LCROSS was a NASA mission that was
sent to the moon. We impacted the south pole of the Moon,
in a permanently shadowed region looking to see if there
is water ice or not on the Moon. So today we're going to
discuss some of the scientific results from LCROSS, and we
have a whole wealth of data that was collected from both
the shepherding space craft that followed behind the impactor
and also from some of the ground-based and space-based observing
assets. So the area where LCROSS hit in the Cabeus proper
crater was on the near side of the Moon so that it could
be seen from Earth. We also had several space-based assets
including the Lunar Reconnaissance Orbiter that was in orbit
around the Moon looking at the impact, the Hubble Space Telescope
was also looking at the impact, and a whole slew of professional
and amateur astronomers here on the Earth that were looking
at the impact as well.
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.
did LCROSS get the pictures?
On the LCROSS payload we had nine different instruments and the two real workhorse instruments for the water measurement were the near infrared spectrometer, and the ultraviolet visible spectrometer. So here you can see the apertures or the openings of those instruments that were pointed down towards the Moon, collecting data as the shepherding spacecraft got closer and closer to the plume and closer and closer to the Moon.
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
we make a crater?
So, as the shepherding spacecraft
descended toward the lunar surface, we collected images with
the near infrared camera, and we were able to image these
permanently shadowed regions, which are areas of the Moon
that have never been seen before because they are in permanent
shadow and they're not illuminated. And also as the shepherding spacecraft came down towards the moon, we were able to take a picture of the impact crater that was created by the Centaur impact. So now we can tell the size of the crater that was created by the LCROSS impact, and we have images of it, which is really great. We know that the crater that we've
identified in the near infrared camera is actually the Centaur
impact crater because we can match the data that we have
with the near infrared camera with the data that we have
in the thermal cameras from the shepherding spacecraft. We
can see the heat created by the impact. That crater is warmer
right after the impact actually formed, and so we can match
up; also based on the navigation information from the shepherding
spacecraft, we can tell exactly where the crater was created
because we know exactly where that impact occurred on the
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.
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.
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.
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.
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.
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.
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.
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.
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
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.
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
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.
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.