Any school kid can tell you that comets are made of ice. That frozen water burning off is what gives comets their characteristic tails. But asteroids were generally thought to be dry. Or at least frost-free. Now two studies published in the journal Nature [Andrew Rivkin and Joshua Emery, http://bit.ly/bkL3DU and Humberto Campins et al, http://bit.ly/bI8Eqa] suggest that notion may be all wet. Because at least one asteroid appears to be coated by a thin layer of ice. And just that kind of asteroidal frosting could have been the source of our water here on Earth.

The asteroid belt just outside Mars is home to a lot of rocky bodies…including a behemoth with a diameter of 129 miles. This asteroid, called 24 Themis, is one of the largest in the belt and has attracted the attention of astronomers because its nearby traveling companions look like comets.

In the latest studies, scientists used an infrared telescope to spectroscopically examine the asteroid’s surface. And the chemical signature they saw was a good match for water ice, which they found all over the asteroid.

The find provides support for the theory that Earth’s water could have been delivered as ice by an asteroid. Lucky for us we could borrow some from the neighbors.

—Karen Hopkin

[The above text is an exact transcript of this podcast.]

See also Water Ice Found on the Surface of an Asteroid for the First Time

[Scientific American is part of Nature Publishing Group.]