Density of liquid ethane at -90.2 °C is 548.0 kg/m3. Data comes from tables at the end of:
The Properties of Gases and Liquids, 4th edition. By Robert C. Reid, John M. Prausnitz, and Bruce E. Poling. McGraw-Hill, Inc.,11 West 19th St., New York, NY 10011. 1987.
API Technical data book shows curve:
Temp (°C) Density (kg/m3)
-100 560
-75 537
-50 500
-25 462
0 420