water ice.

Perhaps the best indication of whether there is ice near the poles of the moon came from the Lunar Prospector which was placed in orbit to search for minerals on the surface of the moon and also to search for hydrogen in the soil. The Lunar Prospector found hydrogen in the soil near both poles, although there's more near the South Pole. However, the hydrogen may be concentrated in mineral hydrates or absorbed solar wind. The mineral hydrates is like Portland cement when you mix it with water you get concrete. So although there may be hydrogen as water in the soil it may be firmly bonded to the rock. If there is ice it's in the soil. The concentrations of hydrogen to techtified Prospector don't tell us how that hydrogen is distributed through the soil.

Let's look now at a map of the hydrogen concentration near the South Pole. In this figure are the latest results from the Lunar Prospector's Neutron Spectrometer instrument presented by Feldman this year. The Lunar Prospector counted the neutron blocks out of the surface of the moon and used that to indicate the presence or absence of hydrogen in the soil of the moon. The soil is called the regalift (?). This map shows you the South polar region. The blue areas are those that contain the highest concentrations of hydrogen while the other colors indicate lower levels of hydrogen. In particular you can see that the blue is concentrated mostly right near the pole, almost all of it south of 85o, although there is a little bit in the upper left quadrant.

Perhaps the most interesting thing about the Lunar Prospector results showing concentrations of hydrogen are that those concentrations occur where there are large permanently shadowed craters at the South Pole. A permanently shadowed crater is one that never sees direct sunlight deep in the bottom of the crater and earth-based radar has been used to look for such craters.

Here are the latest results for radar imaging of South polar region of the moon taken by a group at Cornell, Margot et al. The left is a topographic map showing the altitude relative to a spherical moon, the terrain near the South Pole. So red areas are regions sticking up and purple areas are regions deep down. The areas that are white were not visible from earth so a large fraction of the terrain what couldn't be seen from earth. On the right is a radar back scattering which looks more like a photograph showing what the terrain around the South Pole looks like.

For this figure Margot et al used the topographic data and a ray tracing program to figure out where sunlight was coming from and trace the sunlight rays to see where the sunlight never strikes and those areas are colored on this radar back scattering—colored white. So you can see the bottoms of the five main craters near the South Pole are colored white sort of suggestive of snow being at the bottom of those craters. The areas that are grayish at the bottom of the craters were not actually visible from radar on earth but are colored gray indicating that they're likely regions in permanent shadow. We're particularly interested in the crater about 87o south and 45o west. The middle one in that chain of three craters you see. That crater in particular happens to contain the highest concentrations of hydrogen as indicated by the Lunar Prospector results and is the most likely we feel to contain water ice.

So we realized back around the end of last year that since the Lunar Prospector mission was going to end shortly on July 31 and since the vehicle was the polar orbit that allows it to pass right over the poles twice in each orbit and since its battery was getting weak and it had little fuel left that it might be possible to crash that vehicle right into one of those craters containing ice and to look for a vapor plume of vaporized water with telescopes on earth. Now July 31 is about three days past the full moon and was a pretty good time to try to view any water vapor. Prospector passed directly over that center of three craters I talked about on the previous slide that we felt was the most promising of all craters on the moon and the time of July 31 made it possible for us to use the Hubble Space Telescope to view the impact of that.

We here in the case schematically what the impact trajectory design looked like. The red dotted line indicates the initial orbit around poles of the moon. The earth and the sun are basically off to the left in this figure so the left hand side of the moon is illuminated. The idea was to first raise the orbit to the blue dotted region to put it into it into an intermediate elliptical orbit and then just before impact while the vehicle was on the far side of the moon to fire the retrorockets to slow it down so it would strike the surface right at the South Pole, right at the impact site seen at the bottom.

This radar back scattering image again taken from the Margot paper in Science shows the target crater. It's outlined in a solid red circle and shows the ground track, the thin white line coming from the bottom of the figure and ending at the point of impact that we anticipate in the shadowed region at the base of that center crater.By the way that center crater is as yet officially unnamed.

This figure shows the same ground track over the actual topography as measured by the Arecibo and Goldstone radars. Here we see the spacecraft Lunar Prospector trajectory over the ground on the horizontal axis is the time of day in universal time and on the left axis is an exaggerated vertical height. The blue line is the measured topography and the red line is the trajectory of the spacecraft. The point of showing you this is to show you where the impact occurred at about –87.7o at 42o longitude. The missed distance between the red line and the near rim of the crater was only about 815 meters, about half a mile, pretty close. We had no choice but to come in this shallow and to cut that close to the rim of the crater because we wanted to hit the base at the crater as best we could and the maximum angle of impact we could obtain was about 6.3o.

Well, 800 meters is very very close on a planetary impact scenario but if we were successful in hitting the base of the crater what would we expect? Well the spacecraft weighed about 160 kilograms, that's 340 odd pounds and was coming in at about 1700 meters a second. That's about a mile per second. It had the impact energy of about that of a 4000 pound car crashing at about 1100 miles an hour so we're talking about huge amounts of energy. Its impact angle above a local horizonal would only have been about 6 or 7o so very shallow. And what would we expect? Well we'd expect gross deformation, disintegration, tiny little pieces of spacecraft would be left and possible ricocheting of some of the more sturdy pieces. If we suppose that all of the energy heats the soil inside the small area where the small area gouged out the ground where the spacecraft hit. Let's say that area is about half a cubic meter—not a very big volume. And if we supposed the data from the Prospector itself showing about a 2o concentration of ice mixed in with the soil then we would expect about 18 kilos, about five liquid gallons of water to be produced at about a 127o C so that's just a bit above the boiling temperature of water on your stove. We figure we would have gotten about five gallons of vapor produced out of the impact.

Well what we would look for from that impact? Five gallons of water is not a whole lot to look for. We suggest that the observations of the debris in the plume are not very likely and its looking for sand and dirt, some ice crystals. It didn't seem likely but we had a tremendous interest in the project and we had perhaps thousands of amateurs using their telescopes to look for dust. What we tried to do or thought to do was to look for water vapor in the initial plume of material rising up in the thermal infrared, that is water itself radiates in the infrared and there was a spacecraft available that could look for that. I'll talk a little more about that later. We realized that the water vapor will photo dissociate into hydrogen and OH when it's exposed to sunlight and we could look probably for the OH in the ultraviolet and that's what we did with our three experiments that I'll describe in a little while. And we could look for a thin OH atmosphere or exosphere near the South Pole for a couple of hours after the impact. So we can both look for the rising plume of steam that comes off the pole and after it falls back down to the surface and forms in atmosphere we can look for that atmosphere as well.

We did some preliminary calculations suggesting where to look for the vapor in the rising plume of gas so this is a fairly complicated figure. The axes are in kilometers. The colors on the right hand side of the figure are the brightness of OH, that is the hydroxyl radical at different times, at 100 seconds, 200, 300, all the way up to 1000 seconds. And on the left hand side is the average number density or average number of water molecules per square centimeter as would be seen from earth. We use such a figure to figure out where to point Hubble Space Telescope during the plume phase when the gas was first rising and off the surface and then settling back. We wanted to search for the brightest areas.

This shows our best guess before the impact of what would be the brightness of the OH around the moon. So the black circle is the disc of the moon, the axes, the vertical and horizontal axes are in kilometers measured from the center of the moon and the colors are the brightness of the OH. The red have been just marginally visible to Hubble Space Telescope and so we were hoping to—we anticipated that there would be a possible marginal observation of water around the moon.

We were also interested in determining what might be seen of the dust and debris that came up from the impact. This is a photograph of an impact study done with a 50 caliber low density projectile fired into sand at a very oblique angle. So coming in from the right hand side of the figure is where the bullet was fired. It was a plastic low density bullet. It struck the ground. You can see the debris outlined in the thin line and you could see what happened to the bullet. The only real difference
between this experiment and the impact on the surface of the moon was that the density of our bullet was somewhat too high and the scale was obviously different. We used a fifty caliber bullet instead of a spacecraft which had dimensions aboard her of meters.

There were many observers involved in the observing program looking for evidence of the Prospector impact. We used Hubble Space Telescope, HST. We used the two larger telescopes at McDonald Observatory and the Keck-1 telescope on Mauna Kea in Hawaii. We were looking for OH, that is the photo dissociated byproduct of water using both spectra or taking spectra of the light coming into the telescope, as well as doing imaging. The idea of using a spectrograph is to break the light up into a rainbow of colors and search for the particular color associated with OH. That's at about 3085 angstroms and so we looked for spikes in the spectral data at 3085 angstroms. We also took images with the 2.1 meter telescope at McDonald Observatory to look for—we took images with OH filters that would only pass light around the 3085 angstrom band to try to detect a plume or the atmosphere around the South Pole of the moon.  The group using SWAS, the Submilimeter Wave Astronomy Satellite was looking for water directly in the infrared. That's the satellite usually used for studying water vapor in the intra stellar mediums. Other groups were looking for dust directly or sodium in images or spectra or other perhaps cometary peices like hydrogen cyanide or C2 and there were a large number of amateur astronomers mostly looking for dust.

So what happened? Well the spacecraft impacted the moon one minute later than planned pretty much okay at 9:52 universal time on July 31. We know it impacted the moon because a signal was not reacquired after it came out from behind the moon so we're sure it hit at just about the right time. The navigation team is quite confident they hit inside the crater at the anticipated point. However we have no reliable reports to date of visible observation of any dust or impact debris. We have
no reports as of this point of any water vapor plume from either SWAS or Keck or Hubble or MacDonald. While there were some glitches with the satellite observations with SWAS and HST we apparently did acquire some good data and some very good ground-based data, that is we had good weather over the ground-based observatory. However at this point none of the observatories are reporting having seen any evidence of an impact. You can keep tuned to see what happens in the next few weeks as we continue analyzing the data. You should look at the Web site indicated at the bottom of the page which has been a very popular Web site and you may see later results than I'm able to present here. In particular when I talk about the data which we've been looking at with the Hubble Space Telescope and the Keck we've been looking at the spectra and once we correct the spectra or scattered light off the surface and viewing angle and we normalize the data for the correct frequencies and the correct amplitude there is no apparent signature at this point for water in or near the 3085 angstrom band.

So what do we make of the experiment and preliminary results I just mentioned that we haven't observed any evidence of the impact? Well a spectroscopic detection of water would have been definite proof that there was water in the soil of the moon at the South Pole or at the point of the impact. Our experiment was obviously high stakes gamble. If we were successful in finding water that would have been tremendous. If we're not well it doesn't mean there isn't water there because only a positive result has definite meaning. What do I mean by that? I mean there were several ways we could have failed to observe water. We could have hit a rock right next to a huge pile of snow. If we hit the rock we'd never know the snow was there. We could have hit the near rim of the crater and never got inside the crater. Perhaps there wasn't enough water to vaporize right near the point of impact. Perhaps some of our calculations were wrong. We'll never know. We had only one shot at the event. We had only one spacecraft. And we had no practice runs. So the experiment was fairly difficult. And there were many assumptions made.

There were broader implications to the mission however. If we were successful in finding water or if we are it shows that the Neutron Spectrometer can be used for other missions to other planets and there's substantial science and engineering significance. Significant implications if there's ice there. One science implication relates to how planets are formed, how comets brought volitiles like water to the earth and the moon. The engineering implications that I discussed earlier relate to perhaps using the ice that's at the poles to supply a lunar base.  Our experiment though may not have observed the ice that we were hoping for but it did show that we can arrange a large-scale coordinated program to do such experiments. We had very successful interaction with astronomers around the world and engineers throughout NASA. So we feel that the experiment, although we didn't find ice, was very interesting and very useful. In terms of future work and our search for future funding we would like to obtain funding to look at how comets striking the moon could have left ice at the poles. We do some modeling of low density flows and that would be what would characterize how the ice from a comet
in fact would get to the pole.

And finally I ask the question, can we do it again? Well, you're thinking I just said we just destroyed the spacecraft, how can we do it again? And the answer is what we would like to do is to determine if there are other spacecraft in earth orbit, for example a communications satellite that might perhaps have enough energy to get to the moon but not be useful for its present mission. For example, suppose radio transponders die. Maybe we can identify a spacecraft that would be able to make a controlled impact of a point on the moon and we could execute the same experiment again. We're looking into that possibility now.

I thank you for your time. If you have any questions you can enter them in the way proposed for this meeting. If you'd like to keep up to date with any other results that might come out of the impact event please feel free to look at that Web site. Thank you for your time.

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