1: Mars once had a mild, wet climate similar to early Earth.
2: If Mars was originally Earth-like, then conditions must have been favorable for the origin of life.
3: Where conditions are favorable for the origin of life, life usually emerges.
4: Life originated on Mars at an early date.
5: Once life begins, it becomes extremely versatile, robust, and tenacious.
6: Life exists deep within the Earth's crust.
7: Mars still has much water deep below the surface.
8: Hence, since life once probably existed on Mars, it persists and continues to exist deep within the moist crust of Mars.