JUST HOW much does the Constitution protect your digital data? The Supreme Court is only just coming to grips with that question. On Tuesday, it will consider whether police can search the contents of a mobile phone without a warrant if the phone is on or around a person during an arrest.

California has asked the justices to refrain from a sweeping ruling, particularly one that upsets the old assumption that authorities may search through the effects of suspects at the time of their arrest. Even if the justices are tempted, the state argues, it is hard for judges to assess the implications of new and rapidly changing technologies.

The court would be recklessly modest if it followed California’s advice. Enough of the implications are discernable, even obvious, that the justices can and should provide updated guidelines to police, lawyers and defendants.

They should start by discarding California’s lame argument that exploring the contents of a smartphone — a vast storehouse of digital information — is similar to, say, rifling through a suspect’s purse. The court has ruled that police don’t violate the Fourth Amendment when they sift through the wallet or pocketbook of an arrestee without a warrant. But exploring one’s smartphone is more like entering his or her home. A smartphone may contain an arrestee’s reading history, financial history, medical history and comprehensive records of recent correspondence. The development of “cloud computing,” meanwhile, means that police officers could conceivably access even more information with a few swipes on a touchscreen.

Americans should take steps to protect their digital privacy. But keeping sensitive information on these devices is increasingly a requirement of normal life. Citizens still have a right to expect private documents to remain private and protected by the Constitution’s prohibition on unreasonable searches.

As so often is the case, stating that principle doesn’t ease the challenge of line-drawing. In many cases, it would not be overly onerous for authorities to obtain a warrant to search through phone contents. They could still trump Fourth Amendment protections when facing severe, exigent circumstances, such as the threat of immediate harm, and they could take reasonable measures to ensure that phone data are not erased or altered while a warrant is pending. The court, though, may want to allow room for police to cite situations where they are entitled to more leeway.

But the justices should not swallow California’s argument whole. New, disruptive technology sometimes demands novel applications of the Constitution’s protections. Orin Kerr, a law professor who blogs on The Post’s Volokh Conspiracy, compares the explosion and accessibility of digital information in the 21st century with the establishment of automobile use as a virtual necessity of life in the 20th: The justices had to specify novel rules for the new personal domain of the passenger car then; they must sort out how the Fourth Amendment applies to digital information now.