The passenger, a resident of Scottsdale, Ariz., also texted or emailed his son about what he saw, and the son then called the Massachusetts State Police to report the incident.
Prosecutors say Smith allegedly attempted to erase the images after a flight attendant told him to turn off the computer, according to The Boston Globe.
Massachusetts state troopers met Smith's plane at about 4:30 p.m. Saturday at Logan International Airport.
During an interview, Smith granted written consent for a trooper assigned to the Internet Crimes Against Children Task Force to view his MacBook Pro, which he said was purchased through a research grant from the University of Utah, according to a news release from Suffolk County District Attorney Daniel F. Conley.
Smith allegedly said he was the sole user of the computer.
Troopers recovered from the computer a number of still images depicting young girls, some naked and some engaged in explicit sexual activity with adult males, Conley said in the news release. Based on captions embedded in the images and troopers' estimates, the children depicted were between the ages of 5 and 14 years old.
"These weren't photos of a child in the bath that a parent might keep," Conley said. "These were explicitly sexual and extremely disturbing."
The Associated Press reported that Massachusetts state police spokesman David Procopio said in a statement: "Child pornography is a form of child sexual abuse nothing less. Those who possess it a crime unto itself foster an evil network that sexually abuses and exploits children irreparably."
Smith's laptop and iPhone were seized as evidence. Investigators will seek a search warrant to conduct a thorough forensic examination of their contents, Conley said.
Under Massachusetts law, even possessing child pornography is punishable by up to five years in state prison, Conley said. A person convicted of that offense must also register as a sex offender.
On Monday, Smith was arraigned in East Boston District Court, where Judge Kenneth V. Desmond set bail at $75,000.
Prosecutors had asked the judge for a bail of $150,000. But Smith's attorney, Patrick Murphy, argued for a lower amount, saying Smith has no prior criminal record and has been at the U. for 14 years, The Associated Press reported.
The judge imposed a number of conditions if Smith is freed, including no unsupervised contact with children under 16 and no use of the Internet except for professional purposes. Smith also must allow probation or law enforcement officers to search his personal and work computers and digital media at any time, according to Conley.
Smith's next court date is Dec. 27.
Smith went to Boston to attend the Materials Research Society's weeklong fall meeting and exhibit, scheduled to start Monday, according to The Associated Press.
All of Smith's post-secondary education occurred at the U., where he earned his doctorate in 1990. He is a specialist in molecular dynamics working with the U.'s Center for the Simulation of Accidental Fires and Explosions, or C-SAFE, an interdisciplinary research group funded by the U.S. Department of Energy, as well as his own research lab. His current salary is $170,000, according to Utahsright.com.
A statement from the University of Utah said Smith had been placed on administrative leave pending the outcome of the criminal investigation.
"Professor Smith deserves a full and fair investigation into this issue," the statement said. "The University of Utah, however, has no tolerance for the viewing or possessing of child pornography by any of its employees, regardless of where it occurs."
If the allegations against Smith are discovered to be true, the university will "immediately seek Professor Smith's dismissal," the statement said.
"We are making all appropriate measures to cooperate with the investigation and contain evidence of criminal activity," a university spokesperson said.
The spokesperson said the laptop Smith was using belongs to university if it was bought with grant money. The spokesman said he did not know how Smith's travel arrangements were paid, but noted that the university's policy is not to reimburse for first-class accommodations.
Coincidentally, a Utah judge on Friday signed Smith's divorce decree against his wife. Smith filed for divorce in September, citing irreconcilable differences. The Smiths have two children, ages 8 and 11, according to court documents.
Reporters Brian Maffly and Nate Carlisle contributed to this article.