Many researchers have suggested that Mars once had a great ocean in the north.
Much evidence for this ocean has been gathered over several decades.
 New evidence was published in May 2016.
 A large team of scientists described how some of the surface in Ismenius Lacus quadrangle was altered by two tsunamis.
 The tsunamis were caused by asteroids striking the ocean.
 Both were thought to have been strong enough to create 30 km diameter craters.
 The first tsunami picked up and carried boulders the size of cars or small houses.
 The backwash from the wave formed channels by rearranging the boulders.
 The second came in when the ocean was 300 m lower.
 The second carried a great deal of ice which was dropped in valleys.
 Calculations show that the average height of the waves would have been 50 m, but the heights would vary from 10 m to 120 m.  Numerical simulations show that in this particular part of the ocean two impact craters of the size of 30 km in diameter would form every 30 million years.
 The implication here is that a great northern ocean may have existed for millions of years.
 One argument against an ocean has been the lack of shoreline features.
 These features may have been washed away by these tsunami events.
 The parts of Mars studied in this research are Chryse Planitia and northwestern Arabia Terra.
 These tsunamis affected some surfaces in the Ismenius Lacus quadrangle and in the Mare Acidalium quadrangle.
Many researchers have suggested that Mars once had a great ocean in the north. Much evidence for this ocean has been gathered over several decades.  New evidence was published in May 2016.  A large team of scientists described how some of the surface in Ismenius Lacus quadrangle was altered by two tsunamis.  The tsunamis were caused by asteroids striking the ocean.  Both were thought to have been strong enough to create 30 km diameter craters.  The first tsunami picked up and carried boulders the size of cars or small houses.  The backwash from the wave formed channels by rearranging the boulders.  The second came in when the ocean was 300 m lower.  The second carried a great deal of ice which was dropped in valleys.  Calculations show that the average height of the waves would have been 50 m, but the heights would vary from 10 m to 120 m.  Numerical simulations show that in this particular part of the ocean two impact craters of the size of 30 km in diameter would form every 30 million years.  The implication here is that a great northern ocean may have existed for millions of years.  One argument against an ocean has been the lack of shoreline features.  These features may have been washed away by these tsunami events.  The parts of Mars studied in this research are Chryse Planitia and northwestern Arabia Terra.  These tsunamis affected some surfaces in the Ismenius Lacus quadrangle and in the Mare Acidalium quadrangle.
Ismenius Lacus quadrangle