Supreme Court justices rarely talk about Martians.

But on Wednesday, extraterrestrials were at the heart of a case brought by a schizophrenic teenager who says he killed an Arizona police officer because he thought the lawman was a space alien.

Until now, the high court has avoided challenges to insanity defense laws, even as states around the country toughened their laws following John Hinckley's acquittal by reason of insanity in the 1981 shooting of President Reagan.

It was a surprise when justices agreed to review Eric Michael Clark's case, and they seemed uninterested Wednesday in broadly addressing the constitutional rights of psychotic criminal defendants whose lawyers want them sentenced to psychiatric facilities instead of prisons.

Court members, however, did repeatedly refer to the unusual facts of Clark's case, signaling that they are likely to rule very narrowly. He was a popular football star until he became convinced that aliens had taken over his town, Flagstaff, Ariz., as a "platinum city" and that his own parents were aliens.

Justices David H. Souter and Stephen Breyer both mentioned Martians.

Justices John Paul Stevens questioned whether someone who thought he was on a mission to kill space aliens could receive the death penalty for killing a person instead.

When Arizona lawyer Randall Howe said that the slain officer was wearing a uniform and driving a police cruiser, Justice Ruth Bader Ginsburg said that Clark's lawyer "wants to introduce [evidence] on the other side, 'I had delusions, I thought I killed an alien."'

The Supreme Court has never said mentally ill accused criminals have the right to an insanity defense, and four states do not allow for that: Idaho, Kansas, Montana and Utah.

The remaining states have a variety of standards for proving insanity, and Clark's lawyer argued that Arizona's is almost impossible to meet, violating the constitutional rights of mentally ill defendants.

Texas has a similar law and the court's ruling in Clark's appeal could affect Andrea Yates, the Houston mother who drowned her five children in the bathtub and goes on trial in June.

"The state has the right to define insanity as it sees fit," Howe told the justices.

Under Arizona's law, a defendant "may be found guilty except insane" if lawyers prove the defendant was so mentally ill that he did not know what he did was wrong. Many other states also consider a second factor, if a defendant understood the nature of his acts.

Clark shot Flagstaff police officer Jeff Moritz on June 21, 2000, after the officer pulled over the 17-year-old as he drove around his neighborhood in a truck playing loud rap music. Moritz, 30, was the only police officer ever killed in the line of duty in the mountain community north of Phoenix.

Clark's lawyer, David Goldberg, told justices that his client "was drowning out the voices in his head" with the loud music.

Clark had a trial before a judge in which he was found guilty of first-degree murder and sentenced to life in prison. Part of his appeal turns on whether the judge should have considered Clark's mental illness in weighing whether he intentionally killed the officer.

Justices agreed to hear the case late last year, in the final weeks before the retirement of Arizona native Justice Sandra Day O'Connor. O'Connor was in the courtroom and watched part of the argument.

Most of the questions justices asked Wednesday were technical, and the court might rule in a way that will affect only Clark's case.

Arizona Attorney General Terry Goddard said afterward that he was relieved by the tone of the questions.

"Our concern was they were going to put all the insanity issues on the table," he said.

Deborah Denno, a Fordham Law School professor, said insanity cases are difficult for courts because they involve imprecise matters of science.

"They don't want to get their hands dirty with this psychological evidence," Denno said. `They're going to have to confront this at some time."

Clark's parents were at the court.

"This is beyond our son. The outcome of this case could be far-reaching," said his mother, Terry Clark.

The case is Clark v. Arizona, 05-5966.