Since meteors are so ubiquitous in our nighttime skies (you'll probably see at least a half dozen an hour on a good dark night), it stands to reason that meteors should zip across the Martian sky as well even though the atmosphere there is far more tenuous than Earth's. Theorists have predicted that meteors should streak over Mars at an altitude of roughly 50 miles (80 km).
"So," I can hear you asking, "can the rovers Spirit and Opportunity see them?"They've certainly tried. As Cornell astronomer Jim Bell explains in S&T's August 2006 issue, the rovers' cameras can serve as half-decent low-power telescopes, and they've had modest success recording star fields and even eclipses of the little moons Phobos and Deimos.
But recording meteors is far more challenging. One candidate was the subject of a short report in a 2005 issue of Nature. Since then, a reanalysis by Andrea Domokos (University of Western Ontario) and six others suggests that it was probably just a grazing cosmic ray in the camera. The problem, Bell explains, is that the rover cameras weren't designed for low-light sensitivity, and cosmic rays have a nasty habit of leaving streaks on the detector chips that mimic meteors. Spirit even got up well before dawn on several Martian mornings in 2005 to try to record flashes from gritty debris shed by Halley's Comet as Mars passed through the stream. No luck.
Fortunately, Martian meteors do more than just give off light. They ionize the rarefied air around them, creating a radio-reflecting plasma that has been detected repeatedly with experiments on both the late, great Mars Global Surveyor and the still-operating Mars Express orbiters. Boston University astronomer Paul Withers has slogged through thousands of ionospheric profiles compiled by these spacecraft, and he's found solid evidence for meteoric activity in about 150 of them.
Withers discovered that Martian meteors seem to come in batches, as on Earth, and he's identified 10 candidate meteor showers. Eight of these correspond to comets or asteroids that other astronomers had already flagged as candidate parent bodies for meteor activity on Mars; the other two are still awaiting a match.
The most dramatic meteor outburst occurred in late 2004, but its source is ambiguous. It could be debris from Comet C/1998 U5 (LINEAR), whose orbit was within 200,000 miles of Mars at that time. But that comet has a 1,043-year-long orbit, and any particles it ejected might not hang together for that long. The other candidate is Comet 144P Kushida. It takes just 7.6 years to circle the Sun but doesn't come nearly as close to Mars's orbit.
Now I hear you asking, "Will Phoenix look for meteors once it lands on Mars next May?" No; its robotic eyes won't be very sensitive at night. Much more promising will be the cameras on NASA's forthcoming Mars Science Laboratory, due to touch down in October 2010.