According to Shia Islam prostration is accepted only is done on earth or something that can be considered part of earth. soSo if it is hard or soft is not important. butBut from what it is made is important. somethingsSomethings like leaves of plants, wood, stone, soil and such things that are from earth are accepted. partsParts of planets are considered from earth also. cottonCotton is considered part of earth too. soSo if the mate is made of cotton (although most of shiaShia scholars consider prostration on cotton Makruh ) or leaves of planets like leaves of dates tree is part of earth and so it is accepted. alsoAlso the mat prophet SAWW used for prostration was made of leaves of dates tree. prostrationProstration on soil of Karbala is highly recommended and has many great divine rewards.
Reference: