| 1 | Indeed, two iridium layers have been found in some places. | |
| 2 | They' re also seeking a cheaper catalyst than iridium. | |
| 3 | They' re also seeking a cheaper catalyst than iridium. | |
| 4 | They were especially interested in a 30-ft. layer of sediment just above the iridium layer. | |
| 5 | Change the 20-nanometer iridium to same-sized soot particles and the uptake rate falls by 75 percent. |