We report our Spitzer Space Telescope observations and detections of the binary neutron star merger GW170817 .
At 4.5 \mu m , GW170817 is detected at 21.9 mag AB at +43 days and 23.9 mag AB at +74 days after merger .
At 3.6 \mu m , GW170817 is not detected to a limit of 23.2 mag AB at +43 days and 23.1 mag AB at +74 days .
Our detections constitute the latest and reddest constraints on the kilonova/macronova emission and composition of heavy elements .
The 4.5 \mu m luminosity at this late phase can not be explained by elements exclusively from the first abundance peak of the r-process .
Moreover , the steep decline in the Spitzer band , with a power-law index of 3.4 \pm 0.2 , can be explained by a few of the heaviest isotopes in the third abundance peak with half-life around 14 days dominating the luminosity ( e.g . ^ { 140 } Ba , ^ { 143 } Pr , ^ { 147 } Nd , ^ { 156 } Eu , ^ { 191 } Os , ^ { 223 } Ra , ^ { 225 } Ra , ^ { 233 } Pa , ^ { 234 } Th ) or a model with lower deposition efficiency .
This data offers evidence that the heaviest elements in the second and third r-process abundance peak were indeed synthesized .
Our conclusion is verified by both analytics and network simulations and robust despite intricacies and uncertainties in the nuclear physics .
Future observations with Spitzer and James Webb Space Telescope will further illuminate the relative abundance of the synthesized heavy elements .