New Delhi, Sept. 19: A decision by the Indian Space Research Organisation (Isro) to keep the operations of a science instrument on Chandrayaan-1 as simple as possible has raised doubts about its surprising discovery of carbon dioxide in the lunar atmosphere.
Senior US scientists have challenged the Chandrayaan-1 missions discovery of carbon dioxide in the lunar atmosphere, contending that crucial instrument calibration data to clinch the discovery is missing from the Indian study.
Isro scientists had earlier this year announced their discovery of water and carbon dioxide molecules in the near-vacuum of the lunar atmosphere as reported in The Telegraph on March 22 and September 6.
The findings, based on measurements by an instrument called Chace on the moon impact probe hurled by Chandrayaan-1 towards the moon on November 14, 2008, were published in a peer-reviewed journal Planetary and Space Science.
But some lunar science researchers say the relative abundance of water and carbon dioxide is so large that calibration data to rule out other sources of these molecules becomes crucial.
Calibration would have generated background data allowing scientists to characterise the instruments performance and rule out outgassing —the release of gases embedded within the material of the spacecraft or the instrument in the lunar environment.
Since they didnt do a reliable calibration test ahead of time to rule out outgassing contamination, their results are unfortunately not conclusive, said Kurt Retherford, a senior research scientist at the Southwest Research Institute, Texas, US, and a member of a team exploring the moon through the Lunar Reconnaissance Orbiter (LRO), a robotic spacecraft launched by Nasa in 2009.
Five orbits prior to its 44-minute long descent, Isro scientists turned on the Chace instrument, while the moon impact probe was still on Chandrayaan-1. But this single period was not enough to provide reliable calibration measurements.
The extraordinary claims seem only tentatively supported by data, Retherford told The Telegraph. I would look forward to improved experiments in the Chandrayaan-2 mission.
Isro sources said the decision not to undertake calibration was taken to keep Chace operations as simple as possible as it was part of the moon impact probe and drew its power from a battery and not from the main spacecraft power source.
Repeated calibration while the spacecraft was in orbit would have required additional power and more complex circuitry. This was a first mission and we didnt want to add more complexity than required, said Rajagopal Sridharan, leader of the Isro team that reported the carbon dioxide discovery.
Any space mission involves trade-offs — here, it was between what we wanted to do as scientists and what was possible, given the overall Chandrayaan-1 missions objectives, Sridharan said.
The Isro team believes the abundance and the consistency of the measurements of carbon dioxide at different altitudes in the lunar atmosphere while the impact probe was falling towards the moon rule out outgassing.
Our measurements cannot be explained through simple outgassing from spacecraft components, said Syed Maqbool Ahmed, who was the project manager of the Chace payload, and has since moved to the University of Hyderabad.
The Chace study supports Apollo-era observations that had indicated carbon dioxide on the moon. But the Apollo observations had then been dismissed by US scientists as outgassing or contamination.
Sections of US scientists also question the abundance of carbon dioxide seen by Chace.
The (Indian) work presents intriguing results, said Randy Gladstone, another senior scientist involved in the LRO mission. The Isro results show the total pressure in the lunar atmosphere 100 times higher than expected. If the moon has as much water as indicated by Chace, the ultraviolet sunlight shining on it will produce enough hydrogen atoms per second — similar to a fairly active comet, Gladstone told The Telegraph.
For the moment, I will remain sceptical, he added.
But other space scientists have hailed the finding. The detection of carbon dioxide in the lunar atmosphere is very exciting, said Bernard Foing, lead project scientist for a European Space Agencys lunar orbiter SMART-1.