LCROSS Mission Overview

How did LCROSS see the impact?

We had several thermal cameras on LCROSS, so basically you can see the heat from the impact experiment. What we could see over the first several seconds after the impact is that you could see the heat of the ejecta was blasted out from the impact, expanding and moving over the lunar surface. So here we have data from the ultraviolet visible spectrometer, and what we see here is that as the levels of flux continue to go up, we're seeing the brightness increase in the plume, and as things get brighter, the reason for this is because we have dust now that has been lofted up into the sunlight. So we're seeing that sunlight being reflected off of the dust grains. This is one of the ways that we know that there was a dust cloud that was created by the LCROSS impact.

We have pictures of the dust cloud from the visible camera that was on board the LCROSS Shepherding Spacecraft. So we actually have images of the dust cloud itself, and from that we’re able to tell how big the dust cloud was over time. So we can see how quickly it expanded and how big it actually got. So, at about 20 seconds after impact, we’d measured the cloud, and it was on the order of about 10 to 12 kilometers in diameter. And we can see that cloud in the visible camera for about 42 or 43 seconds after impact.

How did LCROSS get the pictures?

One of the ways to identify water in the LCROSS impact plume is to look at the near infrared spectra. So we're looking at the light in the near infrared wavelengths as it's returned to the spacecraft, and the way that we do this is we look and see water absorbs light at certain wavelengths. So what we do is match those particular wavelengths with the data, and we see if the data is showing absorption features at those same wavelengths that are very diagnostic of water. So here we see data taken from the near infrared spectrometer. You can see the data is in blue with error bars, and then there's a model fit that's in red. What we try to do is we try to model the data as best we can. At the bottom of the graph, you see spectra of different materials that are labeled, whether it be water vapor, water ice, and other components.

So what we do is try to look and see where the features in the library spectra match our data. So what we see in the bottom, in the green line, is what water vapor looks like in the near infrared. So what we do is we try and match the absorption lines shown in green with what we see in the data. So, you see where there's a dip around 1.4 microns in the green line, also matches the dip in the data where you see the red model line so there's a match for water vapor. There's also a match between 1.8 and 1.9 microns, where the green line shows several dips, and also we see several dips in the data at the same wavelengths at 1.8 to 1.9 microns.

One of the things that we found that we absolutely have to have in the model to match the data is water: both water vapor and water ice. So we're confident that there was a significant amount of water that was ejected into the LCROSS plume upon the impact.

Did we make a crater?

What evidence suggests water-ice on the moon?

We have two lines of evidence to support the discovery of water ice in this permanently shadowed region: One line is the near infrared spectra. The other line of evidence is the ultraviolet visible spectra that are showing the presence of OH. What happens is, if you have water ice that’s lofted up above the lunar surface into the sunlight, that sunlight will break that water or H2O apart, and you’ll create OH. So in the UV visible spectra we actually have evidence of that OH. What you can see here is the diagnostic signature of OH emission at about 308 or 309 nanometers. You can see the peek continuing to rise telling us that we have more and more OH that’s being produced. The graph on the right shows the levels of OH before and after impact, so you can see that before impact we have a pre-impact baseline where things are pretty steady, and then right after impact, we see the amount of OH increase. It’s increasing because we have water ice that’s lofted up into the sunlight and then breaks apart and forms this OH. So the discovery of the water ice really relies on two independent lines of evidence: the near infrared spectra showing the water bands, and the ultraviolet visible spectrometer data showing the presence of the OH.

How much water was there?

We’ve done some calculations that determine just how much water was ejected from the LCROSS impact. So we know; we have images of the crater that we created. We sampled one area, that place where the Centaur impacted, and we created a crater on the order of 20 – 30 meters across and several meters deep. We excavated on the order of 250 metric tons of material from the lunar surface that was ejected up into the cloud, but of that material, only about 2200 – 4400 kilograms actually reached the sunlight. So that’s the material that we were actually observing with the LCROSS shepherding spacecraft looking at the dust that got into the sunlight and was reflecting back into the instruments. We also take into account the field of view of the spectrometers: one-degree field of view. And so, looking at the band depth ratios of the near infrared spectra, we estimate that we have on the order of 145 kilograms of water ice. From our second method of determining how much water, which is the ultraviolet visible spectra of the OH emission, we estimate that we have about 110 kilograms of water, so these two numbers are roughly in the same ballpark, which is great to have somewhat consistent numbers from two independent methods, which increases the robustness of the results.

What we’ve done is we’ve looked at the water in the field of view of our instruments and the total amount of dirt in the field of view of our instruments, and we came up with a number of about 5 ½ percent by weight water ice in regolith, in the dirt. So if you were to dig up with a shovel a piece, a patch of the dirt where we impacted, you’d have about five percent water ice. For comparison, the Sahara is maybe two or three percent water, so at the LCROSS impact site, we are almost two times wetter than the Sahara Desert, which is quite impressive.

What was seen from Earth?

For Earth-based observations, we had over 25 professional observatories that were observing the LCROSS impacts. They were collecting data: imagery data, spectral data, trying to corroborate what we learned from the LCROSS shepherding spacecraft. From the ground we had several confirmations of the sodium cloud that was also seen with the LCROSS shepherding spacecraft. We have tentative identifications from the Hubble Space Telescope (these are still pending further analysis because there are still calibration work that's being done on that data), and then also several of the observatories were looking for the dust cloud, but most observatories didn't see the dust, and we're still working to understand why that was the case. Part of the reason is because the LCROSS impact as viewed from Earth occurred behind a large ridge, which blocked the view of the impact site. So that meant that the dust had to rise even higher above the lunar surface in order to be seen. And as we know there's less dust that reaches these higher elevations. Also we know that the dust cloud was not very thick; it was optically thin, so it was not very dense, which gives you fewer particles to reflect sunlight off of, and that way makes it more difficult to see from the ground.

What have we learned?

So in the end, LCROSS discovered water, and we discovered a lot of water on the Moon in this particular area, the Cabeus Crater, where LCROSS impacted. And so, with almost every mission, now we have even more questions then when we started. And so we’re getting smarter, and we’re learning more about the moon. So now we’d like to learn about other areas on the Moon, and look at those other permanently shadowed regions that show us possibly different things. So there are still more things to do in the exploration of the Moon, and we’re just getting started. But for now, we know that the permanently shadowed region at the lunar South Pole does contain significant amounts of water, and we’ve learned that from this LCROSS mission.

What else was found?

One of the other most fascinating things, real surprises for me, was how much other, other than dirt, there was at the impact site where the LCROSS impact occurred. We were looking for the source of the hydrogen, so we were expecting water, maybe some other hydrogen bearing compounds, minerals maybe, but what we found was a lot of other things. Between our measurements on the LCROSS shepherding spacecraft and those on the LRO spacecraft made by the LAMP instrument and the ultraviolet spectrometer, we saw molecular hydrogen, just pure H2 coming up out of the ground, we saw carbon monoxide, we saw sulphur dioxide, hydrogen sulphite, carbon dioxide (did I say carbon dioxide?). We saw methane. We saw ammonia. We saw other lighter hydrocarbons. We saw all kinds of volatiles and other compounds, other than dirt, in the impact site. All in all, when you added everything up, there was as much as 20% other material, other compounds and volatiles where we impacted besides just dry lunar dirt. That was a real surprise.

We also found light metals like mercury. The LAMP instrument on LRO observed a significant amount of mercury coming up out of that crater – about 1 ½ % by weight mercury – that’s remarkable. We found other light metals like silver and sodium coming up also up out of the crater. At first we were all quite startled – Wow. But when you think about it these craters are so cold, 40 degrees Kelvin, that’s minus 220 degrees or more below 0 centigrade, all kinds of things will freeze up there and what we found was these light metals that have a relatively low vapor pressure, meaning they can be vaporized at very low temperatures, relatively low temperatures for a metal, are probably hopping around the lunar surface, or come in on an asteroid or a comet, and actually then condense out, freeze out in these dark craters. That’s what we’re seeing here, and that’s pretty remarkable.

What was the soil like at the impact point?

An impact is a very good experiment actually when you want to learn about a particular surface. One of the most amazing things in my mind is how fluffy the soil appears to be where we impacted. The Centaur rocket, that part that we impacted, was about 12 meters long, 2 ½ meters wide, a couple of tons worth of metal, and it was moving very, very, very fast. We measured how quickly it heated the surface; we measured what’s called the thermal impact flash, that’s how quickly the heat from the compaction of that object, into the dirt occurred and released heat that we could measure. That took a full 3/10ths of a second to occur on the LCROSS impact, meaning, this large rocket was burrowing into the dirt before it started to compact, crush and get hot. That means the surface was very fluffy; we think it was maybe as much as 70 % porous. Most lunar soils are maybe about 50%, so it’s certainly more porous than the typical lunar regolith. And that might be understood in the sense that we saw so many other volatiles and ices there. We saw maybe, as I mentioned, 20% by weight other things besides dirt. So it could be that you have this thin low density frost covering the surface – very low density, high porosity, and then below that you have dirt and ice mixed together.

Is water distributed evenly at the poles?

The other important conclusion we could draw from the concentration of water we’ve seen at the LCROSS impact site, because 5 ½ % by weight is that water is not uniformly distributed around the poles. If you look at the neutron data sets, spectrometers that measure how neutrons come up from the poles, and they look at how the water would have to be distributed at these concentrations to explain those data sets, one has to conclude that the water can’t be uniformly distributed at 5%; you would have a very different picture from a neutron standpoint than what we have now. So what it says is that the water is actually concentrated, certainly in the Cabeus region where we impacted and a relatively small localized pocket. Exactly how small? We’re working on that question right now actually. What it means also is that the rest of the pole of the Moon is probably not any more uniform; it’s probably just as heterogeneous, so you cannot just go to any cold crater and hope to find water. You have to actually go and look for it. There will be these oases throughout the poles with lots of water in them, and lots of other things, but they aren’t everywhere.

Where did these volatiles come from?

When we look at the total complement of species of compounds that we’ve seen: the water, even the form of the water – the fact that it’s relatively pure water ice grains; when we look at the methane, when we look at the carbon dioxide, the sulphur dioxide, and we look at how all those species relate, their total concentration relate to water, meaning their relative abundances to water, we can start to formulate an idea of what’s going on in terms of the source of these species. What we’ve concluded is you’ve got a variety of different species; it’s not all just from the solar wind, protons from the solar wind making water in illuminated parts of the Moon that then migrate to the poles and get trapped can’t explain the concentrations, the other species we see or the relative abundances to each other. What we think we need are other sources like meteoritic or cometary inputs, but we also think that there is in general chemistry, continued chemistry going on here. So that even, regardless of the source of materials, they’re evolving, even in these dark craters. I was really impressed recently when somebody showed a laundry list of compounds they’ve seen at Enceladus, the moon of Saturn. That list of materials, compounds they’ve seen there are very similar to what we’ve seen in the LCROSS impact; and what both groups are talking about is cold ice or cold grain chemistry that you can put in similar blends of materials, chemistry occurs, and you basically come up with the same soup of various compounds.

What did we see on approach before impact?

What we’re learning is that the Moon is much more complicated and interesting than some of us might have thought prior to the LCROSS and LRO missions. One of the things that we saw as we came in was emission lines, lines from various species, compounds as we were looking down on the Moon. We kind of scratched our heads a little bit because nothing had happened yet; we hadn’t yet impacted, yet we were seeing the signatures of these compounds above the dark, cold lunar surface. That suggested to us that there is actually a lunar atmosphere beyond what we’ve ever appreciated before there. Perhaps, actually what we’re seeing there is energetic particles reacting with the cold frost down in that dark crater, creating sputterings, species coming up out of the dark craters and creating a little localized atmosphere or exosphere above the crater itself. This is a brand new finding that we’re really looking into now and trying to finish the analysis on. We had hints of it when we flew by the Moon. About four months prior to impact we flew by the Moon, we turned on our instruments for calibrations, and we saw lots of different species in the lunar atmosphere – lots more than anyone had ever seen before. So, again, it shows how discoveries can be made when you go in with your eyes wide opened. We had an instrument suite that was really designed to do that: to go with its eyes wide opened, not focused on one part over another too much but really open it up so we might be able to discover some new things – and I think that’s what we’ve done.

Has LCROSS changed our view of the Moon?

What we’ve learned over the last year and a half / two years, is that the Moon is not our parents’ Moon. There appears to be a very active, continued, and sustained hydrological, or I should say hydrocycle (water cycle) on the Moon. So what M-Cube, the Moon Mineralogy Mapper on the Chandrayaan spacecraft saw, along with measurements from the Cassini spacecraft and the EPOXI spacecraft is water and hydroxyl (OH) distributed on the sunlit side of the Moon. That was a marvelous discovery, and apparently is indicating some kind of production with solar protons, solar wind coming in, reacting with the lunar surface, making water and hydroxyl. Apparently, too, there’s evidence that it’s actually moving, it’s dynamic -- it depends on the time of day. Then, if you look at the results from LCROSS, where we see water ice and high concentrations, and a variety of other compounds in the permanently shadowed craters, and even go beyond that to some measurements made with radars that have been published about the North Pole of the Moon by the Chandrayaan spacecraft, that are indicative of large blocks of ice, you’re seeing water from all kinds of places and all kinds of concentrations, and all kinds of forms, either absorbed on the grains like in the sunlight, or crystal and water ice in the polar shadows. And what this is really pointing to is a water cycle on the Moon, a water cycle that’s active, dynamic, ongoing, and nothing like what we ever thought was there before. This kind of water cycle I would say is dynamic, and as wet as some of the deserts, or wetter than some of the deserts on Earth. It certainly was not something we were anticipating, but I have really had a lot of fun discovering.

Where do we go from here?

To improve our better understanding of this water cycle on the Moon, we need more measurements. It’s clear that the atmosphere of the Moon, the exosphere, is important to understand how water produced in the sunlight regions as observed migrates its way to the cold traps, and we need to understand just how that works, what time-scales that occurs over, how much is moving, etc. That will help us understand what the distribution of water is within the polar cold traps, or what portion of the water we observed on LCROSS is due to this mechanism.

The LADEE spacecraft, the Lunar Atmospheric and Dust Environment Explorer, will make some of these critical measurements. It will be able to measure hydroxyl in the lunar atmosphere as a function of time of day, so if indeed the sun is actually producing water and producing hydroxyl on the lunar surface, we’ll be able to look for that with the LADEE spacecraft and understand it as a function of time of day.

Why didn't we see a visible flash on impact?

When we first impacted, one of the first things that we were looking for was an optical flash. That’s the flash of light that comes when things get really hot, like 5,000 degrees Kelvin. It’s what produces the visible light when you look at the sun. What we saw was a very, very little visible optical flash. We saw a thermal, a near infrared flash, but we didn’t see the visible flash. This was our first clue that we hit some place pretty interesting. There are a couple of things that can explain why we didn’t see that flash:

1. We obscured the flash with dust. We hit a very low-density target with a low-density rocket. When you do that you create a plume of material that comes up actually faster than the heating occurs down below, so you can actually obscure the dust in front of the flash. We’ve shown this with experiments here on Earth, here at NASA Ames in the vertical gun.

2. If you have even the slightest amount of volatiles, even a half a percent or less, that squashes those high temperatures, because a lot of that energy that would otherwise go into the really high temperatures and visible light goes into changing ice to vapor, and that phase change takes energy. So you just need a little bit of volatiles to squash that flash.

We think we saw between 10 and 20 percent volatiles of all sorts: water and other things, so it’s easy to understand that if we hit someplace fluffy and full of volatiles, we just weren’t going to have any visible flash, and that’s what we saw.