This paper reports an analysis of the thermal properties of ejecta layers of single- and double-lobe impact craters on Mars. First observations of thermal properties were made at low resolution from the Phobos'88 mission and did not allow mapping of variations of thermal properties inside the ejecta layer. The THEMIS instrument on board the Mars Odyssey mission provides new high-resolution thermal mapping of the surface of Mars. From these data we observe a systematic temperature increase at night at the edge of the ejecta. We evaluate first the possible influences on the surface temperatures at night of postimpact modification processes given the topography of an impact crater and its ejecta. We show that the observed thermal signature is more likely related to a particle size distribution inherited immediately after the impact event during the emplacement of ejecta. We propose that the kinetic sieving process, observed in experimental and natural granular flows, like pyroclastic flows, is responsible for the accumulation of larger particles at the ejecta flow front and thus is responsible for the temperature increase. Despite evidence of aeolian activity at Syrtis Major, these craters offer an example of preserved surface physical properties resulting from geological processes which have occurred since the formation of the volcanic shield of Syrtis Major. Further studies are underway with the Mars Express ongoing observations produced by the HRSC and OMEGA instruments in order to explore the interplay between the surface physical properties and the spectroscopic signatures seen at Syrtis Major.