Nasa scientists have been outlining their preliminary results after crashing two unmanned spacecraft into the Moon in a bid to detect water-ice.
A rocket stage slammed into the Moon's south pole at 1231 BST (0731 EDT).
Another craft followed just behind, looking for signs of water in debris kicked up by the first collision.
Instruments on the second spacecraft identified a flash from the initial impact as well as a crater, but the expected debris cloud was not evident.
The $79m (£49m; 53m euro) US space agency mission is known as LCROSS (the Lunar Crater Observation and Sensing Satellite).
The first collision was expected to throw some 350 tonnes of debris up to altitudes of 10km (6.2 miles) or more.
No such dust plume was apparent in images sent back by the spacecraft, however, proving a disappointment to some watchers expecting a space spectacular.
"We need to go back and carefully look at the data to see what it says," Dr Anthony Colaprete, principal investigator on the LCROSS mission, told journalists at a post-impact news conference.
"Exploration has surprises. I'm glad we built our mission plan around all aspects of the impact what's streamed on the video is not at the same fidelity as what we get fresh off the spacecraft. We need to look more closely before we conclude anything about an ejecta cloud, or not."
Wait for water
Dr Colaprete, from Nasa's Ames Research Center in California, added: "I see something in the spectrometer data - the spectrometers are more sensitive than the cameras. But I can't say anything more than that."
Earlier on Friday, Dr Vincent Eke, from Durham University, UK, told the BBC: "The interesting thing is whether the debris that gets thrown out will actually get thrown out of the crater and into the sunlight.
"Nasa predicts that the debris should rise [up to] nine miles, which should certainly take it over the crater rim. But if, for some unfortunate reason, the debris doesn't get into the sunlight, we won't be able to see it, which will be disappointing."
The LCROSS team was able to determine the temperature of the crater punched in the lunar surface.
But questions about the persistence of water-ice on the Moon will have to wait.
Dr Colaprete said: "We saw the impact, we saw the crater. We got good spectroscopic measurements which is what we needed of the impact event."
"We have the data we need to address the questions we set out to address."
The identification of water-ice in the impact plume would be a major discovery, not least because a supply of water on the Moon would be a vital resource for future human exploration.
LCROSS was to have helped pave the way for US astronauts to return to the Moon by 2020.
But these lunar plans have been under scrutiny since President Barack Obama ordered a sweeping review of Nasa's manned spaceflight programme.
Dr Bernard Foing, executive director of the International Lunar Exploration Working Group (ILEWG), said it would be desirable to protect some of the polar ice, if it indeed exists.
"We will have to be careful to keep some areas as 'protected parks' on the Moon. This is so that we could, for instance, send a lander, drill down and obtain a core sample a few metres in depth," he told BBC News.
"Then we could eventually study the history of delivery of water to the Moon and the Earth."
The LCROSS mission consisted of an empty Centaur rocket upper stage and a "shepherding spacecraft".
The shepherding spacecraft was designed to guide the rocket to its target at the Moon's south pole, a shaded 100km-wide depression called Cabeus crater.
The Centaur hit the lunar surface first, at roughly twice the speed of a bullet.
With an energy equivalent to one-and-a-half tonnes of TNT, the collision was expected to carve out another crater inside Cabeus measuring some 20m (66ft) wide and about 4m (13ft) deep.
The shepherding spacecraft ploughed in behind, hitting the Moon four minutes later. Its onboard spectrometers were designed to look for signs of water in the debris kicked up by the Centaur collision.
It was looking for hydroxyl compounds (OH), salts, clays, hydrated minerals and organic molecules in the expected plume.
The impacts were watched by countless professional and amateur astronomers on the ground. They were also observed by Earth-orbiting satellites, including the Hubble Space Observatory and Odin, a Swedish-led astronomy and aeronomy mission.
But the Lunar Reconnaissance Orbiter (LRO), which was launched on the same rocket as LCROSS in June, has been playing a key supporting role.
Dr Jennifer Heldmann, who co-ordinated the LCROSS observation campaign, commented: "LRO has been taking observations during the impact - we have heard that this went well. They are also doing follow-up observations; they are in a great position to collect data that is complementary to that collected by the LCROSS shepherding spacecraft."
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