SCHEMA:
CREATE TABLE table_name_4 (diameter__km_ INTEGER, name VARCHAR, year_named VARCHAR)
USER:
Which Diameter (km) has a Name of alma-merghen planitia, and a Year named smaller than 1997?

QUERY:
SELECT MAX(diameter__km_) FROM table_name_4 WHERE name = "alma-merghen planitia" AND year_named < 1997