Today the Supreme Court heard oral arguments in Florida v. Jardines, which raises the question of whether using a trained dog to sniff for marijuana at the doorstep of a house constitutes a search that must be justified by probable cause. The case involves Joelis Jardines, who was arrested for growing marijuana after Miami-Dade police brought Franky, a drug-sniffing Labrador, to his home based on an anonymous tip. In previous cases, the Court has said probable cause is not necessary for canine inspections of luggage at an airport or the exterior of a car during a traffic stop, on the theory that "a 'sniff test' by a well-trained narcotics detection dog" is minimally invasive, revealing nothing but the presence or absence of contraband. But Fourth Amendment protections are at their strongest in the home and the area immediately surrounding it (the "curtilage"), and this morning Jardines' lawyer, Howard Blumberg, argued that "when a police officer goes up to the front door with a narcotics detection dog" he has "physically trespassed, because there is no consent to do that, onto a constitutionally protected area, the curtilage of the home, and performed a search."

That argument seemed to be crafted with Antonin Scalia in mind. In U.S. v. Jones, the January decision in which the Court ruled that tracking a suspect's car by attaching a GPS device to it amounts to a "search" under the Fourth Amendment, Scalia's majority opinion emphasized the physical trespass required to install the device. "If you…follow the test set forth in Jones and apply it to what happened here," Blumberg said, "it is a trespass." Scalia signaled that he was receptive to this approach even before Blumberg got up to speak, telling Gregory Garre, the lawyer representing Florida:

Police are entitled to use binoculars to look into [a] house if the residents leave the blinds open….[but] they're not entitled to go onto the curtilage of the house, inside the gate, and use the binoculars from that vantage point….Why isn't it the same thing with the dog?… It seems to me crucial that this officer went onto the portion of the house…as to which there is privacy and used a means of discerning what was in the house that should not have been available in that space…. Police officers can come there to knock on the door…[but] when the purpose of the officer's going there is to conduct a search, it's not permitted… He's going there to search, and he shouldn't be on the curtilage to search.

Scalia also wrote the majority opinion in Kyllo v. U.S., the 2001 case in which the Court held that using a thermal imager to measure the heat radiating from a home—evidence of high-intensity lamps used to grow marijuana—requires a warrant. Justice Ruth Bader Ginsburg read to Garre a passage from that decision, which she suggested applies to drug-sniffing dogs as well:

We think that obtaining by sense-enhancing technology any information regarding the interior of the home that could not otherwise have been obtained without physical intrusion into a constitutionally protected area constitutes a search, at least where, as here, the technology in question is not in general public use.

Garre argued that "Franky's nose is not technology," since he is merely "availing himself of God-given senses." But in its natural state, Franky's nose does not tell police when molecules of certain chemicals are floating through the air; that requires human intervention aimed at turning a descendent of wolves into a law enforcement tool. It would be odd if the constitutionality of using a smell detection tool hinged on whether it was made of metal or flesh and blood. The main point, it seems to me, is that Franky, like a thermal imager, enables police to find evidence they could not detect with their own unaided senses. (There is a dispute as to whether Jardines' growing operation was "in plain smell," as Assistant U.S. Solicitor General Nicole Saharsky, arguing for Florida's side, put it. One officer claimed, after the dog alerted to Jardines' door, that he could smell the plants too, but the dog's handler said he smelled nothing.)

Several justices were interested in how long it took Franky to zero in on the smell of marijuana, since people generally do not expect unannounced visitors to walk up the path to their door and, instead of knocking, poke around, with or without the assistance of a specially trained animal, looking (or smelling) for evidence of a crime. The two sides eventually agreed that the whole visit took five to 10 minutes, with the dog's sniffing and "bracketing" taking a minute or two. Justice Stephen Breyer highlighted the unusual nature of this visit:

Would a homeowner resent someone coming with a large animal sitting in front of his front step on his property and sitting there sniffing for 5 to 15 minutes?… You do have an expectation of people coming to your door, perhaps even with animals, perhaps even with binoculars, but not looking into the house, not looking into the house from the front step with the binoculars. Now, why is that unconstitutional? Because it's very unusual that someone would do that, and the homeowner would resent it.

Ginsburg noted that if the Court sides with Florida and the Obama administration, police could bring drug-sniffing dogs to "any home anywhere" at will. Police "could take a dog and go [up to] every house on the street, every apartment in the building." Garre and Saharsky agreed, saying the only restraints would be limited police resources and "the potential for community hostility" (as Saharsky put it).

Should the Court agree that the Constitution allows such olfactory dragnets, there remains one other viable safeguard. Garre conceded that if Jardines had posted a "No Dogs Allowed" sign, bringing Franky to his door would have been a Fourth Amendment violation. But how long will it take for police to start claiming that such signs are evidence of drug activity?

The full transcript of the oral arguments is here (PDF).